Methods for quantitative infrared directional-hemispherical and diffuse reflectance measurements using an FTIR and a commercial integrating sphere.

We have developed methods to measure the directional-hemispherical (ρ) and diffuse (ρd) reflectances of powders, liquids, and disks of powders and solid materials using a commercially available, matte gold-coated integrating sphere and Fourier transform infrared spectrometer. To determine how well the sphere and protocols produce quantitative reflectance data, measurements were made of three diffuse and two specular standards prepared by the National Institute of Standards and Technology (NIST), LabSphere Infragold and Spectralon standards, hand-loaded sulfur and talc powder samples, and water. Relative to the NIST measurements of the NIST standards, our directional hemispherical reflectance values are within ±4% for four of the standards and within ±7% for a low reflectance diffuse standard. For the three diffuse reflectance NIST standards, our diffuse reflectance values are within ±5% of the NIST values. For the two specular NIST standards, our diffuse reflectance values are an order of magnitude larger than those of NIST, pointing to a systematic error in the manner in which diffuse reflectance measurements are made for specular samples using our methods and sphere. Sources of uncertainty are discussed in the paper.