Abstract: A flux averaging sphere having an improved efficiency and a high averaging nd diffusing effectiveness over the visible and ultraviolet ranges. The interior surface of the sphere is coated with a white diffusing material which has a higher reflectance for visible radiation than for ultraviolet radiation. A radiation converting means is disposed inside the sphere for converting incoming ultraviolet radiation to visible radiation before reflections from the sphere wall occur. The radiation converting means is transparent to incoming visible radiation which therefore remains unaffected. The efficiency of the device is further improved by eliminating the small gap between the output area of the sphere and the face of the photomultiplier tube. The sphere is formed with a tapered tubular extension at the output area and the face of the photomultiplier tube is abutted against the end of the extension thereby forming a substantially light tight seal.