Everyone knows that a colored object appears to be one color under a given light source, such as daylight, but appears to change color under another light source, such as an incandescent light bulb. This change in color, whicj occurs frequently with almost all colored objects, has been errpneously called "Metamerism". But what is the true definition of metamerism?

We use the term metamerism when the colors of two objects are perceived to have the same color under one illuminant, such as daylight (CIE D65), but have differnece colors under a different illuminant, such as an incandescent light bulb (CIE A). This is an usually unwanted effect. Imagine that you have a blue jacket and the materials used for sleeves and back exhibit metamerism - the jacket would look perfect in daylight, but in the evening in the light from an electric bulb the sleeves would appear to be a completely different color.

Metamerism can be explained clearly using the color's tristimulus values (X,Y and Z), which represent a color perception. Two samples will appear to have the same color under a given light if their X, Y and Z values calculated for this illuminant are identical. This will always happen when the sampes have identical spectral reflectance curves. However, metameric samples have diifferent spectral reflectant curves. The two curves will have the same X, Y and Z values, and thus the same color under one illuminant, but different X, Y and Z values, and thus different colors, under a different illuminant.