Our lamp power supplies are highly regulated and operate lamps at constant current or power, even if the line voltage changes over a wide range. Even with lamp current constant, the light output of many lamps varies with age, filament or electrode erosion, gas adsorption or desorption, and, particularly for mercury lamps, ambient temperature. Both the total output and the distribution of power between the spectral lines from a mercury lamp change with temperature. These effects cause output variations with time, with different and sometimes pronounced frequency dependence. Even the relatively simple quartz tungsten halogen lamps exhibit output variations: evaporation and halogen cycle redeposition of tungsten constantly change the microstructure of the filament. The 68950 corrects for these lamp output changes, maintaining a constant, long term output, from the lamp.

We recommend that you rod mount the light sensor as shown in Fig. 3, using a 5° Wedge to split off part of the beam. The normal surface reflection off the substrate is enough to "pick-off" a portion of the beam for monitoring by the 68950 sensor. You can also use a beamsplitter at 45°, but you may need to attenuate the light hitting the sensor with an aperture, diffuser or filter, depending upon the reflectance of the beamsplitter. The 7123 Mount holds a 1 inch (25.4 mm) diameter optic at the input of the light sensor. Another method of monitoring the light output, if you have plenty of light to spare, is to use an integrating sphere at the output of the source, and mount the light sensor on one of the sphere’s ports.

For convenience, you can mount the sensor directly to the lamp housing. The Adapter Kit mounts the sensor off our Research Housings, looking transversely at the lamp. The 68952 includes a filter holder with a beam attenuator. The Adapter Kit mounts the sensor on an unused port of the Series Q Housing. It also includes a filter holder and beam attenuator.