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.
Highlights
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
By weighting the correlated colour temperature (CCT) bins of the device under test (DUT) data by the ‘All products’ distribution, shown in figure 5, the BL40 calibration spectrum resulted in the smallest mean absolute spectral mismatch error of 0.30%
The spectral data collected for over 1500 white LED products were used to calculate eight representative spectral power distributions (RSPDs) for DUTs sorted according to their CCTs into 2700 K, 3000 K, 3500 K, 4000 K, 4500 K, 5000 K, 5700 K, and 6500 K bins
Summary
Photometers used for measuring modern LED products are commonly calibrated using tungsten filament incandescent standard lamps with Standard Illuminant A spectrum [7,8,9,10,11], defined by the International Commission on Illumination (CIE) in 1931 [12]. This inevitably leads to spectral mismatch between the calibration source and the LED product under test, which results in increased uncertainty in the quantities measured using the calibrated photometer.
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