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Detector-Based Color Scale
Figure 1

Figure 1. Photograph of the new-generation tristimulus colorimeter.		    The detector-based calibration approach is being extended to colorimetry as well. Based on the spectral responsivity of the channels of a tristimulus colorimeter, a colorimetric scale can be realized and maintained. The low uncertainty of the spectral responsivity measurements can dominate the color measurement uncertainty. Since the successful realization of the first, trap-detector based tristimulus colorimetric scale, new generation, single-photodiode based tristimulus colorimeters are being developed. The channels of the colorimeters can be calibrated on the SIRCUS facility with a responsivity uncertainty of less than 0.15 % (k=2). This responsivity uncertainty will result in a change of 0.0004 in the x, y chromaticity coordinates for a Planckian radiator. This CCG project is funded by the Air Force where the color temperature lamp standards will be calibrated against the NIST developed reference tristimulus colorimeters.
Improvements in detector technology over the past decade have opened a new era in radiometric and photometric calibrations. Based on the spectral responsivity of tristimulus colorimeter channels, a colorimetric scale can be realized [1,2]. The achievable detector-based color measurement uncertainties are smaller than the uncertainties of current primary lamp standards.

A trap-detector based reference tristimulus colorimeter has been developed at NIST [3]. The four channels of the reference colorimeter were calibrated on the SIRCUS for spectral irradiance responsivity.

   Figure 2

Figure 2. SIRCUS measured spectral irradiance responsivities of the four channels of the trap-detector based reference tristimulus colorimeter.

schematic of figure 3

Figure 3. Exploded view of the first design of a new-generation tristimulus colorimeter.		 Second generation reference tristimulus colorimeters are being developed to transfer the NIST reference scale to the Air Force [4]. These colorimeters have been designed with large area single-element silicon photodiodes that are sealed with wedged windows. The filter wheel is temperature controlled and the filter combinations are individually matched to the photodiodes to perform the smallest possible spectral mismatch to the CIE color matching functions.

After the calculation of the channel calibration factors [1] of the reference tristimulus colorimeter and the determination of the tristimulus values of test light sources using these calibration factors, variable reference source models are used [5]. These models can adjust the reference spectral source distribution (used to calculate the channel calibration factors) with iterative calculatiolns until it becomes nearly equal to the test source distribution. Using this approach, spectral mismatch errors can be removed from the tristimulus color measurement of test sources.

Figure 4

Figure 4. Photograph of the final design of the new-generation tristimulus colorimeter.

References

  1. Eppeldauer, G.P., Spectral response based calibration method of tristimulus colorimeters, J. Res. NIST 103(6), 615-619 (1998).

  2. Eppeldauer, G.P., Brown, S.W., Miller, C.C., and Lykke, K.R., Improved accuracy photometric and tristimulus-color scales based on spectral irradiance responsivity, Proc. 25th Session of the CIE 1 (San Diego, CA, 25 June to 2 July, 2003), pp. D2-30 to D2-33.

  3. Eppeldauer, G.P. and Racz, M., Design and characterization of a photometer-colorimeter standard. Appl. Opt. 43(13) 2621-2631 (2004).

  4. Eppeldauer, G.P. and Ohno, Y., Development of the NIST Detector-based Color Temperature Scale (212 kB) PDF, Proc. CIE Expert Symposium on 75 Years of the CIE Standard Colorimetric Observer, CIE x030:2006, 55-60 (2006).

  5. Eppeldauer, G.P., Brown, S.W., Lykke, K.R., and Ohno, Y., Realization and application of a detector-based tristimulus color scale at the National Institute of Standards and Technology USA (686 kB) PDF, Proc. AIC'05, May 9-13, 2005 Granada, Spain (2005), pp. 693-696.
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For technical information or questions, contact:
George Eppeldauer
Phone: (301)-975-2338
Email: geppeldauer@nist.gov

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Online: May 2007