THE USE OF PORTABLE INSTRUMENTS FOR OPTICAL PROPERTY MEASUREMENT OF SELECTIVE SURFACES

Abstract
The operation and accuracy of portable optical measurement equipment, the Willey Alpha Meter (Model 2150) for measuring solar absorptance (α_s) and the Willey McDonald Emissometer (Model 2145) for hemispherical emittance at 100 °C(εh100) are presented. The results obtained with the emissometer for a range of selective materials are compared with calorimetric results and shown to be significantly higher. A model explaining the difference in terms of the reflectivity of the emissometer detector is proposed. A high accuracy modification of the emissometer using a more sensitive detector, is described. Results of α_s, obtained with the Willey Alpha Meter for black surfaces like Maxorb, agree well with those calculated from spectrometer results using AM 2 spectrum. However, because of the non-linear response of the detector used in the alpha meter, misleading results can be obtained when testing coloured surfaces. The magnitude of the errors involved is calculated and the limitations of the instrument discussed.