Topic 8 colour vision

Question 1

what does trichromacy theory state?

Answer 1

Trichomatic theory states that any colour can be expressed using the 3 primary colours – Red Green Blue

Question 2

what is monochromacy?

Answer 2

A person with only 1 photopigment is known as a monochromat or having the condition of monochromacy

true colour blindness, can only see diff intensities

different wavelengths of light can look the same to a monochromat by varying the intensities.

Question 3

what is dichromacy?

Answer 3

A person with 2 photopigments is a dichromat / has the condition of dichromacy.

They have some degree of colour discrimination.

1 wavelength can be matched by another metamer that is made up of 2 different wavelengths

(Metamers are 2 stimuli that appear identical but are physically different)

Question 4

what is trichromacy?

Answer 4

A person with all 3 photopigments present is a trichromat.

Due to the presence of 3 photopigments, a trichromat cannot match 2 different wavelengths by adjusting their intensities.

A wavelength can be matched by another metamer made up of 3 different wavelengths

Question 5

what does grassmans laws describe? What are metamers?

Answer 5

Grassman’s laws of metamers describe the general characteristics of trichromatic vision. Metamers are 2 stimuli that appear identical but are physically different.

Question 6

what are the 3 properties of grassmans laws?

Answer 6

Additive property: when the same radiation is added in an identical manner to 2 metamers, they remain metamers (look identical) but may now be a completely diff colour than the original

Scalar property: if the intensities of 2 metamers are increased or decreased by the same amount, they remain metamers. Only brightness appears to be changed. (intensity increase=brightness increase and vice versa)

Associative property: a match between metamers will be maintained if 1 metamer is substituited for another

Question 7

what is opponent colour theory?

Answer 7

where colour is processed by bipolar red-green and blue-yellow hue channels. Bipolar means that the channel can only signal one colour at a time but not both.

red cancels out green, blue cancels out yellow and vice versa

Question 8

what is hue?

Answer 8

closely associated with wavelength.In lay man terms, hue is the ‘colour’: red, blue, green, yellow etc.

Question 9

what is saturation?

Answer 9

Saturation is the purity or fullness of colour

A desaturated colour appears to have been mixed with white (‘washed out’)

Question 10

at what wavelength will stimulus be the least saturated?

Answer 10

570nm (there will be a dip in the graph)

Question 11

what is brightness?

Answer 11

perceived brightness of a stimulus follows the photopic luminance function. Under photopic conditions, 555nm appears brighter than stimuli of the same energy but different wavelength

Question 12

what is hue discrimination? describe the pattern seen in a hue discrimination function graph

Answer 12

Hue discrimination is the amount of change in wavelength that is required for an observer to detect a change in hue (colour).

For blue and red light, a large change is required whereas for other wavelengths, a change of 2nm can be discerned by an observer with normal colour vision.

Question 13

whatis saturation dicrimination? describe the pattern seen in a saturation discrimination function graph

Answer 13

Saturation discrimination describes the paleness of the colour.

More yellow is needed to make it look ‘not white’. Whereas just a little bit of blue or red will be enough to make the stimulus appear ‘not white’.

Question 14

how does the munsell colour appearance system allow us to describe colour?

Answer 14

It allows us to describe colours based on hue, chroma and value

Hue: related to wavelength: red, yellow, green, blue, purple and in between
(varies around the circle)

Chroma: related to saturation, ranges from 1-14
(the radiating sections)

Value related to brightness, ranges from 0-10 (the pole in the middle)

Question 15

how do we use the CIE diagram to find dominant resultant wavelength?

Answer 15

Locate 510nm and 530nm on the perimeter of the CIE ‘fin’. Draw a line connecting the 2 wavelengths.

Make a mark at the midpoint of this line, since the 2 wavelengths are added together in equal amounts.

Draw a line from W (white) towards the midpoint M, and extend it to the perimeter of the CIE fin again. This identifies the dominant resultant wavelength.

Question 16

how do we use the CIE diagram to find complement wavelength?

Answer 16

Locate the wavelength on the perimeter of the CIE ‘fin’.

Draw a line from this point through W and extend that line to the opposite end of the CIE ‘fin’.

The wavelength that the line reaches is the complementary colour.

Question 17

what are the 3 classifications of abnormal colour vision?

Answer 17

Dichromacy
Anomalous trichromacy
Monochromacy

Question 18

what is dichromacy? what are the 3 dichromacy conditions known as and what photopigments are replaced in said conditions?

Answer 18

Dichromacy is where the retina only has 2 out of 3 types of photopigments.

If erythrolabe is missing and replaced by chlorolabe, it is known as protanopia. A person with this condition is called a protanope.

If chlorolabe is missing and replaced by erythrolabe, it is known as deuteranopia. A person with this condition is called a deuteranope.

If cyanolabe is missing and replaced by either erythrolabe or chlorolabe, it is known as tritanopia. A person with this condition is called a tritanope.

Question 19

what is anomalous trichromacy? what are the 3 anomalous trichromacy conditions known as?

Answer 19

Anomalous trichromacy is where all 3 photopigments are present but the absorption spectrum of 1 of the pigments is displaced.

If erythrolabe spectrum is displaced to the shorter wavelengths, it is known as protanomaly. Such a person is known as a protanomalous trichromat.

If chlorolabe spectrum is displaced, to a longer wavelength it is known as deuteranomaly. A person with deuteranomaly is known as a deuteranomalous trichromat.

If cyanolabe is displaced to a lower position (decreased sensitivity), it is known as tritanomaly. The person with this condition is known as a tritanomalous trichromat.

Question 20

what is monochromacy and what are the 3 conditions of monochromacy?

Answer 20

Monochromacy is where only 1 pigment is present.

Rod monochromacy: only rods present in the retina
Very rare condition resulting in true colour blindness
There is poor VA and nystagmus present.
The patient suffers from photophobia (light sensitivity)

Blue cone monochromacy: only cyanolabe is present
The photopic spectral sensitivity will not be at 555nm, instead it will only have the spectrum of cyanolabe that peaks at 430nm
There is poor VA and nystagmus

Atypical monochromacy
This is where all cone photopigments are present but do not function.
There is normal VA, no nystagmus or photophobia.
The colour vision defect looks like a protan+deutan defect

Question 21

what is the peaks of photopic spectral sensitivity for normal, protanopia, deuteranopia and tritanopia?

Answer 21

Normal peak: 555nm

Protanopia peak is shifted to shorter λ

Deuteranopia peak is shifted to longer λ

Tritanopia peak is similar to normal but peaks at a lower sensitivity

Question 22

what is the effect that CVD can have on hue discrimination?

Answer 22

A person with normal colour vision only needs <2nm difference in wavelength to detect difference over most wavelengths.

Protanopes and Deutanopes have
Good wavelength discrimination in the region of 490nm
Poor wavelength discrimination in the long λ region
Beyond 545nm; unable to discriminate between stimuli based on λ differences alone

Tritans have Good wavelength discrimination at longer λs
Poor wavelength discrimination in the region of 495nm

Question 23

what is the neutral point of saturation discrimination for normal, protanopia, protanomaly, deuteranopia, deuteranopia and tritanopia?

Answer 23

Normals percieve 570nm as least saturated compared to other wavelengths.

Protans’ neutral point at492nm
Protanomals: no neutral point. They have abnormal saturation perception at 492nm

Deutans’ neutral point at 498nm
Deutanomals have no neutral point but an abnormal saturation perception at 498nm

Tritans’ neutral point is at 569nm

Question 24

what are confusion lines in a CIE diagram?

Answer 24

The confusion lines show where each CVD type will confuse colours. Two stimuli, if they have colours that lie on the same confusion line, will appear to be the same colour to the person with CVD.

Question 25

what are the 2 causes of CVD? What is kollners rule for ACVD?

Answer 25

Hereditary defects (Congenital) 
Affects more males, Affects both eyes
Usually Red-green defect (found on X chromosome)

Photopigments are coded by genes
Females have 2 X chromosomes; Males have 1 X and 1 Y chromosome
Congenital RG defect is sex-linked recessive
Found on X chromosome

Recessive: females must have the defective gene on both X chromosomes to manifest RG CVD. Males only have 1 X chromosome, if they carry the abnormal gene they will manifest RG CVD.

Acquired defect
2/3rd of ocular diseases
Can affect one or both eyes
Usually give blue-yellow defect

Kollner’s rule for acquired CVD:
Outer retinal disease and media changes give rise to B-Y defect
Possibly due to higher susceptibility of S-cones and rods to ischaemia and oxidative damage, as well as lower number of S-cones in retina (so loss of a relatively small number of S-cones will result in noticeable CVD)

Inner retina, optic nerve, visual pathway, visual cortex disease will give rise to R-G defect
Exception: glaucoma (B-Y defect initially, R-G later)
Juvenile macular degeneration (R-G defect)
Dominant Optic Atrophy (B-Y defect)

Question 26

What is excitation purity? How is it calculated?

Answer 26

Excitation purity is calculated by the formula a / (a+b) which indicates how ‘pure’ the resultant mixture is.

a and b is the length of the segment of the line connecting W, M and the perimeter of the CIE diagram.

a=w to m
b=m to perimeter

Question 27

What is the number of colour confusion lines for each type of CVD?

Answer 27

Protonopia: 17
Deuternopia: 27
Tritanopia: 11