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.
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