Directional reflectance characterization facility and measurement methodology

A precision reflectance characterization facility, constructed specifically for the measurement of the bidirectional reflectance properties of Spectralon panels planned for use as in-flight calibrators on the NASA Multiangle Imaging Spectroradiometer (MISR) instrument is described. The incident linearly polarized radiation is provided at three laser wavelengths: 442, 632.8, and 859.9 nm. Each beam is collimated when incident on the Spectralon. The illuminated area of the panel is viewed with a silicon photodetector that revolves around the panel (360°) on a 30-cm boom extending from a common rotational axis. The reflected radiance detector signal is ratioed with the signal from a reference detector to minimize the effect of amplitude instabilities in the laser sources. This and other measures adopted to reduce noise have resulted in a bidirectional reflection function (BRF) calibration facility with a measurement precision with regard to a BRF measurement of ±0.002 at the 1ς confidence level. The Spectralon test piece panel is held in a computer-controlled three-axis rotational assembly capable of a full 360° rotation in the horizontal plane and 90° in the vertical. The angular positioning system has repeatability and resolution of 0.001°. Design details and an outline of the measurement methodology are presented.