A double-beam wavelength-modulated reflectometer utilizing a single detector and without a reflectivity reference has been described. Separate electronic channels are used for the two wavelengths at which the reflectometer alternatively operates. The gain of one of the channels is controlled by feedback to eliminate the instrumental background. The derivative and conventional reflectivity are measured simultaneously. The influence of the photomultiplier dark current and scattered light on the accuracy and of some design parameters on the SNR have been evaluated. The system has been checked by measuring the derivative and conventional reflectivity spectrum of GaAs.
W. Kardux and H. J. Krusemeyer, "Double-Beam Two-Channel Wavelength-Modulated Reflectometer," Appl. Opt. 13, 2704-2711 (1974)