Metamerism

Metamerism is perhaps the most challenging issue affecting the capture process. Objects that are perceived to have the same color under an observer model are known as metamers. However, these objects may not have the same perceived color when there is a change in conditions, or in this instance light source and the angle of illumination. This is known as a metameric failure. The consequence of this is that to get the desired result, the conditions under which the capture of the color is acquired during both profiling and reproduction must match very closely.

There are several reasons a set of metamers may have different colors when conditions change:

Observer metamerism
Since the observer model is specified, this is not usually an issue for reproduction. 

Illuminant metamerism
Color changes when the illuminant changes. In practice, one should strive for light sources without any significant spectral peaks for high-quality reproduction. Budget fluorescent tubes, energy-saving bulbs and LED lights can have spectral peaks that distort colors, even if they have the same color temperature (degrees K) as an expensive light source.

Instrument metamerism

This is caused by a mismatch between the observer model and the instrument. This needs to be calibrated, which is achieved using the camera profile in reproduction. Another example is monitor display calibration, using a simple tri-stimulus colorimeter. In this case, it might be necessary to stipulate a “display technology” manually (e.g., CCFL or LED) in the monitor calibration and profiling software. The mismatch between the instrument and observer can be so poor that the instrument fails the calibration process.

Geometric metamerism

This occurs when the geometry of illumination or viewing is changed.

By far the most challenging issue affecting the capture process is geometric metamerism. Ideally, a reflection target should consist of patches with a perfect reflecting diffuser. In this case, the appearance of a patch is unaffected by both the angle of the light and the viewing angle. In many cases, it is unlikely that the same can be said for the objects or materials to be photographed. In practice, it is likely that there will be a significant specular component.

The specular component is highly sensitive to the angle of light and view. For colored materials, the hue of the specular component is usually closer to the hue of the light source than the hue of the diffuse reflection, with the result that patches become brighter (especially dark patches) and colored patches become less saturated (especially highly saturated patches). However, the hue of a patch can often be assessed accurately.

Note that calibrating the processing to a set of test patches is only helpful to obtain accurate colors for materials that react similarly to light. For example, calibrating to a test target is great for calibrating test targets, but unlikely to be helpful to obtain accurate colors for glossy and translucent materials (e.g. paints, metals, and porcelain), non-isotropic materials (e.g. textiles, papyrus, and parchment) or textured surfaces (e.g. art with visible strokes and engravings).

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