A system is described which modifies a scanning spectrometer for the visible and the near ir in a simple manner for double beam operation, particularly for use with small samples. The apparatus utilizes a split pupil of a collimator in conjunction with a rotating sector to separate the two partial beams. The operation of the modulator and the electronic circuitry are discussed in detail. The system offers the advantage of simple construction with a high degree of compensation of the optical path lengths, and particularly, with close proximity of probe and reference specimen.
K. Leutwein, "A Simple Double Beam Accessory for Absorption Spectroscopy," Appl. Opt. 10, 46-50 (1971)