Metamerism is a psychophysical phenomenon commonly defined incorrectly as "two samples which match when illuminated by a particular light source and then do not match when illuminated by a different light source." In actuality, there are several types of metamerism, including sample, illuminant, observer, and geometric. The first two are most commonly referred to and also most commonly confused.
Sample metamerism: When two color samples appear to match under a particular light source but do not match under a different light source, this is "sample metamerism." One can conclude that the spectral reflectance distributions of the two samples differ slightly, and their plotted reflectance curves cross in at least two regions. By illuminating them with lights with considerably differing spectral power distributions, the visual differences between the two samples can be witnessed and even exaggerated.
Example: most people have experienced sample metamerism when putting on two socks that appeared to be black while in the bedroom, which may have incandescent lights, but later finding that one is black and the other is blue upon stepping into the kitchen, which may have fluorescent lights. The differences in the wavelength distribution between the incandescent and fluorescent lights interact with the differences in the spectral reflectance curves of the socks to make them appear the same in one light source and different in another.
Explanation: 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.
Illuminant metamerism: Illuminant metamerism is witnessed when there are a number of spectrally matched — exactly the same — samples, but when each is independently yet simultaneously illuminated and viewed under lights whose spectral power distributions differ, significant variations of the color can be perceived. This phenomenon is rarely witnessed, unless a light box that allows the observer to see both lights separated by a divider is used, and the two identical samples are illuminated by the different light sources.
Example: When visiting a lighting department of a major home improvement store, they will often have a bank of lights with dividers in between. A number of identical sample swatches from the paint chip department may be placed with one identical sample under each light. A observer may be able to see how each illuminant affects the sample.
Observer metamerism: Every individual perceives color slightly differently, assuming the individuals possess adequate color matching aptitude. This can be demonstrated in many ways. Observer metamerism is the reason that 31 individuals tested to derive the 1931 "standard observer" values adopted by the ISO that are still used as the basis for the majority of color science study.
Geometric metamerism: Identical colors appear different when viewed at different angles, distances, light positions, etc. It can be argued that one reason men and women often perceive color differently is that the distance between a woman's eyes is, on average, slightly less than a man's. This slightly different angle of stereoscopic viewpoint falls under the category of geometric metamerism.
Graphic arts and color reproduction considerations: In the printing industry, metamerism is the source of great frustration. It is perceived as a negative characteristic of color; if it did not exist, many believe, color reproduction problems would be eliminated. In actuality, however, it is this phenomenon that allows for mass color reproduction of an artwork.
Explanation: Artists paint with oils, pastels, crayons, and various dyes and pigments, and each medium has unique spectral reflectance curves. The majority of color reproductions utilize cyan, magenta, yellow, and black inks or colorants, although in some cases, printers incorporate a few additional colors to expand their gamut. None of these inks are exact spectral matches to the media originally used to produce the original art, however. Therefore, a printed reproduction of an original artwork reproduction is a metameric match to the original.
Inks used to create a color reproduction can be combined to simulate an artwork, but can only be made to accurately match the reproduction under only one (D50 or D65) light source. Metamerism makes it impossible to generate a color reproduction that can match under every light source. Without the phenomenon of metamerism, however, mass color reproductions would not be possible and the color reproduction industry as we know it simply would not exist.