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 = 2) as compared with that of the traditional filter-based method of 0.42% (k = 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. White light-emitting diodes (LEDs) can be used to define photometric units and offer several advantages over conventional incandescent lamps. Tungsten-filament incandescent lamps are widely used as standard light sources in photometry, but LEDs have higher energy efficiencies, greater robustness, long lifetimes, and good temporal stability. Now, a team of scientists in Finland has demonstrated that a high accuracy can be achieved by using a new primary standard for optical power based on an induced junction photodiode trap as a broadband detector. By measuring the illuminance of a white LED lamp, they show that this detector is more accurate than photometers when LEDs are used as the light source. Furthermore, using LEDs does not require filters as photometric weighting is performed numerically based on the measured relative spectrum of the source.