Metamerism (color theory)
With metamerism , or metamerism ( Greek meta for by, among and Greek meros for part, or "consisting of several parts"), the situation is meant that different composite light spectra the same color stimulus own, so the same color impression cause. Conversely, a change in the assessment conditions as well as a change in the “normal observer” can lead to different color perceptions. The relevant assessment conditions include the light color of the lighting and the type of light selected .
Pairs of colors that show this behavior are called metameric or conditionally equal . This has the practical consequence, especially with body colors , that their dyes or pigments are not monochrome and reflect a (relatively) broad spectral part of the ambient light .
The different color impressions such conditions the same body color under different lighting are using the metamerism index quantitatively described as normal this is daylight used or an artificial light source, the light is similar to daylight.
definition
same color impression under a type of light similar to daylight D65 (left); under illuminant A (center) the upper half of the sheet appears redder, under illuminant TL84 (right) the lower half of the sheet appears
Two samples are referred to as metameric or conditionally the same if, under certain conditions, they produce identical color valences but have different spectral reflection or transmission curves. Samples with the same spectral reflection or transmission curve are never metameric.
Correctly, the term “metamerism” can only be used if the generated color valences of both samples are exactly the same under certain conditions. If this is not the case, but only within the respective tolerance limits , the pair of samples is referred to as paramer . The latter is more often the case, since two samples rarely produce exactly the same color valence, but rather differ slightly. In the German-speaking area, however, a distinction is seldom made between paramerism and metamerism, rather both are summarized under the term metamerism.
Types of metamerism
Five types of metamerism can be distinguished according to the cause.
- Lighting metamerism
- The most common case of metamerism in practice is lighting metamerism. This denotes the correspondence between two colors under one reference illuminant and deviation under other illuminants. The cause of the metamerism here is the different light spectra of the respective types of light.
- Observer geometry metamerism
- With this type of metamerism, the colors match under one observer geometry, for example the 10 ° normal observer , but not under another observer geometry. This is caused by the uneven distribution of rods and cones in the eye.
- Observer metamerism
- This type of metamerism occurs between individual observers. The reason is that the perception of different observers is never exactly the same.
- Geometry metamerism
- In this case, metamerism occurs when changing the viewing angle , so it is caused by surface effects. This type of metamerism is known as the silking effect.
- Device metamerism
- This variant occurs between different color measuring devices and is generated by differences in the built-in components.
Examples of the impact
A garment can give a different color impression when viewed under a different light source. The cause is usually that the other lighting does not contain certain wavelengths that would be reflected by the textile colorants . Such differences exist e.g. B. between lighting with incandescent lamps and lighting with sunlight . The result is a color impression that differs from the previous one; conversely, it also deviates if additional wavelengths are available and reflected (e.g. in daylight). A typical case is when the seller shows the color of two fabrics in the aisle in front of the shop door in daylight instead of under mall light. In the fashion trade this effect is called: evening color.
Another case of metamerism is repainting a car body . The newly painted sheet metal parts no longer look the same as the original parts when extended in daylight, although the selected paint in the artificial light of the workshop led to the same color impression. The fact that both sheet metal parts can look different outside the paint shop is annoying when they are next to each other. The reason is that the paint selected or mixed in the workshop light does not contain the same pigment mixture as the original paint, even if it led to the same color impression with the lighting there. Of the wavelengths contained in daylight, parts of the original or repair paint are reflected, but they do not have the same distribution in both paints.
Physiological basics
Humans perceive light with wavelengths from around 380 nm to around 780 nm as different "colors". The color impression corresponds
- on the one hand a defined wavelength of light, that is to say a spectral color (or a narrow range of the spectrum around a spectral color);
- on the other hand, the same color impression can be produced by a light which is composed of components that are shorter-wave than the light of the spectral color and other components that are longer-wave than the light of the spectral color. Theoretically, this color impression is possible through any number of combinations of light sources with smaller and larger wavelengths and thus through a large number of identical color stimuli.
The cause lies in the retina of the eye , on which the color receptors ( cones ) do not identify individual wavelengths. Instead, the three existing human cone types are shown in light (ie wavelength) sub areas sensitive that overlap each other; only the maximum sensitivity of a cone type is characterized with a wavelength. The distribution width of the wavelength intensities of visible light is interpreted from the sum of signals received from at least two types of cones.
This principle of color vision does not allow any distinction between signal sums with the same value but different composition, or between two colors that are conditionally identical or metamerically. On the other hand, the technically important Additive Color Mixing is only possible because of this basic “deficiency”: the eye can be induced to see all colors of the spectrum, although only combinations of three colored lights are supplied to it.
Animal experiments have shown that for humans, metameric colors do not have to be metameric for other living beings and vice versa. This may be due to a different number of different color receptors, for example usually two in mammals and often four in birds; or the cause are deviating sensitivity curves of the color receptors, which means that the weightings of the cone signals necessary for color perception differ.
Behavior of metameric colors with subtractive color mixing
The different spectral components metameric colors in subtractive color mixing not equally subtracted . The pure cyan (the spectral range called cyan) can pass a yellow filter almost without restriction. A metameric cyan, made up of a narrow spectral range called blue and green, is transformed into green when passing through a yellow filter, because the blue component is filtered out (see figure opposite).
Metamerism index
The color difference Delta E is used as the metamerism index . This is the distance between the color locations that are assigned to the two color impressions in a three-dimensional color space , usually a Lab color space . The CIELab color space from the CIE standard valence system , which has been used since 1976, was incorporated into DIN 6176 as a further developed CIELab color space under the abbreviation DIN99 color space .
A color is considered to be metamerism-free if its Delta E with respect to two agreed light sources is not greater than 0.5, for inexperienced observers not greater than 1.0.
Practical handling of metameric colors
- Textile industry
- The buyer should assess the combinability of items of clothing in daylight because he usually wears them in daylight.
- For production purposes, colorants are sought that reflect the narrowest possible band of the light spectrum. Metamerism is exploited so that woven , knitted or knitted patterns only come to light when there is special lighting. The same colors are deliberately chosen only to a limited extent as colors for the pattern or the background or for both.
- Paint industry
- The color of paints is determined by the pigments they contain. There are many of them that allow an extensive color palette in even more possible combinations. Because this possibility is used to bring a new product onto the market in a new color as possible, more and more paints of the same color are only produced to a limited extent. Partial repainting of such a product - often a car body - only produces the same color impression if the repair paint contains the same combination of pigments. The car manufacturer's paint supplier keeps this secret. The color "replicas" of a paint by other paint manufacturers only have more or less the same pigment combinations as the original paint. The painter doing the repairs should assume that the paint from another manufacturer can be problematic and only use it if the repainting does not deviate from the original sheet in daylight (see figure opposite); a possible different color impression with artificial light is the lesser evil.
- Printing industry
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Spectrophotometers are used in the printing industry to determine the color difference, which is primarily determined by delta-E calculations (see above metamerism index). This is often done as the Euclidean distance in the 1976 formula or, more recently, with the Delta E 2000 formula. Permissible deviations are regulated in ISO standards (such as ISO 12647 ) or in process standards (such as IFRA standard ) - depending on the printing process and the substrate . Since printed products are often viewed under artificial light, D50 is used as the lighting standard.
literature
- Günter Wyszecki, WS Stiles: Color Science: Concepts and Methods, Quantitative Data and Formulas . 2nd edition. John Wiley & Sons, New York 1982.
- Kurt Schläpfer: Colorimetry in the graphic industry. UGRA, St. Gallen 2002, ISBN 3-9520403-1-2 .
Individual evidence
- ↑ DIN 6172: Metamerism index of pairs of samples with a change of light type . Beuth publishing house
- ↑ a b Zhaojian Li, Roy S. Berns: Comparison of Methods of Parameric Correction for Evaluating metamerism. (PDF; 1.2 MB) In: art-si.org. Munsell Color Science Laboratory, August 24, 2006, accessed February 1, 2012 .
- ↑ a b c d e Georg A. Klein: Color physics for industrial applications . 1st edition. Springer-Verlag, Heidelberg 2004, ISBN 978-3-540-40612-9 , p. 96 .
- ↑ Hans G. Völz: Industrial color testing . 1st edition. VCH, Weinheim 1990, ISBN 978-3-527-28083-4 , pp. 96 .
- ↑ DIN 6172: Metamerism index of pairs of samples with a change of light type . Beuth publishing house
- ↑ Paint industry and metamerism
- ↑ Moritz Schwarz: Adaptation of the QUIZ certification to the new ISO newspaper printing standard. WAN-IFRA, December 18, 2014, accessed March 7, 2017 .