The merit of the Hadamard transform active long-path absorption spectrometer was demonstrated by laboratory experiments. The dependence of measurement errors on the codes for the modulation was investigated. The spectral resolution of a spectrometer system and the deformation of the spectrum were tested. The sensitivity of the NO
3 measurement in the 660-nm region was estimated for a system using high intensity LEDs and a room temperature photodiode array. The effect of the ghost of the Hadamard transform due to the nonlinearity of the detector and a method to correct the ghost are discussed.
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