An Optical Null Double-Beam Far Infrared Spectrophotometer

The design, construction, and performance of a compact double-beam far infrared spectrophotometer are described. An optical null system which minimizes the loss of energy is used as the photometric system. Efficient filter combinations not involving a crystal chopper are used for eliminating unwanted radiation. All optical components are mounted in a vacuum tank to eliminate the energy loss due to water vapor absorption. Accordingly, high resolution can be obtained through the entire spectral region from 500 cm−1 to 60 cm−1. In the instrument, provision for fully automatic performance is made through the control system which includes a variety of mechanisms. Ease of change from double-beam to single-beam operation is an additional feature.