CIE Chromaticity Diagram

Does a set of primaries exist that span the space with only positive coefficients?

• Yes, but no pure colours.

• In 1931, the CIE defined three standard primaries (X, Y, Z) . The Y primary was intentionally chosen to be identical to the luminous-efficiency function of human eyes.

  

  Reproducing Visible Colour

• Figure 6.19 shows the amounts of X, Y, Z needed to exactly reproduce any visible colour via the formulae:

  
  
  
  

• All visible colours are in a horseshoe shaped cone in the X-Y-Z space. Consider the plane X+Y+Z=1 and project it onto the X-Y plane, we get the CIE chromaticity diagram as shown in Fig. 6.20.

  

  CIE Chromaticity Diagram

• The edges represent the pure colours (sine waves at the appropriate frequency)

• White (a blackbody radiating at 6447 kelvin) is at the dot

• When added, any two colours (points on the CIE diagram) produce a point on the line between them.

L*a*b (Lab) Colour Model

• A refined CIE model, named CIE L*a*b in 1976

• Luminance: L Chrominance: a - ranges from green to red, b - ranges from blue to yellow (Fig, 6.21)

  

  LAB Colour Model

• Used by Photoshop