Improved Techniques For The NPL Hemispherical Reflectometer

The NPL hemispherical reflectometer/transmissometer can be used to measure a complete set of spectral radiometric properties from 2.5 to 56 micrometres for any kind of sample, whether or not it has a regular component of reflectance or transmittance. A rotatable hemispherical copper mirror focusses radiation from a cylindrical source on to the sample. Systematic errors are caused by radiometric non-linearity from interreflection effects and by temperature changes of the source arising from interreflection and from the necessary purging air current. A custom-built power supply is operated in a constant-load-resistance mode to maintain the source at a constant temperature to within 0.1 K. This allows simplification of the correction procedures, but the use of standard reference samples is still required for calibration service work. A new method of use for absolute measurements employs a supplementary mirror for additional measurements to let the monochromator optics view the source directly, without and with the sample present. The ratio of these readings then provides a simple correction factor for the reflectometer value, allowing immediate calculation of absolute spectral reflectance. Experimental tests show that the method is valid from 2.5 to 36 micrometres with a grating spectrophotometer, but becomes impracticable at longer wavelengths. A Fourier transform instrument has been modified radically to accommodate the hemisphere reflectometer for further tests.