Room temperature hemispherical and bi-directional reflectances

The emissivity of a material is conveniently determined from its reflectance, which is turn can be used to validate optical models for estimating emissivity at high temperatures. The Optical Technology Division of the National Institute of Standards and Technology maintains the national reference instruments for measurements of reflectance. Within the Division, the Spectrophotometry project is responsible for reflectance measurements at wavelengths from 250 nm to 2500 nm on non-fluorescent materials at room temperature. The reflecting properties of a material can be categorized as either specular (mirror-like) or diffuse. In addition, the reflectance depends upon the geometry of the measurement. In general, the reflectance of a specular material is measured using a bi-directional geometry (incident and reflected radiant flux at fixed directions) while a diffuse material is measured using a directional-hemispherical geometry (incident or reflected radiant flux at a fixed direction, the other flux at all directions). The Spectrophotometry project has reference and commercial instruments for measurements at both geometries, with uncertainties (k=2) in reflectance of approximately 0.2% for specular materials and 0.4% for diffuse materials.