Polarization characteristics of Spectralon illuminated by coherent light.

The Multiangle Imaging Spectroradiometer makes use of an onboard calibration system that includes two Spectralon panels that are used to reflect sunlight into the cameras. During preflight testing, these panels were quantified in terms of their bidirectional reflectance distribution function, which was measured as a function of the source-incident and detector view angles and at laser wavelengths of 442.0, 632.8, and 859.9 nm. Principal plane measurements are presented in which polarizations of the source and detector are analyzed. These data are unique and valuable in modeling Spectralon reflectance properties and for experiments in which polarization sensitivities are important.

Directional-hemispherical reflectance for spectralon by integration of its bidirectional reflectance.

The directional-hemispherical reflectance is obtained for Spectralon, the material chosen for onboard radiometric calibration of the multiangle imaging spectroradiometer, at laser wavelengths of 442, 632.8, and 859.9 nm. With p- and s-polarized incident light and for an angle of incidence of 45 degrees , the bidirectional reflectance distribution function was measured over a polar angle range of 1-85 degrees and a range of azimuthal angles of 0-180 degrees in 10 degrees increments. The resultant directional-hemispherical reflectance is found by integration to be 1.00 ? 0.01 at 442 nm, 0.953 ? 0.01 at 632.8 nm, and 0.956 ? 0.01 at 859.9 nm. The experimental methodology and the data analysis are presented together with a full discussion of the primary experimental errors.

Multiangle Imaging Spectroradiometer: optical characterization of the calibration panels.

The reflectance properties of an engineering model (EM) of the Spectralon panel intended for use within an on-board calibrator (OBC) on the NASA Multiangle Imaging Spectroradiometer (MISR) instrument have been fully characterized with regard to panel uniformity and isotropy in response to three incident laser wavelengths of 442, 632.8, and 859.9 nm. A regional variation in the relative bidirectional reflectance factor (RBRF) across the surface of the EM panel, which contributes to spatial nonuniformity at the +/-2% level, has been measured at all three laser wavelengths. Further, a reflectance anisotropy has been identified. The mechanism causing these departures from the ideal Lambertian surface may originate in the sanding of the Spectralon surface in the final stage of preparation. This supposition is corroborated by measurements made on a pressed polytetrafluoroethylene (PTFE) panel in which a greatly reduced anisotropy in panel RBRF is measured. The EM panel RBRF exhibits a deviation from Lambertian characteristics as an off-specular peak in the forward scattering direction. A common crossover point at an angle of reflection of approximately 37 degrees at which the BRF is constant within +/-0.4% for an illumination angle range of theta(i) = 30 degrees-60 degrees is observed at all three wavelengths. Two Spectralon protoflight panels that were fabricated after the EM was studied were also the subject of a uniformity study over part of the area of the Spectralon panels at the 442-nm wavelength. The analysis indicated that the panel uniformity satisfies the +/-0.5% criterion, which indicates improved panel preparation. However, the off-specular peak in the forward scattering direction was essentially unchanged, with the crossover point at approximately 37 degrees.

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.

Integrating sphere depolarization at 10 µm for a non-Lambertian wall surface: application to lidar calibration.

The relative Stokes vectors at the detector exit port of a sandblasted and gold-plated integrating sphere are determined for four different polarizations incident on five unique surfaces. The results indicate in all cases that the integrating sphere is a depolarizer. These results validate assumptions used in hard-target calibration methodology for infrared lidars.

Reflectance characteristics of reference materials used in lidar hard target calibration.

Lidar hard target calibration is discussed, emphasizing the transfer target methodology. Characteristics of example calibration target surfaces are described in light of the four reflectance mechanisms: specular, diffuse, retroreflection, and off-specular reflectance. This ideal transfer target is one which can be described as entirely diffusely reflecting, i.e., Lambertian. Correction for retroreflection is required when using the flowers of sulfur transfer target at CO(2) laser wavelengths. Corrections for specular and retroreflection for the integrating sphere are negligible; however, the off-specular reflectance of rough metal surfaces used in the integrating spheres is only qualitatively compared with the diffuse reflectance of flowers of sulfur.