Multiangle Imaging Spectroradiometer: optical characterization of the calibration panels.

Area of Science:

• Optical engineering
• Remote sensing instrumentation

Background:

• Spectralon panels are crucial for on-board calibrators (OBC) in instruments like NASA's Multiangle Imaging Spectrometer (MISR).
• Ideal calibration panels should exhibit uniform and isotropic reflectance properties for accurate measurements.

Purpose of the Study:

• To characterize the reflectance properties (uniformity and isotropy) of an engineering model (EM) Spectralon panel for the MISR OBC.
• To identify deviations from Lambertian behavior and assess panel preparation methods.

Main Methods:

• Characterized reflectance properties using three laser wavelengths (442, 632.8, 859.9 nm).
• Measured relative bidirectional reflectance factor (RBRF) across the panel surface.
• Compared EM panel results with a pressed polytetrafluoroethylene (PTFE) panel and later protoflight panels.

Main Results:

• Measured regional variations in RBRF (up to +/-2% nonuniformity) and identified reflectance anisotropy.
• Observed an off-specular peak in the forward scattering direction, deviating from Lambertian characteristics.
• Identified a common crossover point at ~37 degrees reflection angle where BRF is stable across illumination angles.

Conclusions:

• The EM Spectralon panel exhibits spatial nonuniformity and anisotropy, potentially due to surface sanding.
• Improved panel preparation for protoflight models met uniformity criteria, but the forward scattering peak persisted.
• The identified crossover point offers a stable reflectance characteristic for calibration despite deviations from ideal Lambertian behavior.