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Home | Book Supplements | Metamerism

Metamerism

Source: This article is provided by Wikipedia and licensed under the GNU Free Documentation License.

Metamerism is a psychophysical phenomenon commonly defined incompletely 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, of which the first two listed below are most commonly referred to and also most commonly confused:

  • Sample metamerism
  • Illuminant metamerism
  • Observer metamerism
  • Geometric metamerism

Sample metamerism

Example: most people experience sample metamerism when putting on a pair of socks that appeared to be black 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 spectral power distributions between the incandescent and fluorescent lights each 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.

When two color samples appear to match under a particular light source, and then do not match under a different light source, this is an example of "sample metamerism." One can conclude that the spectral reflectance distributions of the 2 samples differ, and their plotted reflectance curves cross in at least 2 regions.

The two socks are called a "metameric match."

Illuminant metamerism

Illuminant metamerism is witnessed when you have 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, you can perceive significant variations of the color. This phenomenon is rarely witnessed, unless you have a light box that allows you to see both lights separated by a divider, and your two identical samples illuminated by the different light sources. The lights are metameric matches. The sample does not matter in this case.

Example: When you visit a lighting department of a major home improvement store they will have a bank of lights with dividers in between. Grab a number of identical sample swatches from the paint chip department and place one identical sample under each light. Stand back to witness how each illuminant affects the sample.

Observer metamerism

Every individual perceives colour slightly differently. (Assuming the individuals possess adequate color matching aptitude.) This can be demonstrated in many ways, but suffice it to say, observer metamerism is the reason there were 17 individuals tested to derive the 1931 "standard observer" values adopted by the CIE [1] and are still used as the basis for the majority of color science study today.

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 woman's eyes is, on average, slightly less than a man's, and that slightly different angle of stereoscopic viewpoint also falls under the category of geometric metamerism.

Graphic arts and color reproduction considerations

In the printing industry, metamerism is often considered to be a source of great frustration.

Explanation: Artists paint with oils, pastels, crayons, and various dyes and pigments, and each medium has unique spectral reflectance curves. The majority of colour reproductions utilize combinations of cyan, magenta, yellow, and black inks or colourants. No combination of these inks can generate spectral matches to the colorants originally used to produce the original art. Therefore, a printed reproduction of an original artwork is only a metameric match to the original, and accurate viewing is thus dependent upon the spectral characteristics of the illumination used.


This information is provided as a supplement to:

Spring into Digital Photography
by Joseph T. Jaynes & Rip Noël
Paperback: 336 pages
Publisher: Addison-Wesley Professional
ISBN: 0131853538

If you don't yet have your own copy of this popular book, you can buy it now. [Enjoy the PhotoGain.com preferred discount price & FREE domestic shipping!]

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