Why color matching is inconsistent under different light sources - metamerism

In the color matching process, the following phenomenon is often found: under one light source, the target color sample and the experimental color sample are matched or the visual color difference is small, and under another light source, the target and the experimental color sample are not matched. Or the visual colors vary greatly. This phenomenon is called "me tamerism". Metamerism, as the name suggests, means that two color samples with different spectral reflectances have the same or very close visual color perception (or CIEXYZ, CIELAB values) under one viewing condition. Metamerism is one of the most commonly used and important concepts in color science. Its significance is as follows: 1 It is possible to make color reproduction possible. The visual evaluation of 2 color samples must be carried out under the same conditions. Taking automotive interiors as an example, many components are composed of different materials, and strict spectral reproduction between different materials is impossible, so it is necessary to ensure that the color difference of different components under various light sources is small.


Principle of metamerism and spectral reproduction

The figure below shows the spectral reflectance curves of two green color samples. You can see that there are multiple intersections between the two curves. When the two color samples are the same under a light source (daylight), the calculated CIEXYZ values ​​are the same (where S(λ) is the spectral power distribution of the light source, and R(λ) is the spectral reflectance of the color sample. x(λ), y(λ), z(λ) standard observer color matching function); when the visual perception difference is large under the tungsten halogen lamp source, the calculated CIEXYZ value difference is large.

Spectral reflectance curve of a pair of color samples

The same color perception of the two color samples in daylight


The visual perception of the two color samples is different under the tungsten halogen lamp

From the above calculation formula, the spectral reflectance R(λ) of the object is fixed, so that the source of the metamerism can only be two, the difference of the light source S(λ) and x(λ), y(λ), The z(λ) standard observer color matching function is different, that is, the observer isomerism and the source isomerism. The most common in industrial applications is the source isomerism, as in the following example, two colors in daylight. The samples are matched, and the two color samples vary greatly under the tungsten halogen light source. By the way, the current spectral recurrence is a research hotspot in the academic world, because if the spectral reflectance of the target and the experimental color sample are exactly the same, the two color samples are consistent under any light source, essentially eliminating the problem of metamerism. . Spectral reappearance is of great value to industries such as cultural relics that have extremely high color requirements.


Metamerism example two


Light source metamerism and daylight simulator

In addition, color matching staff often encounter this situation: the color sample confirmed in the laboratory was returned to the customer. Most of this is due to the metamerism of the source. In the case of D65, the D65 of the laboratory light box is different from the D65 of the customer light box. For example, X-Rite's two light boxes Judge II and Spectralight QC, Judge II's D65 simulator uses fluorescent light, while Spectralight QC uses Tungsten lamp + filter mode; the measured spectrum of the two and the standard D65 spectral power distribution curve are shown in the figure below. Obviously, the actual emission spectra of Judge II and SpectralightQC's daylight simulators are significantly different, which also results in a color sample confirmed by the laboratory under the J65's D65 lighting, which may be rejected under the customer's Spectralight QC D65. In addition, whether it is Spectralight QC or Judge II's daylight simulator, it is different from the standard daylight D65, which results in the CIELAB value measured by the measuring device (commonly used standard D65 SPD calculation), and visually perceived color ( Lightbox D65 actual SPD) is inconsistent. In addition to the commonly used color rendering index CIE Ra, the International Commission on Illumination specifically targets the lightbox's daylight simulator with the introduction of the metamerism index MI for evaluating the performance of the lightbox's daylight simulator.

Metamerism index me tamerismIndex (MI)

The ISO2360 CIE S 012E standard defines an index for evaluating the spectral quality of daylight simulators D50, D55, D65 and D75, ie the metamerism index, which is divided into the metameric index MIVIS of the visible region and the metameric index MIUV of the ultraviolet region.

For MIVIS, each solar simulator uses 5 sets of virtual non-fluorescent color pairs. The figure below shows the spectral reflectance of the 5 sets of virtual color samples used to calculate the MIVIS of D65. Calculate the 5 sets of color samples in real The average of the color difference in the case of daylight simulator illumination and the CIE1964 standard color observer is the MIVIS of the daylight simulator. If it is a standard daylight SPD, the MIVIS is zero.

For MIUV, each daylight simulator uses three sets of virtual fluorescent color pairs, including spectral reflectance and fluorescence reflection characteristics, to calculate the three sets of color samples for real daylight simulator illumination and CIE1964 standard color observers. The average color difference is the MIUV of the daylight simulator. If it is a standard daylight SPD, the MIUV is 0.



The MIVIS and MIUV ratings are shown in the table below. The international standard for MI for commercial daylight simulations for color light boxes is BC, while the measured light quality parameters for color light boxes on the market rarely reach BC levels, especially for UV bands, most of which are UV. The MIUV of the fluorescent tube is D grade. At present, the best daylight simulator mode is to simulate the standard daylight for multi-channel LEDs, and its MI parameters can reach AB or even AA grade, which is the performance that traditional fluorescent light boxes can't achieve!

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