Objects (Level 8)

MetaCow as viewed by a human under daylight and incandescent light
This is another example from the MetaCow test image. The front half of each cow is a metameric match to the back half when viewed by an average human with normal color vision under typical daylight illumination. This is illustrated in the left half of the image. Metameric matches are two stimuli that match in color, but are different in their spectral power distributions (or in this case their spectral reflectances). When those same cows are viewed by the same human observer under incandescent light (right half of image), then the matches break down. The differences in spectral reflectances that were invisible under daylight become readily apparent under incandescent light. In other words, the metameric matches no longer hold under incandescent illumination.

How Can Two Objects Match in One Lighting and Not Match in Another?

Metamerism. A simple, one-word answer. Metamerism describes the phenomenon of metameric color matches. Isomeric (or identical) color matches occur when two stimuli have identical spectral power distributions (identical emission or reflection of light at every visible wavelength). However, since we only have three types of cone photoreceptors, it is possible for two stimuli to match in color without having identical spectral power distributions. As long as the integrated responses of the three cone types are equal, then the stimuli will match in color. Basically, a little less energy at one wavelength can be compensated for at other wavelengths. When color matches happen despite differences in spectral power distributions (or spectral reflectance of objects), they are called metameric color matches and they exhibit the property of metamerism.

Isomeric color matches hold for any observer and any viewing conditions. The two objects are physically identical and we cannot perceive a difference between them as long as they are viewed together under the same conditions. However, metameric color matches (sometimes called conditional matches) depend on both the illumination and the observer. Changes in the illumination can highlight, or diminish, certain differences in spectral reflectance properties and make them more or less pronounced. Likewise different observers have slightly different visual sensitivities and a nice metameric match for me might look like a significant mismatch for you. On top of that, our visual systems change as we age, so a match for me today might not have looked like a match to me 30 years ago.

Metamerism is one of the most fundamental concepts of color science and presents us with many opportunities (such as trichromatic color reproduction systems) and many challenges (such as materials that change appearance with changes in lighting or observer). It is an endlessly fascinating topic and it is interesting to ponder how important it would be in the natural world. Or is it just an artifact of man-made color systems?

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Updated: Dec. 31, 2010