Establishment of absolute diffuse reflectance scales using the NPL Reference Reflectometer Original Research Article

Absolute scales of radiance factor and diffuse reflectance are being established at the NPL using the Reference Reflectometer. In the basic instrument, a reflecting sample and a detector can be rotated about a common vertical axis to any angular positions relative to an incident light beam. The sample can also be displaced horizontally and vertically, allowing examination of its uniformity and removal for direct measurement of the incident beam. The polarization of the incident light is defined by a Rochon prism, and wavelengths are selected by means of interference filters or a double monochromator. The wavelength range has been extended into the near ultraviolet and the near infrared using UV-enhanced silicon and Peltier-cooled InGaAs photodiodes. For absolute measurement of radiance factor, the light reaching the detector passes through a circular aperture whose diameter and distance from the reflecting surface are determined. The flux distribution is made closely similar when observing the incident beam and the reflected beam. Diffuse reflectance is then derived by numerical integration of results over a full hemisphere, avoiding errors due to nonuniformity of an integrating sphere and giving a direct link between radiance factor and diffuse reflectance scales.