Metamerism refers to two samples that appear to match when illuminated by one light source, but do not match when illuminated by another.
Metamerism is a psychophysical phenomenon commonly incorrectly defined as “two samples that match when illuminated by a specific light source and do not match when illuminated by a different light source”. There are actually several types of metamerism, including sampler, illuminant, observer, and geometric. The first two are more commonly mentioned and also more commonly confused.
Sample Metamerism – When two color samples appear to match in a specific light source but do not match in a different light source, this is “sample metamerism”. It can be concluded that the spectral reflectance distributions of the two samples differ slightly and their plotted reflectance curves intersect in at least two regions. By illuminating them with lights with significantly different spectral power distributions, the visual differences between the two samples can be observed and even exaggerated.
Example: Most people have sampled metamerism by putting on two socks that appeared black while in the bedroom, which may have incandescent lights, but then found one to be black and the other blue when walking into the kitchen, which may have fluorescent lights. Differences in wavelength distribution between incandescent and fluorescent lamps interact with differences in the spectral reflectance curves of socks to make them look the same in one light source and different in another.
Explanation: Incandescent lamps contain relatively little light in shorter blue wavelengths and therefore it would be more difficult to distinguish blue colors under such lighting conditions. Fluorescent kitchen lighting emits more short-wavelength light, so dark blue can be more easily distinguished from black. In incandescent light, the socks are a “metameric combination”; in fluorescent light, they don’t match.
Illuminant metamerism: Illuminant metamerism is observed when there are several spectrally matching (exactly the same) samples, but when each is illuminated independently but simultaneously and viewed under lights with differing spectral power distributions, color variations can be perceived significant. This phenomenon is rarely observed unless a light box is used that allows the observer to see the two lights separated by a divider, and the two identical samples are illuminated by different light sources.
Example: When you visit the lighting department of a large home improvement store, they often have a set of lights with dividers between them. Multiple identical samples from the ink chip department can be placed with an identical sample below each light. An observer can see how each illuminant affects the sample.
Observer Metamerism: Each individual perceives color slightly differently, assuming individuals have the appropriate aptitude for color matching. This can be shown in several ways. Observer metamerism is the reason 31 subjects were tested to derive the ISO-adopted 1931 “standard observer” values that are still used as the basis for most color science studies.
Geometric Metamerism: Identical colors appear different when viewed at different angles, distances, light positions, etc. One could argue that one reason men and women often perceive colors differently is that the distance between a woman’s eyes is, on average, slightly shorter than a man’s. This slightly different angle from a stereoscopic point of view falls into the category of geometric metamerism.
Graphic arts and color reproduction considerations: In the printing industry, metamerism is a source of great frustration. It is perceived as a negative color characteristic; many believe that if it did not exist, color rendering problems would be eliminated. In fact, however, it is this phenomenon that allows the massive reproduction of the colors of a work of art.
Explanation: Artists paint with oils, pastels, crayons, and various dyes and pigments, and each medium has unique spectral reflectance curves. Most color reproductions use cyan, magenta, yellow and black inks or dyes, although in some cases printers will incorporate some additional colors to extend their gamut. However, none of these inks are exact spectral matches for the media originally used to produce the original artwork. Therefore, a print reproduction of a reproduction of an original work of art is a metameric correspondence to the original.
The inks used to create a color reproduction can be combined to simulate a work of art, but can only be made to accurately match reproduction under a single light source (D50 or D65). Metamerism makes it impossible to generate color rendering that can be the same for all light sources. However, without the phenomenon of metamerism, mass color reproductions would not be possible and the color reproduction industry as we know it would simply not exist.