An automatic recording spectrophotometer for the infrared is described, which employs a double-beam source, Littrow spectrometer, fast bolometer, and high speed pen- and drum-recording system. The sample and reference beams are alternately chopped at 10 cycles/sec. at the entrance slit to the spectrometer. The amplified a.c. signal on the bolometer is used to drive a variable comb diaphragm in the reference beam. The spectra are registered on a recorder by a pen mechanically linked to this comb.
The spectral region covered is between 2 microns and 16 microns with a sodium chloride prism. The range can be extended to about 26 microns by the use of a potassium bromide prism. The instrument is designed for high speed analytical work, producing permanently recorded spectrograms of gases, liquids, and solids on accurate charts at intervals of 18 minutes. Percent transmission is recorded with an accuracy of ±1 percent, and wave-length with an accuracy of ±0.025 micron. Records are reproducible to within ±1 percent transmission and ±0.01 micron.
Special features are its high recording speed, stability and freedom from drift with the “optical-null” principle, linearity and ease of operation.
W. S. BAIRD, H. M. O’BRYAN, GEORGE OGDEN, and DOROTHY LEE, "An Automatic Recording Infra-Red Spectrophotometer," J. Opt. Soc. Am. 37, 754-759 (1947)