Notes on the design, adjustment and calibration of spectrophotometers

Abstract

Techniques of calibration and adjustment of spectrophotometers, made possible by the high sensitivity of photomultipliers, together with some general principles of design, are illustrated with reference to the Dobson spectrophotometer for measurements in the near ultra-violet. This instrument is primarily intended to investigate atmospheric ozone, but the methods described are of general application. Techniques discussed include (1) the use of null-balancing to measure intensity ratio of nearby wavebands, whose variance, both in the instrumental measurement and in the light source itself, is less than that of direct intensity recording; (2) the use of synchronous commutator rectification of the chopped light signals to discriminate against random fluctuations, giving a centre-zero deflexion linear with intensity; (3) new methods of optical alinement to ensure that the maximum possible aperture is used but that the effective stops act on undispersed light only, giving all wavelengths of the same incident beam an equal chance to pass to the exit slit; (4) determination of photomultiplier characteristics and methods of setting to obtain repeatable outputs; (5) an accurate null-method for calibration of the optical wedge.