human visual system eye perception of colour display types display terminology

The Science of Digital Media

• Human Visual System– Eye– Perception of colour

• Display types• Display terminology

6 May 2010Metropolia University of Applied Sciences,

Jutta Jokela

Human eye and Displays


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Human Visual System–Eye–Brains

• Camera and HVS• HVS and displays

Introduction


Jutta Jokela


36 May 2010 3Metropolia University of Applied Sciences,

Jutta Jokela

The eye

http://www.eyemdlink.com/Anatomy.asp


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Seeing process


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Seeing process


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Seeing process

• http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.aspx


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Eye

• Retina– Three layers – one converts light to

neural signals, one does the image processing and one transfers information to the brain

– Rods and cones• Rods see in black and white• Cones create the colour images

– Red cone detects light around 564 nm, green at 533 nm and blue at 437 nm


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http://askabiologist.asu.edu/rods-and-cones


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Eye

• Fovea– In the center of the retina

• 1 mm in diameter

– Used for high resolution vision– No rods around

• Makes it easier to see in dark when looking to the side rather than forward


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Summary of the eye


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http://www1.curriculum.edu.au/sciencepd/readings/ligh_how.htm#1


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Camera versus HVS (1)

• Camera and eye are built the same way– Image based on light– Lens and image sensor

• Two adjustable parameters– Focus

• With camera: manually moving lens• With eye: Cornea let's light in but lens

behind it changes in shape and size

– Aperture


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11

Brightness

• Eye– Subjective measurement

• Display– Amount of light a display is capable of

emitting– Black should look black – can change as

ambient light varies– With LCD displays usually affects to

backlight luminance– Measured with candelas per square metre


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Contrast

• Luminance levels within an image – difference between brightness of objects in an image

• In displays contrast affects to the luminance that is meant for reproducing a full white pixel

• Contrast and brightness must be in decent level to make image viewable


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Dynamic range and Contrast ratio (1)

• In display means the maximum and minimum intensities that a display can generate– Brightest white and darkest black a display

can display

• No common way of measuring – measures are not comparable


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Dynamic range and Contrast ratio (2)

• Dynamic contrast ratio is difference in intensity over time

• Static contrast ratio is a difference between darkest and brightest pixels displayed by the display

• Intensity of the backlight affects as well– Different areas of the screen may be

illuminated differently

• With OLED only static contrast ratio matters6 May 2010

Metropolia University of Applied Sciences, Jutta Jokela


15

Visual acuity• Eye

– Clearness of the vision– Light enters the eye Through pupil, then lens and

is projected on retina Muscles move eyeball so that image is in the

fovea

• Display– Level of detail eye can actually see from

pattern of pixels on a screen– Not usually counted separately – 200 ppi

good enough6 May 2010Metropolia University of Applied Sciences,

Jutta Jokela


16

Angle of viewing• Eye

– The angle in which eye tries to get the point of interest reflected to the foval area• Usually around 155 degrees vertically and

185 degrees horizontally

• Camera– Means the scene that can be imaged

• Display– Angle in which you are still able to see the

content of the display – usually 120-170 degrees with LCD displays


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17

Perception of colour

• Very often a subjective matter• Hue – dominant wavelength of the colour• Brightness/lightness – luminosity of the

colour– If an object reflects light, this is lightness, if

it emits light, this is brightness

• Saturation – pureness of the colour– Can be called as white light mixed with hue


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Colour

• In additive colour mixing light is added to a dark background– Example RGB

• In subtractive colour mixing a certain method is used for blocking white pixels– Example CMYK


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Colour gamut

• A certain part of the colour map that a device can present

• In 1931 Commission Internationale de l'Eclaraige (CIE) developed the chromaticity diagram

http://www.pcmonitors.org/articles/monitor-technology


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20

Colour in displays

• Displays emit or reflect light with different strengths of three primary colours– Ambient light has always an effect as well

as it reflects off the display

• With LCD displays the gamut is related to the emitted spectrum of the backlight


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21

Colour depth

• The number of colours the display can display– Bits

• LCDs 6-bits per pixel causing a 18-bit colour image

• Dithering increases the amount of colours – Dithering – artificially changing shade of a

colour during a refresh of the display– Causes 24-bit image


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Refresh rate (1)

• This means the number of times a display is repainted per second– Expressed in Hertz

• Should be twice as much as the viewer's refresh rate


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Refresh rate (2)

• Eye– Detects and processes a moving object in

about 120 ms – requires 20 images per second for blurring to start

• Display– Too slow refresh rate causes flicker effect– Meaningful with CRTs– With LCDs flickering does not occur

because liquid crystals merely act as shutters against the backlight


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•Typical display types•Terminology related to displays and visual system

Introduction


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Cathode Ray Tube (CRT)

• The most common display until last years– No fixed amount of pixels– Image can be very good– Usually big in size– Not as bright as LCD displays

• Technology– Electron guns send beams of electrons

• Stimulating a layer of phosphor that are ”the pixels” of a CRT (red, green and blue)

• Magnetic field and deflection of the electrons form the image


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Liquid Crystal Display (LCD)

• Liquid crystals are sandwiched between two polarizing filters– Backlight travels through the first filter

while electrodes create a current that travels through liquid crystals and forces those to align in a certain way

• Alignment of the liquid crystals creates the intensity and the colour of light visible finally on the screen


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Plasma Display Panel

• High contrast ratio, panel can be big– Popular in public places

• Contains cells that are filled with ionized gas– Glows when electrical field around it is

charged

• Pixels can be only on or off so no grey levels are possible to create


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28

Organic Light Emitting Diode (OLED)

• Yet quite small displays– Can be even flexible

• Do not use backlight – Self illuminated materials– Less power consumption– Very good contrast ratio, colour

reproductions and response time


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Resolution

• CRT – no resolution• LCD, OLED, Plasma – amount of pixels is

fixed (pixels horizontally and vertically available)

• HDTV has made this especially important• Resolution is not fixed quantity

– For that reason displays with same resolution but with different size are not comparable


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Response time

• A time it takes for pixels to move from one state to another– Faster response time means smoother

image transitions e.g. In sports– Before this measure measured time for

turning display on, off and on again– 2005 manufacturers changed this for

considering grey-to-grey changes– Good LCD displays claim to have 2 ms

response time6 May 2010

Metropolia University of Applied Sciences, Jutta Jokela


31

Screen size

• Diagonal size of the screen in inches– With CRTs there was screen size and

viewable area separately– Different LCD display manufacturers

handle this differently• EITHER viewable area• OR actual screen size


Jutta Jokela6 May 2010Metropolia University of Applied Sciences,

Jutta Jokela

The Science of Digital MediaSummary

• Eye is a very complex system• Camera is a simpler but similar system

• Different aspects have to be taken into account when designing displays• Depends on technology


Jutta Jokela6 May 2010Metropolia University of Applied Sciences,

Jutta Jokela

human visual system eye perception of colour display types display terminology

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