Lecture 13 Colour

Question 1

What indicated that different colours on the spectrum have different physical properties?

Answer 1

the fact that the degree to which beams from each part of the spectrum were bent by the second prism differed

Question 2

When are chromatic colours (e.g. blue, green, red) perceived? Provide an example.

Answer 2

• when certain wavelengths are reflected by objects more than others –> SELECTIVE REFLECTION
• e.g. red paper reflecting long wavelengths (and absorbing short/medium)

Question 3

When are achromatic colours (grey, black, white) perceived? Provide an example.

Answer 3

• when light is reflected equally across the spectrum
• e.g. white paper reflecting all wavelengths equally

Question 4

What do reflectance curves plot?

Answer 4

percentage of light transmitted at each wavelength

Question 5

What is the colour of transparent objects created by?

Answer 5

selective transmission

Question 6

When does selective transmission occur? What kind of curve can be used to plot it?

Answer 6

• when only certain wavelengths pass THROUGH objects (e.g. cranberry juice selectively transmitting long wavelengths results in a reddish appearance)
• transmission curves

Question 7

What colour is perceived with short, medium, long and medium, and long wavelengths?

Answer 7

• short = blue
• medium = green
• long and medium = yellow
• long = red

Question 8

What is subtractive colour mixing?

Answer 8

• occurs when mixing together paints that have different pigments
• adding more pigments to a mixture results in fewer wavelengths being reflected (and more being absorbed)

Question 9

Why does a mixture of blue and yellow paint appear green?

Answer 9

• blue reflects short wavelength (and SOME MEDIUM) and ABSORBS OTHERS
• yellow reflects long wavelength (and SOME MEDIUM) and ABSORBS OTHERS
• after combined, both pigments continue to reflect the same wavelengths they did on their own
• therefore, the only wavelengths that are reflected from a mixture of blue and yellow are MEDIUM wavelengths (i.e. green)

(SUBTRACTIVE)

Question 10

What is additive colour mixing?

Answer 10

• occurs when mixing lights of different wavelengths
• all of the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed

Question 11

Why does superimposing blue and yellow lights onto a whiteboard lead to the perception of white?

Answer 11

short (blue) + medium and long (yellow) wavelengths are ALL reflected back to our eyes, creating white

Question 12

What are spectral colours?

Answer 12

those that appear on the spectrum

Question 13

What are nonspectral colours?

Answer 13

can only be created by mixing spectral colours in various combinations (e.g. magenta by mixing red and blue)

Question 14

what is hue?

Answer 14

another term for chromatic colour (blue, red, etc.) or ‘pure’ colour

Question 15

What is value?

Answer 15

refers to the light-to-dark dimension

Question 16

What is saturation?

Answer 16

determined by the amount of WHITE in a hue

Question 17

TRUE or FALSE: more white = value decreasing, and more dark = desaturation

Answer 17

FALSE: more white = desaturation, and more dark = value decreasing

Question 18

_______________ can be used to determine additive colour combinations.

Answer 18

HSV (hue/satuation/value) colour solids

Question 19

What is the trichromatic theory?

Answer 19

states that all human colour vision is based on 3 principle colours (red, blue, green)

Question 20

Describe the behavioural evidence for the trichromatic theory.

Answer 20

• observers with normal colour vision could match any reference wavelength by mixing the proportion of 3 wavelengths (red, green, and blue)
• > 3 were NEVER needed and 2 was sufficient to match some colours
• observers with colour deficiencies could match their perception of any colour with 2 wavelengths
• this meant that ‘normal’ colour vision depends on 3 receptor mechanisms (i.e. 3 types of cones)

Question 21

What is the physiological evidence for trichromatic theory?

Answer 21

• visual pigments were found that respond maximally to short, medium, and long wavelengths
• colour perception is based on the combined responses of the 3 different types of cones, which vary depending on the wavelengths available

Question 22

TRUE or FALSE: colour matching experiments show that colours that are perceptually similar can be caused by different combinations of various physical wavelengths.

Answer 22

TRUE

Question 23

TRUE or FALSE: the cone pigments fire in an all or nothing response.

Answer 23

FALSE: cone pigment types differ in the PROPORTION OF LIGHT they absorb from particular parts of the light spectrum (i.e. short cones do not ONLY respond to blue)

Question 24

Can one receptor type lead to colour vision?

Answer 24

NO - because any 2 wavelengths can cause same response in a monochromat simply by altering the intensity

Question 25

What is the principle of univariance?

Answer 25

- absorption of a photon causes the same effect in all receptors, no matter what the stimulating wavelength is - once a photon of light is absorbed by a visual pigment molecule, the identity of the light's wavelength is lost - THE ONLY INFO THE NEURAL SYSTEM ENCODES IS THE TOTAL AMOUNT OF LIGHT ABSORBED

Question 26

review slides 24-25

Answer 26

slides 24-28 (pay attention to the curve)

Question 27

TRUE or FALSE: the ratio of responses of 2 different pigments (associated with 2 different cone types) to 2 different wavelengths changes depending on intensity/brightness

Answer 27

FALSE: ratio is always constant

Question 28

In terms of colur vision and receptors, what helps us encode diffrent wavelengths as being distinct from each other?

Answer 28

the relative difference in activity across receptor types (i.e. the ratio of responses of 2 different pigments)