PHOTOMETRIC AND COLORIMETRIC STABILITY OF LEDS

The aging of LEDs is well known, although the detailed mechanism of the aging is not well enough understood. Most of the investigations deal with long term aging, i.e. life time expectancy of LEDs. In the present paper results of shorter time (several hundred hours to 1,000 hours) stability of LEDs is reported, as this is of importance if LEDs are to be used in standard light sources. The latter is important if general purpose light measurement instrumentation is to be used for evaluating LED light. In two experimental series not only of the luminous intensity change, but also the colour change of the LEDs has been recorded. Experiments show that to be able to get standards with 0.1% long term stability LEDs have to be selected both regarding luminous output and colour stability.