New reference spectrophotometer.

A new single beam spectrophotometer is described in which transmittance is measured by placing samples normal to a parallel beam of light. Collimation and focusing of the main beam are achieved by means of off-axis parabolic mirrors. The wavelength at which the transmittance is to be measured is selected by a plane grating monochromator having off-axis parabolic mirrors and circular holes as entrance and exit apertures. The instrument has an inherent accuracy estimated to be 0.0001 transmittance unit. Its precision is characterized by a repeatability of 0.00004 transmittance units for neutral-density filters with transmittances between 10% and 30%. The design philosophy used to achieve these results is presented. A discussion of some systematic errors commonly neglected in routine spectrophotometric, measurements is given. Systematic errors such as detector nonlinearity and stray radiant energy are measured.