Last Updated: 24/6/98
In order to make the necessary long term measurements of a large number of irradiated LEDs feasible, an automatic measurement system, called the scanning machine, was built with space and driver electronics for the simultaneous operation of up to 448 LEDs or VCSELs.
In the scanning machine, the LEDs are mounted without any further packaging in groups of eight on ceramic boards, which ensure a good thermal contact. The VCSELs were produced as twentyfold arrays which are mounted on the same kind of ceramic boards but providing 16 connector pads at half the pitch for an alternate operation of eight devices. Electrical contacts are made by wire-bonding. The light output is measured with a moving PIN diode at a distance of a few millimetres in front of the devices. The LEDs can be operated at forward currents up to 100mA, and, using Peltier elements, the temperature can be set in the range from -10oC to 50oC. For each LED or VCSEL, the forward current and voltage, the light output, and the fall and rise time can be measured automatically under computer control. For measurements at low temperatures, the scanning machine provides a tight volume with a nitrogen atmosphere.