Methods for decreasing uncertainties in LED photometry

¹ VTT Technical Research Centre of Finland Ltd, Centre for Metrology MIKES, P.O. Box 1000, FI-02044 VTT, Finland
² Metrology Research Institute, Aalto University, P.O. Box 13000, FI-00076 Aalto, Finland

^a Corresponding author: timo.donsberg@vtt.fi
^b Present address: Tallinn University of Technology, Estonia

Abstract

Even though energy-efficient and sustainable solutions, such as light emitting diodes (LEDs), have become popular in general lighting, mainly incandescent lamps are used as measurement standards in photometry. Optical properties of the LED lamps together with the often unstable built-in power converters bring challenges to NMIs and testing laboratories. Due to the narrow and complicated spectra of the LED lamps, the uncertainties of traditional photometers calibrated by incandescent lamps tend to increase when LED lamps are measured. Switching from an incandescent lamp to an LED-based calibration source would decrease the uncertainty related to the spectral mismatch correction. LED-based photometric standard lamps would also have other benefits, such as long lifetime and good temporal stability. Moreover, as spectra of white LED lamps are limited to the visible wavelength range, a novel method for illuminance measurements based on the Predictable Quantum Efficient Detector (PQED) can be used to characterize these standard lamps with luminous flux uncertainties significantly below 1 % (k = 2) at NMIs. The method eliminates the need of photometric filters in realization of the illuminance unit. Instead, the photometric weighting is carried out numerically using a separately measured relative spectrum of the source. Well characterized LED-based calibration lamps, together with improved electrical power measurement, would reduce measurement uncertainties of illuminance, luminous intensity, luminous flux and luminous efficacy measurements of LED lamps at NMIs and testing laboratories. This would have a high impact on the development of energy-efficient LED lamps and on the assessment of the energy saving potential of solid state lighting. It is also shown, that recent advances in illuminance and electrical power measurement will enable luminous efficacy measurements of LED lamps with uncertainty well below the present state-of-the-art level of about 1 % (k = 2).