Advantages of white LED lamps and new detector technology in photometry

Abstract

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.

Methods for decreasing uncertainties in LED photometry

“…In a test measurement, the illuminance produced by a white LED was determined using the new method and a conventional reference photometer [1]. The values obtained by the two methods deviated only by 0.03 % from each other.…”

“…Furthermore, various properties of LED lamps can be customized to fit different applications. These include the dimensions of the illuminating area, angular distribution, and the shape of the emission spectrum [1].…”

“…As the industry and the know-how needed for the manufacturing of traditional lamps might eventually be lost, the availability of incandescent standards in the future is unknown. Moreover, if the majority of the lamps to be measured are LED-based, the use of tungsten filament lamps as photometric standard might not be mandatory or even justified, as the spectra of the two differ drastically [1].…”

“…The latter is caused by the uncertainty of the reference photometer illuminance responsivity. In addition, the narrow and complicated spectra of LEDs tend to increase the uncertainty associated with spectral mismatch when measured with traditional filtered photometers [1][2]4].…”

“…This is the wavelength range where the responsivity of the PQED is accurately known. In addition to improved uncertainty, the new method simplifies the traceability chain of photometric measurements considerably [1].…”