Photometry and colorimetry of reference LEDs by using a compact goniophotometer
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In the development, production control, quality assurance and during the selection of appropriate LEDs for very different applications, it is essential to know their photometric and colorimetric quantities very precisely. The calibration of the measuring devices required for this purpose is performed via LED transfer standards. These LED standards transfer photometric, spectroradiometric and colorimetric units and characteristics, from the National Metrology Institute (NMI) to the user. Finally, LED standards serve as references for the user. Usually a set of LED reference standards consists of at least four different LED colours (blue, green, red and white), sometimes even more. Further more it is very often necessary to have more than only one set of reference standards as a backup. This generally causes a large number of calibrations. To handle this work load, a special reference standard and a compact goniophotometer prove to be a considerable advantage. PTB developed a special LED transfer standard which operates under controlled conditions: a controlled LED chip temperature and a controlled chip current help to keep the photometric and colorimetric properties constant. On the other hand, a compact goniophotometer was designed to measure luminous flux, luminous intensity and colorimetric quantities of LED transfer standards in a relatively short time with low measurement uncertainties.