Development of a Collimated Large Area Uniform Light Source for the Measurement of Solar Diffuser BRDF in Support of NASA Satellite Instrument Programs

We report the development of a collimated large area uniform light source, which is used to acquire diffuser BRDF measurements in support of the pre-launch calibration of NASA Earth observing satellite instrument. In accordance with the goal of “testing as you fly,” this large area light source permits the measurement of diffuser BRDF using illumination geometrically similar to that realized on orbit. In the design and testing of this source, several approaches using different light sources and collimating optics were examined with the overarching goal of producing a monochromatic, unpolarized large area, uniform, collimated beam with sufficient throughput power to enable BRDF to be measured. The major components of the collimated large area uniform light source employ a series of high-power LEDs from UV-VIS to SWIR with or without an integrating sphere. Light from this source is coupled to either a 30.48 cm diameter size off-axis parabolic mirror (OAP) or a 45.72 cm diameter spherical concave mirror. A large beam uniformity evaluation system employing a scanning detector was used to measure light source uniformity. In this presentation, we describe our approaches to produce large area uniform solar diffuser illumination, and discuss potential technical difficulties. Since technical challenges exist in achieving the 1% uniformity in a collimated large area light source, we also propose a correction method to mitigate non-uniformity using a laser scan method. The characterization of the collimated large area uniform light source and preliminary BRDF results are presented.