A 5-m integrating sphere.

An integrating sphere with an inner diameter of 5 m was constructed with special attention in order to make a true sphere by welding together steel plates of thickness of 12 mm. The deviations of radii from a true sphere are within +/-0.3%. A special lift, which is installed below the sphere, is used for changing a lamp or painting the sphere. The inner surface of the sphere was coated with a water paint of BaSO(4). Its spectral reflectance was measured by a method, in which a part of the spherical surface was substituted by a surface with low reflectance and spectrophotometric measurements were made on the luminance of the measurement window of the sphere before and after the substitution, operating a tungsten lamp in the sphere. The values of spectral reflectance obtained by this method were from 0.93 to 0.96 in the visible region, ten months after the coating.