Hadamard transform spectroscopy for the 1990s

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Abstract
Two Hadamard transform instruments designed for the 1990's using a fixed Hadamard encodement mask are presented. One is a dispersive spectrometer for doing near-infrared (NIR) Hadamard transform Raman spectrometry. The other is a stationary interferometer for doing visible (VIS) and near-infrared (NIR) emission and absorption spectrometry. The encodement mask displays the Hadamard encodement generated by external command - in this case, a computer. In the dispersive spectrometer the encodement determines specific spectral resolution element combinations that are allowed to impinge upon the single detector. The irradiance for each such combination is measured at the single detector and the Hadamard transform of this data set represents the irradiance (i.e. intensity) of each spectral resolution element. The complete collection of these intensities compares favorably to the spectrum collected on a conventional dispersive spectrometer. The stationary interferometer uses a fixed encodement mask and appropriate optics to allow specific combinations of sections (i.e. spatial resolution elements) of the spatial interferogram to impinge on a single detector. The irradiance for each such combination is measured at the single detector and the Hadamard transform of this data set represents the irradiance (i.e. intensity) of each section (i.e. spatial resolution element) of the spatial interferogram. The advantage of encoding the spatial interferogram from the stationary interferometer is the ability to use the single fixed detector rather than an array of many fixed detectors or a single moving detector. Some general advantages of Hadamard transform instruments are: (1) multiplexing the signal to one detector (2) no continuously moving parts allow frequency precision and good spectral subtraction (3) reasonably high optical etendue' or throughput (4) selective elimination of certain areas of the spectra and the rejection of Rayleighline in Raman experiments

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