Introduction to Colour

• Light sources emit intensity:

  

  assigns intensity to each wavelength of light

• Humans perceive as a colour - navy blue, light green, etc.
• Exeriments show that there are distinct I with are perceived as the same colour (metamers)
• Normal human retina have three types of colour receptor which respond most strongly to short, medium, or long wavelengths.

  (picture)

• Note the low response to blue.

  One theory is that sensitivity to blue is recently evolved.

• Different animals have different number of wavelengths that they are sensitive to:
  • Dogs: 1
  • Primates: 2 or 3
  • Pigeon: 4
  • Birds: up to 18 (hummingbird?)
• Different Regions of the eye are "designed" for different purposes:
  • Center - fine grain colour vision
  • Sides - night vision and motion detection

Tri-Stimulus Colour Theory

  

• Tri-stimulus Colour Theory models visual system as a linear map:

  

  where is a continuous function of wavelength

  C is a three dimensional vector space (colour space)

  

  where

  

  where L, M, and S are weight functions.

• Monitor has Red, Green, Blue additive colour system
• Most colour creation is guess work.

Colour Systems

  

• RGB (Red, Green, Blue) Additive
• CMY (Cyan, Magenta, Yellow) Subtractive (complement of RGB)

  Often add K (blacK) to get better black

• HSV (Hue, Saturation, Value) Cone shaped colour space

  Also HSL (double cone)

• CIE XYZ (Colour by committee) More complete colour space

  Also L*u*v and L*a*b

• YIQ (Y == Luminence == CIE Y; IQ encode colour)

  Backwards compatible with black-and-white TV (only show Y)

Readings: Hearn and Baker, Chapter 15, 14-5. Glassner, Volume 1, Chapter 1, 2.