Reflectance measurements of aluminium surfaces using integrating spheres

In the investigation of the considerable absorption of visible light in industrially rolled aluminium surfaces, a thorough knowledge of the total reflectance measurement method is required. In this paper a general introduction to the integrating sphere method is given, with emphasis on the current understanding of instrumental artefacts and ways of correcting them. Selected aluminium surfaces were measured employing two spheres; a single-beam instrument equipped with a white-light source and a Si-photoelement detector, and a double-beam sphere, which measures reflectance properties with spectral resolution. It was found necessary to take precautions concerning the orientation of rolled samples relative instrument geometry, to avoid artificial losses from the sphere. The use of a specular reference standard is assumed to minimise the effect of several sphere artefacts, since it produces similar angular distribution of reflected light as the rather glossy aluminium samples. Measurements with spectral resolution show that the total reflectance of aluminium is somewhat red shifted after rolling, a tendency that cannot be revealed in ordinary white-light measurements.