january 22, 2002psyc202-005, term 2, copyright jason harrison 20021 perception of brightness the...

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002

1

Perception of Brightness

The physics and psychophysics…

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 2

Perception of brightness• psychophysics:

relate psychological measures to physical ones

• perception of brightness is one of the simplest aspects of vision to study

• what is the relationship between – psychological perception of brightness– physical parameters of light intensity


Units: watts

Property:

intensity

= (amplitude)2

= energy/sec

= powerλ

Amp

Physical measures1. Light Ray


Units: watts, lumens

(1 lumen ≈ 1mW)

Property:radiance

= total energy / sec

(all light produced)

Physical measures2. Light Source

120 volt, 100 watt "standard" bulb produces ~1750 lumens


Units: watts/m2,lumens/m2

Property:irradiance

= power / area (total falling on

surface from all directions)

Physical measures3. Light Received


Units: fraction (between 0 and 1)- 0 = total absorption (black)- 1 = total reflection (white)

Property:reflectance (albedo)

= outgoing light incoming light

Physical measures4. Light Reflected


Psychological measures• Brightness subjective estimate of…

– intensity?– radiance?– irradiance?– reflectance?

• of light from a light source or from a reflecting surface


Psychological measures• Lightness subjective estimate of the

whiteness of a surface:– intensity?– radiance?– irradiance?– reflectance?


Relating physical and psychological measures1. brightness versus intensity2. brightness versus wavelength3. brightness depends on surroundings4. lightness depends on illumination


Light 1 Light 2

Intensity = 10 W Intensity = 10 W

Brightness versus intensity• standard light at fixed intensity• test light with adjustable intensity• adjust power of test until just begins to differ• just noticeable difference: JND


Light 1 Light 2


Light 1 Light 2


Light 1 Light 2


Light 1 Light 2


Standard Test

A just noticeable difference (JND) at 11W

1 W above standard

Brightness versus intensity



Forced-choice Response• A bit more rigorous• Q: brighter light on left or right?



• Analyse accuracy of response versus intensity of lights



.

50%

100%

Intensit y of test light

Standard = 10W

10 11 12 13

Psychometric function• accuracy (probability correct) as a

function of intensity of test light

Pro

bab

ility

of

corr

ect

resp

on

se




.

50%

100%


Standard = 10W

10 11 12 13

75%

Threshold (75%) = 10.8 W


.

50%

100%


Standard = 10W

10 11 12 13

75%

Threshold (75%) = 10.8 W

jnd = Δ = 0.8 I W




For I = 10 W, jnd ΔI = 0.8WFor I = 30 W, jnd ΔI = 2.5WFor I = 50 W, jnd ΔI = 4.0W

ΔI / I = .08

ΔI / I = .08

ΔI / I = .08

Weber’s Law• for a large range of intensities:

ΔI / I = constant


Weber’s Law• for a large range of intensities:

ΔI / I = k• k = Weber constant• different value of k for different senses

– light intensity: k = 0.08– sound intensity: k = 0.05


Relating physical and psychological measures1. brightness versus intensity2. brightness versus wavelength3. brightness depends on surroundings


Light 1 Light 2

Power = 10 W Power = 10 W

Brightness depends on wavelength• Light 1: at one wavelength• Light 2: at different wavelength

Adjust power of second light until its brightness is the same as the first


Light 1 Light 2





Light 1 Light 2


Light 1 Light 2


Light 1 Light 2


Light 1 Light 2





Result: Luminosity Curve• different wavelengths have different

luminosities• some wavelengths are brighter than

others


Luminosity Curve

wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700

Sensitivityin daylight


wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700


Sensitivityat night

Luminosity Curve


wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700


Sensitivityat night

Luminosity Curve

Photopic vision(cones)

Scotopic vision (rods)


.

wavelength (nm)

1.0

0.8

0.6

0.4

0.2

0400 500 600 700


wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700


Sensitivityat night

Purkinje shift

Luminosity Curve

Photopic vision(cones)

Scotopic vision (rods)


Pulfrich Effect


Simultaneous brightness contrast:two squares of the same intensity


Simultaneous brightness contrast:left one looks brighter


Simultaneous brightness contrast:pattern increases difference


Recall structuralism• perception of each patch = atom• atoms are independent• perception of patch should not be

influenced by surroundings• but it is…• Visual system always takes context into

account– visual intelligence


V1 cell

photoreceptors (eye)

V1 cell


100100

output= center 100 +neighbors - 0 100

V1 cell


70100

output= center 100 +neighbors -30 70

(darker)

V1 cell


20100

output= center 100 +neighbors -80 20

(even darker)

Is this lateral inhibition?• reduction of signal when neighboring

areas are active


Context matters!• simultaneous brightness contrast

– affected by immediate surrounding

• and– context further away

• the right context can make this effect even stronger…


The snake illusion (Adelson)


Lightness depends on illumination• one patch (on a simple background)• estimate lightness of patch when

different illumination is used


Light fromsquare

Light frombackground

Incominglight

Light fromsquare

Light frombackground

Incominglight

Result?• Lightness unaffected by illumination

– lightness constancy

• Visual system computes lightness as a

ratio = intensity of light from square intensity of light from background


Lightness is invariant under illumination level• Thus, obtain an estimate that is a

property of the world (object surface) – doesn’t change with lighting

• rather than the image (light on retina)– does change with lighting


Finally• What was the purpose of this

presentation?

• Which question remains unanswered?

january 22, 2002psyc202-005, term 2, copyright jason harrison 20021 perception of brightness the...

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