The present invention relates generally to spectroscopy, and more specifically the invention pertains to a Hadamard spectrograph configuration which will significantly increase the throughput, and hence the sensitivity, of a conventional, single-slit, planar detector array electronic spectrograph.
Recently, a technique known as Hadamard spectroscopy has been developed. The basic idea of conventional Hadamard spectrometers as follows. In order to determine the spectrum of a beam of light, instead of measuring the intensity at each wavelength separately, the spectral components are combined in groups and the total intensity of each group is measured. The different wavelength components are thereby multiplexed onto a single detector. As a result, the spectrum is determined much more accurately, than with conventional spectrometers. Further details regarding the distinction between Hadamard spectroscopy and conventional spectroscopy are discussed briefly below.
In conventional spectrometers, a beam of energy composed of the separate wavelengths to be analyzed is passed through an entrance slit, collimated and then passed through a dispersive element to disperse the band into a spectrum and decollimated so that the separate wavelengths are spatially spread out on the exit plane. An exit slit is used to pass only a narrow band of the wavelengths to a detector and the individual wavelengths are analyzed or scanned by mechanically moving either the dispersive element or the slit. The slit needs to be relatively narrow to achieve fine resolution so that the energy passed by the slit is relatively small in comparison to the energy of the whole spectrum being scanned. The detector thus measures only the relatively small signal in the passband for a relatively short time compared with the total time of observation so that the signal-to-noise ratio is relatively low.
The conventional photodiode array electronic spectrograph uses a monochromator with a single entrance slit but many detectors in the exit plane. Usually the width of the entrance slit is the same as the width of a single element on the detector array to achieve maximum spectral resolution with the particular array. The advantage of this system is that all wavelengths are measured simultaneously. The disadvantage is that the small aperture of the entrance slit severely limits the amount of light which may pass into the instrument.
The basis of conventional Hadamard transform spectroscopy is to measure groups of wavelengths simultaneously on a single detector in order to improve the signal-to-noise ratio. This could be implemented by placing a series of masks at the exit plane of spectrometer, however a much more efficient design, and therefore the one usually implemented, is to use a cyclic mask and then step the mask, making a measurement at each step of the mask. This reduces the number of mask elements which are required from n.sup.2 to 2n-1. For each mask or mask position a different combination of spectral elements falls on the detector. Intensities measured with n different masks can be used to compute the intensities of n different spectral elements.
Doubly encoded Hadamard transform spectrometers use masks at both the entrance and exit plane and offer advantages of both multiplexing and wide aperture.
The task of applying Hadamard mask technology to conventional photodiode array spectrograph systems is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,007,989 issued to Wajda;
U.S. Pat. No. 4,580,162 issued to Mori;
U.S. Pat. No. 4,442,454 issued to Powell;
U.S. Pat. No. 4,435,838 issued to Gourlay;
U.S. Pat. No. 3,955,891 issued to Knight et al.; and
U.S. Pat. No. 3,578,980 issued to Decker.
While the above cited references are instructive, the ongoing task remains to increase the performance of conventional photodiode array spectrograph systems, and the present invention is intended to satisfy that need.