Colour
Colour: a private experience...
Definition:
• Additive mixture: light
• Subtractive mixture: ink or paint
Photoreceptors:
Rods: monochromatic
Cones: trichromatic
—trichromatic theory:
Young and Helmholtz
vs
—principle of univariance
Physiology
Ganglion cells and LGN
Concentric single opponent cells
Concentric non-opponent cells
Striate cortex (Hering)
Purely opponent cells
Double opponent cells
Psychophysical evidences for physiology
Negative afterimages
Colour constancy
Additive colour mixing
light
1.0
0.5
0.0
400
1.0
500
600
Wavelength
700
Intensity
Intensity
Blue Light
Yellow Light
0.5
0.0
400
Intensity
1.0
White
(blue light plus
yellow light)
0.5
0.0
400
500
600
Wavelength
700
500
600
Wavelength
700
Subtractive colour mixing
paint
Blue Ink
0.5
0.0
400
500
600
Wavelength
700
Transmission
Transmission
1.0
1.0
Yellow Ink
0.5
Transmission
0.0
400
1.0
Green
(blue ink plus yellow ink)
0.5
0.0
400
500
600
Wavelength
700
500
600
Wavelength
filters
700
Paint reflects colours
(wavelength)
Green
Red
Blue
Photoreceptors absorb “colours”
Light of different wavelengths
Photoreceptors absorb “colours”
Absorbtion of Photons
Light of different wavelengths
Rods
Red
Blue
Green
400
500
Wavelength
600
700
Colour deficiencies
Monochromatism
One receptors wavelength selectivity
Dichromatism
Two receptors wavelength selectivity
Tritanope
Protanope
Deuteranope
Ishihara Test
The world.
How the world looks to
a person with a
red/green color deficit
(deuteranopia).
How the world looks to a
person with a blue/yellow
color deficit (tritanopia).
http://www.vischeck.com/showme.shtml
Cones sensitivity
NORMAL FATHER
CHILDREN
CARRIER
MOTHER
X
Y
GIRLS
BOYS
X'
Carrier X'X
X
XX
Color
Blind
X'Y
XY
Relative Response
UNIVARIANCE
G Cone
400
500
600
Wavelength
Principle of univariance
531 nm
100% absorbance
Electrical signal
G cone
photon
2 times brighter
590 nm
SAME
2 X 50% absorbance
Electrical signal
G cone
One wavelength
selectivity
NO COLOUR
Percent Absorbance
photon
G Cone
100
50
400
472
531
590
Wavelength (nanometers)
Percent Absorbance
100
Cone 1
Cone 2
45
35
500
600
Wavelength (nanometers)
wavelength (nm)
Cone 1
Cone 2
500
100%
45%
600
35%
100%
Trichromatic Theory:
Young & Helmholtz
CONES
Retina
r
Opponent Theory:
Hering
PATHWAYS
LGN and striate cortex
Luminance
Opponent
Color
g
R/G
b
B/Y
Wavelength
Opponent and non-opponent receptive
fields of cells in the retina and LGN
Red/Green
Opponent cells
+r
-r
-g
+g
+g
-g
-r
+r
Opponent and non-opponent receptive
fields of cells in the retina and LGN
Blue/Yellow
Opponent cells
Non-Opponent cells
+b
-b
-r-g
+r+g
+r+g
-r-g
-r-g
+r+g
Colour cells
Opponent receptive fields in
striate cortex
Purely opponent cells
Double opponent cells
+r -g
-r +g
Afterimages
Light
-r +g
Firing rate
+r -g
Firing rate
White
Red
White
baseline
0
baseline
0
Fatigue Model
time
Constancy
+r -g
Gain
-r +g
Gain
Automatic Gain Control