Mechanisms of Colour Vision

Question 1

What is trichromatic theory?

Answer 1

The idea that there are 3 types of particles in the retina to detect red, green and blue and all other hues are a mixture of these primaries

Question 2

What is colour matching?

Answer 2

The basic idea that you can match any colour with a combination of blue, green and ref

Question 3

What is opponent colours?

Answer 3

The idea that there are 2 pairs of opposite colours - blue- yellow, green-red and that they are mutually exclusive

Question 4

Can we explain colour sensation by the synthesis of trichromatic theory and opponent processing?

Answer 4

No, it is more complicated than that

• The light reflected from an object surface will differentially affect the three sets of cones according to the SSR
• Light does not normally appear coloured but there is variation in its spectral quality
• This means that under different illuminants, light reflected from the object will appear ‘distorted’ frmo the SSR
• Therefore the relative cone responses will change the different lighting conditions

Question 5

What is the Macbeth Colour Checker?

Answer 5

• The industry standard matte-coloured surfaces
• SSR for plates 11, 12, 14, 15
• SSR shows relative reflectance at each wavelength
• Actual reflected light will depend on the quality of the illuminating light

Question 6

How will the light reflected from plate 11 change under different illuminants?

Answer 6

• first column is the spectrum of illuminant, the second is the SSR of the colours surface and third is the spectrum of reflected light reachign the eye
• for each wavelength: Y values of col 1 x col 2 = col 3
• event thought SSR of object constant, the reflected light reaching the retina has different spectral composition under diff wavelengths
  ◦ YET COLOUR APPEARANCE DOES NOT CHANGE
• spectrum of light reaching the eye depends on both SSR and the spectrum of the illuminant
• the spectrum of reflected light which actually reaches the ye is very diff under the 2 diff illuminants

Question 7

What are some of the implications of colour constancy?

Answer 7

• for a given surface, wavelength compositing light reaching the eye will change if wavelength composition of illuminant changes
• if the real-world property of colour is SSR, then the job of the visual system may be to estimate real SSR
  ◦ but can only be done if spectrum of the illuminant is known
  ‣ c = i s
  * where c is the spectrum of light reaching the eye, i is the illuminant and s is the SSR
  * if brain only knows c then the problem is not solvable
• wavelength composition of light reaching the eye is coded by the relative response of the three cone classes
• for a given object, wavelength composition of light reaching the eye (and therefore relative cone responses) will change id wavelength composition of the illuminant changes
• however, perception of object colour appearance does not normally change
  ◦ therefore colour appearance cannot be the same as wavelength composition
• the converse can also be true - wavelength composition of light reaching the eye can remain the same in situations where colour appearance does change