Reflectance: transmittance accessory for absolute angle dependent spectrophotometric optical properties measurements

摘要

To enable accurate estimation of the total solar gain through glazed apertures, it is important to know the optical properties of the surfaces for all angles of incidence rather than simply near-normal properties. The need to know the angular dependant optical properties of glass and window products has increased with the use of advanced glazing materials such as solar gain control and low emittance coatings. This paper presents the design and performance of a unique variable angle reflectance and transmittance accessory developed within our laboratory. The accessory is coupled to a Fourier Transform uv/vis/nir/ir spectrophotometer and is capable of measurement of the absolute spectral optical properties of non-scattering samples at both solar and thermal wavelengths. Measurements at wavelengths from the ultraviolet to the thermal infrared have been validated through a series of inter laboratory comparisons within the European Union ADOPT programme and with the UK National Physical Laboratory. Common sources of error, calibration materials and recommendations for the accurate measurement of optical properties at near normal and oblique incidence are identified.