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How good are high-power LEDs?

A new facility serves to submit modern highpower LEDs to long-term tests over periods from two to three years. The tests are aimed at investigating the spectral aging behaviour of the LEDs and, thus, at evaluating their possible use in modern medical lighting engineering.

Facility for long-term tests of LEDs (on the left: revolver table with four colour measuring heads, on the right: horizontal rotating table with a capacity of up to 40 LEDs).

Due to their small size, their efficiency, their longlife cycle and the diversity of their spectral colour, high-power LEDs are today an excellent replacement for "classical" light sources in special lighting fields. They shall, for example, replace the halogen and xenon lamps so far used in the operating department of hospitals. In addition to their lifetime of up to approx. 50 000 hours and efficiencies of typically more than 100 lm/W, the stability of the adjustable luminous colour is of great interest.

Within the scope of a technology transfer project performed in cooperation with the company Zett Optics GmbH, Braunschweig, a completely automated aging measuring system has been developed at PTB for the photometric and colorimetric quantities of high-power LEDs. It can determine both the luminous intensity and the chromaticity coordinate of the test objects. These test objects are mounted on a horizontal rotating table and measured periodically with four measuring heads of different spectral sensitivity arranged in the form of turrets. The horizontal rotating table can accommodate up to 40 LEDs at the same time, whereupon the temperature and the current of the LEDs are stabilized individually. The LEDs are mounted separately on special holders which allow a simple change or exchange.

After observation series over periods of up to 13 months it turned out that many specimens furnish – after an individual burn-in time – a very stable luminous intensity. Their relative degradation lies below 10–5/h. However, great differences within one delivery batch are possible. Although the chromaticity coordinate is in many cases extremely stable, it may – due to aging of the implemented luminescence converters – slightly drift in the case of white high-power LEDs.

In addition to these performance tests on LEDs, the facility also serves to validate existing theoretical aging models and to select LED transfer standards.