Light metamerism

Clearly, standardized light sources are desirable for color matching, particularly in view of the phenomenon of illuminant metamerism described below. Over the years CIE has defined several standard illuminants, some of which can be closely approximated by practical sources. In 1931 there was Source A, defined as a tungsten filament incandescent lamp at a color temperature of 2854 K. Sources B and C used filtering of A to simulate noon sunlight and north sky daylight, respectively. Subsequently a series of D illuminants was estabUshed to better represent natural daylight. Of these the most important is Illuminant E). ... 

When Munsell devised his color space, he did so on the basis of minimum observable color perception steps. But the problem with the Munsell System was one of reproducibility, which the CIE Standard Observer cured. In formulating a color match, one wants to be able to predict the correct concentration of colorants required, whose scattering and absorption properties are known, i.e.- the lightness, so as to match the sample submitted, starting with their spectrophotometric curves. In practice, this is not so simple, since two colors must have identical spectrophotometric curves to be exactly equal. It turns out that the human eye will identify the two colors to be equal if their spectrophotometric reflectances are reasonably close. Two colors may appear to be equal under Daylight illumination, but quite different under incandescent lamp illumination. These colors are known zus "metamers" and the phenomenon "metamerism". 

G. Anders, E. Ganz Metameric White Samples for Testing the Relative UV Content of Light Sources and of Daylight , Appl. Opt. 18 (1979) 1067-1072. 

The importance of the light source and other conditions that affect viewing of samples cannot be overemphasized. Many substances are metameric that is, they may have equal transmittance or reflectance at a certain wavelength but possess noticeably different colors when viewed under illuminant C. 

Metamerism is minimized in calculations (spectral matching ensures quality under all lighting conditions, not just default conditions). 

How does the color match trial appear relative to the color standard under different light sources Is there metamerism ... 

Metamerism The condition that exists when two colored articles appear to match under one light source but not another. The three common light sources used in color matching are simulated daylight, incandescent, and cool white flourescent. 

Let us consider color match quality. The color formulator should ask the customer if the closeness of a match to the color target is the highest priority. Since there are hundreds of colorants available for the color formulator to select from, more than one possible combination of colorants may produce an acceptable match. Is the best, nonmetameric match of most importance to the customer and application If a perfect, nonmetameric match is not possible, which is often the case when trying to match plastic colorant systems to color targets in paint, ink, ceramic systems, or plastic media, will the customer accept some metamerism Also what light source will be the customer s preference when judging a slightly metameric match ... 

The easiest part of the color-matching process is blending the various colors and additives to make the match. The selected colorants and additives should (1) match the hue of the target (watching for metamerism), (2) match the color saturation (chroma value), (3) match the lightness/darkness value, and (4) match the desired level of transparency or opacity. The colorist should consider and allow for the effect of any required additives on the color and evaluate the finished formulation for cost. 

Where assessments take place on a production line, good consistent conditions for viewing the sample or matching to a memorised or formal colour standard need to be made available. Market researchers often find themselves in conditions unsuitable for the making of reliable appearance assessments. A specification of the environment of the test, including types of illumination and colour of the test surrounds, should be noted in the final report. This will make it easier for the occurrence of colour metamerism problems to be minimised. If the consumer s views of product colour and appearance are important to the project, standard lighting and viewing conditions should be provided. 

It should always be borne in mind, however, that if a subjective assessment of the colour of a particular food is normally carried out under, say, a tungsten filament light or under direct sunlight, then it may be preferable to carry out the instramental measurement under similar lighting conditions. It should be noted also that a pair of samples which colour-match another under one illuminant may not match under a different illuminant. This phenomenon is known as metamerism and is of great importance to those in the textile and paint industries. 

The function of the light booths is not simply to shine light on a sample. It is to produce a particular spectrum of light that will reflect the proper colors off the sample. If there are any shifts in the spectral output, the samples may or may not match. This is due to an effect known as Metamerism. This effect will cause samples to match under one lighting condition, but not... 

Formulating to minimize shrinkage differences has its drawbacks as well. The first is metamerism. Metamerism occurs when two color samples (a standard and batch) match under one light source but no longer match when the light source is changed. This is important in automotive interiors, for example, where the OEM wants all of the materials in the vehicle to... 

Metameric color match n. A color match between two materials in which the colors are identical under some lighting conditions but not under others. Metameric color matches are common when different pigments or dyestuffs are used to color the two materials. 

Metameric pair n. A pair of colors, which match when viewed in a described way but which do not match if the viewing conditions are changed, thus, a metameric pair of samples exhibit the same tristimulus values for a described set of viewing conditions (observer, light source, geometry of the illumination, and viewing arrangement) but have different spectral distributions. Hence, they exhibit a match, which is conditional only. 

There are, necessarily, limitations to the above. Normally, the specular component of the reflected light is eliminated, and since the color matches are metameric, viewing conditions must be specified. Also, the greatest usefulness lies in the measurement of surface colors. The object should be opaque and nonfluorescent. The nature of the surface of tomato pur4e is far from ideal. Although many difficulties arise in borderline cases, and the result is dependent on the skill and color vision of the observer, the method has served a useful purpose. If however we examine the wording of the PMA specification, a sample shall contain as much or more red than that produced by spinning the specified Munsell discs. . . , it... 

If the match is to be metameric, the customer and supplier must agree on the light source(s) under which the color is to be evaluated. 

The colorist not only functions as a service to manufacturing colleagues but even more importantly provides a sales and customer service, dealing with all technical enquiries and special requirements, e.g. to match a particular fashion shade with a combination of dyes that will match the pattern provided by the customer. Not least among the colorist s difficulties is to choose a dye combination which will give a match under daylight and artificial light alike (metameric match). Computer methods have been developed by which dye combinations can be selected which have closely similar absorption characteristics in the visible to those of the pattern. 

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