Infrared diffuse re ̄ectance instrumentation and standards at NIST

A spectrophotometer system for spectral characterization of materials in the infrared has been built around a bench-top Fourier transform instrument. Its capabilities include the measurement of directional-hemispherical re ̄ectance from 1 to 18 mm. The spectral re ̄ectance measurement is performed with an integrating sphere with an incidence angle of 88. Both relative and absolute measurements can be made. Several methods can be used to determine the absolute value of the directionalhemispherical re ̄ectance of samples. The primary method used is independent of the integrating sphere theory and the requisite assumptions associated with its use. The calibration of a standard reference material (SRM) is described. This SRM has a re ̄ectance value near 0.9 over the complete calibration range 2±18 mm. As part of the calibration procedure the spatial uniformity of the sphere throughput and the bi-directional re ̄ectance distribution function (BRDF) of the SRM material are evaluated. # 1999 Published by Elsevier Science B.V. All rights reserved.