Optical Sphere Paint and a Working Standard of Reflectance

TLDR

Specially prepared barium sulfate powders and their polyvinyl alcohol coatings serve as working reflectance standards and as coatings for integrating spheres, performing satisfactorily across 0.20–2.00 µm. Because barium sulfate has a low refractive index, thick layers are required to achieve high reflectance. The barium sulfate powder exhibits an absolute luminous reflectance of 0.995 ± 0.001, while the paint reaches 0.992 ± 0.001; its spectral reflectance exceeds that of magnesium oxide, especially in the UV, enabling reliable measurements down to 200 nm and improving instrument sensitivity in the shortwave region.

Abstract

Specially prepared barium sulfate powders and coatings of these powders with polyvinyl alcohol have been used as working standards of reflectance and for coating integrating spheres. These materials are satisfactory for use in the wavelength range 0.20–2.00 μ. Since the refractive index of barium sulfate is rather low, it is necessary to use fairly thick layers in order to obtain good results. The absolute value of luminous reflectance of the barium sulfate powder, described in this paper, is 0.995 ± 0.001, and that of the paint, when properly applied, is 0.992 ± 0.001. The spectral reflectance of this material is higher than that of magnesium oxide, particularly in the uv region of the spectrum. By using barium sulfate as reference standard and as a coating on the integrating spheres of spectrophotometers, it is now possible to measure spectral reflectance reliably in the uv region to 200 nm. Because the reflectance of barium sulfate paint is significantly greater than that of magnesium oxide in the shortwave region, there is a corresponding gain in instrument sensitivity when barium sulfate is used as a sphere coating and as a comparison material.