The embodiment of the subject apparatus disclosed in the present application employs an additional infrared detector which is positioned to detect scattered infrared energy. An infrared filter having a passband including the wavelength of the reference beam but not that of the absorption beam is positioned in front of this detector. The signal derived at this additional detector is thus based only upon scattered infrared energy at the reference wavelength, and is used to oppose the signal derived at the standard detector at the reference wavelength. Further adjustment of the outputs of the detectors is also obtained so that the accuracy of measurement of film thickness is not affected by scatterers in the film or by a change in the number of such scatterers.
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