2016 Volume 100 Issue 10 Pages 455-461
An integrating sphere is commonly used for a total luminous flux measurement of LED lighting. According to the theory of the integrating sphere, its interior coating needs to be a perfect reflecting diffuser. Although JIS C7801-2009 and IES-LM79, in which the measurement method of the total luminous flux of a light source is standardized, recommend that the reflectance of an interior coating of an integrating sphere be 90 % or higher, its deviation from the Lambertian is not discussed. This report measures and compares the bidirectional reflectance distribution functions (BRDF) of two commercial BaSO4 coatings, made by different manufacturers, and a PTFE coating for the integrating sphere. All samples had characteristics similar to the perfect diffuse reflection if the angle of incidence was close to the normal of the surface. At an incident angle of 60 degrees or more, BRDF of some of the sample was grater out-of-perfect diffusion. It affects the spatial response distribution function (SRDF) of the integrating sphere when a 2π geometry light source mounted on the sphere wall is tested and when part of the light from the source is reflected on the sphere wall closest to the source.