PNNL

Abstract

Accurate and calibrated directional-hemispherical reflectance spectra of solids are important for both in situ and remote sensing. Many solids are in the form of powders or granules and in order to measure their diffuse reflectance spectra in the laboratory, it is often necessary to place the samples behind a transparent medium such as glass for the UV, visible or near-infrared spectral regions. Using both experimental and theoretical methods we have found that the glass (fused quartz in our case) leads to artifacts in the reflectance values. We report for the first time that the measured reflectance intensity values, for both hemispherical and diffuse reflectance, are distorted by the additional reflectances arising at the air-quartz and sample-quartz interfaces. The values are dependent on the sample reflectance and are vertically shifted with intensity offsets in the hemispherical case leading to measured values up to ?6% too high for a 2% reflectance surface, ?3.8% too high for 10% reflecting materials, approximately correct for 40%- to 60%-diffuse reflecting surfaces, and ?1.5% too low for 99% reflecting Spectralon surfaces. For the case of diffuse-only reflectance, the measured values are uniformly too low due to the polished glass, with differences of nearly 6% for 99%-reflecting matte surfaces. The deviations arise from the added reflections from the quartz surfaces as verified by both theory and experiment, and have some dependence on sphere design. Empirical correction factors were implemented into post-processing software to redress the artifact for hemispherical and diffuse reflectance data across the 300 to 2300 nm range.