human colour vision - linköping universitysasgo26/tnm011/humancolourvision.pdfthe munsell system 37...
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Contents
INTRODUCTION 3A Brief History of Colour Vision 3Trichromatic Theory of Colour Vision 4Opponent-Process Theory of Colour Vision 4CIE System 5
LIGHT, THE VISUAL STIMULUS 8Electromagnetic Spectrum 8Photons 8Light Sources 9CIE Illuminants 10Light Material Interaction 11BidirectionalReflectance Distribution Functions 12
COLOUR STIMULUS 13Surface Reflectance 13Stimuli 14Metamerism 15Fluorescence 15
THE EYE 16Cornea 16Iris 16
Lens 17Retina 17
VISUAL SIGNAL PROCESSING 20Signal Processing In the Retina 20Lateral Geniculate Nucleus (LGN) 22Visual Receiving Area 22
SENSITIVITY CONTROL 23Spectral Sensitivity of Rods and Cones 23Light and Dark Adaptation 24Chromatic Adaptation 25Dark Adaptation of Rods and Cones 25
LIGHTNESS AND COLOUR CONSTANCY 27Lightness Constancy 27Colour Constancy 27Chromatic Adaptation 27Memory Colours 28
SPATIAL AND TEMPORAL PROPERTIESOF COLOUR VISION 29Spatial and Temporal Frequency 29Contrast Sensitivity Functions (CSF) 30The Oblique Effect 31Mach Bands 31Flicker 31
COLOUR-VISION DEFICIENCY 32Monochromats 32Dichromats 32Anomalous Trichromats 33Trichromats 33Colour Vision Tests 33
SUBJECTIVE COLOUR PHENOMENA 34Simultaneous Contrast 34Crispening 34Spreading 35Luminance Phenomena 35Hue Phenomena 36Surround Phenomena 36
COLOUR ORDER SYSTEM 37The Munsell System 37NCS 38DIN 39OSA 40
TERMINOLOGY 41Colour 41Hue 41Brightness and Lightness 41Colourfulness and Chroma 42Saturation 43Related and Unrelated Colours 43Achromatic and Chromatic Colours 43
REFERENCES 44
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1. IntroductionThe question of how, through the sense of vision, we are able to perceivecolour of remote objects has been raised repeatedly throughout recordedhistory. Early philosophers and scientists held very different viewsregarding vision and colour perception than those now accepted incontemporary vision science.
1.1 A Brief History of Colour Vision
Among the Greek philosophers it was widely believed that rays weredischarged from the eyes (emanation theory) and that tiny replicas ofperceived objects could be released by such rays, to be deliveredthrough the pupil of the eye and from there flushed through theoptic nerve to the sensorium in the brain.
In the fifth century BCE, Empedocles (493-433BCE) wrote thatthe eye functioned like a lantern, that light from the eye shiningoutwards would interact with the “outer rays” and thereby allowobjects to be seen.
Aristotles (384-322 BCE) propagated a different notion of colourvision. He thought that colour was based on the interaction ofstimulus brightness and ambient light level. He based this view onthe perception that the colour of a sunset changed as darkness set in.
In the Middle Ages the Arab scholar Alhazen (965-1040 CE)rejected the emanation theory. He correctly proposed that the eyespassively receive light reflected from objects, rather than emanatinglight rays themselves. He proposed a camera obscura model for thetransmission of light in the eye, but did not speculate on the basis ofcolour vision.
Leonardo da Vinci (1452–1519) came close to a full understandingof visual optics but was still convinced that the retinal image couldnot be inverted.
The German astronomer Johannes Kepler (1571–1630) was thefirst to understand the basis of image formation by positive lensesand was, thereby, able to conclude that there must be an invertedretinal image.
Sir Isaac Newton (1642–1727) demonstrated that the colours ofobjects relate to their spectral reflectance. He also stated correctlythat the rays of light are not themselves coloured; rather theycontain a disposition to elicit colour perceptions in an observer. Itwas Newton who gave the name spectrum to a strip of light shownthrough a prism and divided it into seven colours. The relationshipbetween light and colour was revealed by Newton’s experiment. Healso showed that the colours that compose white light could not befurther subdivided but they could be recombined to form whitelight. His conclusion was that colour is not the product of theexternal objects we see, but is a property of the eye itself. Thisprovided the foundation for modern theories of colour vision.
Alhazen (965-1040 CE)
Kepler (1571-1630)
Newton (1642-1727)
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1.2 Trichromatic Theory of Colour Vision
In the later half of the nineteenth century, the trichromatic theory ofcolour vision was developed based on the work of James Clerk Maxwell(1831-1879), Thomas Young (1773-1829), and Hermann vonHelmholtz (1821-1894). The trichromatic theory of colour vision proposesthat colour vision depends on three receptor mechanisms, each withdifferent spectral sensitivities [4][5]. The pattern of activity in the threemechanisms results in the perception of a colour. It was based on theresults of a psychophysical procedure called colour matching. One of themore important empirical aspects of this theory is that it is possible tomatch all of the colours in the visible spectrum by appropriate mixing ofthree primary colours. Which primary colours are used is not criticallyimportant as long as mixing two of them do not produce the third.
The trichromatic theory correctly explains one part of the colour visionprocess but the theory fails to explain several visually observedphenomena.
1.3 Opponent-Process Theory of Colour Vision
The opponent-process theory of colour vision was proposed by EwaldHering (1834-1918), who stated that colour vision is caused byopposing responses generated by blue and yellow, and by red and green[4]. It was based on the results of phenomenological observationsinvolving afterimages, simultaneous contrast, colour visualization, andobservations of the effect of colour blindness. These observations couldnot be accounted for by the trichromatic theory. For example, he notedthat there are certain pairs of colours one never sees together at the sameplace and at the same time. For example, a colour perception is neverdescribed as reddish-green or yellowish blue. He also observed that therewas a distinct pattern to the colour of the afterimages we see. Forexample that a red field generates a green afterimage and that viewing agreen field generates a red afterimage, and that analogous results occurfor blue and yellow. Hering also observed that people who are colour-blind to red also are colour blind to green, and that people who can’t seeblue also can’t see yellow. All these observations led to the conclusionthat red and green are paired and that blue and yellow are paired [4][5].
It was popular in the first half of the 20th century for authors to pit thetrichromatic theory against the opponent processes theory, but both thetrichromatic and the opponent-process theories were proved to becorrect. The reason for this is that the psycho-physical findings on whicheach theory was based were each reflecting physiological activity atdifferent places in the visual system. The trichromatic theory operates atthe receptor level and the opponent processes theory applies to thesubsequent neural level of colour vision processing. The modernopponent-colour theory of colour vision explains how the two theorieswork together. The first stage of colour vision, the receptors, is indeedtrichromatic, as hypothesized by Maxwell, Young, and Helmholtz [4][5].However, contrary to simple thrichromatic theory, the three signals arenot transmitted directly to the brain. Instead, the neurons of the retinaencode the colour into opponent signals. The outputs of all the threecone types are summed (L+M+S) to produce an achromatic response anddifferencing of the cone signals allows construction of red-green(L–M+S) and yellow-blue (L+M–S) opponent signals [4]. Thetransformation from LMS signals to the opponent signals serves to
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decorrelate the colour information carried in the three channels, thusallowing more efficient signal transmission and reducing difficulties withnoise. The three opponent pathways also have distinct spatial andtemporal characteristics that are important for predicting colourappearance.
1.4 CIE System
Until 1931, there were no way to get a quantitative measurementdescription of colour and colours could only be specified by appeal tophysical samples. In that year, the CIE (International Commission onIllumination) adopted a system of colour specification, which has lastedto present time. CIE has developed several colour systems based on anumber of extensive measurements and experiments on how humansperceive colours. The sensitivity functions for the three cones (L, M, andS) were obtained through an experiment were an observer looked at asplit screen with 100% reflection, i.e. a white surface. One half of thescreen was illuminated by a reference light source, and the other half wasilluminated by three light sources with red, green and blue light. Theobservers then tried to match the colour perceived from the referencelight source with the colour perceived from the three monochromaticlight sources by mixing them so that the two halves of the screen wereperceived identical. The experiment was repeated with different referencewavelengths, but the same intensity and different observers. This is thesimple form of colour matching and can be described by the followingequation:
Eq. 1.1: M *P *w = M * t
where M is the measurement matrix, P is the primary spectra matrix,w=(w1…wk) is the weight vector and t=(t1…tN) is the test spectrumvector.
Some of the reference colours could not be matched by any combinationof the three primaries. In these cases, light from one or more of theprimaries is added to the light of the reference colour. A match can thenbe achieved by adjusting the primaries in this configuration. Light that isadded to the reference colour can be considered to have been subtractedfrom the mixture of the primaries. The advanced colour matchingequation is given by:
Eq. 1.2: M *P *w1 = M * (t + P *q)
where w1 and q are the new weight vectors.
The mean value from these tests constitutes the CIE colour matchingfunctions, x( ), y( ) and z( ), which represents a standard observer, seeFigure 1.1. From the colour matching functions, the tristimulus values,CIE XYZ can be calculated, see Eq. 1.3. These values are normalized forthe current illumination so that a completely white surface always givesY=100:
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Eq. 1.3:
X = k R( )I( )x( )d
Y = k R( )I( )y( )d
Z = k R( )I( )z( )d
k =100
I( )y( )d
I( ) is the spectral power distribution of the incident light and R( ) isthe spectral reflectance of the object.
Figure 1.1. CIE colour matching functions, x( ), y( ) and z( ).
The colours can be represented in two dimensions by two chromaticitycoordinates, x and y, that are independent of lightness (a definition ofbrightness and lightness can be found in 12.3), see equation 1.4.
Eq. 1.4: x =X
X +Y + Zy =
Y
X +Y + Z
These coordinates can be plotted in a chromaticity diagram, see figure1.2. If all xy-coordinates for the pure wavelengths in the visible spectrumare plotted in the diagram, all will fall on a horseshoe shaped line, calledthe spectrum locus. The line that connects the end points of the spectrumlocus is called the purple line. The colours on this line are a mixture ofpure 380 nm (blue) and 770 nm (red) light. There is a white point in themiddle of the chromaticity diagram where x=y=1/3.
WAVELENGTH (nm)
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Figure 1.2. CIE chromaticity diagram.
Although very useful, there are many limitations to this system. For onething, the colour matches that it predicts apply only to a hypotheticalstandard observer, and not exactly to any particular human being. Foranother, it is valid only for restricted conditions of viewing with smallfields that are neither too bright nor too dim. And, finally, thechromaticity diagram does not represent colour appearance very well,and although there is really no reason why it should, it has often beenused for this purpose [5].
x CHROMATICITY
500nm
600nm
550nm
700nm
Purple Line
White Point
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2. Light, the Visual StimulusLight provides the electromagnetic energy required to initiate visualresponses, i.e. it is the visual stimulus. Since the perception of colourbegins with light, the colours that are perceived are influenced by thecharacteristics of the light source.
2.1 Electromagnetic Spectrum
The receptors in our eyes are designed to receive and processelectromagnetic energy from a very narrow band of energy within theelectromagnetic spectrum that encompasses wavelengths between about380 and 750 nm [5]. The wavelengths within this interval and theirmixtures is called light, and light is the primary stimulus for colourvision. The energy in this spectrum can be described by its wavelength,i.e. the distance between the peaks of the electromagnetic waves. Thewavelengths are associated with the different colours of the spectrum, seeFigure 2.1.
Figure 2.1. Wavelengths and associated colours of the electromagnetic spectrum.
2.2 Photons
Light consists of photons, which are indivisible units of radiant energy.The amount of energy associated with a photon of wavelength is:
Eq. 2.1: E =hc
where E=energy, h=6.626x10-34J·s (Planck’s Constant), c=2.997x108m·s-1
(velocity of light), and =wavelength.
The brighter a light is, the more photons are contained in it. Becausephotons are discrete packets of energy it is not possible to absorb afraction of a photon. When a photon is emitted from a source itimmediately moves at the speed of light [5].
If a photon moves with frequency v and in a plane perpendicular to itsdirection of travel at the speed of light c, then some distance will betraversed during the time required for the particle to move through onecycle. This distance is called wavelength and it is inversely proportional tothe frequency:
Eq. 2.2: =c
where c=2.997x108m·s-1(speed of light) and v=frequency.
400nm 500nm 600nm 700nm
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2.3 Light Sources
All visual perception requires a source of illumination that irradiates theobjects that are seen. Because colour begins with light, the colours thatare seen are influenced by the characteristics of the light source used forillumination. Two important concepts are Correlated Colour Temperature(CCT) and Colour Rendering Index (CRI). Colour temperature is asimplified way to characterize the spectral properties of a light source,while colour rendering index is a way to determine its quality.
The Correlated Colour Temperature (CCT) of a light source is defined asthe absolute temperature of a blackbody radiator (an “ideal”,hypothetical, body which absorbs all radiation falling on it) whichproduces the chromaticity nearest to that emitted by the light test source.It is measured in Kelvin (K) [4]. The CCT rating is an indication of how"warm" or "cool" the light source is. Low colour temperature implieswarmer (more yellow/red) light while high colour temperature implies acolder (more blue) light. Some different colour temperatures and theircorresponding colours are shown in Figure 2.2.
Figure 2.2. Different colour temperatures and their corresponding colourappearance.
CRI (Colour Rendering Index) is a measure of how well the colours arereproduced by different light sources in comparison with a referencelight source (typically a black body) at the same colour temperature. CIEhave defined a method for how to determine CRI for light sources wherethe measure is graded from 1-100. Within this scale a CRI of 100(optimal CRI) means that a sample illuminated with the light source isperceived to have the same colour as when illuminated with a referencelight source. If a light source have a low CRI (50-60) it can cause severecolour distortions. It is preferable to have a light source with a CRI over90 [2][6].
The natural illumination, sunlight, is the most important source ofillumination. The solar energy is emitted from the sun and afterinteraction with the earth’s atmosphere it reaches the earth. Parameterssuch as solar elevation angle and atmospheric conditions will affect theoverall intensity and spectral characteristics of direct solar illuminationthat, under normal conditions is the dominant source of illumination[5].
The production of artificial illumination originally required thatsomething be burned in open air, for example the flame of a candle. Itsspectral output in the shorter wavelengths of the visible spectrum isdeficient, relative to that of daylight. In general, the same is true forincandescent lamps (ordinary light bulbs) in which a filament is heateduntil it glows [5]. When operated at very low current, no visibleradiation is produced by such a lamp. As the applied voltage is increased,
8000 K
7000 K
6000 K
5000 K
4000 K
3000 K
2000 K
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causing an increase in current flow through the filament, its temperatureis raised and the spectral distribution of the emitted light changes so thatthe level of short wavelength energy relative to long wavelength increases[5].
2.4 CIE Illuminants
The CIE has established a number of spectral power distributions as CIEilluminants for colorimetry. These include CIE illuminants A, C, D65,D50, F2, F8, and F11.
CIE Illuminant A is a mathematical representation of tungsten halogen(incandescent) having a colour temperature of 2 856 K. The colour ofthe light source is yellow/orange [4].
CIE Illuminant C is the spectral power distribution of illuminant A asmodified by particular liquid filters defined by the CIE. It represents adaylight simulator with a CCT of 6 774 K. The colour of the lightsource is bluish [4].
CIE Illuminants D65 and D50 are part of the CIE D-series ofilluminants that have been statistically defined based upon a largenumber of measurements of real daylight. Illuminant D65 represents anaverage daylight with a CCT of 6 504 K (neutral colour tone), and D50(yellowish colour tone) represents an average daylight with a CCT of 5003 K. D65 is commonly used in colorimetric applications, while D50 isoften used in graphic arts applications [4].
Figure 2.3. Relative spectral power distributions for CIE illuminants A, C, D50,D65 and F11.
CIE F Illuminants (12 in all) represent typical spectral powerdistributions for various types of fluorescent sources. CIE illuminant F2represents cool-white fluorescent with a CCT of 4 230 K. Illuminant F8represents a fluorescent D50 simulator with a CCT of 5 000 K, andilluminant F11 represents a triband fluorescent source with a CCT of4 000 K [4].
Wavelength (nm)
Rela
tive S
pectr
al
Po
wer
CIE A
CIE CCIE D65CIE D50
CIE F11
11
A light source can be described by the spectral power distribution, i.e. thepower of its electromagnetic radiation as a function of wavelength or thenumber of photons as a function of wavelength. Figure 2.3 shows thespectral power distribution for two daylight illuminations (D50, D65), atungsten lamp and a fluorescent lamp.
2.5 Light Material Interaction
When light travels and encounters a medium other than that throughwhich it has been travelling the light can be affected in many differentways. For example when light is incident upon a colour print, some ofthe light passes through the outer glossy surface and through the layers ofselectively absorbing dyes. The spectral distribution of the light is alteredby the double transfer through the dyes, both before and after reflectionfrom the white paper surface beneath. There may also be internalreflections, which is a problem of scatter. Figure 2.4 illustrates thedifferent paths a photon can take when it encounters a medium.
Figure 2.4. The various ways in which light rays interact when encountering atransparent medium.
Glossy surfaces reflect the light at the same angle of incidence andwithout any change of colour [5]. Whereas glossy surfaces are verysmooth, matte surfaces have tiny surface imperfections that cause light toscatter and somehow have the ability to change colour. The mostimportant property of a surface for perceiving its colour is diffuse spectralreflectance. This is a statement about how the probability of a photonbeing reflected from a surface, in an unpredictable direction, variesdepending on the wavelength of the incident photon.
When a beam of light enters some medium, not all of it will emerge outof the other side. Some of the light is absorbed by the medium and isconverted to heat. The more transparent the medium is, the lessabsorption will take place. The extent to which absorption takes place iswavelength dependent [5].
Refraction refers to a change in the direction of light as it passes from onemedium to another.
Scatter occurs whenever the reradiation of photons by the molecules oftransmitting medium is other than in the forward direction [5]. Thelight that we see when a beam pierces a smoky room is visible onlybecause of scatter. When scattering particles are large, as they are in theeye, scatter is largely independent of wavelength and is concentrated in a
AirIncident rays Reflected ray
Refracted ray
Transmitted ray
Scattered rays
Absorbed ray
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forward direction. When the particles are small, as in the atmosphere ona clear day; shortwave photons are much more likely to be scattered thanlong-wave ones; this is the physical basis for the blue colour of the sky.
When light moves through a medium it is being transmitted. Except forvacuum, there is no such thing as a perfectly transparent medium.Transparent media have an effect on light because they contain atomsthat interact with photons.
If a light passes near the edge of a surface, it will appear to bend aroundthe edge. This is called diffraction.
2.6 Bidirectional Reflectance Distribution Function (BRDF)
Reflectance characteristics of objects can be described by a BidirectionalReflectance Distribution Function (BRDF). This function describes whatwe all observe every day: that objects look differently when viewed fromdifferent angles, and when illuminated from different directions. Thefunction describes the geometrical reflectance properties of a surface, i.e.how much light is reflected of a surface as a function of illuminationgeometry and viewing geometry at the light interaction point.
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3. Colour StimuliIn colour science, a ”colour” that is to be viewed or measured is calledmore correctly a colour stimulus. This colour stimulus always consists oflight. In some cases, that light might come directly from a light sourceitself, such as when a CRT screen or the flame of a lighted candle isviewed directly. But more typically, colour stimuli are the result of lightthat has been reflected from or transmitted through various objects.
3.1 Surface Reflectance
How an object reacts to incident light depends upon various microscopicphysical characteristics of its surface that determine the probability thatan incident photon will be reflected in a particular direction dependingupon its wavelength. It is important to distinguish clearly between twolimiting aspects of surface reflection: diffuse, which mediates surfacecolour perception and specular, which usually does not [5]. An exampleof specular reflectance is provided by a dust-free mirror. If an idealmirror’s edges are suitably disguised, its surface will be invisible. Lightreflected from it does so at an angle equal to the angle of incidence,which is the geometrical property that allows a plane mirror to provideperfect virtual images. Objects are seen by reflection “in the mirror” as ifthey were located behind it; this happens because the complex, three-dimensional flux of light reaching the eye is identical to what it would beif the perceived objects actually were located where they seem to be.
Most surfaces exhibit reflectance components of both kinds. In veryhighly polished surfaces the outermost smooth layer of a hard surface canact as a mirror. But unlike a real mirror, such a surface is not totallyreflecting. A significant fraction of the incident light penetrates thesurface and is diffusely reflected by a substrate that contains dye and/orpigment particles collectively known as colorants. Diffuse reflectionvaries as a function of wavelength depending upon the nature of thecolorants, and it is also affected to some extent by the type of binder thatcontains them [5]. Diffuse reflectance provides the physical basis for thecolours of most objects.
When light reaches an object, that light is absorbed, reflected, ortransmitted. Depending on the chemical makeup of the object andcertain other factors, the amount of light that is reflected or transmittedgenerally will vary at different wavelengths. This variation can bedescribed physically by a spectral reflectance curve or a s p e c t r a ltransmittance curve. These curves respectively describe the fraction of theincident power reflected or transmitted as a function of wavelength, seeFigure 3.1 [3]. Note that the reflectance spectrum is has values between0 and 1, while the illumination spectrum has values between 0 and .
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Figure 3.1. Spectral reflectance for a red object.
In most cases, an object’s spectral characteristics will correlate in astraightforward way with the colour normally associated with the object.For example, the spectral reflectance shown in Figure 3.1 is for a redobject. A red object (generally) is seen as red because it reflects a greaterfraction of red light (longer visible wavelengths) than of green light(middle visible wavelengths) or blue light (shorter visible wavelengths).Sometimes, however, the correlation of colour and spectral reflectance isless obvious.
3.2 Stimuli
If the object in Figure 3.1 is illuminated with the light source to the leftin Figure 3.2 the colour stimulus will have the spectral powerdistribution shown to the right in Figure 3.2. The spectral powerdistribution of this stimulus is the product of the spectral powerdistribution of the light source and the spectral reflectance of the object.The spectral power distribution of the stimulus is calculated bymultiplying the power of the light source times the reflectance of theobject at each wavelength. For a reflective or transmissive object, thecolour stimulus results from both the object and the light source. Ifanother light source is used, the colour stimulus will change. A “red”object can be made to appear almost any colour, depending on how it isilluminated.
Figure 3.2. Calculation of the spectral distribution of a colour stimulus.
Note that the human eye is insensitive to light of wavelength greater than650nm and less than 400nm [4]. This shows that even though the colour
Wavelength (nm)
Refl
ecta
nce
Rela
tive P
ow
er
Rela
tive P
ow
er
Wavelength (nm)Wavelength (nm)
X =
Wavelength (nm)
Refl
ecta
nce
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stimuli of an object suggest one colour, the human perception mayperceive a completely different colour, based on the sensitivity of thephotoreceptors in the retina of the eye. The visual process is verycomplex and not fully understood.
How a stimulus appears does not only depend on the spectral propertiesof the stimulus and the light source in which it is viewed. It also dependson many other factors like for example the size, shape and spatialproperties and relationships of the stimulus, the background andsurround, observer experience and the adapted state of the observer.
3.3 Metamerism
Because of the trichromatic nature of the human vision, it is possible thattwo colour stimuli that are physically different (i.e., having differentspectral power distribution) will appear identical to the human eye. Thisis called metamerism and two such stimuli are called a metameric pair [3].The reason metamers look alike is that they both result in the samepattern of response in the three cone photoreceptors. Thus as far as thevisual system is concerned, these stimuli are identical. In colourreproduction, metamerism is what makes colour encoding possible. It isbecause of metamerism that there is no need to reproduce the exactspectrum of a stimulus, rather it is sufficient to produce a stimulus that isa visually equivalent of the original one [3]. Note that, metamerisminvolves matching visual appearances of two colour stimuli, and not twoobjects. Hence, two different objects with different reflectance propertiescan form a metameric pair, under some special lighting conditions. Twostimuli that physically match, and for that reason also look identical arecalled isomers [5].
3.4 Fluorescence
An important topic in colorimetric analysis of materials is fluorescence.Fluorescent materials absorb energy in one region of wavelengths andthen re-emit this energy at another, usually longer, region of wavelengths[1]. For example, a fluorescent orange material might absorb bluephotons and emit orange photons, i.e. some of the absorbed energy isemitted at longer wavelengths.
A fluorescent material is characterized by its total radiance factor, which isthe sum of the reflected and emitted energy at each wavelength relativeto the energy that would be reflected by a PRD (Perfect ReflectingDiffuser) (A PRD is a theoretical material that is both a perfect reflector,i.e. it has 100% reflectance, and perfectly Lambertian, i.e. its radiance isequal in all directions) [1]. This definition allows for total radiancefactors greater than 1.0, which is often the case. It is important to notethat the total radiance factor will depend on the light source used in themeasuring instrument, since the amount of emitted energy is directlyproportional to the amount of absorbed energy in the excitationwavelengths. Spectrophotometric measurements of reflectance ortransmittance of nonfluorescent materials are insensitive to the lightsource in the instrument, since its characteristics are normalized in theratio calculations. This important difference highlights the majordifficulty in measuring fluorescent materials [1].
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4. The EyeHuman vision is a complex process that involves the interaction of thetwo eyes and the brain through a network of neurons, receptors, andother specialized cells. The human eye is equipped with a variety ofoptical components including the cornea, iris, pupil, a variable-focuslens, and the retina. Together, these elements work to form images of theobjects that fall into the field of view for each eye. When an object isobserved, it is first focused through the cornea and lens, forming aninverted image on the surface of the retina, a multi-layered membranethat contains millions of light-sensitive cells that detect the image andtranslate it into a series of electrical signals for transmission via the opticnerves to the brain. In the brain, the optic nerves from both eyes join atthe optic chaisma where information from their retinas is correlated. Thevisual information is then processed through several steps, eventuallyreaching the visual cortex, which is located on the lower rear section ofeach half of the cerebrum. Figure 4.1 shows a schematic representationof the optical structure of the human eye.
4.1 Cornea
The cornea is the transparent outer surface of the front of the eyethrough which light enters the eye. It serves as the most significantimage-forming element of the eye. This is because its refraction index(1.37) is substantially greater than that of air [5]. Thus, the smoothnessof the corneal surface, and its index of refraction, are very important.Vision is unclear under water because water (1.33) and the cornea havenearly the same refractive index [5]. The optical power of the cornea isnearly lost and severe “farsightedness” (hyperopia) results [1].
4.2 Iris
The iris is the muscle that controls the pupil size, and thus the amount oflight entering the eye and reaching the retina. It is pigmented whichgives the eye its specific colour. The size of the pupil depends mostly onthe overall level of illumination, but it also depends on many otherfactors including the size and region of the retina stimulated, spectral andtemporal characteristics of the light, and emotional reactions. Thus it isdifficult to accurately predict the pupil size from the prevailingillumination. In practical situations, pupil diameter varies from about 3
IRIS
PUPIL
CORNEA
LENS
FOVEA
OPTIC NERVE
RETINA
OPTIC NERVE FIBERS
RETINA
RODCONE
PIGMENT EPITELEUM
RECEPTOR CELLS(RODS AND CONES)
Figure 4.1. The eye [4].
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mm to about 7 mm [1]. This change in pupil diameter results inapproximately a five-fold change in pupil area and therefore in retinalilluminance. The pupils of both eyes change size together, calledconsensual papillary response, which means that both pupils grow smallerwhen light is delivered to only one of the eyes.
4.3 Lens
The lens is a flexible structure with varying index of refraction. It ishigher in the centre of the lens and lower at the edges. The shape of thelens is controlled by the ciliary muscles, and is called accommodation.When we gaze at a nearby object, the lens becomes “fatter” and thus hasincreased optical power to allow us to focus on the nearby object, seeFigure 4.2. When we gaze at a distant object, the lens becomes “flatter”,thereby resulting in the decreased optical power required to bring faraway objects into sharp focus. However, in some instances thecomponents do not work correctly or the eye is slightly altered in shapeand the focal point does not intersect with the retina. As people age, forinstance, the lenses of their eyes become harder and loose their flexibility,which results in poor vision. If the point of an eye’s focus is short of theretina the condition is called nearsightedness or myopia. People with thisaffliction are unable to focus on distant objects. In cases where the eye’sfocal point is behind the retina people have trouble focusing on nearbyobjects, which is a condition called hypermetropia, commonly known asfarsightedness. These malfunctions of the eye can usually be correctedthrough the use of glasses, with concave lenses correcting myopia andconvex lenses rectifying hypermetropia. The lenses can also becomecloudy as one ages, called cataracts.
Figure 4.2. Focusing Lens.
Concurrent with the hardening of the lens is an increase in its opticaldensity. The lens absorbs and scatters short wavelength (blue and violet)energy. As it hardens, the level of this absorption and scattering increases.In other words, the lens becomes more and more yellow with age.Various mechanisms of chromatic adaption generally make us unawareof these gradual changes. However, we are all looking at the worldthrough a yellow filter that not only changes with age but alsosignificantly differs from observer to observer [1].
4.4 Retina
The optical image formed by the eye is projected onto the retina. Theretina is less than half a millimeter thick and contains a total area ofabout 1100 mm2. This area contains about 200 million neural cells thatare directly involved with the processing of visual information [6].
DISTANT OBJECT
FOCUS ON RETINA
NEARBY OBJECT
FOCUS ON RETINA FOCUS BEHIND RETINA
NEARBY OBJECT
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The are two classes of photoreceptors in the human retina: rods andcones, see Figure 4.3, which transform light energy into electrical energy.
Rods function under very low luminance levels (e.g., less than 1 cd/m2,where 1 cd correspond to the light emitted from one candle), while conesare used for high or daylight levels (e.g., greater than 100 cd/m2) and forseeing fine details [1].
Figure 4.3. Rod and Cone distribution on the retina [4].
The rods are most sensitive to green wavelengths of light (about 510nm), although they display a broad range of response throughout thevisible spectrum [1]. Each eye contains about 120 million rods ascompared to the number of cones that is only about 7 million [1]. Thelight sensitivity of rod cells is about 1000 times that of cone cells.However, the images generated by rod stimulation alone are relativelyunsharp and confined to shades of grey. Rod vision is commonly referredto as scotopic vision.
There are three types of cone receptors that absorb light from threedifferent portions of the visible spectrum. The L-cones absorb long-wavelength light (red), the M-cones absorb middle-wavelength light(green) and the S-cones absorb short-wavelength light (blue). Sometimesthe cones are denoted with other symbols such as RGB or . Thestimulation of the three types of cone receptors allows the human visualsystem to distinguish very small colour differences. It has been estimatedthat stimulation to various levels and ratios can give rise to about tenmillion distinguishable colour sensations.
The relative distribution of the different cone types (L:M:S) on the retinais approximately 40:20:1 [1]. Stimulation of these visual receptors resultsin what is known as true colour vision. Cone vision is referred to asphotopic vision.
There is a difference in peak spectral sensitivity between scotopic andphotopic vision. With scotopic vision, we are more sensitive to shorterwavelengths. This effect is known as the Purkinje shift and it can beobserved by finding two objects, one blue and the other red, that appearthe same lightness when viewed in daylight [1]. When the two objectsare viewed under very low luminance levels, the blue object will appearquite light while the red object will appear nearly black because of thescotopic spectral sensitivity function.
Near the centre of the retina is the area of sharpest vision, fovea centralis,that subtends about two degrees of visual angle (see section 8.1 fordefinition of visual angle). One method to measure the visual angle is the“Thumb method” where you fully extend your arm and look at your
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thumb. The approximate visual angle of the thumb at arms length is 2degrees [4]. The retina is less than half as thick in the fovea as in theremainder of the eye, and this change in thickness creates the depressionfrom which the term fovea derives. The anatomy of the fovea pit hasimportant implications for the resolution of fine visual detail. In order tosee details (visual acuity) the eye needs to be focused on the fovea, whichcontains only high-density tightly packed cone cells. The density level ofcone cells decreases outside of the fovea centralis and the ratio of rod cellsto cone cells gradually increases. At the periphery of the retina, the totalnumber of both types of light receptors decreases substantially, causing adramatic loss of visual sensitivity. To have a retinal image of excellentoptical quality formed upon the photoreceptors, it is necessary to reducethe scattered light within the retina as much as possible; this is neatlyaccomplished with the foveal depression. The improved spatialresolution that results is not accomplished at the expense of sensitivity tolight. On the contrary, the fovea of the light-adapted retina is its regionof highest sensitivity. There are no rods whatever in the central fovea.
Located around 12°-15° from the fovea is the blind spot see Figure 4.3.This is the area where the optic nerve is formed and there is no room forphotoreceptors.
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5. Visual Signal ProcessingThe following section explains the neural processing of visualinformation from the retina to the brain, i.e., the encoding of colour.Visual signal processing is a field of intense research and not all partshave been fully understood. All explanations given in the literature areconsidered to be of a more or less speculative nature. The followingdescription of the process is somewhat simplified.
5.1 Signal Processing In the Retina
Colour vision starts in the eye with the absorption of light by the outersegments of the photoreceptors, which contain visual pigment moleculesthat trigger electrical signals. These molecules have two components: alarge protein called opsin and a small light-sensitive molecule calledretinal [4]. Retinal, which is attached to the opsin reacts to light and istherefore responsible for the transformation of light energy into electricalenergy (visual transduction). The transduction process begins when thelight-sensitive retinal absorbs one photon of light. When the retinalabsorbs this photon it changes its shape, a process called isomerization[4]. The electrical signals are then processed through a network of retinalneurons, which consists of four types of cells: bipolar cells, horizontalcells, amacrine cells, and ganglion cells, see Figure 5.1.
Figure 5.1. The figure illustrates the signal processing from the photoreceptors to theganglion cells [4].
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The rods and cones connect differently to other neurons in the retina,they differ in the amount of convergence [4]. Convergence occurs whenmore than one neuron synapses on another neuron. In the retina 126million receptors converge on 1 million ganglion cells [4]. Since there are120 million rods but only 6 million cones, the rods must converge muchmore than the cones. On the average, about 120 rods pool their signalsto one ganglion cell, but only about six cones send signals to a singleganglion cell. This difference between rod and cone convergencebecomes even greater because of the fact that many of the foveal coneshave “private lines” to ganglion cells. In these situations, with eachganglion cell receiving signals from only one cone, there is noconvergence. The greater convergence of the rods compared to the conestranslates into two differences in perception: the rods are more sensitivein the dark than the cones, and the cones result in better detail visionthan the rods [4].
The receptors are connected to bipolar and horizontal cells by synapses.Together they form receptive fields where signals from a number ofdifferent photoreceptor cells are compared (Small receptive fields, i.e.fewer photoreceptor cells, provides greater visual acuity and largereceptive fields, i.e. more photoreceptor cells, provides greatersensitivity). This causes an effect called center-surround antagonism. Thereare two basic types of bipolar cells: ON-center and OFF-center, see Figure5.2. ON-center bipolar cells are activated by bright spots on darksurroundings, whereas OFF-cells are activated by black spots on lightbackground.
Figure 5.2. Receptive fields for bipolar cells: on-centre (left) and off-centre (right).
The center-surround receptive fields are sensitive to contrast. If bothcentre and surround are illuminated at the same time, the antagonisticeffects almost cancel each other out. A consequence of this is thatbipolars respond poorly to overall illumination levels, but are verysensitive to local differences in intensity, i.e. they are sensitive tocontrast, not intensity.
The impulses from the bipolar and horizontal cells are then transferreddirectly, or indirectly via amacrine cells, to the ganglion cells, that alsohave receptive fields with a centre-surround organization, just like thebipolar cells. It is the amacrine cells that add the surround signal to theganglion cells and together they also form receptive fields. The ganglioncells have axons that leave the retina via the optic nerve, and connectthem to the brain.
The ganglion cells are of two major types: parvocellular ganglion cells (Pcells) and magnocellular ganglion cells (M cells) that form two majorparallel processing streams. P cells respond best to hight contrast, smallobjects (high spatial resolution) and slowly flashing stimuli (lowtemporal resolution). M cells respond best to the opposite, that is, lowcontrast, large objects (low spatial resolution) and fast flashing stimuli(high temporal resolution). The receptive fields of the M cells are also
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larger compared to the P cells. Larger receptive field means moreconnections to photoreceptors and consequently the nerve impulsesreach the brain more quickly.
Figure 5.3. Colour opponent ganglion cells.
The final difference in behaviour between M and P cells is their responseto light of different wavelengths. M cells are not wavelength selective andwill respond to light of any colour. P cells do care about what colour thelight is and are sensitive to wavelength in a ”colour opponent” way.Different types of colour opponent ganglion cells are shown in Figure5.3. For example, they may have a centre that is excited by only greenlight (input from M-cones), and a surround that is inhibited by only redlight (negative input from L-cones), see Figure 5.3 (left). Blue versusyellow centre-surround antagonism may also be found, see Figure 5.3(right). P cells are the cells that form the basis of colour processing in thevisual system.
5.2 Lateral Geniculate Nucleus (LGN)
The optic nerve fibers enter the Lateral Geniculate Nucleus (LGN) in alayered structure with cells that respond to form, motion, and color.Here the process of co-ordinating vision from the two eyes starts. TheLGN consists of six layers with each alternating layer receiving inputsfrom a different eye: 3 layers for the left eye and 3 layers for the right. The outer 4 layers (parvocellular layers) receive inputs from the Pganglion cells and the inner two layers (magnocellular layers) receivetheir input from the M ganglion cells. The result is three signals whichare sent to the brain: one corresponding to the amount of green-or-red,one corresponding to the amount of blue-or-yellow, and onecooresponding to the lightness. The LGN cells then project to visual areaone (V1) in the occipital lobe of the cortex. At this point, theinformation processing begins to become very complex.
5.3 Visual Receiving Area
Most of the fibers from the LGN project to a region of the occipitalcortex (outer layers of the brain at the back of the head) known as V1,primary visual cortex, or striate cortex. From V1 nerve fibres carryinformation to many other cortical areas. Approximately 30 visual areashave been defined in the cortex with names such as V2, V3, V4, andMT. The encoding of visual information becomes significantly morecomplex. Much as the outputs of various photoreceptors are combinedand compared to produce ganglion cell responses, the outputs of variousLGN cells are compared and combined to produce cortical responses.Beyond V1, there are two general streams of information processing: onefor motion and location, and the other for colour and form. These areknown as the ventral and dorsal streams, respectively. And in the end ofthis network of information, our ultimate perceptions are formed.
YELLOW ON BLUE ONRED ON GREEN ON
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6. Sensitivity ControlThe human visual system is capable of functioning across vast changes inviewing conditions while providing relatively stable perceptions. Themechanism that allows the visual system to do this is known asadaptation. There are three types of adaptation: light, dark and chromatic.Light and dark adaptation describe the human visual system’s capabilityof functioning across large changes in luminance levels and chromaticadaptation is the ability of the human visual system to adjust to changesin the colour of illumination.
6.1 Spectral Sensitivity of Rods and Cones
Perception is determined by the properties of the visual pigments. Thiscan be shown by comparing the rods and cones spectral sensitivity, i.e.,an observer’s sensitivity to light at each wavelength across the visiblespectrum. The cone and rod spectral sensitivity curves are shown inFigure 6.1. The curves show that the rods are more sensitive to short-wavelength light than are the cones, with the rods being most sensitive tolight of 500 nm and the cones being most sensitive to light of 560 nm[4]. The spectral sensitivity curves also show that the sensitivity of thehuman visual system rapidly decreases above 650 nm. That is whyobjects that have a high reflectance at longer visible wavelengths canappear having a certain colour even if the objects reflectance spectra tellsotherwise. The human visual system also has very little sensitivity towavelengths below 400 nm. This difference in the sensitivity of the rodsand the cones to different wavelengths means that, as vision shifts fromthe cones to the rods during dark adaptation, we become relatively moresensitive to short-wavelength light, that is, light nearer the blue andgreen end of the spectrum [4]. The shift from cone vision to rod visionthat causes the enhanced perception of short wavelengths during darkadaptation is called the Purkinje shift, after Johann Purkinje, whodescribed this effect.
Figure 6.1. Spectral sensitivity for rod vision and cone vision [4].
The difference between the rods and cone spectral sensitivity curves iscaused by differences in the absorption spectra of the rod and cone visualpigments [4]. The absorption spectra of the rod and cone pigment areshown in Figure 6.2.
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Figure 6.2. Absorption spectra of the human rod pigment and the three human conepigments [4].
The rod pigment absorbs best at 500 nm, the blue-green area of thespectrum. There are three absorption spectra for the cones because thereare three different cone pigments, each contained in its own receptor.The short-wavelength pigment absorbs light best at about 419 nm; themedium-wavelength pigment absorbs light best at about 531 nm; andthe long-wavelength pigment absorbs light best at about 558 nm [4].The absorption of the rod visual pigment closely matches the rod spectralsensitivity curve, and the short-, medium-, and long-wavelength conepigments add together to result in a psychophysical spectral sensitivitycurve that peaks at 560 nm [4]. Since there are fewer short-wavelengthreceptors and therefore much less of the short-wavelength pigment, thespectral sensitivity curve is determined mainly by the medium- and long-wavelength pigments [4]. It is clear that the rates of rod and cone darkadaptation and the shapes of the rod and cone spectral sensitivity curvesare determined by the properties of the rod and cone visual pigments.
6.2 Light and Dark Adaptation
Light adaptation is the decrease in visual sensitivity as a function of theoverall amount of illumination [1]. The more light illuminating a scene,the less sensitive the human visual system becomes to light.
Dark adaptation is the opposite of light adaptation, i.e., the change invisual sensitivity that occurs when prevailing level of illumination isdecreased, opposite to light adaptation. The human visual systembecomes more sensitive to light as the overall amount of illuminationdecreases. This can be thought of as walking from the sunny afternoonlight into a darkened room. After several minutes, objects becomerecognizable as your visual system adapts. The visual sensitivity willgradually improve and eventually (in about 30 minutes) reach a state thatis optimal for that amount of illumination. This happens because thevisual system is responding to the lack of illumination by becoming moresensitive and therefore capable of producing s meaningful visual responseat the lower illumination level.
Light and dark adaptation function at different speeds. The speed ofadaptation is called the time-course for full adaptation. Light adaptationworks at a much faster rate than dark adaptation, being on the order of 5minutes compared to 30 minutes for dark adaptation [4].
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6.3 Chromatic Adaptation
Chromatic adaptation refers to the human visual system’s ability to adjustto the colour of overall illumination rather than the absolute levels of theillumination. Consider a white object such as a piece of white paper.This paper can be viewed under a variety of light sources such asdaylight, incandescent, and fluorescent. Despite the large change in thecolour of these sources (ranging from blue to orange), the paper willalways retain an approximate white appearance. This is because the S-cone system becomes relatively less sensitive under daylight tocompensate for the additional short-wavelength energy, while the L-conesystem becomes relatively less sensitive under incandescent illuminationto compensate for the additional long-wavelength energy.
6.4 Dark Adaptation of Rods and Cones
Dark adaptation is a two-stage process where the increased lightsensitivity takes place in two distinct stages: an initial rapid stage and alater, slower stage. Figure 6.3 shows the dark adaptation curves (the eye’slight sensitivity over time). The curves indicate that the observer’ssensitivity increases in two phases. It increases rapidly for the first 3 to 4minutes after the light is extinguished and then levels off; then, afterabout 7 to 10 minutes, sensitivity begins to increase further andcontinues to do so for another 20 to 30 minutes [4]. The sensitivity atthe end of dark adaptation, labelled dark-adapted sensitivity, is about100000 times greater than the light-adapted sensitivity measured beforedark adaptation began [4]. The initial rapid stage is due to adaptation ofthe cone receptors and the second slower stage is due to adaptation of therod receptors.
Figure 6.3. Dark-adaptation curves [4].
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Both the rods and cones begin gaining in sensitivity as soon as the lightsare extinguished, but since the cones are more sensitive at the beginningof dark adaptation, they determine the early part of the dark-adaptationcurve. After about 3 to 5 minutes, the cones finish their adaptation, andthe curve levels off. However, by about 7 minutes after the beginning ofdark adaptation, the rods finally catch up to the cones and then becomemore sensitive. When this occurs, the curve starts down again, creatingthe rod-cone break, which is where the sensitivity of the rods begins todetermine the dark adaptation curve. As the rods continue theiradaptation, the dark adaptation curve continues downward for about 15more minutes. The rods reach their maximum sensitivity about 20 to 30minutes from the beginning of dark adaptation, compared to only 3 to 4minutes for the cones [4].
These differences in the rate of adaptation can be traced to a processcalled visual pigment regeneration that occurs with different speeds in therods and the cones [4]. When the visual pigment absorbs light, the light-sensitive retinal molecule changes shape and triggers the transductionprocess. It then separates from the larger opsin molecule, and thisseparation causes the retina to become lighter in colour, a process calledpigment bleaching [4]. Before the visual pigment can again change lightenergy into electrical energy, the retinal and the opsin must be rejoined.This process, which is called pigment regeneration, occurs in the dark withthe aid of enzymes supplied to the visual pigments by the nearbypigment epithelium. As the retinal and opsin components of the visualpigment recombine in the dark, the pigment begins to become darkeragain.
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7. Lightness and ColourConstancyLightness and colour constancy helps to keep our perception ofachromatic and chromatic colours constant even when the illuminationchanges. This means that we can perceive the actual properties of objectswithout too much interference from different light sources.
7.1 Lightness Constancy
Lightness constancy refers to how our perception of lightness remainsrelatively constant even when objects are viewed under differentintensities of light [4]. If a brighter light hits an object more light hits theobject and therefore much more light is also reflected. But the perceptionof the shade of lightness remains the same regardless of the changes inthe amount of light reflected into the eyes. Our perception of an object’slightness is related not to the amount of light that is reflected from theobject, which can change depending on the illumination. But on thepercentage of light reflected from the object, which remains the same nomatter what the illumination. Objects that look black reflect about 5percent of the light, objects that look grey reflect about 10 to 70 percentof the light, and objects that look white reflect about 80 to 90 percent ofthe light [4].
7.2 Colour Constancy
Colour constancy refers to how our perception of colour remainsrelatively constant even when objects are viewed under differentilluminations [4]. As an example the colour of objects do not changewhen moving from indoors to outdoors, even though the illuminationcondition has changed dramatically. Figure 2.3 showed the wavelengthsthat are contained in light from a lightbulb (CIE Illuminant A) and thewavelengths contained in sunlight (CIE Illuminant D50 and D65). Thesunlight contains approximately equal amounts of energy at allwavelengths, which is a characteristic of white light. The bulb containsmuch more energy at long wavelengths. Even though there is a bigdifference between the wavelength distribution of the sunlight and thelightbulb, we do not notice much change in how we perceive the coloursof objects under these two different light sources. Although small shiftsof colour perception sometimes occur when the illumination changes,our overwhelming experience is that colours remain at leastapproximately constant under most natural conditions. Colourconstancy is due to a number of factors including chromatic adaption,the effect of surrounds, and memory colour.
7.3 Chromatic Adaptation
One of the mechanisms that contributes to colour constancy is chromaticadaptation, i.e. prolonged exposure to a chromatic colour. When we walkinto a room illuminated with a tungsten light, the eye adapts to the long-
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wavelength-rich tungsten light, which decreases the eye’s sensitivity tolong wavelengths. This decreased sensitivity causes the long-wavelengthlight reflected from objects to have less effect than before the adaptation,and this compensates for the greater amount of long-wavelengthtungsten light that is reflected from everything in the room. The result isjust a small change in the perception of colour. The eye is adjusting itssensitivity to different wavelengths in order to keep colour perceptionapproximately constant under different illuminations [4].
7.4 Memory Colours
An object’s perceived colour is affected not only by the observer’s state ofadaptation. Another small effect is that past knowledge can have someeffect on colour perception through the operation of a phenomenoncalled memory colour, in which an objetc’s characteristic colour influencesour perception of its colour. Research has shown that since people knowthe colours of familiar objects, like a red stop sign, or a green tree, theyjudge these familiar objects as having richer, more saturated colours thanunfamiliar objects that reflect the same wavelengths [4]. Thus, our abilityto remember the colours of familiar objects may help us perceive thesecolours under different illuminations.
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8. Spatial and TemporalProperties of Colour VisionOur eyes are constantly sampling information of images projected ontothe retina. Information is then integrated so objects around us (overallshapes and small details) appear clearly visible and also appear to bestable or move smoothly. Since there is a finite amount of field of viewand time required to collect and process information, there arelimitations to the responsiveness of our visual system to details and ratesof change.
8.1 Spatial and Temporal Frequency
Spatial frequency is how rapidly a stimulus changes across space, withhigh spatial frequencies corresponding to small details in theenvironment and low spatial frequencies corresponding to larger forms[4]. For example, the grating to the right in Figure 8.1 has a higherspatial frequency than the one to the left, because it has more bars perunit distance.
Figure 8.1. Two gratings. The one to the right has a higher spatial frequency thanthe one to the left.
Spatial frequency is measured in terms of cycles per degree of visualangle. The visual angle is the angle of an object relative to the observer’seye, see figure 8.2. The visual angle depends on both the size of thestimulus and on its distance from the observer. If the distance isincreased, the visual angle becomes smaller. The term “cycles per degree”means the number of cycles in a grating that fit within an angle of onedegree on the retina, where one cycle is a dark bar and a light bar.
Figure 8.2. Visual angle, the angle of an object relative to the observer’s eye.
The experimental procedure used in studying the spatial characteristics ofthe visual system typically involves a visual stimulus that is displayed inthe form of a sine-wave grating, that is, a regular stripe pattern whoseluminance across the pattern varies sinusodially [4]. The observer isasked to determine the threshold for detecting the pattern. Presentedwith a sine-wave grating of given spatial frequency, the observer adjuststhe amplitude of the luminance variation until he or she can just see thepresence of the grating, or just distinguish it from a perfectly uniformfield.
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In the temporal domain, the same principle applies, except now thestimulus is separated in time, i.e. temporal frequency is how rapidly astimulus is changes over time. Temporal frequency is measured in Hz.
The experimental procedure used in studying the temporal responsecharacteristics of the visual system typically involves a visual stimulus ofsome specified size whose luminance is varied sinusoidally as a functionof time over a range of frequencies and luminance amplitudes. For agiven frequency and mean luminance, the observer adjusts the luminanceamplitude until the imposed sinusoidal variation is just large enough thatthe field does not appear steady in brightness.
8.2 Contrast Sensitivity Functions (CSF)
The spatial and temporal characteristics of the human visual system canbe measured as so-called contrast sensitivity functions (CSF). A CSF is aplot of contrast sensitivity vs. spatial or temporal frequency [1]. Contrastis typically defined as the difference between maximum and minimumluminance in a stimulus divided by the sum of the maximum andminimum luminances, and CSFs are typically measured with stimuli thatvary sinusodially across space or time.
Figure 8.3 (left) illustrates typical spatial CSFs for luminance (black-white) contrast and chromatic (red-green and yellow-blue at constantluminance) contrast. The luminance CSF has band-pass characteristics,with a peak-sensitivity around 5 cycles per degree. This functionapproaches zero at zero cycles per degree, thus illustrating the tendencyfor the visual system to be insensitive to uniform fields. It alsoapproaches zero at about 60 cycles per degree, the point at which detailcan no longer be resolved by the eye. The band-pass CSF correlates withthe concept of center-surround antagonistic receptive fields that wouldbe most sensitive to an intermediate range of spatial frequency. Thechromatic mechanisms have low-pass characteristics and havesignificantly lower cutoff frequencies. This indicates the reducedavailability of chromatic information for fine details (high spectralfrequencies) that is often taken advantage of in image coding andcompression schemes (e.g. JPEG). The low-pass characteristic of thechromatic mechanisms also illustrate that edge detection/enhancementdoes not occur along these dimensions. The blue-yellow chromatic CSFhas a lower cutoff frequency than does the red-green chromatic CSF dueto the scarcity of S cones in the retina. The luminance CSF issignificantly higher than the chromatic CSFs. This indicates that thevisual system is more sensitive to small changes in luminance contrastcompared to chromatic contrast.
Figure 8.3 (right) illustrates typical temporal CSFs for luminance andchromatic contrast. They share many characteristics with the spatialCSFs. Again, the luminance temporal CSF is higher in both sensitivityand cutoff frequency (close to 60 Hz) than are the chromatic temporalCSFs. Also, it shows band-pass characteristics that suggest theenhancement of temporal transients in the human visual system.
The spatial and temporal CSFs interact with one another. A spatial CSFmeasured at different temporal frequencies will vary tremendously, aswill a temporal CSF measured at various spatial frequencies.
Many visual scientists have directed their attention to visual phenomenathat are mainly associated with temporal and spatial variations of the
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observed stimuli. In particular, the objective has been to quantify thetemporal and spatial response characteristics of the visual system. Thetool of Fourier analysis have been applied effectively to this task yieldingtemporal and spatial modulation transfer functions of the visual system.
8.3 The Oblique Effect
Humans are more sensitive to horizontally or vertically oriented gratingsthan to other, oblique, orientations. This enhanced sensitivity for verticaland horizontal gratings is called the oblique effect. This phenomenon isconsidered in the design of rotated halftone screens that are set up suchthat the most visible pattern is oriented at 45°.
8.4 Mach Bands
Mach Bands are light or dark narrow bands that are perceived near theborder of two adjacent fields, one field being darker than the other, seeFigure 8.4. Between two regions of different intensity a thin bright bandappears at the lighter side and a thin dark band appears on the darkerside. These bands are not physically present they are just illusions.
Figure 8.4. Mach Band.
The phenomenon in all its complexities is not fully understood but it isgenerally agreed that lateral interactions in the neural network of thevisual system account for it.
8.5 Flicker
When intermittent stimuli are presented to the eye they are perceived asseparate if the rate at which they are presented is below a certain value.Depending on the speed the intermittent stimulation of the observer’svisual system results in the sensation of flicker. At a very slow rate itappears to flash on and off in a discrete but regular fashion. If the rate isincreased, then, above a certain critical rate, the flicker ceases. This pointis called the critical flicker frequency (CFF) and is influenced by a numberof factors. The phenomenon of the disappearance of flicker at thatfrequency is called flicker fusion [6].
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Figure 8.3. Spatial contrast sensitivity functions (left) and temporal contrastsensitivity functions (right) for luminance and chromatic contrast [1].
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9. Colour-Vision DeficiencyColour-vision deficiency is an inability to perceive some of the coloursthat people with normal colour vision can perceive. People with colourdeficiency (dichromats) and colour blindness (monochromats) needfewer wavelengths than a normal trichromat to match any wavelength inthe spectrum. There are three types of colour deficiency: monochromatswho need only one wavelength to match any colour in the spectrum,dichromats who need only two wavelengths to match all otherwavelengths in the spectrum and anomalous trichromats who need threewavelengths to match any wavelength, just as a normal trichromat does,but the anomalous trichromat mixes these wavelengths in differentproportions from a trichromat. An anomalous trichromat also havedifficulties in discriminating between wavelengths that are close together.
Colour-vision deficiencies are not rare, particularly in the malepopulation where about 8% have some type of colour-vision deficiencyas compared to the female population where the number is only 0.4%[1]. The reason for this disparity is genetic. The genes for photopigmentsare present on the X chromosome. Since males (XY) have only one Xchromosome, a defect in the visual pigment gene on this chromosomecauses colour deficiency. Females (XX) with their two X chromosomesare less likely to become colour deficient, since only one normal gene isrequired for normal colour vision. If a female is colour-deficient, itmeans she has two deficient X chromosomes and all male children aredestined to have colour-vision deficiency [1][4].
9.1 Monochromats
People with no functioning cones (i.e., only rod vision in both dim andbright light) are called rod monochromats or achromats. Mono-chromatssee everything in shades of lightness and can therefore be called colour-blind. Only 0.001% of the population are monochromats and it ishereditary [4].
Another group of people who are truly colour-blind are those that haverods and only one class of cone receptors. At photopic levels suchobservers would not be able to distinguish one colour from another.These observers are called cone monochromats.
9.2 Dichromats
Those who have two classes of functioning cones are called dichromats.Dichromats experience some colours, though a lesser range thantrichromats. There are three different forms of dichromats depending onwhich one of the three normal photopigments (L, M, S) is missing. Anobserver with tritanopia (0.002% of males, 0.001% of females) is missingthe S-cone photopigment and therefore cannot discriminate yellowishand bluish hues [4]. A deuteranope (1% of males, 0.01% of females) ismissing the M-cone photopigment and therefore cannot distinguishreddish from greenish hues [4]. And a protanope (1% of males, 0.02% offemales) is missing the L-cone photopigment and therefore is also unableto discriminate reddish and greenish hues [4].
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9.3 Anomalous Trichromats
Observers who have three classes of cones but don’t see the world as so-called colour normal observers are called anomalous trichromats(abnormal trichromatic vision). In this case, the ability to discriminateparticular hues is reduced either due to shifts in the spectral sensitivitiesof the photopigments or the contamination of photopigments (e.g.,some L-cone photopigment in the M-cones, and so on) [1]. Among theanomalous trichromats are those with any of the following: protanomaly,that is, either they are weak in L-cone photopigment or the L-coneabsorption is shifted toward shorter wavelengths, deuteranomaly, that is,either they are weak in M-cone photopigment or the M-cone absorptionis shifted toward longer wavelengths and tritanomaly, that is, either theyare weak in S-cone photopigment or the S-cone absorption is shiftedtoward longer wavelengths [1].
9.4 Trichromats
There are also colour vision variations among observers with normalcolour vision, trichromats. There can for example be differences in theproportion of the different cone types or variances in the peak spectralabsorbance of the cone photopigments.
9.5 Colour Vision Tests
There are many different types of colour vision tests available. Some testsare very quick and makes it possible to differentiate colour normals fromthose who clearly have a colour vision deficiency in just a few minutes,while other tests takes considerably longer to administer. One of thewell-known quick tests uses the Ishihara Plates, which belong to acategory of tests called pseudoisochromatic plates. Another common testthat measures the observer’s ability to make very subtle colourdiscrimination is the Farnsworth-Munsell 100 Hue test.
Pseudoisochromatic plates are colour plates made up of dots of variouscolours. The test takes advantage of one of the Gestalt laws oforganization, the law of similarity. According to this principle, elementshaving the same appearance tend to be apprehended as a pattern. Bymanipulating the chromaticities of such elements at constant luminance,they can form a figure and a background. The plates are presented underproperly controlled illumination to observers who are asked to respondby either tracing the pattern or reporting the number observed. Variousplates are designed with colour combinations that would be difficult todiscriminate for observers with the different types of colour-visiondeficiencies.
The Farnsworth-Munsell 100-Hue Test consists of four sets of chips thatmust be arranged in an orderly progression of hue. Observers withvarious types of colour-vision deficiencies will make errors in thearrangement of the chips at various locations around the hue circle. Thetest can be used to distinguish between the different types of deficienciesand also to evaluate the severity of colour discrimination problems. Italso can be used to identify observers who have normal colour vision butpoor colour discrimination for all colours.
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10. Subjective Colour PhenomenaColour vision is a very complex phenomenon and the colour of an objectdepends not only on the nature of the paint on its surface, but also onthe colour of the light used to illuminate it, the intensity of that light,and the chromatic characteristics of other surfaces located nearby. Thecolours we perceive can be separated into objective colours and subjectivecolours. Objective colours are the perception of colour consistent withwhat is expected in response to a particular spectral distribution ofenergy. In contrast, subjective colours are produced within the visualsystem without being related directly to specific wavelengths of light.Objective colours are initiated by the differential activation of the threekinds of cone photoreceptors. Whereas subjective colours are seen whenthis initial receptor stage of vision is bypassed. Here follows someexamples of subjective colours.
10.1 Simultaneous Contrast
Simultaneous contrast causes a stimulus to shift in colour appearancewhen the colour of its background changes. A light background inducesa stimulus to appear darker, a dark background induces a lighterappearance, red induces green, green induces red, yellow induces blue,and blue induces yellow, see Figure 10.1.
Figure 10.1. An example of simultaneous contrast. All the grey patches to the left arephysically identical, and all the red and green patches are identical.
10.2 Crispening
Crispening is the increase in perceived colour difference between twostimuli when the background of the stimuli is close to the colour of thestimuli itself. The figure below illustrates crispening for a pair of greysamples. The two grey stimuli appear to be of greater lightness differenceon the grey background than on either the white or the blackbackground, see Figure 10.2.
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Figure 10.2. An example of crispening. The pairs of grey patches are physicallyidentical on all three backgrounds.
10.3 Spreading
Spreading is the mixture of a colour stimulus with its surround. Whenthe stimuli increase in spatial frequency, or become smaller, thesimultaneous contrast effect disappears and is replaced with a spreadingeffect, see Figure 10.3.
Figure 10.3. An example of spreading.
The effect works as if it is combining the adjacent colours rather thanaccentuating the differences between adjacent colours as contrast does.
10.4 Luminance Phenomena
Hunt effect (Colourfulness increases with luminance) – As the luminanceof a given colour increases, its perceived colourfulness also increases [1].Objects appear much more vivid, or colourful, when viewed in brightsunny environment.
Stevens effect (Contrast increases with luminance) – As the luminancelevel increases, so too does the brightness contrast [1]. As the adaptingluminance level increases, the rate of change between the brightness ofthe dark and light colour increases. This rate of change is oftenconsidered to be the contrast of the scene.
Helmholtz-Kolrausch effect (Brightness depends on luminance andchromaticity) – Brightness changes as a function of saturation, i.e., as astimulus becomes more saturated at constant luminance, its perceived
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brightness also increases [1]. A chromatic stimulus will appear brighterthan an achromatic stimulus at the same luminance level.
10.5 Hue Phenomena
Bezold-Brücke hue shift (Hue changes with luminance) – Illustrates thatthe wavelength of monochromatic light sources is not a good indicator ofperceived hue [1]. As luminance levels change the perceived hue can alsochange.
Abney effect (Hue changes with colorimetric purity) – States that adding”white” light to a mono-chromatic light does not preserve constant hue[1]. Straight lines in a chromaticity diagram radiating from thechromaticity of the white point to the spectral locus, are not lines ofconstant hue. Unlike the Bezold-Brücke hue shift, this effect is valid forrelated as well as unrelated colours.
Helson-Judd effect (Hue of nonselective samples) – Illustrates thatnonselective (grey) stimuli viewed under highly chromatic illuminationtake on the hue of the light source if they are lighter than thebackground, and they take on the complementary hue if they are darkerthan the background [1].
10.6 Surround Phenomena
Bartleson-Braneman Equations (Image contrast changes with surround) –Perceived contrast in images increases as the luminance of the surroundincreases [1]. When an image is viewed in a dark surround, the blackcolours look lighter while the light colours remain relatively constant. Asthe surround luminance increases, the blacks begin to look darker,causing overall image contrast to increase.
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11. Colour Order SystemsColour order systems arrange colours in a “space” – one that exists onlyin our imaginations – within which colours change continuously. Mostof these systems arrange colours shading from dark at the bottom to lightat the top, with hues arranged circumferentially and saturation increasingoutward from a central achromatic axis. At the vertical extremes, withwhite above and black below, no saturation variation is possible. Themaximum chromatic variation occurs at intermediate lightness levels.The outermost shell of the space resembles two lopsided cones joined attheir bases with apices opposites.
11.1 The Munsell System
The American colour teacher Albert H. Munsell (1858-1918) developedthe Munsell Colour System in 1905 [5]. He wanted to create a system inwhich the spacing between each colour and its neighbour could beperceived as equal, i.e. a perceptually uniform system.
Figure 11.1. Munsell colour system (Images from www.adobe.com).
There are ten basic hues in the system. Five primary colours: red (R),yellow (Y), green (G), blue (B) and purple (P). And five intermediatecolours: yellow-red (YR), green-yellow (GY), blue-green (BG), purple-blue(PB), and red-purple (RP) placed in between. Each of these ten hues arefurther subdivided by four decimal numbers: 2.5, 5, 7.5 and10, giving40 hues in total. These hues are arranged in a circle around a centralvertical neutral grey-value (N) axis where all have equal distances and areselected in a way that opposing pairs result in an achromatic mixture, seeFigure 11.1 (left).
Each colour is characterised by three attributes: Munsell Hue (describedabove), Munsell Value (N) and Munsell Chroma (C). The Munsell Valueindicates the index of brightness in terms of a neutral grey scale andranges from 0N for pure black to 10N for pure white.
The Munsell Chroma is the gradation of saturation. The scale starts at 0for neutral, but there is no arbitrary end to the scale. Maximum chromacan be somewhere in between 10 and 26 depending on the hue. Thus,
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different hues have different number of chromatic steps. That is why theshape of the colour space is asymmetric.
The notation for a colour in the Munsell System is written H u eValue/Chroma. For example 7.5YR 7/12 (an orange colour), where 7.5YRis the Munsell Hue, 7 is the Munsell Value and 12 is the MunsellChroma.
The colours are arranged in a colour atlas, The Munsell Book of Colour,from 1929. This edition is still in use today and contains 1200-1500colour chips.
11.2 NCS
The Swedish Natural Colour System (NCS) was introduced in 1979 by ateam led by Anders Hård. The objective with NCS was to establish acolour system with which a user with normal colour vision coulddetermine colours without the need for colour measuring instruments orcolour samples. The NCS system is designed as an aid to defining, forexample, the colour of a wall in a room purely on the basis of itsperception.
The system is based on six elementary colours: white (W), black (S),yellow (Y), red (R), blue (B) and green (G). And all other colours are thendescribed in terms of these. The system possesses the external shape of adouble-cone where Y, R, B and G occupy the circular base with evenlyspaced positions. The tips of the double-cone are W (above) or S(below). In this three-dimensional model, called the NCS colour space,all imaginable surface colours can be placed. The double cone is alsodivided into two two-dimensional models, the NCS colour circle (ahorizontal section through the colour space) and the NCS colour triangle(a vertical section through the colour space), see Figure 11.2.
Figure 11.2. The NCS colour circle and the NCS colour triangle (www.ncs.se).
The colours in the system are characterized by three attributes: NCSColour Hue (H), NCS Blackness (S) and NCS Chromaticity (C). The NCS ColourHues are defined on the basis of the basic colours yellow, red, blue and greenshown in the colour circle. Each of the quadrants in the circle is furthersubdivided between two basic colours by a scale that expresses theportion of each colour as a percentage. For example, Y40R implies ayellow with 40% red, and B20 G implies a blue with 20% green. Thisallocation is based on the principle of similarity, that each colour issimilar to a maximum of two chromatic elementary colours (in addition
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to white and black) and that such a match can be quantitatively assesseddown to an accuracy of 5%.
The NCS Blackness indicates the proportion of black and the scale rangesfrom 0 (white) to 100 (black). And the NCS Chromaticity indicates thedegree of chromaticity and also varies from 0 (achromatic colour) to 100(full chromatic colour). This is shown in the colour triangle where allcolours, which lie on the vertical lines, contain equal chromaticproportions. In the same way, all colours in the rows running parallel tothe line between white and the observed colour contain equalproportions of black.
NCS colour notations are based on how much a given colour seems toresemble the six elementary colours. In the NCS notation S 2030-Y90R,for example, 2030 indicates the nuance, i.e. the degree of resemblance toblack (S) and to the maximum chromaticness (C); in this case, 20%blackness and 30% chromaticness. The hue Y90R indicates the portionof each colour as a percentage; in this case a yellow (Y) with 90% redness(90R). Purely grey colours lack colour hue and are only given nuancenotations followed by -N as neutral. 0500-N is white and this is followedby 1000-N, 1500-N, 2000-N and so on to 9000-N which is black.
11.3 DIN
The Deutsche Institut für Normung (DIN) system was developed inGermany by Manfred Richter and introduced in 1953. The objectivewas to create a colour system operating with the explicit variables ofcolour hue, saturation and brightness and as perceptively equidistant aspossible.
The DIN system has three variables: DIN Colour Hue (T), DINSaturation (S) and DIN Darkness (D). They provide the coordinates forthe three dimensional system that has the shape of a cone.
The DIN Colour Hue is defined by means of a colour circle with 24gradations. Hue varies from a value of T=1 (yellow) via red (7), blue(16), and green (22) to a green-yellow that has a value of T=24.
Within the DIN colour-circle the DIN Saturation gradations commencewith S = 6 and end at an achromatic point S = 0, and both colour-hueand saturation together form the colour type.
The DIN Darkness is related to the luminous reflectance of the samplerelative to an ideal sample (a sample that either reflects all or none if theincident energy at each wavelength) of the same chromaticity [1]. Thisenables the DIN system to associate colours not of the same brightnessbut of the same relative brightness. In terms of perception, this is moreappropriate, since we tend to sense colours of differing colour-hue asbeing of equal value. The scale ranges from a value of 0 (white) to 10(black).
The notation for colours in the DIN system is written in the sequenceT:S:D. For example 22.5:3.2:1.7 (a green colour), where 22.5 is the DINColour Hue, 3.2 is the DIN Saturation and 1.7 is the DIN Darkness.
The colours are arranged in a colour atlas, the DIN Colour Chart 6164,which contains 600 colour samples (20 x28 mm).
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11.4 OSA UCS
The Optical Society of America Uniform Color Scales (OSA UCS) systemwas introduced in 1960. The aim in developing the OSA UCS colourorder system was to determine a set of colour samples that, underappropriate viewing conditions, defined a perceptual uniform colourspace. The OSA colour system has the form of a cubo-octohedron,which is the form resulting from slicing off all corners of a cube down tothe midpoint of each edge yielding 12 corner points. The colours of thecubo-octahedron have been selected so that the distances between acolour sample and each of its 12 nearest neighbours are perceived asequally large colour differences.
The position of a sample within this space is defined by the coordinatesof three axes, which intersect each other at right angles: Lightness (L),Yellowness-Blueness (j) and Greenness-Redness (g). The j-axis does notexactly correspond to a yellow-blue axis. The reference j representsyellow at high lightness values. For negative values of j the axis separatesblue from the violet region. Correspondingly, the positive values for gwill not indicate green, instead this parameter separates the blue andgreen colours. And red does not lie at the end of the negative g scale, butpink.
The OSA Lightness value is zero when the brightness corresponds to thebackground generally recommended for viewing the samples, it ispositive when a colour is brighter than the background, and it is negativefor a colour that is darker.
The samples are arranged in an array along a vertical axis running fromblack to white, orthogonal to two chromatic axes, one of which runsroughly from red to green, the other from blue to yellow. In the 1978report issued by the Committee for Uniform Color Scales, a total of 558samples were colorimetrically specified, together with their exactcoordinates.
The objective of equal colour differences in all directions results in a verydifferent type of colour order system. Perhaps due to its complexgeometry, the OSA UCS is not very popular [1].
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12. TerminologyHere follows some important definitions of our perceptions of colourstimuli. A complete specification of a colour appearance requires fiveperceptual dimensions: brightness, lightness, colourfulness, chroma andhue.
12.1 Colour
Definition of colour: Attribute of visual perception consisting of anycombinations of chromatic and achromatic content. This attribute can bedescribed by chromatic colour names such as yellow, orange, brown, red,pink, etc., or by achromatic colour names such as white, grey, black, etc., andqualified by bright, dim, light, dark, etc., or by combinations of such names.
Note: Perceived colour depends on the spectral distribution of the colourstimulus, on the size, shape and structure, and surround of the stimulusarea, on the state of adaption of the observer’s visual system and on theobserver’s experience of the prevailing and similar situation ofobservations.
There are eleven basic colour terms that can be subdivided into threecategories:
1. achromatic colour terms (white, grey, black); and two varieties ofchromatic colour terms
2. primary (red, yellow, green, blue) and
3. secondary (orange, purple, pink, brown)
We can describe all the colours we can discriminate by using thechromatic primary colour terms red, yellow, green and blue, and theircombinations.
12.2 Hue
Definition of hue: Attribute of a visual sensation according to which anarea appears to be similar to one of the perceived colours: red, yellow, green,and blue, or a combination of the two of them.
Definition of achromatic colour: Perceived colour devoid of hue.
Definition of chromatic colour: Perceived colour possessing a hue.
Hue is often described with a ”hue circle”. In which one can find theunique hues; red, yellow, green and blue and their combinations.
12.3 Brightness and Lightness
The attributes of brightness and lightness are very often interchanged,despite the fact that they have very different definitions.
Definition of brightness: Attribute of a visual sensation according to whichan area appears to emit more or less light.
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Definition of lightness: The brightness of an area judged relative to thebrightness of a similarly illuminated area that appears to be white or highlytransmitting.
Note: Only related colours exhibit lightness.
The standard definition of lightness is given by the equation below:
Lightness =Brightness
Brightness (white)
Brightness refers to the absolute perception of the amount of light of astimulus, while lightness can be thought of as the relative brightness. Thevisual system generally behaves as a lightness detector.
Example: A newspaper when read indoors would have a certainbrightness and lightness. When viewed side by side with standard officepaper, the newspaper often looks slightly grey, while the office paperappears white. When the newspaper and office paper are broughtoutdoors on a sunny summer day, they would then have much higherbrightness. Yet the newspaper still appears darker than the office paper asit has a lower lightness. The physical amount of light reflected from thenewspaper might be more than a hundred times greater than the officepaper was indoors, yet the relative amount of light reflected has notchanged. Thus, the relative appearance between the two papers has notchanged.
12.4 Colourfulness and Chroma
Definition of colourfulness: Attribute of a visual sensation according towhich the perceived colour of an area appears to be more or less chromatic.
Note: For a colour stimulus of a given chromaticity and, in the case ofrelated colours, of a given luminance factor, this attribute usuallyincreases as the luminance is raised, except when the brightness is veryhigh.
Definition of chroma: Colourfullness of an area judged as a proportion ofthe brightness of a similarly illuminated area that appears white or highlytransmitting.
Note: For given viewing conditions and at luminance levels within therange of photopic vision, a colour stimulus perceived as a related colour,of a given chromaticity, and from a surface having a given luminancefactor, exhibits approximately constant chroma for all levels of luminanceexcept when the brightness is very high. In the same circumstances, at agiven level of illuminance, if the luminance factor increases, the chromausually increases.
The standard definition of chroma is given by the equation below:
Chroma =Colourfulness
Brightness (white)
Colourfulness describes the amount or intensity of the hue of a colourstimulus and thus is an absolute perception. And chroma can be thoughtof as relative colourfulness just as lightness can be thought of as relative
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brightness. The human visual system generally behaves as a chromadetector.
12.5 Saturation
Definition of saturation: Colourfulness of an area judged in proportion toits brightness.
Note: For given viewing conditions and at luminance levels within therange of photopic vision, a colour stimulus of a given chromaticityexhibits approximately constant saturation for all luminance levels,except when brightness is very high.
The standard definition of saturation is given by the equation below:
Saturation=Colourfulness
Brightness=Chroma
Lightness
Like chroma, saturation can be thought of as relative colourfulness.However, saturation is the colourfulness of a stimulus relative to its ownbrightness, while chroma is colourfulness relative to the brightness of asimilarly illuminated area that appears white. For a stimulus to havechroma it must be judged in relation to other colours, while a stimulusseen completely in isolation can have saturation.
12.6 Related and Unrelated Colours
The definition of colour is further enhanced with the notion of relatedand unrelated colours.
Definition of related colour: Colour perceived to belong to an area of objectseen in relation to other colours.
Definition of unrelated colour: Colour perceived to belong to an area ofobject seen in isolation from other colours.
The colours brown and grey only exists as related colours. It is impossibleto find an isolated brown or grey stimulus, as evidenced by the lack of abrown or grey light source. These lights would appear either orange orwhite when viewed in isolation.
12.7 Achromatic and Chromatic Colours
Definition of achromatic colours: When light reflection is flat across thespectrum, such as white, black or grey.
Definition of chromatic colours: When some wavelengths are reflectedmore than others, as for example blue pigment.
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13. References[1] Fairchild, M. Colour Appearance Models, First Edition, Addison-
Wesley, Massachusetts (1998).
[2] Field, G. Color and its Reproduction, Second edition, SewickleyGatfPress, (1999).
[3] Giorgianni, E.J. and Madden, T.E. Digital Color Management –Encoding Solutions, Addison-Wesley, Massachusetts (1998).
[4] Goldstein, E. B. Sensation and Perception, Sixth edition,Wadsworth publishing Company, Belmont, CA. (1998).
[5] Kaiser, P.K. and Boynton, R.M. Human Colour Vision, SecondEdition, Optical Society of America, Washington, D.C. (1996).
[6] Wyszecki, G., and Stiles, W.S. Colour Science, Second Edition,John Wiley and Sons, New York (2000).