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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 3

Units: watts

Property:

intensity

= (amplitude)2

= energy/sec

= powerλ

Amp

Physical measures1. Light Ray

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 4

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 5

Units: watts/m2,lumens/m2

Property:irradiance

= power / area (total falling on

surface from all directions)

Physical measures3. Light Received

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 6

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

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Psychological measures• Brightness subjective estimate of…

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

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

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Psychological measures• Lightness subjective estimate of the

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

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 10

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 11

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 12

Light 1 Light 2

Intensity = 10 W Intensity = 10 W

Light 1 Light 2

Intensity = 10 W Intensity = 11 W

Light 1 Light 2

Intensity = 10 W Intensity = 12 W

Light 1 Light 2

Intensity = 10 W Intensity = 13 W

Standard Test

A just noticeable difference (JND) at 11W

1 W above standard

Brightness versus intensity

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Intensity = 10 W Intensity = 15 W

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

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Intensity = 10 W Intensity = 15 W

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 15

Intensity = 13 W Intensity = 10 W

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

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Intensity = 13 W Intensity = 10 W

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 17

Intensity = 11 W Intensity = 10 W

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 18

Intensity = 11 W Intensity = 10 W

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 19

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

• Analyse accuracy of response versus intensity of lights

Intensity = 11 W Intensity = 10 W

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 20

.

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 21

Psychometric function• accuracy (probability correct) as a

function of intensity of test light

.

50%

100%

Intensit y of test light

Standard = 10W

10 11 12 13

75%

Threshold (75%) = 10.8 W

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

.

50%

100%

Intensit y of test light

Standard = 10W

10 11 12 13

75%

Threshold (75%) = 10.8 W

jnd = Δ = 0.8 I W

Psychometric function• accuracy (probability correct) as a

function of intensity of test light

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 23

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 24

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 25

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 26

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 27

Light 1 Light 2

Power = 10 W Power = 6 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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 28

Light 1 Light 2

Power = 10 W Power = 4 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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 29

Light 1 Light 2

Power = 10 W Power = 10 W

Light 1 Light 2

Power = 10 W Power = 6 W

Light 1 Light 2

Power = 10 W Power = 4 W

Light 1 Light 2

Power = 10 W Power = 3 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

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Result: Luminosity Curve• different wavelengths have different

luminosities• some wavelengths are brighter than

others

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Luminosity Curve

wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700

Sensitivityin daylight

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wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700

Sensitivityin daylight

Sensitivityat night

Luminosity Curve

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 33

wavelength (nm)

Lum

inosi

ty

1.0

0.8

0.6

0.4

0.2

0400 500 600 700

Sensitivityin daylight

Sensitivityat night

Luminosity Curve

Photopic vision(cones)

Scotopic vision (rods)

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 34

.

wavelength (nm)

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

Sensitivityin daylight

Sensitivityat night

Purkinje shift

Luminosity Curve

Photopic vision(cones)

Scotopic vision (rods)

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Pulfrich Effect

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 36

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 37

Simultaneous brightness contrast:two squares of the same intensity

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Simultaneous brightness contrast:left one looks brighter

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 39

Simultaneous brightness contrast:pattern increases difference

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 40

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 41

V1 cell

photoreceptors (eye)

V1 cell

photoreceptors (eye)

100100

output= center 100 +neighbors - 0 100

V1 cell

photoreceptors (eye)

70100

output= center 100 +neighbors -30 70

(darker)

V1 cell

photoreceptors (eye)

20100

output= center 100 +neighbors -80 20

(even darker)

Is this lateral inhibition?• reduction of signal when neighboring

areas are active

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 42

Context matters!• simultaneous brightness contrast

– affected by immediate surrounding

• and– context further away

• the right context can make this effect even stronger…

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 43

The snake illusion (Adelson)

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 44

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 45

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

different illumination is used

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 46

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 47

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

January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 48

Finally• What was the purpose of this

presentation?

• Which question remains unanswered?