Source Test and Characterization Program

Several recently completed and ongoing programs have demonstrated the value of stable internal source of illumination for on-orbit responsivity trending and calibration update processes. A prime example was the SPIRIT III sensor on the Midcourse Space Experiment in the mid-90s which used grain of wheat bulbs as stimulation sources for the infrared focal plane arrays. As more recent programs have continued to prove, being able to monitor the responses of each pixel in an array over time enables calibration responsivity coefficients and dead pixel mask updates. As the internal sources often use very little of the optical path, while external sources (e.g. stars and the moon) utilize the full optical path, the combination of responsivity trending data on both can enable the analyst to ascertain which (if any) of the focal plane, internal source, or optical path is changing. Thus, contamination effects can usually be separated from optical degradation, focal plane responsivity from source changes, etc., especially if the sensor is capable of providing some spectral information. Unfortunately, there is a paucity of data supporting the space worthiness and operational capabilities of such sources. The Chicago Miniature Lamps CM-7220 used for SPIRIT III are a notable exception, as well as a few isolated LEDs used in some existing sensors. Duane Miles, Space Dynamics Laboratory, did a major characterization of stability and lifetime for the CM-7220 bulbs, but only at one power setting and one temperature. At The Aerospace Corporation, an Independent Research and Development program has been started to identify, test, and publish comprehensive data sets on a range of sources including Nichrome filaments, the CM-7220 bulbs, and both visible and IR LEDs. This program has the ambitious goals of measuring spectral content as well as uniformity of illumination, repeatability, and lifetime as a function of drive current (or similar power parameter). Effects of temperature, vacuum, vibration, etc., will be documented during the expected three year program completion time. However, preliminary data on two LEDs at ambient conditions will be shown. Experience with existing sensors and sources has shown that a significant non-uniformity (up to ~2X, when dynamic range effects can become important) of the illumination pattern can be readily tolerated, as long as the pattern has been characterized on the ground prior to launch and it is stable over mission life. Modest changes in absolute level can be calibrated out through the use of additional calibration sources, whether they be carried along (such as an on-board black body or Spectralon screen) or are celestial (stars, planets, the Moon), but this may limit the knowledge of total trended responsivity changes. Inputs from the community have been solicited for types of sources and database parameters that would be desirable to help guide the course of the test program. This work is supported at The Aerospace Corporation by the Independent Research and Development program