Radiance calibrations are performed on our radiance bench (Figure 1). The instrument to be calibrated points to a Spectralon plaque that is in turn illuminated by a 1000 W FEL standard lamp, mounted 290 cm away from the plaque. The plaque's reflectivity is regularly being calibrated by Labsphere.
Figure 1: Radiance calibration bench. The plaque is at left, the FEL lamp at the far right. The OXR radiometer points at plaque at an angle of 45°. The front wall of the bench is removed for clarity.
The radiance calibration bench features several apertures made of black acrylic plastic, which are mounted between the lamp and plaque to minimize stray light. Two instrument holders with kinematic mounts point to the center of the plaque from opposite sides at 45° angles. The setup permits the installation of two radiometers at the same time, with one radiometer being a monitoring device (in this case an OXR) and the other the instrument to be calibrated. The lamp is mounted on an optical rail to permit accurate determination of the calibration distances. Depending on the application, other distances can be configured.
One unique feature of the bench is that it features a 3-m long fine pitch lead screw under stepping motor control. This allows a radiometer measuring irradiance to be mounted on a platform driven by the lead screw. The distance between a standard lamp and radiometer can them be continuously varied while acquiring data permitting an multichannel radiometer like C-OPS to measure a spectrally constant light source while changing intensity gradually. This permits identification of anomalies caused by gain switching, ADC nonlinearity, and other causes to be easily identified.
For more on BSI's calibration activities, read Instrument and Calibration Standards Intercomparisons.