Inverse-fourth apparatus for photometric calibrations

A new photometric device is described which can perform direct, as opposed to comparison, measurements of optical transmittance, without bootstrapping, over a range of nearly six orders of magnitude. According to the demonstrated principle of operation, the amount of light flux received at a detector is proportional to the inverse-fourth power of the effective source-to-detector distance. As the device is configured, the adjacent source and detector are stationary while a flexible light guide, which collects light from the source and re-emits it toward the detector, moves on a carriage. The transmittance, T, of a filter is measured in terms of the ratio of carriage positions, for constant detector output, with and without the filter in the light path; the optical density is then D=-logT. The position of the carriage, relative to the source-detector plane, is measured over the 3 m range of travel with a resolution of 0.01 mm. Operated with a 1000 W tungsten-halogen lamp as a source, a photomultiplier as a detector, and either an integrating sphere or opal glass for diffuse collection at the detection plane, the system is designed to attain an overall accuracy of +/-0.5% for ANSI PH2.19 diffuse transmission densities up to 6.0 density units, corresponding to +/-0.1% for transmittances above 0.5000 transmittance units and to +/-3% for transmittances between 0.000001 and 0.0001 transmittance units.