Abstract: Light to be measured is input to a diffraction grating after is has been oscillated by a wavelength modulation device with a constant frequency F. The light is incident to the diffraction grating at an angle of incidence, .theta., and is oscillated with a frequency F with a center angle of incidence, .theta..sub.0, as a center. An output spectrum of said diffraction grating is received by a photoelectric converter. An electric signal from the photoelectric converter is oscillated with a frequency F over a wavelength range of .lambda..sub.0 .+-..DELTA..lambda. where .lambda..sub.0 is a center wavelength measured at the center angle of incidence .theta..sub.0. The oscillated spectrum signal is synchronously detected, by a synchronous detector, with a frequency 2F through a high-pass filter, obtaining a variation spectrum at the measured center wavelength .lambda..sub.0.

Abstract: A wavelength-modulated derivative spectrometer is disclosed which enables continuous measurement of trace amounts of NH.sub.3 in stack gas which includes a high concentration of SO.sub.2, and which in general enables measurement of low amounts of a substance having a relatively broad-band absorption spectrum present in a gas together with a high concentration of a substance having a relatively narrow-band periodic spectrum structure, with interference resulting from the latter substance being substantially eliminated.