Measurement of LEDs
|Publication Date:||1 January 2007|
Semiconductor devices which emit optical radiation can be divided into two distinct groups, luminescent diodes, usually known as Light Emitting Diodes or LEDs, and laser diodes. The present report is concerned only with the first group, LEDs. This report deals with measurement of individual LEDs only and does not cover clusters or arrays of LEDs, fixtures using LEDs, nor large area surface emitters such as organic light emitting diodes (OLEDs). This report covers measurement of photometric, radiometric, and colorimetric quantities of LEDs, to be performed in calibrating laboratories; it does not cover measurement procedures in production lines which require other considerations. It is the responsibility of the manufacturers and users to ensure that, after obtaining well characterised working standards from their laboratory, the test set-up used for production control will measure the defined quantities properly. The production line measurement recommendations will be dealt with in another report. The deviations from laboratory measurement conditions and possible sources of error have to be carefully examined when the test equipment is designed and installed.
Purpose of the report
LEDs are produced in enormous quantities and in a wide range of different types to meet the very different specifications of a variety of applications. When a wide range of different types of LEDs is measured, the multi-dimensional properties of the emitted optical radiation must be considered during a measurement, not only in relation to the emitting diode but also as they affect the receiving detector. The range of possible influences on the result of a measurement is considerable and the related measurement uncertainty becomes correspondingly high. The low level of the radiant power emitted by some LEDs can limit the resolution of the spectral and spatial distribution measurements; in order to increase the signal of the detector, it has become common practice to measure, for example, the luminous intensity of LEDs at relatively short distances at a fairly large solid angle of the radiation coming from the LED. In this case LEDs are not measured as a point source and measured results vary depending on the geometrical conditions used. To minimize such variation of results, this report standardizes such geometrical conditions so that measured values can be comparable and reproducible among different users.