Editing Metamerism (color) (section)

== Metameric failure ==
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 [[Prepress proofing|proofs]] are best viewed under a 5000K [[color temperature]] lighting source, with good color rendering properties,<ref>{{Cite journal |last=Royer |first=Michael P. |date=2022-04-03 |title=Tutorial: Background and Guidance for Using the ANSI/IES TM-30 Method for Evaluating Light Source Color Rendition |url=https://www.tandfonline.com/doi/full/10.1080/15502724.2020.1860771 |journal=LEUKOS |language=en |volume=18 |issue=2 |pages=191–231 |doi=10.1080/15502724.2020.1860771 |issn=1550-2724}}</ref> for color accuracy.<ref>{{cite web |url=http://www.newsandtecharchives.com/issues/2003/12-03/pt/12-03_cshelp.htm |title=Color Source Help Desk |last=Nate |first=John |publisher=Newspapers & Technology |date=2003-12-01 |access-date=2018-12-15 |quote=compare the inkjet proof to the printed piece under 5000 K lighting conditions }}</ref>

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 coatings|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 [[cone cell|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.<ref>{{cite book |last1=G.A. |first1=Klein |title=Farbenphysik für industrielle Anwendungen |date=2004 |publisher=Springer}}</ref>

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.