Metamerism

Experienced color matchers can achieve a good color match by trial and error without using any instmmentation. In some cases, however, this technique can be a lengthy process, and should the desired match be outside the color space defined by the available color standards, the technician might spend too much time just to determine that the match is not possible. To get the most cost-effective match using a low metamerism in the shortest possible time, the use of a computet color matching system is preferable. 

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). ... 

Quantification of metamerism is difficult and its avoidance is a principal aim in color technology. The car where body color, upholstery, and plastic parts match in the Ht showroom should not show clashing colors in daylight. The obvious but rarely appHcable solution is to use the same pigments in all parts. Some illuminant metamerism is almost unavoidable, the aim of the color expert being to keep it within acceptable limits, ie, to achieve adequate color consistency. 

Color-order systems, such as the many MunseU collections available from Macbeth, have been described previously. Essential for visual color matching is a color-matching booth. A typical one, such as the Macbeth Spectrahte, may have available a filtered 7500 K incandescent source equivalent to north-sky daylight, 2300 K incandescent illumination as horizon sunlight, a cool-white fluorescent lamp at 4150 K, and an ultraviolet lamp. By using the various illuminants, singly or in combination, the effects of metamerism and fluorescence can readily be demonstrated and measured. Every user should be checked for color vision deficiencies. 

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". 

Metamerism is a change of colour under different illuminants. 

Ionisation Metamerism.—Numerous examples of this type of isomerism occur in the cobalt-ammine series, such as the two metameric substances, ehloro-nitrito-tetrammino-cobaltic chloride, [Co(NH3)4Cl. NOJC1, and dichloro-tetrammino-cobaltic nitrite, [Co(NH3)4C12]N02.1... 

Ammino-derivatives op Cobalt Salts—Cobaltous Salt Ammines—Cobaltic Salt Ammines—Mononuclear Cobalt-ammines containing One Atom of Cobalt in the Molecule—Cobaltic Salts with Trivalent Cation—Cobalt-ammines Containing Divalent Cation—Cobalt-ammines containing Monovalent Cation—Cobalt-ammines consisting of Non-dissociable Complex— Cobalt-ammines containing Monovalent Anion—Cobalt Salts containing Trivalent Anion—Polynuclear Cobalt-ammines containing Two or more Cobalt Atoms in the Molecule—Cobalt-ammines of Unknown Constitution— Ionisation Metamerism—Polymerisation Isomerism—Valency Isomerism —Co-ordination Position Isomerism—Isomerism due to Asymmetric Cobalt Atoms. 

METAMERIC Pertaining to one of the more or less similar segments or parts arranged in a longitudinal series composing the body, or to the condition where the body is composed of similar segments. 

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. 

Figure 7.5 A phylogenetic tree of animal phyla based on 18S ribosomal DNA from several laboratories (Raff, 1996). The most significant innovations are the appearance of (1) multicellularity, (2) tissues, (3) the anterior-posterior axis and central nervous system, (5) metameric segmentation, (6) pentameral symmetry, (7) neural crest and amplification of Hox clusters.

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. 

There is a second type of metamerism that is important to discuss and that is observer metamerism. This is where two objects appear to one observer to be a... 

Is there metamerism involved Do the samples have the same spectral curves or do they cross each other ... 

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. 

---