Overview of colour vision

Question 1

Describe light as electromagnetic radiation

Answer 1

• Light as a wave and equation:
  wavelength = speed of light / frequenc

Question 2

What are photons

Answer 2

Radiation is emitted from a source in small packets of energy = photons

Question 3

what speed do photons travel at?

Answer 3

speed of light

Question 4

how do photons vibrate?

Answer 4

Photons vibrate at a frequency that increases with their energy

Question 5

what is frequency?

Answer 5

the number of cycles per second pass a given point

Question 6

describe the electromagnetic spectrum

Answer 6

as you go right wavelength increase and as you go left frequency increases

Question 7

high energy photons vibrate…

Answer 7

at high frequency; shorter wavelength

Question 8

what is visible light on the EM spectrum

Answer 8

narrow band

Question 9

how does light quality vary?

Answer 9

-along 2 dimensions
-intensity and wavelength or chromaticity

Question 10

what is intensity?

Answer 10

-can be measured as energy (watts/unit area)
-relates to brightness in visible spectrum

Question 11

what is wavelength/chromaticity?

Answer 11

relates to colour

Question 12

how to measure light as radiant energy?

Answer 12

can be measured in watts/m^2

Question 13

how to measure light as luminance?

Answer 13

candela/m^2
-scaled by interaction conventions according to spectral sensitivity of human eye

Question 14

how to measure light as photon flux (quantal flux)?

Answer 14

• photons per unit area per unit time
• on the longer wavelength end
• stimulus intensity

Question 15

Brightness is the same as luminance. TRUE OR FALSE

Answer 15

FALSE

Question 16

what is hue?

Answer 16

Hue = colour appearance, which changes from blue to green to yellow to red as wavelength increases

Question 17

what are spectral colours?

Answer 17

Spectral colours are those that can be elicited by single wavelength

Question 18

what are non-spectral colours?

Answer 18

Many colours cannot be equated with specific wavelengths: purple, pink, brown… black, white (non-spectral)

Question 19

what does colour depend on?

Answer 19

Colour depends on wavelength but cannot simply be equated with wavelength

Question 20

what does colour vision entail?

Answer 20

Colour vision entails the ability to see colours and discriminate objects on the basis of colour

Question 21

describe colour as surface spectral reflectance

Answer 21

-Objects appear coloured due to the wavelength composition of the light reflected from the surface
- The SSR is a fixed physical/chemical property of a surface
- SSR describes the proportion of light reflected at each wavelength of the visible spectrum

Question 22

Is SSR the physical property that corresponds to colour?

Answer 22

Coloured objects do not reflect a single wavelength corresponding to a spectral colour

Question 23

colour appearance depends…?

Answer 23

Colour appearance depends on the relative amount of different wavelength

Question 24

Quality of the visible spectrum depends on …?

Answer 24

• an organisms spectral sensitivity
• e.g. humans have rather low sensitivity to wavelengths <400 nm
• Wavelength. Chromaticity or perceived hue, changes with wavelength
• signalled by comparison of cone signals

Question 25

what does human colour vision depend on?

Answer 25

3 spectral classes of cone

Question 26

what are the 3 spectral classes of cone

Answer 26

Blue, green and red; more correctly termed SW, MW and LW cones

Question 27

Explain photoreceptor spectral sensitivity

Answer 27

• Summed photoreceptor signals determine luminous* sensitivity over visible spectrum
  
  • • determined by sums of photoreceptor signals across classes
• Brightness increases when total signal summed from photoreceptor increases

Question 28

Vertebrates have a duplex retina. What are rods and cones for?

Answer 28

• rods for scotopic vision
• cones for photopic vision

Question 29

The quality of light varies along two dimensions. State them.

Answer 29

• Intensity - other things being equal, luminance and perceived brightness increase with intensity
• Wavelength - chromaticity or perceived hue, changes with wavelength

Question 30

Wavelength discrimination is a prerequisite for colour vision. Explain.

Answer 30

• The ability to discriminate changes in wavelength independently from changes in intensity
• need to show discrimination based purely on wavelength differences
• control for achromatic differences in intensity

Question 31

Is wavelength discrimination possible with a single spectrally-tuned photoreceptor?

Answer 31

• No, e.g. a low intensity at 560 could produce the same response as a higher intensity 450
• In theory, any two stimuli in the spectral range could be adjusted in relative intensity to produce the same response
• cannot disentangle wavelength and intensity - a single photoreceptor is colour blind

Question 32

Principle of univariance: a single cone is colour blind

Answer 32

• Phototransduction signal depends on rate of photon absorption (quantal catch)
• Probability of absorption depends on wavelength

Question 33

Wavelength discrimination requires comparison of photoreceptor responses

Answer 33

• Two spectral classes:
  
  • LW peak 560 nm
  • SW peak 440 nm
• A decrease in intensity at 550 would decrease activity in both photoreceptors
  
  • but a decrease in wavelength from 550 would decrease response of LW but increase response of SW responses

Question 34

Photoreceptors spectral classes

Answer 34

mono-, di-, tri-, tetra-chromats

Question 35

MONOCHROMATS:

Answer 35

animals with only a single spectral class of photoreceptors

Question 36

DICHROMATS:

Answer 36

two spectral classes: SW plus MW or LW, most mammals; some humans with colour deficiency

Question 37

TRICHROMATS:

Answer 37

three spectral classes: SW, MW, LW, old world primates (humans); some invertebrates - bees

Question 38

TETRACHROMATS

Answer 38

4 spectral classes, most vertebrates (birds, reptiles, fish); some invertebrates - butterflies