Efficient averaging spheres for visible and ultraviolet wavelengths

On the basis of a theoretical analysis that identifies the design criteria for optimization of efficiency, an averaging sphere with a barium-sulfate target was constructed that has an efficiency of 80-90% in the visible. In spite of the decreased reflectance of the sphere walls in the uv, this sphere is still 30% efficient at 300 nm and, thus, can be used for near-uv work. For applications below 300 nm, a cell containing a fluorescent dye is placed inside the sphere in order to shift the incident radiation into the longer wavelength region in which the reflectance of the sphere walls is higher. The use of this fluorescent wavelength converter, which was designed so that the sphere remains usable in the visible, resulted in a sphere efficiency of about 20% in the region between 200 nm and 300 nm and 50% in the visible. The averaging properties of these spheres were found to be adequate for high-accuracy photometric measurements.