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Spectrally Tunable Radiometric Sources

 

1.   LED-based Spectrally Tunable Source
2.   Spatial Light Modulator-based Advanced Radiometric Source (forthcoming)

  
Figure 1

Figure 1. LED-based spectrally tunable source developed at NIST.		    A spectrally tunable radiometric calibration source has been developed using an integrating sphere illuminated by a large number of LEDs and LED clusters. The source can simulate various light spectra in the visible region including traditional light sources as well as solid-state lighting sources and theoretical spectra such as D65. The spectral matching is achieved by feedback control of the individual LEDs based on spectroradiometer reading. The source is used as a transfer calibration source for calibration or characterizing photometers, colorimeters, and radiometric instruments to measure various light spectra that are dissimilar to incandescent standard lamps.

We are also developing a spectrally tunable source utilizing a spatial light modulator, which has further advantages compared to LED-based tunable source.

1.   LED-based Spectrally Tunable Source

Spectral matching of light sources is often required, e.g., to simulate daylight illuminants for object color measurements, visual inspection, or vision experiments. Such spectrally tunable source would also be useful to calibrate or characterize photometric and colorimetric instruments measuring a variety of light sources, while these instruments are normally calibrated with incandescent standard lamps. Such instruments have much larger uncertainties when measuring sources other than the calibration source. Spectroradiometers (particularly, array spectrometers) are also liable to larger errors when measuring spectra dissimilar to that of the calibration source (normally tungsten halogen lamps) due to stray light within the instrument and other sources of error. Such errors can be minimized if the instruments are calibrated against light spectra that are to be measured. A spectrally tunable source would be useful to calibrate or characterize instruments under various light spectra that the instruments are used for.

Figure 2

Figure 2. LEDs with a variety of peak wavelengths, used for the NIST spectrally tunable source.

A wide variety of Light emitting diode (LED) are now available with high efficiency and at peak wavelengths covering the entire visible region (see Figure 2) Utilizing such LEDs with different spectral power distributions, and controlling the radiometric output of each type of LED, it is possible to make a spectrally tunable light source that is able to produce various light spectra matched to specific "target" spectral power distribution.

Figure 3

Figure 3. Tunable source spectrum simulating D65.

Utilizing a large number of LEDs with different spectral power distributions, and controlling the radiometric output of each type of LED, it is possible to make a spectrally tunable light source that is able to produce various light spectra matched to specific "target" spectral power distribution. Such a spectrally tunable source was developed at NIST using a 30 cm integrating sphere and utilizing about 200 LEDs (5 mm epoxy type) and 6 Chip-on-board LED packages covering the visible spectral region from 380 nm to 780 nm. The spectral matching is achieved with a feedback control of individual LEDs from the spectroradiometer readings. This source is designed for photometric and colorimetric applications, and have maximum luminance of about 1200 cd/m2 on a 5 cm circular exit port of the integrating sphere. The source is used as a transfer calibration source for photometers, colorimeters and radiometers that are used to measure light sources of specific spectral distributions that are dissimilar to incandescent standard lamps. Figure 4 shows the electrical configuration of the spectrally tunable source and Figure 5 shows photographs of the sphere source and temperature controlled LED heads.

Figure 4

Figure 4. Electrical configuration of the NIST spectrally tunable source.

Figure 5

Figure 5. Photographs of the back of the sphere source and temper.

References

  1. S.W. Brown, C. Santana, and G.P. Eppeldauer, Development of a tunable LED-based colorimetric source, J. Res. Natl. Inst. Stand. Technol. 107, 363-371 (2002).

  2. I. Fryc, S.W. Brown, and Y. Ohno, Spectral Matching with and LED-Based Spectrally Tunable Source (563 kB) PDF, Proc. SPIE 5941, 59411I (2005).

  3. I. Fryc, S.W. Brown, G.P. Eppeldauer, and Y. Ohno, LED Base Spectrally Tunable Source for Radiometric, Photometric and Colorimetric Applications (840 kB) PDF, Opt. Eng. 44, 111309 (Nov. 2005).

  4. I. Fryc, S. Brown, and Y. Ohno, Development of a spectrally tunable calibration source for display colorimeters, Proc, Lightmetry, 20-22 October 2004, Warsaw, Poland.
For technical information or questions, contact:
Steven Brown
Phone: (301)-975-5167
Fax: (301)-840-8551
Email: steven.brown@nist.gov		 Yoshi Ohno
Phone: (301)-975-2321
Fax: (301)-840-8551
Email: ohno@nist.gov

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Online: May 2007   -   Last updated: February 2008