Lecture 9: The perception of colour

Question 1

6What does the colour of an object depend on

Answer 1

which wavelength it reflects

Question 2

Spectral reflectance

Answer 2

proportion of wavelengths reflected by an object

Question 3

How many types of photoreceptors in the retina

Answer 3

4
Rods
Cones: S, M, L
each receptor is sensitive to different ranges of wavelengths

Question 4

Principle of univariance

Answer 4

different wavelength intensity combinations can produce the same response from a photoreceptor photoreceptor signals only the amount of light absorbed (number of protons), not which wavelength is absorbed.
Single cone response won’t give colour vision.

Question 5

photoreceptor response

Answer 5

• Maximum for preferred wavelength, BUT
• same for other pairs of wavelengths (e.g., 450 nm and 625 nm)
• lower for preferred wavelength at lower density

Question 6

seeing colour at night

Answer 6

• The night scene is drained of colour as rods alone do not allow colour vision, see a grey scale as only rods are responding as they alone wont give colour. Even though they also have a peak wavelength in which they respond to.
• Need a combination of cones to see colour

Question 7

trichromacy

Answer 7

Colour vision depends on the ratio of three cone outputs
The colour we see is based on the ratio of the responses of the three cone types in the retina. Perceived colour depends on the three cone type to object spectral reflectance

Question 8

Importance of primary colours

Answer 8

any colour can be matched using a combination of three primary colours
led to discovery of the cones

Question 9

Trichromatic theory of colour vision:

Answer 9

• Young-Helmholtz theory (1800’s)
• Based on behaviour observed in colour matching experiments
• Maxwell found a proof of the theory (1800’s)
• Discovery of three cone types came much later

Question 10

Seeing white or grey

Answer 10

perceived when the three cone types are stimulated equally
no single wavelength excites the cones equally
as you increase the light level it is going to become more of a whitish colour

Question 11

What are metamers

Answer 11

a consequence of univariance and trichromacy
physically different, perceptually equivalent
could be: two different wavelengths or one wavelength resulting in the same response as when two cone types were mixed

Question 12

Dichromats

Answer 12

Have two cone types

• Have a neutral point: a monochromatic wavelength of light that is confused with white light, produces same response for L and M cones
• Can match all colours using two primary colours, but see a smaller variety of colours

Question 13

How common colour deficiency is

Answer 13

8%, 0.5% female
M & L pigments on X chromosome
tritanope is rare as not coded in the same was as the other two types

Question 14

Types of colour blindness: colour anomalous

Answer 14

Similar photopigments in different cone types (L and M have very similar photopigments in the two cone types, should be different, as a result it seems as though they are missing that cone as they appear so similar

Question 15

Types of colour blindness: colour monochromat

Answer 15

only one cone type not see colour

Question 16

Types of colour blindness: rod monochromat

Answer 16

rods only, no cones, not see colour and poor visual acuity

Question 17

Types of colour blindness: tetrachromacy

Answer 17

4 cone pigments (more common for women see a broader range of colour)

Question 18

Types of colour blindness: cortical colour deficiencies - achromatopsia

Answer 18

can perceive colour boundaries, but not the colour themselves (eyes see colour fine but the brain can’t perceive colour, is failing)

Question 19

vision in bees

Answer 19

tetrachromatic

Question 20

Mantis shrimp vision

Answer 20

16 photoreceptor types

Question 21

three primaries of defining colours: LMS

Answer 21

perceptual colour space based on 3 cone outputs

Question 22

three primaries of defining colours: RGB

Answer 22

used in computer applications; red, green, blue

Question 23

three primaries of defining colours: HSB

Answer 23

a rearrangement of RGB

Question 24

three primaries of defining colours: CIE XYZ

Answer 24

internationally-used transformation of RGB

Question 25

Tristimulus values

Answer 25

percentage of each primary used to produce a given colour

Question 26

Additive mixtures of light - RGB colour space

Answer 26

three primaries add up to white

Question 27

Subtractive mixtures of pigment - CMYK colour space

Answer 27

inks subtract from white to produce black

Question 28

Hue

Answer 28

wavelength

Question 29

Saturation

Answer 29

richness vs. whiteness or paleness

Question 30

Brightness:

Answer 30

intensity (zero brightness = black)

Question 31

non spectral hues

Answer 31

mixture of wavelengths

Question 32

Opponent colour theory: Hering, 1800's

Answer 32

observed that red and green don't blend mutually exclusive hues red vs. green blue vs. yellow + black and white for brightness known as opposing colour channels

Question 33

perceptual phenomena that support opponent colour theory

Answer 33

mutually existing hues colour afterimages colour contrast

Question 34

Colour afterimages

Answer 34

Adaptation effects caused by neurons as they will first be excited and fire but can’t keep it up (take the stimulus/colour away) and give it something else that’s neutral. So all the other stuff that’s firing instead is gonna be what you see. What you see depends on which neurons are firing.

Question 35

opponent colour model (or the colour channel model)

Answer 35

colour perception is controlled by the activity of two opponent systems If there's a lot of activation in the M and L cones it means they have received a lot of light. S doesn’t contribute much. L and M respond to a similar range of wavelengths different from S. L-M cones tell us if there if more greenish wavelengths or reddish wavelengths

Question 36

Opponent colours and HSB: Hue channels (represents the different colours)

Answer 36

- L-M… red/green - S-(L+M)…blue/yellow

Question 37

Opponent colours and HSB: Brightness channel

Answer 37

L + M (more activation = brighter)

Question 38

Opponent colours and HSB: Saturation

Answer 38

ratio of hue channels to brightness channel: more hue = more saturated * L-M / L+M * ( [L+M] -S ) / L+M less saturated means the colour becomes less intense

Question 39

Opponent colours and HSB Neutral point

Answer 39

Midpoint of opponent mechanism , if RG and YB channels are at their neutral point, colour appears grey

Question 40

Opponent colour theory compared to trichromatic theory

Answer 40

trichromatic theory applies to colour detection at the photoreceptor level opponent theory applied to colour perception brought about by neurons at later stages compare cone response colour appearance is based on comparison of cone responses

Question 41

Colour opponency in LGN and cortex

Answer 41

Cone opponent cells: retinal ganglion cells, LGN Centre-surround organisation - L-cone centre, M-cone surround = L-M channel - M-cone centre, L-cone surround = M-L

Question 42

Colour opponency in LGN and cortex: single opponent cell

Answer 42

broad patches of colour

Question 43

Colour opponency in LGN and cortex: double opponent cell

Answer 43

chromatic edges, Vi onwards, L+ M- in the centre and then something else in the surround

Question 44

Colour appearance and colour constancy: colour contrast

Answer 44

When the colour of a given region takes on chromatic attributes opposite to the surround The way you percieve the colour patch in the centre depends on the surround, that means the cone response alone is not just affecting how you see colour the other stuff around is also affecting it

Question 45

Colour appearance and colour constancy: colour assimilation

Answer 45

when colours take on the chromatic attributes of adjacent region

Question 46

colour constancy

Answer 46

the ability to percieve the true colour of an object despite the changes in the lighting conditions or illumination

Question 47

lightness constancy

Answer 47

perception of the reflectance of an object despite changes in illumination white is white in dark or light

Question 48

illuminance

Answer 48

amount of light reflected on a surface

Question 49

reflectance

Answer 49

proportion of light reflected by a surface

Question 50

luminance

Answer 50

reflected light power weighted by eye sensitivity to different wavelengths

Question 51

lightness

Answer 51

perceived reflectance, e.g., what shade of grey is the object, the surface property

Question 52

brightness

Answer 52

perceived luminance, e.g., how much light is the object reflecting

Question 53

problems of colour constancy

Answer 53

our visual system strives to perceive the true color of objects despite changes in illumination, which makes the task of color perception mathematically complex and challenging

Question 54

solving the problem of colour constancy:

Answer 54

discounting the illuminant Inferences and assumptions that help to remove lighting from the equation

Question 55

What does colour constancy involve

Answer 55

knowledge or assumptions of the illuminant

Question 56

why does colour constancy not require knowledge of the object

Answer 56

Not a cognitive effect: not because you know what colour strawberries are, not based on beliefs * A perceptual inference from assumptions of lighting conditions * Constancy illusions occur when unusual illumination disrupts constancy mechanisms * visual system discounts the greenish illuminant and incorrectly concludes the strawberries are red

Question 57

Colour constancy vs. colour contrast

Answer 57

Constancy: same colour perceived despite different spectral content, and different cone outputs * Contrast: different colour perceived despite same spectral reflectance (of the test patch), and same cone outputs (for the test patch)