PSYB51H3 Chapter Notes - Chapter 5: Trichromacy

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12 May 2011
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Chapter 5: The Perception of Color
Basic Principles of Color Perception
The colour we see is related to wavelength
We see things that reflect light
Darker surfaces absorb light
Colour is result of interaction btw physical w/ nervous system
The Problem of Univariance
photoreceptors respond to different wavelengths with different strengths
however, we can get the same response from more than one wavelength
when intensity varies, we can also get the same response
Therefore, output of photoreceptor itself doesnt tell us colour
oProblem of univariance: combinations of intensity and wavelengths produce the
same response, so we cant tell colour
Trichromacy
Rods and Cones
Rods are sensitive to low light levels
oContain photopigment rhodopsin
Scoptic: Light level bright enough for rods but too dim for cones
oi.e. low light levels
owe can tell light from dark, but still cant see colour
cone receptors sensitive to daylight levels
photopic: bright enough to stimulate cones and saturate(max out) rods
cones have one of 3 different wavelength senstivies:
odepends on peak response
oS-cones: sensitive to short wavelgneths (blue)
peak at 440
oM-cones: sensitive to medium wavelengths (green)
Oeak at 535
oL-cones: senesitve to long wavelengths (red)
Peak at 565
Because we have 3 types of cones, each gets diff response for a wavelength, allows us to
distinguish colours
oIf we change intensity, ratio of response btw cones is same
Trichromaitc theory of colour vison (trichromacy): Colour is understood by relation
btw responses from 3 cones
Metamers
Actually see mix of wavelengths at same time
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If we mix 2 wavelengths and consider 2 cones; sum response together, we can get the
same response from a signle wavelength
oE.g. red for L-cone, green for M-cone = Yellow
Metamers: mixtures of wavelengths that look the same
oIndividual wavelengths dont change when mixed
oExact amounts are needed
Lights, Filters, and Finger Paints
Additive colour mixture: mix of lights when light reflects together onto surface of eye
oe.g. computer monitor is mix of dots up close, red + green light = yellow
opossible with painting by using small spots
Subtractive colour mixture: When 2 pigments mix, light shining is subtracted by both.
We see the result of the subtraction.
oe.g. mix red & green paint, see brown
ocolour filters act like this, removing specified colour
whats left is what we see
Three Numbers, Many Colours
can make same colour without same physical properities b/c of metamers
Colour space: 3D space, b/c colour is based on 3 cone types
oE.g. hue, saturation, brightness
Different dimensions for colour
Hue: colour aspect of colour
Saturation: Strength of hue
oe.g. white is zero
Brightness: distance from black
oI.e. Intensity of light
Nonspectral hues are colours only possible by combining wavelengths
History of Trichromatic Theory
Basis was Newtons discopery that prism would break sunlight into spectrum of hues
oKnew colour was production of mind
Maxwell found we could match colours with just 3 lights (RGB)
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