Chapter 5: Perceiving Colour

Question 1

Colour is a perceptual experience

Answer 1

Our experience of different wavelengths as being differently colours is a result of the interaction between receptors in the eyes and the wavelengths of light reflected from the surfaces of objects

Question 2

Monochromatic

Answer 2

light consists of a single wavelength

Question 3

Heterochromatic

Answer 3

light having a wide range of wavlengths

Question 4

Achromatic

Answer 4

white light… means without, colourless

Question 5

3 dimensions of colour

Answer 5

hue, saturation, brightness

Question 6

Hue

Answer 6

• is a spectral colour (red, green) that has one wavelength
• Non-spectral is a combo of wavelengths (like pink and purple)
• the “colour” of the target
• wavelength

Question 7

saturation

Answer 7

• richness of colour (dark blue, baby blue, light white-ish blue)
• the degree of whiteness in the target
• spectral purity

Question 8

brightness

Answer 8

• grey levels
• the perceived intensity of the target
• luminance

Question 9

Subtractive Mixing

Answer 9

• paints
• gets darker, loses more wavelengths
• the source produces a wide range of wavelengths, some of which are eliminated (absorbed)

Question 10

Additive Mixing

Answer 10

• adding more wavelengths, gaining energy,
• gets whiter increase luminance
• TV, spotlights
• wavelengths from different sources are combined
• final result is lighter than the components (less saturated because we are reducing the purity of the light – adding broader range of wavelengths)

Question 11

Blue/Yellow/Green paint with subtractive mixing

Answer 11

B: Absorbed (Y, O, R) Reflected (B, G)
Y: Absorbed (B, G) Reflected (Y, O, R)
G: Absorbed (B, Y, O, R) Reflected (G)

Question 12

Two Stage Model

Answer 12

• Trichromatic Theory: 3 cones S(B) M(G) L(R)

- Opponency Theory: colour after effects, G+R and B+Y and W+B seem to go together this is the retnial ganglion cell

Question 13

Trichromatic Theory of Color Vision

Answer 13

• Proposed by Young and Helmholtz (1800s)
• Three different receptor mechanisms are responsible for color vision
• Behavioral evidence:
• Color-matching experiments Observers adjusted amounts of three wavelengths to match a comparison field to a test field
• later tested and found pigments that responded maximally to Short med and long wavelengths

Question 14

Metameric Color Matching Experiment

Answer 14

Results showed that:
It is possible to perform the matching task
Stimuli are metamers (i.e., stimuli are physically different but are perceived as identical)
Observers with normal color vision need at least 3 wavelengths to make the matches
Observers with color deficiencies can match colors by using only 2 wavelengths

Question 15

Principle of Univariance

Answer 15

• Each cone mechanism has only one dimension of response (i.e., firing rate or number of action potentials) and that reflects the amount of light absorbed by the cone
• As such is unable to distinguish changes in intensity of a light from changes in the spectral composition (i.e., wavelength)
• It is impossible to work backward from the response of a single cone to determine the wavelength of the light that caused the response
• The output of a photoreceptor is a product of the intensity of the light and the sensitivity of the receptor to that particular wavelength:
• Response = Stimulus Intensity * Relative Sensitivity to Wavelength