The reflection factor of magnesium oxide

Abstract

This paper describes the design and use of a small integrating reflectometer for determining the total reflection factor of a smoked magnesium oxide surface under diffuse illumination. It also shows how the reflectometer may be used for the absolute determination of apparent reflection factors (reflectances) under parallel illumination. The instrument is based on theory given by A. H. Taylor, J.O.S.A. 4, 1920 (14), and depends on the change in brightness of the walls of a small integrating sphere which occurs on removal of a test plate covering a circular aperture of known area in the sphere. The theory is extended to show how the interior of the sphere may be used as a standard of brightness when the diffuse reflection factor of the coating and the flux entering the sphere are known. This affords a new means for the standardisation of photometric test plates. A second method of determining the diffuse reflection factor of the reflectometer coating is described depending on measurements of the brightness of the interior of the sphere and of an artificial sky used to illuminate it through the aperture. In practice it is shown that the surface on which the oxide is smoked may influence the results except in the case of very thick coatings. Polished silver was finally selected for this purpose and the results given are for coatings on this surface. They include the total reflection factor under diffuse illumination and the apparent factor for normal incidence, 45° angle of view, and 45° incidence, normal view. Finally the Helmholtz reciprocal relations as developed by H. J. McNicholas, B. of S. Journal of Research, 1, No 1, are utilised to calculate the apparent factors at normal and 45° angle of view and diffuse illumination from polar measurements of brightness under conditions of parallel illumination.