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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002
1
Perception of Brightness
The physics and psychophysics…
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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
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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
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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
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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
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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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 7
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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 8
Psychological measures• Lightness subjective estimate of the
whiteness of a surface:– intensity?– radiance?– irradiance?– reflectance?
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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 9
Relating physical and psychological measures1. brightness versus intensity2. brightness versus wavelength3. brightness depends on surroundings4. lightness depends on illumination
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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
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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
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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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 13
Intensity = 10 W Intensity = 15 W
Forced-choice Response• A bit more rigorous• Q: brighter light on left or right?
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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 14
Intensity = 10 W Intensity = 15 W
Forced-choice Response• A bit more rigorous• Q: brighter light on left or right?
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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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 16
Intensity = 13 W Intensity = 10 W
Forced-choice Response• A bit more rigorous• Q: brighter light on left or right?
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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?
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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?
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 30
Result: Luminosity Curve• different wavelengths have different
luminosities• some wavelengths are brighter than
others
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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 31
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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 32
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
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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)
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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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 35
Pulfrich Effect
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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
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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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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 38
Simultaneous brightness contrast:left one looks brighter
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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 39
Simultaneous brightness contrast:pattern increases difference
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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
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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
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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…
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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 43
The snake illusion (Adelson)
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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
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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
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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
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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
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January 22, 2002 Psyc202-005, Term 2, Copyright Jason Harrison 2002 48
Finally• What was the purpose of this
presentation?
• Which question remains unanswered?