Optical properties measurements of solar absorber surfaces - Results of an International round-robin programme

Results are presented of a round-robin programme of optical properties measurements carried out on a commercial chemically oxidised stainless steel spectrally selective solar absorber surface and two commonly employed reference calibration standards. Measurements of solar absorptance and thermal emittance were obtained using a number of different spectrophotometric broad-band and radiometric techniques. The results are analysed for each technique employed and comparisons are also made of results obtained from different methods. The results highlight differences which can occur when supposedly identical methods are employed using different commercially available instruments and reflectance attachments. The importance of understanding calibration techniques, the use of diffuse and specular reference standards, calculation methods for optical properties determinations and sample dependent observable effects are clearly demonstrated. Lessons have been learned regarding 'routine' measuremen ts which will assist in developing better practices and refining recommended procedures for commonly quoted surface optical properties.