The term
illuminant metameric failure or
illuminant metamerism is sometimes used to describe situations in which two material samples match when viewed under one light source but not another. Most types of fluorescent lights produce an irregular or peaky spectral emittance curve, so that two materials under fluorescent light might not match, even though they are a metameric match to an incandescent "white" light source with a nearly flat or smooth emittance curve. Material colors that match under one source will often appear different under the other.
Inkjet printing is particularly susceptible, and inkjet
proofs are best viewed under a 5000K
color temperature lighting source, with good color rendering properties, for color accuracy. Normally, material attributes such as translucency, gloss or surface texture are not considered in color matching. However
geometric metameric failure or
geometric metamerism can occur when two samples match when viewed from one angle, but then fail to match when viewed from a different angle. A common example is the color variation that appears in
pearlescent automobile finishes or "metallic" paper; e.g.,
Kodak Endura Metallic,
Fujicolor Crystal Archive Digital Pearl.
Observer metameric failure or
observer metamerism can occur because of differences in
color vision between observers. The common source of observer metameric failure is
colorblindness, but it can also occur among "normal" observers. In all cases, the proportion of long-wavelength-sensitive
cones to medium-wavelength-sensitive cones in the retina, the profile of light sensitivity in each type of cone, and the amount of yellowing in the lens and macular pigment of the eye, differs from one person to the next. This alters the relative importance of different wavelengths in a spectral power distribution to each observer's color perception. As a result, two spectrally dissimilar lights or surfaces may produce a color match for one observer but fail to match when viewed by a second observer.
Field-size metameric failure or
field-size metamerism occurs because the relative proportions of the three cone types in the retina vary from the center of the visual field to the periphery, so that colors that match when viewed as very small, centrally fixated areas may appear different when presented as large color areas. In many industrial applications, large-field color matches are used to define color tolerances. Finally,
device metamerism comes up due to the lack of consistency of colorimeters of the same or different manufacturers. Colorimeters basically consist of a combination of a matrix of sensor cells and optical filters, which present an unavoidable variance in their measurements. Moreover, devices built by different manufacturers can differ in their construction. The difference in the spectral compositions of two metameric stimuli is often referred to as the
degree of metamerism. The sensitivity of a metameric match to any changes in the spectral elements that form the colors depend on the degree of metamerism. Two stimuli with a high degree of metamerism are likely to be very sensitive to any changes in the illuminant, material composition, observer, field of view, and so on. The word
metamerism is often used to indicate a metameric failure rather than a match, or used to describe a situation in which a metameric match is easily degraded by a slight change in conditions, such as a change in the illuminant. == Measuring metamerism ==