A method of rapid, programmable filtering of spectral transmittance, reflectance, or fluorescence data to measure the concentrations of chemical species has been proposed. By "programmable" is meant that a variety of spectral analyses can readily be performed and modified in software, firmware, and/or electronic hardware, without need to change optical filters or other optical hardware of the associated spectrometers. The method is intended to enable real-time identification of single or multiple target chemical species in applications that involve high-throughput screening of multiple samples. Examples of such applications include (but are not limited to) combinatorial chemistry, flow cytometry, bead assays, testing drugs, remote sensing, and identification of targets.
The basic concept of the proposed method is to perform real-time cross-correlations of a measured spectrum with one or more analytical function(s) of wavelength that could be, for example, the known spectra of target species. Assuming that measured spectral intensities are proportional to concentrations of target species plus background spectral intensities, then after subtraction of background levels, it should be possible to determine target-species concentrations from cross-correlation values. Of course, the problem of determining the concentrations is more complex when spectra of different species overlap, but the problem can be solved by use of multiple analytical functions in combination with computational techniques that have been developed previously for analyses of this type.
The method is applicable to the design and operation of a spectrometer in which spectrally dispersed light is measured by means of an active-pixel sensor (APS) array. The row or column dimension of such an array is generally chosen to be aligned along the spectral-dispersion dimension, so that each pixel intercepts light in a narrow spectral band centered on a wavelength that is a known function of the pixel position. The proposed method admits of two hardware implementations for computing cross-correlations in real time. One hardware implementation would exploit programmable circuitry within each pixel of an APS array. The analog spectral-intensity reading of the photodetector in each pixel would be multiplied by a gain proportional to value of the analytical function for the wavelength that corresponds to the pixel position. As a result, the output from each pixel would be proportional to contribution of the pixel to the cross-correlation (plus background).
The outputs of all the pixels along the spectral-dispersion dimension would be summed to obtain the value of the cross-correlation (plus background). Such on-chip cross-correlation could be performed rapidly because the analytical function could be statically programmed into the APS array and the multiplications could be done simultaneously or nearly so. All of the additions could be done simultaneously by means of a single binning instruction. The charge wells of all the pixels could be connected simultaneously, collecting all the charge outputs from multiplication operations into one "superpixel," the single readout value of which would constitute the cross-correlation value for the given analytical function.
For an instrument in which the APS rows were aligned along the spectral-dispersion dimension and in which the image of a spectrograph slit was aligned along the pixel columns and spanned multiple pixel rows, it would be possible to perform simultaneous cross-correlations for multiple target species by applying, to each pixel row, the analytical function corresponding to one of the target species. A separate readout would be needed for each target species. In the other hardware implementation, cross-correlations would be computed externally to the APS array. The multiplications and additions would be performed in pipeline fashion. If the APS-array outputs were analog, then programmable analog signals representing the analytical functions would be synthesized in phase with the corresponding stream of analog APS-array outputs and the multiplications and additions would be performed by relatively inexpensive, commercially available analog mixing and filtering circuits, respectively. If the APS-array outputs were digital, the cross-correlations could be computed by a digital signal processor.
Ordinarily, the analog approach would be preferable because the analog operations can be performed much more rapidly than can the corresponding digital multiplications and additions.
This work was done by Gregory Bearman, Michael Pelletier, and Suresh Seshadri of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on- line at www.techbriefs.com/tsp under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention.
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Refer to NPO-30912,volume and number of this NASA Tech Briefs issue,and the page number.