Light emitting diode (LED) lighting is becoming more and more popular, as incandescent lamps are being phased out globally. LEDs have several advantages over incandescent lamps, including energy efficiency, robustness, long lifetime, and good temporal stability. The three latter features make LEDs attractive candidates as new photometric standards. Because the spectra of white LEDs are limited to the visible wavelength range, a novel method for the realization of photometric units based on the predictable quantum efficient detector (PQED) can be utilized. The method eliminates the need of photometric filters that are traditionally used in photometry, and instead relies on carrying out the photometric weighting numerically based on the measured relative spectrum of the source. The PQED-based realization simplifies the traceability chain of photometric measurements significantly as compared with the traditional filter-based method. The measured illuminance values of a white LED deviate by only 0.03% when determined by the new and the traditional methods. The new PQED method has significantly lower expanded uncertainty of 0.26% (k 5 2) as compared with that of the traditional filter-based method of 0.42% (k 5 2). Furthermore, when filtered photometers that measure LED lighting are calibrated using LED lamps as calibration sources instead of incandescent lamps, a significant decrease in the uncertainty related to the spectral mismatch correction can be obtained. The maximum spectral mismatch errors of LED measurements decreased on average by a factor of 3 when switching from an incandescent lamp to an LED calibration source.
We present the design and construction of a new compact room temperature predictable quantum efficient detector (PQED). It consists of two custom-made induced-junction photodiodes mounted in a wedge trap configuration and a window aligned at Brewster's angle for high transmission of p polarized light. The window can also be removed, in which case a dry nitrogen flow system is utilized to prevent dust contamination of the photodiodes. Measurements of individual detectors at the wavelength of 488 nm indicate that reflectance and spectral responsivity are consistent within 4 ppm and 13 ppm peak-to-peak variation, respectively, and agree with the predicted values. The spatial non-uniformity of the responsivity of the PQED is an order of magnitude lower than that of single photodiodes. The internal quantum efficiency of the photodiodes is concluded to be spatially uniform within 50 ppm. These measurement results-together with the responsivity predictable by fundamental laws of physics-provide evidence that the room temperature PQED may replace the cryogenic radiometer as a primary standard of optical power in the visible wavelength range of 380 nm to 780 nm.
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