Development of a New Reference Spectrophotometer

Abstract

A new reference spectrophotometer is being developed at the National Research Council of Canada (NRCC) for high-accuracy transmittance measurements over the spectral range 200 to 2500 nm. This computerized instrument is a single-beam design based upon a servomotor-driven double monochromator with a wavelength resolution of 0.022 nm. The other main components are: (1) two interchangeable sources (deuterium and tungsten-halogen), (2) two computer-selectable TE-cooled detectors (GaAs photomultiplier tube and PbS cell), (3) all-reflective input and exit optics, and (4) a large sample compartment. The significant feature of the optical system is a large, well-collimated measurement beam: the angle of convergence is 0.7° for a slit height of 7 mm, and the maximal beam size is 37x20 mm. This beam geometry eliminates the need for polarization corrections (using linearly polarized light) or compensation for spatially non-uniform detector responsivity (using averaging sphere). The paper describes the instrument design and presents data from preliminary performance tests. The systematic and random sources of error that have been investigated include: wavelength accuracy and reproducibility, bandpass, beam uniformity, polarization, stray light, system drift and linearity. A new linearity tester has been developed for checking the photometric accuracy. This automated device is based upon the double-aperture method but takes advantage of high precision piezoelectric motors to give a single adjustable aperture. Transmittance measurements of several neutral-density glass filters at 546 nm demonstrate that the photometric precision is better than 0.01% of the measured value and that the photometric accuracy is a few parts in 104. The wavelength scale is accurate to better than ±0.1 nm from 300 to 2500 nm, and is reproducible within ±0.03 nm.