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. TutorialColor Managing InstrumentsWorkers in digital imaging and publishing use color management to achieve consistency throughout a workflow. The goal is to preserve the quality and accuracy of each image from capture to final reproduction. Each device in the workflow supports a different color space. The available color management systems profile the gamut capabilities of each device, and then limit the working color space to the color gamut that is shared by all of them. The workflow used by color metrologists is different. Many labs employ more than one instrument to measure color or appearance and to assure quality control. Here we'll focus on measurements of spectral reflectance factors ("SRF") for uniformly colored samples. A spectrophotometer is used to measure SRF data throughout the visible spectrum, which extends from 400 - 700 nm. Different instruments may not output the same SRF data based on measurements of the same color standard. In this situation tests of inter-instrument agreement may be used to measure the consistency between the participating instruments. Depending on the outcome of these tests, color management may be needed to enable the meaningful sharing of SRF data. Inter-instrument agreement cannot be achieved unless the test instruments employ the same optical geometry. In printing and the graphic arts metrologists who use sphere-based instruments employ the (d/8°) diffuse hemispherical specular-excluded ("SPEX") geometry. Others prefer the (0°/45°) or the (45°/0°) bi-directional geometry. Because gloss is important in the manufacture of paints and architectural ceramics, in those industries metrologists prefer the (d/8°) total hemispherical specular-included ("SPIN") geometry. Other factors may lead to differences in the measured SRF data among comparable instruments that employ the same geometry. Before listing these factors, we stipulate that the instruments to be compared must be maintained in good working order, and that suitable procedures are used to achieve consistent sample quality and to measure accurate SRF data. Differences in the SRF values measured by comparable instruments may result from the following types of wavelength-dependent error: (1) Photometric scale. (2) Wavelength scale. (3) Bandwidth. The standard multivariate regression procedure that is used to characterize these errors was first described in 1988 by R.S. Berns and K.H. Peterson. (Note: Consult the References page for a list of relevant publications.) This multivariate regression procedure detects and measures the following errors: (1) Photometric zero (black point). (2) Photometric linear scale (100% reflectance line). (3) Photometric nonlinear scale. (4) Wavelength scale. (5) Bandwidth. This method cannot compensate for different instrument geometries or for the errors that result from translucency or thermochromism. During 2009 Mt. Baker Research introduced the Diagnostic Tile Sets, which offer a complementary approach for detecting and characterizing instrument errors. CyberChrome's OnColor Profiler® color management system can analyze the errors listed above on most of the spectrophotometers that are in use today. This system includes software and 32 reduced-translucency color standards that are optimized for profiling. The OnColor Profiler® software compares the SRF data measured on each test instrument with data from a master instrument. The software generates correction factors that may then be used to improve inter-instrument agreement with the master instrument. Linked Resources
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Phone: (360) 650-0771 | E-mail: <jackroot@mtbakerresearch.com>
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