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Propagation of Detector-Based Photometric and Color Scales Using a Spectrally Tunable LED-Source

Published

Author(s)

George P. Eppeldauer, Steven W. Brown

Abstract

Improved accuracy photometric and tristimulus color scales are being developed at NIST utilizing the very low uncertainty of novel spectral responsivity determinations [1]. The scales are realized on a filtered tunnel-trap detector [2]. During scale realizations, monochromatic point sources are used [3] to calibrate the four channels (Y, X1, X2, and Z) of the filtered tunnel-trap detector against Si tunnel-trap transfer standard detectors. The channels are calibrated in irradiance measurement mode. The Si transfer standard detectors were calibrated against a cryogenic radiometer [4]. From the measured spectral irradiance responsivities, the (broadband) illuminance or luminance responsivities can be determined for the four channels. The broadband calibration factors of the channels can be calculated for the spectral distributions of standard sources such as incandescent lamps and special test sources such as display monitors using phosphors or liquid crystals, light emitting diodes (LEDs), and gas discharge lamps.In order to transfer the scales with minimal uncertainty increase from the NIST standard (the filtered tunnel-trap detector) to field-level photo/colorimeters, a transfer source is needed with a spectral distribution equal to the distribution of the test source to be measured [5].The design and characterization of a tunable output LED transfer-source is described here that can satisfy the increased accuracy requirements of photometric and colorimetric scale transfers. In this prototype source, 40 LEDs with 10 different spectral distributions are mounted onto an integrating sphere source. A voltage-to-current control circuit was designed and implemented, enabling independent control of the current feeding each set of 4 LED's. The effect of LED seasoning for luminance and chromaticity was studied. Also, the luminance, chromaticity, and spectral distribution changes versus LED drive current were analyzed for representative red, green, and blue LEDs. The prototype source demonstrates the feasibility of development of a spectrally tunable LED source with up to 40 different LED's. Simulations demonstrate that such a source would be able to approximate standard light-source distributions over the visible spectral range - from 380 nm to 780 nm - with deviations smaller than 2 %. The tunable LED source can also simulate the spectral distributions of special test sources. With this tunable transfer source, a test instrument can be rapidly calibrated for a variety of different source distributions tailored to the anticipated uses of the artifact. The first tests on increased luminous intensity LEDs show that extension of the maximum LED drive current from 50 mA to 1 A will be necessary. Also, pulsed mode light source operation is planned for the tunable LED source. Target uncertainties for the calibration of test artifacts are less than 1 % (k=2) in luminance and 0.002 in chromaticity for any source distribution.[1] Eppeldauer G., Spectral Response Based Calibration Method of Tristimulus Colorimeters, J. Res. Natl. Inst. Stand. Technol. Vol. 103, No. 6. p. 615, 1998.[2] Eppeldauer G. and Racz M., Design and Characterization of a Photometer and Colorimeter Standard, accepted poster presentation for the NEWRAD-02 Conference in Gaithersburg, Maryland, May, 2002.[3] Lykke K. R., Shaw P. S., Hanssen L. M., and Eppeldauer G. P., Development of a Monochromatic Uniform Source Facility for Calibration of Radiance and Irradiance Detectors from 0.2 m to 12 m, Metrologia, Vol. 36, p. 141-146, 1999.[4] To be published.[5] Steven W. Brown, C. Santana, and George P. Eppeldauer, Development of a Tunable LED-based Colorimetric Source, WERB approved for the J. Res. NIST.
Proceedings Title
Proceedings of the AIC Color 2002 Conference
Conference Dates
August 29-31, 2002
Conference Title
AIC Color Conference

Keywords

color scale transfer, light emitting diodes (LED), photometry, spectrally tunable source, tristimulus color measurement

Citation

Eppeldauer, G. and Brown, S. (2002), Propagation of Detector-Based Photometric and Color Scales Using a Spectrally Tunable LED-Source, Proceedings of the AIC Color 2002 Conference (Accessed June 20, 2024)

Issues

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Created May 1, 2002, Updated February 17, 2017