In the course of infrared studies on polymers and other materials, one is often interested in weak absorption bands that may be significant in the structural determination of the sample. In order to increase the intensity of these bands, the operator generally increases the sample thickness and thereby frequently decreases the background transmission. However, some samples such as certain mulls or samples containing inorganic fillers or even inorganic samples themselves are quite dense and require the attenuation of the reference beam in order to record the spectrum properly. Thus it is often desirable to compensate for the fixed energy losses in the sample beam (or in effect, to expand the scale by attenuation of the reference beam). This has been accomplished through the use of wire mesh screens or slotted shutters. These methods are inconvenient since they either produce a "step-by-step" attenuation, have only a limited attenuation range, or the degree of attenuation can not be easily adjusted. A reference beam attenuator that will compensate the energy losses in the sample beam from 0 to 100% transmission in a continuous manner and can be adjusted to energy differences as small as 1% is shown in Figure 1. This attenuator was constructed for use with the Perkin-Elmer Model 21 Spectrophotometer and its mounting is shown in Figure 2.
J. P. Luongo, "Continuous Reference Beam Attenuator for Infrared Spectrophotometry," Appl. Spectrosc. 14, 24-25 (1960)