BRDF Measurements using a Large-area Uniform Illumination Scatterometer (LUIS) in Support of NASA Remote Sensing Programs

There are two newly developed scatterometers at the Diffuser Calibration Lab (DCL) NASA GSFC. One of them is a laser/lamp-based table-top scatterometer (TTS) covering the spectral range from 300 nm to 2300 nm with a relative small beam size of 5 to 10 mm in diameter. The spatial non-uniformity of BRDF is requested when a large diffuse sample is measured on TTS. In order to test the BRDF along with spatial non-uniformity, a Large-area Uniform Illumination Scatterometer (LUIS) was built to meet the requirements for different large samples, especially volume diffuse samples. The LUIS system at NASA GSFC is capable of generating a 6” dia. uniform collimated beam using a 16” dia. 2.5 m long telescope, high power LEDs from 340 nm to 1650 nm, and different field-of-view detectors, and applies the same measurement equation as that used in flight. BRDF results of selected white and black diffuse samples measured using LUIS are presented, including surface diffusers and volume diffusers, and compared with those from TTS. Several new capabilities of BRDF measurements of LUIS are also demonstrated. In addition, another approach of BRDF tests for a large-area sample was attempted to simulate a large-area uniform illumination by rastering a 532 nm pencil laser across it. These BRDF results are compared with that from LUIS, showing a good agreement. To extend the spectral coverage of LUIS beyond 2000 nm, a high power SWIR tunable laser is used to generate the large-area uniform collimated light. Some related results will be reported. Also discussed in this presentation are the BRDF validation and uncertainty budget.