An Optical Null Double-Beam Double-Pass Infrared Spectrophotometer

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Abstract
A novel signal treatment for an optical null double-beam double-pass system is developed and used in a vacuum-type prism infrared spectrophotometer. A feature of this instrument is the arrangement of the specially-shaped beam switch and the second chopper working in conjunction with the phase-sensitive rectifier. The beam switch is made so that the lower half of the reference beam and the upper half of the sample beam are sent to the monochromator optics during one half of the cycle; the other parts of reference and sample beams reach the monochromator optics during the other half of the cycle. The second chopper alternately blocks the upper and lower halves of the image formed at the focal point of single-pass beam in proper synchronism with the beam switch. Fairly good cancellation of the scattered radiation was achieved through adjustment of the phase relation between the beam choppers and the phase-sensitive rectification system. In addition to reporting the general design and construction, detailed descriptions of the control and recording systems which provide the unique features are also included.
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