NBS specular reflectometer-spectrophotometer

A specular reflectometer has been constructed and tested for calibrating the reflectance of mirror standards over the 250-2500-nm spectral range. This instrument is a measurement accessory to a reference spectrophotometer, which is also used for diffuse hemispherical spectral reflectance and 45 degrees /0 degrees spectral reflectance. The specular reflectometer is designed to measure mirror reflectances at angles of incidence between 5 and 80 degrees using both vertically and horizontally polarized radiation. Absolute reflectance measurements are obtained by an optical system, which provides for direct measurement of the incident beam and for the sample mirror reflectance using the same beam. This is accomplished by means of a beam tracking system through which the beam is directed into a signal averaging sphere. The sphere rotates with the beam tracking optics, and the stationary detector views the interior of the sphere. Control of the beam tracking optical system is accomplished by a computer-controlled stepping-motor-driven precision turntable. Uncertainties of the reflectance measurements obtained with this system are estimated to be +/-0.2% of the measured value.