Mt. Baker Research L.L.C.
2921 Sylvan Street/P.O. Box 28370
Bellingham, Washington 98228-0370
Defining Color Standards
Phone: (360) 650-0771 E-mail: <jackroot@mtbakerresearch.com>
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Mt. Baker Research L.L.C. Defining Color Standards Phone: (360) 650-0771 E-mail: <jackroot@mtbakerresearch.com> |
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Mt. Baker Research L.L.C. TutorialTranslucency Measurements
Consult the instruction manuals for the Diagnostic Tile Set and the Profiling Tile Set for references to the published literature. These manuals also present background information about translucency, explicit procedures for testing, validating and color managing spectrophotometers, and typical spectral reflectance factor data in 3 optical geometries. A translucent solid is not perfectly opaque. Part of the light incident on a translucent sample penetrates the surface where it undergoes internal scattering and lateral diffusion away from the point of entry. As shown in the diagram, these processes reduce the intensity of reflected light. As a result of lateral diffusion, the reflectivity of a translucent solid decreases as the size of the instrument's sample port is reduced. This effect introduces systematic errors in measured spectral reflectance factor ("SRF") data. The magnitude of these errors depends on the following: (1) Instrument geometry. (2) Surface characteristics of the sample. (3) Sizes of illuminated and measured areas of the sample. In commercial spectrophotometers that support diffuse illumination the entire exposed surface of the sample is illuminated and an optical system controls the area viewed by the detector. Over-illumination is achieved by configuring the viewed area to be smaller than the illuminated area. Experiments in which over-illumination is varied can be used to measure the fractional reflectance loss ("FRL") that results from lateral diffusion. During 2008 and 2009 Dr. Jack Root adapted this technique for use with the X-Rite ColorEye 7000A ("CE7000A") spectrophotometer. Next he measured FRL values for many ceramic color standards, glasses and plastics (see below). The CE7000A supports a wide range of over-illumination. Maximum over-illumination is achieved with the LAV/VSAV configuration in which the LAV sample port is combined with the VSAV lens setting. The VSAV/VSAV configuration minimizes over-illumination. The typical FRL results reported below were calculated from SRF data measured using the LAV/VSAV and VSAV/VSAV configurations of a newly purchased CE7000A (S/N 37132651108). The success of this method requires that the instrument's white calibration tile exhibit negligible translucency. This was true for the new CE7000A, but not for an older instrument (S/N 37116190602). Excel charts #1 - #8 (pages 9 - 16) in this certificate of translucency show typical FRL measurements for the 16 color standards included in Diagnostic Tile Set A. The experimental procedure is described on pages 2 - 3 of the certificate. The Excel charts are interpreted on pages 4 - 6. Although other spectrophotometers may be used to measure FRL values, the numerical results will depend on the following: (1) Optical geometry. (2) Translucency of the instrument's white calibration tile. (3) Dimensions of the LAV and VSAV sample ports. Translucency in Color StandardsMany optical materials exhibit translucency. The FRL values listed below are based on measurements of one sample of each material. (Note: The typical 2-sigma standard error of estimate is ± 0.03%.)
White Carrera® glass, 25.6% Conclusions and RecommendationsMeasurements of inter-instrument agreement between instruments that support large vs. very small sample ports should be based on opaque color standards. If the color standards are translucent, inter-instrument agreement cannot be achieved. Instrument profiling cannot mitigate the errors that result from the following sample characteristics: (1) Translucency. (2) Thermochromism. (3) Surface inhomogeneity. The guidelines listed below are based on the sample port sizes of the CE7000A. They are recommended for color standards that are used for testing and profiling instruments in the total hemispherical specular-included geometry. (1) FRL values should be less than 3.5% for instruments that support over-illumination and a LAV, MAV or SAV sample port. (2) FRL values should not exceed 1.5% for instruments that support a VSAV sample port. Linked Resources
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Phone: (360) 650-0771 | E-mail: <jackroot@mtbakerresearch.com>
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