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A simple scheme by which any spectrometer is readily converted to a direct ratio-recording spectrophotometer is described. The optics consist of a double-beam “slit illuminator” which passes light from a common source area alternately over two identical paths to the spectrometer entrance slit. The optical arrangement allows great flexibility in the use of samples since it includes foci for small samples and sufficient space for low temperature cells, gas cells, etc. Common paths make the use of filters, polarizers, etc., convenient and accurate. Spectrometer optics, detector and amplifier are common to both signals and hence non-critical. The electrical signals from the two beams are amplified, “sorted” and rectified by synchronous switches, compared potentiometrically and their ratio recorded. Details of the amplifier and ratio circuits are included. A simple method is described by which errors in the ratio caused by the frequency characteristics of the detector, input transformer, or amplifier are eliminated. The energy in the comparison beam is kept constant at any predetermined level while a spectrum is scanned by means of a simple servo slit-width control. This arrangement is much more convenient and flexible than cams, particularly when interchangeable prisms are used.
© 1950 Optical Society of America
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