Performance of a Double-Beam Recording Infra-Red Spectrophotometer

Abstract

The performance of the Perkin-Elmer Model 21 Recording Infra-Red Spectrophotometer has been tested by measuring known spectra under a wide variety of operating conditions. A complete infra-red spectrum in the rock salt region can be recorded as a graph of percent transmittance vs. wave-length in less than 10 minutes. At slower speeds resolving power can be varied from the normal of 30 percent to 40 percent up to about 60 percent of the Rayleigh limit. For extreme resolution or stability, the r.m.s. noise level of 10−9 volts or 2×10−10 watts at the fastest response speed can be reduced as far as 2×10−10 volts or 4×10−11 watts by increasing the response time. The extreme ranges of the operating conditions are: response time, 4.5 to 110 seconds for full scale; scanning rate, 0.5 to 400 minutes per micron; chart scale length, 1 to 50 inches per micron; slit width, to give constant energy levels between. 01 and 2 r.m.s. microvolts full scale. The effects of scattered light and atmospheric absorption bands are less than 3 percent throughout. Operation is automatic after the conditions are selected and the instrument turned on. These features are illustrated in photographic reproductions of original records with more detailed data on the independent performance of the instrument’s more critical parts.