A Null-Balance Vidicon Spectrophotometer

Abstract

Automatic control theory was applied to the null-balance processing of spectral data generated by a vidicon television camera. The spectrum, after passing through reference and sample cells, was focused on the vidicon faceplate. The camera detects a single wavelength with each horizontal scan. The scan generates monochromatic dark reference, source reference, and sample signals. The voltage signals (video) are processed by a nonlinear null-balance loop. The loop computes the ratio (I/I(0))(gamma) at each scanned wavelength. gamma is a constant introduced by the vidicon light transfer characteristic. The train of horizontal scans generated in one vertical vidicon scan represents the scanned spectrum. The entire spectrum is thereby scanned without mechanical motion. The null-balance loop renders the spectrophotometer insensitive to changes in line voltage, amplifier drift, or shift in spectral sensitivity. The oscilloscope readout displays two horizontal lines representing 100% transmission and dark level as a function of wavelength. The sample absorption spectrum is traced between these two lines. (I/I(0))(gamma) is obtained by linear interpolation between these limits. gamma has a numerical value of less than unity and therefore tends to compress the photometric scale (I/I(0)). That is, the low light level signals are of larger amplitude than those of a linear scale. Gamma becomes a constant coefficient of the extinction coefficient and presents no problem in quantitative chemical analysis.