Definition of metamerism

Metamerism results when two objects have different spectral reflectance curves but the same colour coordinates for at least one illuminant. This is a limited definition and three more definitions can be added:

1. Illuminant metamerism results when two objects, which match under one set of conditions, no longer match when the illuminant is changed.
   
2. Observer metamerism results when two objects, which match under one set of conditions by one observer, do not appear to match by another observer.
   
3. Geometric metamerism results when two objects, which match under one set of conditions, no longer match when the geometry of illumination, or viewing, or both is changed.
   
4. Instrument metamerism results when two objects, which give the same colour coordinates under one set of instrument conditions, no longer do so when measured on another instrument.

The last form is that relating to instruments, commonly colorimeters but also to spectrophotometers of differing viewing geometries.

The problem of metamerism is a constant one in colour matching. Often it is impossible to eliminate. While it is a simple matter to obtain a metameric match for a single observer in a specified illuminant it must be realised that not only will the match not hold good in a different illuminant  but also that it will be unlikely to satisfy many observers in the original illumination for which it was matched. Differing retinal response characteristics will give different results in the summation products that create the sensation of colour just as will differing proportions of the spectral energy entering the eye.

If the differing values of R₁ and R₂, the reflectances of the metameric pair are such that the above equation holds then a change either of Ec to say Ea or x or y or z to any different shape will destroy the equality and so give a mismatch. A simplified representation of this CIE analogy with human colour vision is shown below. The weighting functions for D₆₅ and A have been modified by multipliers for the sake of scaling clarity only. It will be seen that the areas under x, y and z are equal for illuminant D₆₅ but unequal for illuminant A.