This article is about metamerism as it relates to color theory. For other meanings see the disambiguation page.

Metamerism is a psychophysical phenomenon commonly defined as the situation when two samples match in color under one condition, but fail to match under another condition.

The underlying basis of metamerism is that visual color matches are possible between two samples even though the spectral reflectance factors of the two samples are different. Thus many color matches are conditional. Conversely, if two samples have identical spectral reflectance factors they unconditionally match when viewed under the same conditions. (...assuming all else being equal, that is.)

Types of metamerism

Illuminant metamerism is the most typical kind of metamerism. It occurs when two samples match if viewed under one light source, but then do not match under a different light source.

Geometric metamerism occurs when two samples match when viewed from one angle, but then fail to match when viewed from a different angle. The reason for geometric metamerism is that the spectral reflectance factors of samples often vary when measured at different angles.

Observer metamerism occurs because of differences in color vision between observers. Often these differences have a biological source so that, for example, the ratio of long-wavelength-sensitive cones to medium-wavelength-sensitive cones may differ from one person to the next. Thus, two spectrally dissimilar surfaces may match for one observer when viewed under a certain light source but then fail to match when viewed by a second observer under the same light source.

Field-size metamerism occurs because the ratios of the three cone types in the retina vary not only between observers but also within a single observer depending upon the position on the retina. Light from a small object, for example, may illuminate only the central part of the retina where there are no short-wavelength-sensitive cones at all whereas light from a similarly illuminated object viewed from closer up will illuminate a greater area of the retina and will activate the short-wavelength-sensitive cones. It is quite possible, therefore, for two objects to match when viewed from one distance but then fail to match when viewed from a different distance.

Metamerism and color constancy

The phenomenon of metamerism is sometimes confused with color inconstancy. However, the former refers to a phenomenon with a pair of samples whereas the latter refers to a single sample. Color constancy refers to the observation that samples tend to retain their approximate daylight appearance when viewed under a wide range of different light sources. When samples fail to behave in this way we refer to this as color inconstancy.

Relevance of metamerism and applications

In the printing industry, metamerism is often considered to be a source of great frustration. Artists paint with oil pastels, pastels, crayons, and various dyes and pigments, and each medium has unique spectral reflectance curves. The majority of color reproductions utilize combinations of cyan, magenta, yellow, and black inks or colorants. 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.

Understanding what causes metamerism and knowing how to spot and control metamerism in paper and graphic design applications is crucial to effective graphic design and commercial printing.

Color

Metamerie (Beleuchtungstechnik) | Metamerismo (color) | Metamerie