Metamerism is a psychophysical phenomenon with several types, including sample, illuminant, observer, and geometric. Sample metamerism is when two color samples match under one light source but not another due to differences in spectral reflectance. Illuminant metamerism is when spectrally matched samples appear different under different light sources. Observer metamerism is when individuals perceive color differently. Geometric metamerism is when identical colors appear different from different angles or positions of light. In the printing industry, metamerism allows for mass color reproduction, but accurate color matching under all light sources is impossible due to metamerism.
Metamerism is a psychophysical phenomenon commonly erroneously defined as “two samples that match when illuminated by a particular light source and then do not match when illuminated by a different light source”. In fact, there are several types of metamerism, including sample, illuminant, observer, and geometric. The first two are more commonly referred to and also more 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”. It can be concluded 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 markedly different spectral power distributions, the visual differences between the two samples can be observed and even exaggerated.
Example: Most people have experienced sample metamerism when putting on two socks that look black while in the bedroom, which may have incandescent lights, but later discover that one is black and the other is blue upon entering in the kitchen which could be fluorescent lights. The differences in the wavelength distribution between 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 at shorter wavelengths — blue — and therefore it would be more difficult to distinguish blue colors under such lighting conditions. Fluorescent lighting in the kitchen emits shorter wavelength light and therefore dark blue can be more easily distinguished from black. In incandescent light, socks are a “metameric match”; under fluorescent light, they do not match.
Illuminating metamerism:
Illuminant metamerism is witnessed when there are a number of spectrally matched samples that are exactly the same, but when each is illuminated independently but simultaneously and viewed under lights whose spectral power distributions differ, significant color changes can be perceived. This phenomenon is rarely seen, unless a light box is used that allows the observer to see both lights separated by a divider, and the two identical samples are illuminated by different light sources.
Example: When you visit a lighting department of a major home improvement store, they often have a string of lights with dividers in between. A number of identical samples from the paint chip department can be placed with an identical sample under each light. An observer may be able to see how each illuminant affects the sample.
Observer metamerism:
Each individual perceives color slightly differently, assuming that individuals possess adequate color-matching aptitude. This can be demonstrated in many ways. Observer metamerism is the reason 31 individuals attempted to derive the 1931 “observer standard” values adopted by ISO which are still used as the basis for most color science studies.
Geometric metamerism:
Identical colors appear different when viewed from different angles, distances, positions of light, etc. It can be argued that one of the reasons 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 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 characteristic of color; if it didn’t exist, many believe, color reproduction problems would be eliminated. In reality, however, it is this phenomenon that allows for the mass reproduction of color in a work of art.
Explanation: Artists paint with oils, pastels, pastels, 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 incorporate some additional colors to expand the gamut. However, none of these inks exactly match the spectrum of media originally used to produce the original art. Therefore, a printed reproduction of an original artwork is a metameric match 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 the reproduction under a single light source (D50 or D65). Metamerism makes it impossible to generate a color reproduction that can match any 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.
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