Parameters for an infrared diffuse reflectance standard

Abstract

The determination of the optical properties of a scattering material requires the measurement of either its emittance or diffuse reflectance. This can be performed through use of a bidirectional reflectance distribution function (BRDF) instrument, emission measurement instrument, or diffuse reflectance (generally directional-hemi spheric al or hemispherical-directional reflectance) measurement instrument. If spectral information is required, these instruments can be coupled with a prism/grating monochromator or a Fourier transform spectrometer. For those who measure diffuse reflectance in the UV, visible, and near-IR spectral regions (250 nm through 2.5 rim), standards for calibration purposes are available from the National Institute of Standards and Technology (NIST).* However, for the JR region beyond 2.5 urn, no satisfactory recourse is presently available. To satisfy this unmet need in the IR, NIST has initiated an investigation of JR-reflecting diffuser materials for eventual use as standard reference materials (SRMs) that would be available to the JR community. These standards will be intended for use with diffuse reflectance and emittance measurement instrumentation working within the JR region from ,-2 to ' 16 jim. The instruments that could be calibrated with such standards include laser-based narrow wavelength band and broad-band instruments. The standards will be provided with the calibration values of directional hemispherical reflectance; this will provide an absolute basis for relative BRDF measurements made with BRDF instrumentation. Separate BRDF standards will be developed for calibration of BRDF at specific input and output angles. Many potential users have expressed their need to NJST for diffuse reflectance standards for the JR. Many have also