Lecture 8: Colour Vision

Question 1

describe the 3 chromacies

Answer 1

monochromacy: 1 cone pigment, cannot discriminate colour at all, only differentiate objects by intensity
dichromacy: 2 cone pigments, some colour perception, can match colours if given 2 wavelengths of colours and instructed to adjust relative intensity of each wavelength until matches
trichromacy: 3 cone pigments, can match sample colour if given 3 wavelengths of colour and instructed to adjust relative intensity (amount) of each wavelength until matches

Question 2

state the two theories of CV

Answer 2

trichromatic theory & opponent colour theory

Question 3

explain trichromatic theory

Answer 3

explains colour vision on a photoreceptor layer. states that any colour can be expressed using the 3 primary colours

[when a cone pigment absorbs light, a biochemical reaction occurs that sets off a signal to the brain. eg erythrolabe has strongest response to reds

each colour we see is a mix of different relative activities of erythrolabe, chlorolabe, cyanolabe in L,M,S cones]

Question 4

explain opponent colour theory

Answer 4

explains CV beyond photoreceptors. explains that cones are paired together

such that only 1 cone can create a signal to the brain at a time through bipolar red-green and blue-yellow channels (NOTE: yellow channel is red+green cones combined)

eg. if red active, green supressed
if blue inactive, red+green still work to perceive yellow
VICE VERSA

Question 5

explain grassman’s law

Answer 5

states that colour vision system is linear by way of 3 properties

NOTE: metamers are 2 colours that look identical

(1) additive: if the same wavelength is added to 2 metamers, remain metamers
(2) scalar: if intensity is changed by equal amounts to 2 metamers, remain metamers
(3) associative: if one metamer substituted for another metamer, remain metamers

Question 6

describe 3 terms to describe colour

Answer 6

hue: colour, associative with wavelengths
saturation: purity eg. 100% saturated is 100% pure/rich, lesser saturated, less pure/faded
brightness: linked to photopic spectral sensitivity ie. 555nm is perceived to be brightest

Question 7

state what hue discrimination is

Answer 7

the amount of change in wavelength needed for one to detect a change in colour

REMEMBER ‘W’ shape with dip at 450nm

normal CV needs about 2nm change to detect different colour at 500nm

Question 8

state what saturation discrimination function is

Answer 8

it is the paleness of a colour (mountainous cone looking shape)

ie. only little amount of red or green (edges of shape) is needed for sample to turn ‘not white’ BUT a lot of yellow (570nm) can be added until sample turns ‘not white’

Question 9

state 2 systems for specifying colour

Answer 9

munsell colour appearance system and CIE

Question 10

explain munsell

Answer 10

each colour is designated a colour notation eg. 5R 7/2

hue is on the perimeter

saturation/chroma is on radius [range 1-14, 14 being purest]

brightness/value is on the vertical [range 0-10, 10 being brightest]

Question 11

explain CIE

Answer 11

to get dominant wavelength when 2 colours added together: locate 2 colours on fin and join using a line, then draw another line from W to midline of first line and extend to fin

to get excitation purity of dominant wavelength: measure line a and b, use formula a divide by (a+b)

to get complementary colour of sample: draw line from sample fin through W to opposite fin

Question 12

explain different types of abnormal vision

Answer 12

monochromat

protanope (red): L replaced by M,
protanomalous trichromat: L displaced to shorter wavelength

deutanope (green) M replaced by L
deutanomalous trichromat: M displace to longer wavelength

tritanope (blue): S replaced by M or L
tritanomalous trichromat: same wavelength but displaced to lower sensitivity

Question 13

explain hue discrimination graphs for CVDs

Answer 13

protanopia & deutanopia: good discrimination around 490nm (deu>pro), poor discrimination at long wavelengths **beyond 540nm, cannot discriminate based on hue (maybe intensity)

tritanopia: good discrimination at longer wavelengths but poor around 495nm

Question 14

explain saturation discrimination for CVDs

Answer 14

NOTE: neutral point = look white (y=0)

protan **492nm: -opia neutral point, -omal least saturated

deutan **498nm: -opia neutral point, -omal least saturated

tritanopia: 569nm neutral point

Question 15

explain confusion lines

Answer 15

CVD people will struggle with hue discrimination for colours lying on this line.

if same intensity, look identical. if different intensity, can still tell they are different colours

Question 16

name the clinical CV tests and purpose

Answer 16

ishihara - RG

city university - RG & BY

album tritan - BY

100 hue D15 (arrangement) - RG & BY

Question 17

explain kollner’s rule

Answer 17

typically..

RG defect: inner retina, optic nerve, visual pathway, visual cortex

BY defect: outer retinal diseases and media changes (cataract)

exceptions: glaucoma (BY initally, RG later), juvenile macular degeneration (RG defect), dominant optic atrophy (BY defect)

Question 18

compare/contrast main differences between congenital and acquired CVD

Answer 18

C vs A

bilateral vs uni/bi

M>F vs M=F

tends to be RG vs RG/BY

stable vs progressive