A rapid-scan spectrometer using a TGS detector typically operates with a mirror velocity of 0.15 cm/s producing a modulation frequency of 5 kHz at the reference laser wavelength, 1.25 kHz at 4000 cm^-1 and 125 Hz at 400 cm^-1. The corresponding separation between adjacent spectral elements is 1.3 Hz. If a cryogenic detector is used, a higher mirror velocity may be used, producing modulation at the laser wavelength as high as 100 or 125 kHz, with a separation of 25 to 30 Hz between adjacent spectral elements.