New photometry approach more accurately measures luminous efficacy of LEDs

Sept. 21, 2015
LED reference lamp and new predictable quantum efficient detector (PQED) lead to lower measurement uncertainties.
A PQED consists of a Brewster window (left) protecting the detector elements from impurities, adjustable bellows, and the detector chamber itself. To further decrease uncertainties in measurement, the window was removed and the detector was protected from impurities using nitrogen flow. (Image: Aalto University)

Researchers at Aalto University and VTT Technical Research Centre of Finland (both in Espoo, Finland) have improved on the standard method of photometrically measuring the luminous efficacy of LEDs by replacing the incandescent reference lamp plus filters with a precision LED that requires no filters.1 The new method lowers the expanded measurement uncertainty from 0.42% to 0.26% and the relative uncertainty from 5% to 1%.

The photometers that lamp manufacturers traditionally use for calibrating their devices are produced and calibrated for incandescent lamps, resulting in errors when measuring the efficacy of LEDs. In the new research, an LED lamp with a well-defined spectrum was used along with a predictable quantum efficient detector (PQED), which was developed at Aalto University together with VTT MIKES Metrology and European partners, and whose spectral responsivity can be determined highly accurately. As a result, there was no need for the problematic optical filters normally used in applications based on incandescent lamps.

Indeed, accurately determining and analyzing the spectrum of the LED was the most challenging and crucial part of the research, says Tomi Pulli, a doctoral student at Aalto University. The detector used in measurements by Pulli and his coresearchers measures the illuminance of LEDs in a very small area. According to Professor Erkki Ikonen, the head of research, the next step will be to move on to measurements corresponding to real-life conditions for lighting.



1. Tomi Pulli et al., Light: Science & Applications (2015); doi: 10.1038/lsa.2015.105

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