Metamerism

Metamerism is observed when two color samples appear to match under a particular light source, and then do not match under a light source with a different "spectral power distribution."

For example, one may experience color metamerism in putting on two socks that appear to be black while in the bedroom (which may have incandescent lights), but finding that one is black and the other is blue upon stepping into the kitchen (which may have fluorescent lights). The differences in light quality between incandescent and fluorescent lights causes the metamerism.

Incandescent light bulbs contain relatively little light in shorter (blue) wavelengths, and thus it would be more difficult to distinguish blue colors in such lighting conditions. The fluorescent illumination in the kitchen emits more short-wavelength light, and thus the dark blue can be more easily distinguished from black. In incandescent light, the socks are a "metameric match"; in fluorescent light, they do not match.

Technically speaking, these socks may have had "spectral reflectance curves" which differed slightly in the medium to short wavelength region. The curves must also have crossed each other at least twice on their spectral reflectances.

Metamerism may seem like a bad thing; however, metamerism is important in the graphic arts reproduction industry. A printed reproduction of an original artwork is almost always a metameric match to the original, since the inks used to create the reproduction are made to match the reproduction under only one (D50 or D65) light source. It would be rare for an artwork and its reproduction to match under each of many differing light sources.

Artists paint with oils, pastels, crayons, and other pigments, none of which are exact spectral matches of the inks used to reproduce the art. A spectral match is when the spectral reflectance curves match exactly, which also means they would match each other under any light source.