Development of white LED illuminants for colorimetry and recommendation of white LED reference spectrum for photometry

Abstract

General lighting is undergoing a revolutionary change towards LED-based technologies. To provide firm scientific basis for the related colorimetric and photometric measurements, this paper presents the development of new white-LED-based illuminants for colorimetry, and their evaluation to recommend a new reference spectrum for calibration of photometers. Spectra of 1516 LED products were measured and used to calculate eight representative spectral power distributions for LED sources of different correlated colour temperature categories. The suitability of the calculated representative spectra for photometer calibration was studied by comparing average spectral mismatch errors with CIE Standard Illuminant A, which has been used for decades as the reference spectrum for incandescent standard lamps in calibration of photometers. It was found that in general, when compared with Standard Illuminant A, all the potential LED calibration spectra reduced spectral mismatch errors when measuring LED products. Out of the potential LED calibration spectra tested, the white LED spectrum with correlated colour temperature of 4103 K was found to be the most suitable candidate to complement Standard Illuminant A in luminous responsivity calibrations of photometers. When compared with Standard Illuminant A, employing the 4103 K reference spectrum reduced the spectral mismatch errors, on average, by approximately a factor of two in measurements of LED products and lighting. Furthermore, the new LED reference spectrum was found to reduce the spectral mismatch errors in measurements of daylight, and many types of fluorescent and discharge lamps, indicating that the proposed reference spectrum is a viable alternative to Standard Illuminant A for calibration of photometers.

Definition of a Spectral Mismatch Index for Spectral Power Distributions

“…To find out the optimal way to describe the differences between pR(λ) and pT(λ), 1-norm and 2-norm distances of 1298 LED sources to the reference SPD were compared with the results obtained for the average values of |1 -F*| for the same source and reference SPD pairs over 77 different spectral responsivities srel(λ) [Kokka 2018] (giving in total around 100000 data points). The calculated values of |1 -F*| were ranging from 1.5×10 -4 to 1.5×10 -2 , thus covering two decades of correction factor values.…”

“…For the SPDs and spectral responsivities from [Kokka 2018], the 95 th and 50 th percentiles of |1 -F*| (P50 and P95, respectively) are used to estimate the factors f50 and f95 for which:…”

“…The technical committee TC 2-90 ("LED Reference spectrum for photometer calibration") has been recently created by CIE to investigate, select and publish a LED reference spectrum [Kokka 2018] to complement the CIE Standard Illuminant A in photometric calibrations. This task additionally requires defining a spectral mismatch index to quantify the match between spectral power distributions (SPDs) of real sources and the selected reference spectrum.…”

“…Since the reference SPD was proposed for photometric calibrations [Kokka 2018], the spectral mismatch index of the SPD of a real source should be directly related to the spectral mismatch systematic error introduced when the photometric quantity is measured by a photometer calibrated with respect to a light source with the reference SPD. This error is related to the spectral mismatch correction factor (SMCF).…”

“…f95 were calculated for each class, using the LED illuminant BL40 from[Kokka 2018], a white LED spectrum with a CCT of about 4100 K which was recently proposed as LED reference spectrum for photometric calibration by CIE TC2-90. 10 of the available photometers are 4*, 32 are 3*, 23 are 2* and 12 are 1*.…”