Overview of Colour Vision

Question 1

What is light?

Answer 1

• electromagnetic radiation
  ◦ radiation is emitted from a source in small packets of energy called photons which travel at the speed of light
  ◦ photons vibrate at a frequency that increases w their energy
  ◦ can think of light as a wave - where speed of light, wavelength and frequency are related

Question 2

How does the electromagnetic spectrum work?

Answer 2

• high energy photons vibrate at high freq - shorter wavelength
• visible light is a narrow band of the EM spectrum
• light quality varies along 2 dimensions
  ◦ intensity - can be measured as anergy (watts/unit area) - relates to brightness in visible spectrum
  ◦ wavelength or chromaticity relates to colour

Question 3

How do you measure light intensity?

Answer 3

• light as radiant energy
  ◦ can be measures in watts/m^2
• light as luminance
  ◦ candela/m^2
  ◦ scaled by international conventions, according to spectral sensitivity of human eye
• light as photon flux (quantal flux)
  ◦ photons per unit area per unit time

Question 4

What does human colour vision depend on?

Answer 4

3 spectral classes of cone
* blue, green and red - more correctly termed SW, MW and LW cones
* summed photoreceptor signals determine luminous sensitivity over the visible spectrum
* colour depends on wavelength but cannot be equated with wavelength

Question 5

What is hue?

Answer 5

term for colour appearance, which changes from blue to green to yellow to red as wavelength increases

Question 6

What are spectral colours?

Answer 6

those that can be elicited by single wavelengths

Question 7

What is spectral surface reflection (SSR)?

Answer 7

• 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 8

What does the quality of light depend on?

Answer 8

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

Question 9

What is wavelength discrimination?

Answer 9

◦ 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 10

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

Answer 10

No, for example, a low intensity at 560nm could produce the same response as a high intensity at 450nm

• 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 o a single photoreceptor is colour blind

Question 11

What is the Principle fo Univariance?

Answer 11

a single photoreceptor is colourblind
* phototransduction signal depends on rate of photon absorbance (quantal catch)
* probability of absorption depends on wavelength