An automatic two wavelength pyrometer without a drift was achieved by using zero method The application of zero method permits the use of photosensors whose stability in spectral responses is rather poor. A feedback system including two variables was newly developed for this instrument. In order to detect the differences between object and reference lights at two wavelengths, the instrument is equipped with a rotating mirror which guides object and reference radiations alternately to a pair of

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... hotosensors, one for a short wavelength and the other for a long wavelength.