Devices and method for spectral measurements Number:7,180,588 from the United States Patent and Trademark Office (PTO) owispatent

Title: Devices and method for spectral measurements

Abstract: A spectral measurement device comprising an entrance aperture for receiving an electromagnetic energy and a mask located at the entrance aperture in the form of a two-dimensional encodement pattern. An optical element conditions the electromagnetic energy received from the mask for presentation to the spectral dispersion element and the and a spectral dispersion element disperses the electromagnetic energy in one or more dimensions. Additionally, the optical element conditions the dispersed electromagnetic energy onto an array of detector elements.

Patent Number: 7,180,588 Issued on 02/20/2007 to Geshwind,   et al.

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Inventors:	Geshwind; Frank (Madison, CT), Coifman; Ronald R. (North Haven, CT), Coppi; Andreas (Groton, CT), Deverse; Richard A. (Kailua Kona, HI), Fateley; William G. (Manhattan, KS)
Assignee:	Plain Sight Systems, Inc. (Hamden, CT)
Appl. No.:	11/075,114
Filed:	March 7, 2005

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Related U.S. Patent Documents

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	Application Number	Filing Date	Patent Number	Issue Date
	10832684	Apr., 2004
	09798860	Mar., 2001	6859275
	09672257	Sep., 2000	6392748
	09502758	Feb., 2000	6128078
	09289482	Apr., 1999	6046808
	60550966	Mar., 2004

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Current U.S. Class:	356/310 ; 356/330
Current International Class:	G01J 3/28 (20060101)
Field of Search:	356/310,330

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References Cited [Referenced By]

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U.S. Patent Documents

	4193691		March 1980		Fjarlie
	4448529		May 1984		Krause
	4790654		December 1988		Clarke
	5061049		October 1991		Hornbeck
	5323002		June 1994		Sampsell et al.
	5483335		January 1996		Tobias
	5504575		April 1996		Stafford
	5506676		April 1996		Hendler et al.
	5627639		May 1997		Mende et al.
	5737075		April 1998		Koch et al.
	5748308		May 1998		Lindberg et al.
	5828066		October 1998		Messerschmidt

Primary Examiner: Evans; F. L.
Attorney, Agent or Firm: Fulbright & Jaworski LLP

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Parent Case Text

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RELATED APPLICATION

This application claims priority benefit of provisional patent application No. 60/550,966, filed Mar. 6, 2004, which is incorporated by reference in its entirety and is a continuation-in-part of application Ser. No. 10/832,684, filed Apr. 26, 2004, which is a divisional of application Ser. No. 09/798,860, filed Mar. 1, 2001, now U.S. Pat. No. 6,859,275, which is a continuation-in-part of application Ser. No. 09/672,257, filed Sep. 28, 2000, now U.S. Pat. No. 6,392,748, which is a continuation of application Ser. No. 09/502,758 filed Feb. 11, 2000, now U.S. Pat. No. 6,128,078, which is a continuation of application Ser. No. 09/289,482 filed Apr. 9, 1999, now U.S. Pat. No. 6,046,808, each of which is incorporated by reference in its entirety.

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Claims

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We claim:

1. A spectral measurement device comprising: an entrance aperture for receiving an electromagnetic energy; a mask located at the entrance aperture in the form of a two-dimensional encodement pattern; a spectral dispersion element for dispersing said electromagnetic energy in one or more dimensions to provide a dispersed electromagnetic energy; an array of detector elements; and an optical element for conditioning said electromagnetic energy received from said mask for presentation to said spectral dispersion element and conditioning said dispersed electromagnetic energy onto said array of detector elements; and wherein said mask comprises spatially differentiated mask elements; and wherein said spectral dispersion element comprises line densities exposed to a plurality of spatially differentiated electromagnetic energy propagating from said spatially differentiated mask elements such that the magnitude of diffraction is varied for said spatially differentiated mask elements.

2. The spectral measurement device of claim 1, wherein said line densities are programmable line densities.

3. A spectral measurement device comprising: an entrance aperture for receiving an electromagnetic energy; a mask located at the entrance aperture in the form of a two-dimensional encodement pattern; a spectral dispersion element for dispersing said electromagnetic energy in one or more dimensions to provide a dispersed electromagnetic energy; an array of detector elements; and an optical element for conditioning said electromagnetic energy received from said mask for presentation to said spectral dispersion element and conditioning said dispersed electromagnetic energy onto said array of detector elements; and wherein said array of detector elements in a linear array of detector elements or a two dimensional (2D) array of detector elements; and wherein said array of detector elements is a linear array of detector elements; and wherein each detector element is set to receive light from a corresponding row of said two-dimensional encodment pattern of said mask.

4. A spectral measurement device comprising: an entrance aperture for receiving an electromagnetic energy; a mask located at the entrance aperture in the form of a two-dimensional encodement pattern; a spectral dispersion element for dispersing said electromagnetic energy in one or more dimensions to provide a dispersed electromagnetic energy; an array of detector elements; and an optical element for conditioning said electromagnetic energy received from said mask for presentation to said spectral dispersion element and conditioning said dispersed electromagnetic energy onto said array of detector elements; and wherein said mask is encoded with arbitrary fixed weighted combinations of spectral components selected to directly measure predetermined spectral features.

5. The spectral measurement device of claim 4, wherein said predetermined spectral features are the positive and negative parts of one or more features from a set consisting of regression vectors, principal component vectors of a spectral model, and local discriminative basis (LDB) vectors.

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Description

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FIELD OF THE INVENTION

The present invention relates generally to multiplexed, encoded and weighted spectral measurements and devices, more particularly to an improved spectral measurement method and system utilizing a compact, field deployable solid state device with no moving parts and employing weighted combinations of resolution elements, Hadamard, principal component analysis and other encoding methods to provide real-time high-throughput spectrometric, chemometric and/or other analytical measurements.

BACKGROUND OF THE INVENTION

Imagers employ either a two-dimensional (2D) multichannel detector array or a single element detector. Imagers using a 2D detector array measure the intensity distribution of all spatial resolution elements simultaneously during the entire period of data acquisition. Imagers using a single detector require that the individual spatial resolution elements be measured consecutively via a raster scan so that each one is observed for a small fraction of the period of data acquisition. Prior art imagers using a plurality of detectors at the image plane can exhibit serious signal-to-noise ratio problems. Prior art imagers using a single element detector can exhibit more serious signal-to-noise ratio problems. Signal-to-noise ratio problems limit the utility of imagers applied to chemical imaging applications where subtle differences between a sample's constituents become important.

Spectrometers are commonly used to analyze the chemical composition of samples by determining the absorption or attenuation of certain wavelengths of electromagnetic radiation by the sample or samples. Because it is typically necessary to analyze the absorption characteristics of more than one wavelength of radiation to identify a compound, and because each wavelength must be separately detected to distinguish the wavelengths, prior art spectrometers utilize a plurality of detectors, have a moving grating, or use a set of filter elements. However, the use of a plurality of detectors or the use of a macro moving grating has signal-to-noise limitations. The signal-to-noise ratio largely dictates the ability of the spectrometer to analyze with accuracy all of the constituents of a sample, especially when some of the constituents of the sample account for an extremely small proportion of the sample. There is, therefore, a need for imagers and spectrometers with improved signal-to-noise ratios.

Prior art variable band pass filter spectrometers, variable band reject filter spectrometers, variable multiple band pass filter spectrometers or variable multiple band reject filter spectrometers typically employ a multitude of filters that require macro moving parts or other physical manipulation in order to switch between individual filter elements or sets of filter elements for each measurement. Each filter element employed can be very expensive, difficult to manufacture and all are permanently set at the time of manufacture in the wavelengths (bands) of radiation that they pass or reject. Physical human handling of the filter elements can damage them and it is time consuming to change filter elements. There is, therefore, a need for variable band pass filter spectrometers, variable band reject filter spectrometers, variable multiple band pass filter spectrometers or variable multiple band reject filter spectrometers without a requirement for discrete (individual) filter elements that have permanently set band pass or band reject properties. There is also a need for variable band pass filter spectrometers, variable band reject filter spectrometers, variable multiple band pass filter spectrometers or variable multiple band reject filter spectrometers to be able to change the filters corresponding to the bands of radiation that are passed or rejected rapidly, without macro moving parts and without human interaction.

In several practical applications it is required that an object be irradiated with radiation having particularly shaped spectrum. In the simplest case when only a few spectrum lines (or bands) are necessary, one can use a combination of corresponding sources, each centered near a required spectrum band. Clearly, however, this approach does not work in a more general case, and therefore it is desirable to have a controllable radiation source capable of providing arbitrary spectrum shapes and intensities. Several types of prior art devices are known that are capable of providing controllable radiation. Earlier prior art devices primarily relied upon various "masking" techniques, such as electronically alterable masks interposed in the optical pathway between a light source and a detector. More recent prior art devices use a combination of two or more light-emitting diodes (LEDs) as radiation sources. In such cases, an array of LEDs or light-emitting lasers is configured for activation using a particular encoding pattern, and can be used as a controllable light source. A disadvantage of these systems is that they rely on an array of different LED elements (or lasers), each operating in a different, relatively narrow spectrum band. In addition, there are technological problems associated with having an array of discrete radiation elements with different characteristics. Accordingly, there is a need for a controllable radiation source, where virtually arbitrary spectrum shape and characteristics can be designed, and where disadvantages associated with the prior art are obviated. Further, it is desirable not only to shape the spectrum of the radiation source, but also encode its components differently, which feature can be used to readily perform several signal processing functions useful in a number of practical applications. The phrase "a spectrum shape" in this disclosure refers not to a mathematical abstraction but rather to configurable spectrum shapes having range(s) and resolution necessarily limited by practical considerations.

In addition to the signal-to-noise issues discussed above, one can consider the tradeoff between signal-to-noise and, for example, one or more of the following resources: system cost, time to measure a scene, and inter-pixel calibration. Thus, in certain prior art systems, a single sensor system may cost less to produce, but will take longer to fully measure an object under study. In prior art multi-sensor systems, one often encounters a problem in which the different sensor elements have different response characteristics, and it is necessary to add components to the system to calibrate for this. It is desirable to have a system with which one gains the lower-cost, better signal-to-noise, and automatic inter-pixel calibration advantages of a single-sensor system while not suffering all of the time loss usually associated with using single sensors.

In a conventional monochromater, white light or broadband optical energy emerging from an entrance slit is collimated onto a diffraction grating, angularly dispersed according to wavelength and then focused onto an exit slit. In this way, relatively monochromatic light or optical energy confined to some narrow band defined by the geometry and physical properties of optical elements and their arrangement emerges from the exit slit. By moving or translation of one or more of the slits left and right or rotating the grating while leaving the slits stationary, the wavelength of the monochromatic output light scans through the wavelength range of the device. The resulting output sequence interacts with the matter, and the results of each are measured. Alternatively, the light interacts with the matter prior to passing through the spectrometer and the resulting output sequence is then measured. In either case, the resulting sequence of measurements gives a spectral signature of the matter. However, such devices are deficient in that the two slits required to help define the bandpass located both at the object and image plane of the optical system work to severely limit the amount of light that passes through the system, and therefore limit the amount of light that is measured. For this reason, it is difficult or impossible to rapidly obtain good signal to noise ratios with monochromaters where high resolution is desired or in situations where there is a limit to the amount of light energy available.

Conventional linear array spectrometers can be viewed as an improvement over the monochromaters. In a conventional linear array spectrometer, the output slit located at the image plane or focal plane of the optical system is replaced by a linear array of detectors situated such that there is a detector at each exit slit position to receive the light. For such spectrometers, light interacts with the matter prior to passing into the monochromater, and the linear array of detectors simultaneously measures the resulting sequence of spectral data. The intensity of the set of bands of wavelength that impinge upon the linear array of detectors during the integration time of the measurement provides the spectral signature of the matter. Linear array spectrometers have an advantage over monochromaters in that the linear detector array collects all of the data simultaneously such that fluctuations in the source energy are not interpreted as features of the spectral signature of the matter. Additionally, unlike the monochrometers, the linear array spectrometers have no moving parts and can make instantaneous measurements. Further, during the time it takes to collect a spectrum using a scanning type monochrometer, the linear array spectrometers can collect multiple spectra. Conversely, the linear array spectrometer can collect an entire spectrum in the time it takes the scanning monochrometer to collect one spectral resolution element. However, the entrance slit still limits the amount light entering the system to each detector element in the linear detector array.

Conventional Hadamard spectrometers can be viewed as an improvement over both the scanning monochromaters and linear array spectrometers. In conventional Hadamard spectrometers, one or both of the slits of a monochromater are replaced by a coded array of slits (or mask). Thus, the exit light is no longer monochromatic in nature, but is an encoded mixture of wavelengths of light where the encodement is determined by the optical masks that can be located at the object or image planes of the optical system. The conventional Hadamard spectrometer operates by changing or moving one or both of the mask(s) through a pre-determined sequence of changes or moves. In this way, a full encodement library of exit light is produced. The light entering or exiting the optical system interacts with the sample or matter, and the results of each of the encodements are measured. The measurements of the light resulting from the interaction with the sample or matter and sequence of encoding combinations dictated by mask positions or encodements, is then mathematically inverted, so that one reconstructs the spectral signature of the sample or matter. Since the Hadamard spectrometer has many more slits than the monchrometer, more light is available at the exit aperture. However, the conventional Hadamard spectrometers have changing or moving parts to move or translate the encoded aperture through the requisite combinations of encodements to be measured. Such motion or change due to physical limitation of the conventional Hadamard spectrometer is generally subject to some variation, error and/or distortion over time, and is especially susceptible to errors in the presence of noise, heat, and other environmental or mechanical disturbances.

As noted herein, conventional spectral measurement systems, such as the scanning monochromaters suffer from these attributes noted herein. The linear array spectrometers and scanning monochromaters suffer from a lack of light throughput. Conventional spectral measurement systems, such as the monochromaters, Hadamard spectrometers and Fourier transform spectrometers suffer from a complexity and instability due to the presence of moving or changing parts. Since the latter spectral measurement systems make a series of measurements over time, rather than instantaneously, each suffers from errors when it is looking at light sources or sample/matter that is changing during the time of measure or scan. A further disadvantage of the scanning monochrometers, linear array spectrometers and Fourier transform spectrometer systems is that a contiguous regular interval of wavelength spectral data are collected. In many spectrometric applications, such contiguous spectral data generally contains no relevant or useful information with respect to the spectral signature of the sample or matter. Hence, it is desirable to collect non-contiguous variable band pass spectral resolution element data that comprise only those spectral bands that are deemed of significance to the spectrometric measure or analysis of the desired sample or matter.

Accordingly, there is a need for a spectral measurement system that offers the advantages of both the linear array spectrometers and Hadamard or Fourier spectrometers. Additionally, there is a need for a spectral measurement system capable of collecting only the non-contiguous and non-uniform band pass spectral data necessary for the desired analysis. The spectral measurement system of the present invention comprises multi-detector and no moving parts and provides instantaneous measurements.

In prior-art dispersive systems, as one moves to high resolution spectrometric measures one must decrease the slit width when preserving spectral bandwidth (spectral range) or increase the angular dispersion of the diffraction element and this results in a decrease of spectral range. Both the decrease in slit width and increase in angular dispersion decrease the photon flux through the spectrometer for any given measure of individual spectral resolution elements. This decrease in photonic flux limits sensitivity, analytical capability and the ability of the detector to measure subtle differences in spectral energy at each data resolution element or related spectral resolution element. Hadamard transform measurements allow for multiplexing in which a multitude of slits are opened simultaneously for each measurement. Fourier transform measurements allow for multiplexing in which there is no slit in the system, and each wavelength is measured with a Fourier weight that varies over time. The advantage of these multiplexing techniques is that photonic flux improves over single slit methods as the demand for resolution or data resolution elements increase for a given spectral bandwidth of operation. The drawback to increasing the spectral resolution requirement that can apply to these conventional methods, even Hadamard and Fourier Transform multiplexed measurements, is that the scan times may need to be increased so that the integration times for each of the data resolution elements in the scan is sufficient to collect enough signal to rise above the noise floor of the system and to achieve the desired measure of the requisite signal-to-noise. The conventional multiplexing methods require measuring a series of encodements over time T which dictates a maximum detector integration time of T/N where N is the number of resolution elements. This requires an increase in scan time and can be problematic if the source energy or sample changes over the time T of the scan. The spectral measurement system of the present invention based on multiplexing spectrometry proceeds upon the desirability of eliminating such problem as each detector views the source or sample fluctuations simultaneously.

In the spectral measurement system of the present invention, the scan is not intermodulated by these changes and the intermodulations do not show up as noise in the spectral data. This has been the impetus for fast scan type Fourier Transform (FT) spectrometer systems. Despite the speed increase in scan times, these fast scan systems are inadequate for many measures such as looking at spark or combustion products during the life of a burn cycle. The spectral measurement system of the present invention based on static spectrometer is capable of looking at rapidly changing sources, samples and environments while taking advantage of the high throughput afforded by multiplexed measurements.

Many point spectrometers whether multiplexed or not utilize single detectors. Historically this has in large part been due to the lack of availability, reliability, or affordability of appropriate detector arrays. Most applications that did not overtly require linear or two-dimensional arrays were hastily assigned to single detector solutions. However, detector arrays, when properly employed, can enable advantages such as faster collection rates, oversampling, and better signal to noise. One of the first to realize this was S. Mende in "Hadamard spectroscopy with a two-dimensional detecting array", S. B. Mende; E. S. Claflin; R. L. Rairden; G. R. Swenson, Applied Optics 32 (34): 7095 7105 (Dec. 1, 1993), in which he made the clever observation that for diffuse sources one could illuminate a two-dimensional coded aperture spectrograph and to obtain accurate and high resolution measurements. Each row of the mask consisted of a Hadamard sequence parallel to the direction of dispersion. The dispersed output from that row was recorded across the corresponding row of a two-dimensional focal plane array. Using that data, he proposed a scheme to invert the data and reconstruct the spectrum of the uniform light illuminating that row. Different Hadamard sequences of varying lengths were put on each row providing redundant information for averaging, or, if the source were uniformly illuminating across rows but spatially coherent across columns, 1-D spectral imaging could be obtained. In either case, the measurement required no mechanical scanning and only one frame of acquisition. His invention was limited by the numerical conditioning of the mathematical inverse required to reconstruct the spectra from the data collected. He also neglected to consider the possibilities of a linear array in similar configurations.

The present invention differs from Mende's in several ways. Each row of the coded aperture contains a Hadamard basis vector generated by the same Hadamard sequence. In one embodiment, this would mean that each row would contain a cyclic shift of a particular Hadamard sequence. Dispersion is still parallel to the rows, but the detector no longer needs to measure across the corresponding row to compute the spectra of the impinging light but rather across the columns. This removes the mathematical issues of Mende's approach and also no longer requires a two-dimensional array which can still difficult to afford or obtain for many wavelength ranges. Furthermore with a linear array perpendicular to the rows, acquisition can be even faster. A two-dimensional array can still be employed in the current invention, yielding redundant information to improve SNR for example by averaging.

It should be pointed out that in the subsequent U.S. Pat. No. 5,627,639, Mende discloses various coded aperture approaches to spectral imaging, and does consider one configuration in which each row is a generated by shifts of the same sequence, as in some embodiments of the present invention. However, he only discloses this in the context of spectral imaging and with the added requirement that the mask (or scene) be scanned. Again, the present invention is for diffuse input only and does not resolve spatial information, i.e. image. It requires no scanning and yields an instantaneous measurement that Mende's scanning method does not. Furthermore, other embodiments of the present invention are not designed to recover the full spectrum but rather "filters" or weighted combinations of wavelengths derived from simple or sophisticated mathematical and chemometric models. These measurements rapidly yield quantities of interest at reduced data rates. None of Mende's disclosures have any provision for this; the full spectrum is always being measured.

While there are more and more intelligent spectral devices being developed that are capable of measuring quantities of interest as opposed to a full raw spectrum which is analyzed post acquisition, most either involve some kind of clever active illumination or scanning filters and or a mechanically moving part--in the best case a MEMS device.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a spectral measurement system which overcomes the above-noted shortcomings of conventional spectral measurement systems.

An object of the present invention is to provide a spectral measurement system as aforesaid, which allows for a faster collection of spectral data during a single integration time of a detector or array of detectors. This faster collection is enabled by multiplexing.

An object of the present invention is to provide a spectral measurement system as aforesaid, which allows for the collection of spectral data with better signal-to-noise ratio (SNR) than similar multiplexed instrumentation that requires scanning and integration time of t.sub.i at every data collection point. The Fourier transform (FT) or Hadamard transform (HT) instruments need to move a mirror (FT) or a mask (HT) and then integrate for integration time t.sub.i such that the total data collection time is t.sub.i times the number of data resolution elements (RE) or mirror or mask positions. The static HT multiplexing spectrometer of the present invention can collect an entire spectrum of RE during t.sub.i. These spectra can then be averaged to realize an additional improvement in SNR over scanning FT or scanning HT instruments that is equivalent to the SQRT of the number of RE. This faster collection is enabled by having a detector array, rather than a single detector.

An object of the present invention is to provide a spectral measurement system as aforesaid, which provides better SNR than conventional multiplexing spectrometers. This better SNR is accomplished by making multiplexing measurements with a multi-element detector array, rather than making non-multiplexed or single detector measurements.

An object of the present invention is to provide a spectral measurement system as aforesaid, which is a solid state device suitable for rugged field use with no moving parts. This is enabled by making multiplexing measurements with a multi-element detector array. Since each quantity to be measured is present at an element of the detector array at any given time, it is not necessary to have any moving parts.

An object of the present invention is to provide a spectral measurement system as aforesaid, which is more compact than conventional spectral measurement systems.

An object of the present invention is to provide a spectral measurement system as aforesaid which does not require scanning of mirrors (FT) or masks (HT), thereby enabling real-time spectral data collection. This is enabled by making multiplexing measurements with a multi-element detector array. Since each quantity to be measured is present at an element of the detector array at any given time, it is not necessary to have any moving parts.

An object of the present invention is to provide a spectral measurement system as aforesaid which improves the measurement of biological living samples. In the spectral measurement system of the present invention, the scan is not intermodulated by changes in the samples, and so the typical prior art noise in the spectral data caused by such intermodulation are not present. As a result, the spectral measurement system of the present invention based on a static spectrometer is capable of looking at rapidly changing sources, samples and environments while taking advantage of the high throughput afforded by multiplexed measurements.

An object of the present invention is to provide a spectral measurement system as aforesaid which improves Raman spectrometry such that source fluctuations are no longer an issue. This is due to the lack of intermodulations in the present invention. Since all detectors measure their respective data all of the time, source fluctuation do not distort the measurements.

An object of the present invention is to provide a spectral measurement system as aforesaid wherein designers and manufacturers can set the accuracy and precision of the spectral measurement system at the time of manufacture. In the present invention this is accomplished by determining the resolution and accuracy of the mask elements, and other elements of the system, and by computing calibration data for the system as described herein.

An object of the present invention is to provide a spectral measurement system as aforesaid utilizing less expensive optical elements. Indeed because of the other advantages described, the static spectrometer disclosed herein will not need, for example, the precision parts required for interferometric motion, or the other components standard in the art and used to compensate for the shortcomings of prior art spectrometers.

An object of the present invention is to provide a spectral measurement system as aforesaid in which the alignment of the optical system is not as critical at time of manufacture. Indeed, the calibration at time of manufacture allows the alignment issues to be incorporated into the mask design or detector data processing such that performance is maintained. Such calibration is accomplished by measuring a series of known spectral features with the device, at the time of manufacture, and incorporating inverse data into the software accompanying the disclosed spectral measurement system.

An object of the present invention is to provide a spectral measurement system wherein the smallest spectral data collection time equals the lowest detector integration time, thereby providing faster multiplexed measurements.

An object of the present invention is to provide the spectral measurement system as aforesaid which does not require scanning, thereby eliminating scanning errors. Indeed, one skilled in the art will readily see that the present invention teaches a spectrometer that does not need to scan, and can instead be used in a mode in which the device stares at the sample, material, scene or object of interest, and hence is a staring spectral measurement device.

An object of the present invention is to provide a spectral measurement system as aforesaid wherein all detectors see the same source fluctuations in time, thereby eliminating noise contributions from changes in the source over time.

An object of the present invention is to provide a spectral measurement system as aforesaid, wherein principal components can be encoded into the aperture to affect a direct measure of components of interest. This is accomplished simply by selecting the appropriate mask for the system, as disclosed herein. Of course the system can be designed so that the mask can be changed in the field, thus allowing a base system to be adapted to various field uses.

An object of the present invention is to provide a spectral measurement system as aforesaid which simultaneously encodes multiple spectral ranges. This is accomplished by using different regions on the mask for different spectral ranges, and, in some embodiments, using multiple gratings and/or detector types for different regions of the mask.

In some embodiments of the present invention, one can employ a spectral measurement system as disclosed which employs dual beam spectrometric measures thereby resulting in instantaneous transmission or absorption measures.

An object of some embodiments of the present invention is to provide a spectral measurement system as aforesaid which employs an all reflective design to enable an achromatic operation.

An object of some embodiments of the present invention is to provide a spectral measurement system as aforesaid which measures non-contiguous spectral data.

An object of some embodiments of the present invention is to provide a spectral measurement system as aforesaid which measures variable band pass spectral resolution elements. This is accomplished as further disclosed herein, by selecting appropriate mask designs.

In accordance with an embodiment of the present invention, the spectral measurement system has no moving parts of any kind. In certain other embodiments, the system has no parts that move during measurement, but can move during reconfiguration.

In accordance with an embodiment of the present invention, the spectral measurement system comprises a two-dimensional (2D) detector array for collecting multiple samples of the encoded spectral data which can be used to improve SNR or oversample the spectral data.

In accordance with an embodiment of the present invention, the spectral measurement system comprises a fixed series of fixed masks corresponding to the full encodement library and a detector for each mask (e.g. in a linear array). The masks and detectors are fixed, and the system simultaneously measures each of the encoded combinations of input light. Since a detector is associated with each mask and the full encodement library is represented simultaneously in the system, the present invention eliminates the need to move or change the mask(s) in any way. That is, the spectral measurement system of present invention has the advantages of both the linear array spectrometers and Hadamard spectrometers without their disadvantages.

In accordance with an embodiment of the present invention, the spectral measurement system comprises predetermined and known fixed series of masks, each mask corresponding to one spectral weighting combination, and a detector for each mask. That is, in accordance with an embodiment of the present invention, the fixed series of masks correspond to a subset of the full encodement library chosen to measure any fixed set of spectrally weighted combinations. In accordance with an aspect of the present invention, the encodement can relate to a certain spectral signature attribute of the sample under study, the type or class of the sample, or a particular spectral signature attribute. Additionally, a combination of spectral principal components can also serve as an encodement sequence for the fixed mask.

In accordance with an embodiment of the present invention, the spectral measurement system comprises encoded masks and detectors that are fixed, and the spectral measurement system simultaneously measures each of the desired spectrally weighted combinations of input light, thereby providing advantages over conventional linear array spectrometers and conventional Hadamard spectrometers. Preferably, since the encodement sequence allows combinations of contiguous or non-contiguous wavelengths and variable band pass over the spectral range of operation, the spectral measurement system of the present invention measures only those elements of the spectral signature that have value to the desired analysis.

In accordance with an embodiment of the present invention, the spectral measurement system can act as a spectrometer and measure energy as a function of wavelength or frequency using linear detector arrays.

In accordance with an embodiment of the present invention, the spectral measurement system can act as a direct measurement device and measure the interaction of radiation with matter to compute the desired linear functions of the wavelength energy profile of the input radiation. Preferably, the spectral measurement system utilizes a small number of detectors, arranged in an array or separated from each other.

In accordance with an embodiment of the present invention, the spectral measurement system employing the real-time simultaneously measured multiplexed encodement method disclosed herein, is capable of looking at rapidly changing sources, samples and environments while taking advantage of the high throughput afforded by multiplexed measurements. As events in time become shorter, there is an ever increasing need for throughput of photons because less are available during a given integration period of the detector. In such case, the spectral measurement system of the present invention suffers no loss of speed or increase in integration time for spectral data collection as spectral resolution or number of data resolution elements increase.

In accordance with an embodiment of the present invention, a method for spectral measurement comprises the steps of receiving an electromagnetic energy through a mask located at the entrance aperture in the form of a two-dimensional encodement pattern, dispersing said electromagnetic energy in one or more dimensions, conditioning the electromagnetic energy received from said mask for presentation to a spectral dispersion element and conditioning the dispersed electromagnetic energy onto an array of detector elements.

In accordance with an embodiment of the present invention, a spectral measurement device comprises an entrance aperture for receiving electromagnetic energy, a mask located at the entrance aperture in the form of a two-dimensional encodement pattern, a spectral dispersion element for dispersing the electromagnetic energy in one or more dimensions, an optical element for conditioning the electromagnetic energy received from the mask for presentation to said spectral dispersion element and conditioning the dispersed electromagnetic energy onto an array of detector elements.

In accordance with an embodiment of the present invention, a spectral measurement device comprises an entrance aperture for receiving electromagnetic energy, a mask located at the entrance aperture in the form of a two-dimensional encodement pattern, a concave grating for conditioning and dispersing the electromagnetic energy in one or more dimensions; and an array of detector elements for receiving the dispersed electromagnetic energy from the concave grating.

It is intended that the devices and methods in this application in general are capable of operating in various ranges of electromagnetic radiation, including the ultraviolet, visible, infrared, and microwave spectrum portions. Further, it will be appreciated by those of skill in the art of signal processing, be it acoustic, electric, magnetic, etc., that the devices and techniques disclosed herein for optical signal processing can be applied in a straightforward way to those other signals as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

FIGS. 1A and 1B are schematic diagrams illustrating a spectrometer constructed in accordance with two embodiments of the invention;

FIG. 2 is a plan view of a micro-mirror array used in the present invention;

FIG. 3 is a schematic diagram of two micro-mirrors illustrating the modulations of the mirrors of the micro-mirror device of FIG. 2;

FIG. 4 is a graph illustrating an output signal of the spectrometer when used to analyze the composition of a sample;

FIG. 5 is a graph illustrating an output signal of the imager when used for imaging purposes;

FIG. 6 is a schematic diagram illustrating an imager constructed in accordance with a preferred embodiment of the invention; FIG. 6A illustrates spatio-spectral distribution of a DMA, where individual elements can be modulated;

FIG. 7 is an illustration of the input to the DMA Filter Spectrometer and its use to pass or reject wavelengths of radiation specific to constituents in a sample;

FIG. 8 illustrates the design of a band pass filter in accordance with the present invention (top portion) and the profile of the radiation passing through the filter (bottom portion);

FIG. 9 illustrates the design of multi-modal band-pass or band-reject filters with corresponding intensity plots, in accordance with the present invention;

FIG. 10 illustrates the means for the intensity variation of a spectral filter built in accordance with this invention;

FIGS. 11 14 illustrate alternative embodiments of a modulating spectrometer in accordance with this invention; FIGS. 11A and 11B show embodiments in which the DMA is replaced with concave mirrors; FIG. 12 illustrates an embodiment of a complete modulating spectrometer in which the DMA element is replaced by the concave mirrors of FIG. 11. FIG. 13 illustrates a modulating lens spectrometer using lenses instead of DMA, and a "barber pole" arrangement of mirrors to implement variable modulation. FIG. 14 illustrates a "barber pole" modulator arrangement;

FIGS. 15 and 16 illustrate an embodiment of this invention in which one or more light sources provide several modulated spectral bands using a fiber optic bundle;

FIG. 17 illustrates in diagram form an apparatus using controllable radiation source;

FIGS. 18A and 18B illustrate in a diagram form an optical synapse processing unit (OSPU) used as a processing element in accordance with the present invention;

FIG. 19 illustrates in a diagram form the design of a spectrograph using OSPU;

FIG. 20 illustrates in a diagram form an embodiment of a tunable light source;

FIG. 21 illustrates in a diagram form an embodiment of the spectral imaging device, which is built using two OSPUs;

FIGS. 22 and 23 illustrate different devices built using OSPUs;

FIGS. 24 26 are flow charts of various scans used in accordance with the present invention. Specifically, FIG. 24 is a flow chart of a raster-scan used in one embodiment of the present invention; FIG. 25 is a flowchart of a Walsh-Hadamard scan used in accordance with another embodiment of the invention. FIG. 26 is a flowchart of a multi-scale scan, used in a different embodiment; FIG. 26A illustrates a multi-scale tracking algorithm in a preferred embodiment of the present invention;

FIG. 27 is a block diagram of a spectrometer with two detectors;

FIG. 28 illustrates a Walsh packet library of patterns for N=8.

FIG. 29 is a generalized block diagram of hyperspectral processing in accordance with the invention;

FIG. 30 illustrates the difference in two spectral components (red and green) of a data cube produced by imaging the same object in different spectral bands;

FIGS. 31A E illustrate different embodiments of an imaging spectrograph used in accordance with this invention in de-dispersive mode;

FIG. 32 shows an axial and a cross-sectional views of a fiber optic assembly;

FIG. 33 shows a physical arrangement of the fiber optic cable, detector and the slit; FIG. 34 illustrates a fiber optic surface contact probe head abutting tissue to be examined;

FIGS. 35A and 35B illustrate a fiber optic c-Probe for pierced ears that can be used for medical monitoring applications in accordance with the present invention;

FIGS. 36A, 36B and 36C illustrate different configurations of a hyperspectral adaptive wavelength advanced illuminating imaging spectrograph (HAWAIIS) in accordance with this invention;

FIG. 37 illustrates a DMA search by splitting the scene;

FIG. 38 illustrates wheat spectra data (training) and wavelet spectrum in an example of determining protein content in wheat;

FIG. 39 illustrates the top 10 wavelet packets in local regression basis selected using 50 training samples in the example of FIG. 38; FIG. 39A shows a typical wheat spectrum together with one of the top 4 Walsh packets; FIG. 40 is a scatter plot of protein content (test data) vs. correlation with top wavelet packet; FIG. 41 illustrates PLS regression of protein content of test data; FIG. 41A shows a plot of regression error versus the percentage noise energy;

FIG. 42 illustrates the advantage of DNA-based Hadamard Spectroscopy used in accordance with the present invention over the regular raster scan;

FIGS. 43 47(A D) illustrate hyperspectral processing in accordance with the present invention;

FIG. 48 illustrates how, in accordance an embodiment of the present invention, a Hadamard encoded aperture of length N=3 in a de-dispersive imaging spectrograph results in a combination of spectral resolution impinging upon the detector array;

FIG. 49 illustrates a top view of schematic of a Hadamard encoded aperture of length N=3 in a de-dispersive imaging spectrograph in accordance with an embodiment of the present invention;

FIG. 50 illustrates a typical spectrum together with an individual wavelet packet function;

FIG. 51 is a schematic diagram of an exemplary spectral measurement system in accordance with an embodiment of the present invention;

FIG. 52 is a schematic of a conventional imaging spectrograph system function where the entrance aperture is masked by a slit opening;

FIG. 53 is a schematic of a conventional scanning slit imaging spectrograph using a linear array detector;

FIG. 54 is a schematic of a conventional scanning Hadamard transform imaging spectrograph operating in a de-dispersive modality;

FIG. 55 is a schematic of a spectral measurement imaging spectrograph in accordance with an embodiment of the present invention;

FIG. 56 is a schematic of a spectral measurement imaging spectrograph in accordance with an embodiment of the present invention;

FIG. 57 illustrates the path of light in an optimized optical model of a spectral measurement system in accordance with an embodiment of the present invention;

FIG. 58 illustrates an exemplary mask utilized in accordance with an embodiment of the present invention for a non-contiguous non-equally spaced band pass or principal component type analysis;

FIG. 59 illustrates an exemplary optical mask in accordance with an embodiment of the present invention;

FIG. 60 illustrates the simulated output of a two dimensional detector array using 2D array of detector elements in accordance with an embodiment of the present invention and the optical model of FIG. 57;

FIG. 61 illustrates exemplary results of the detector output and transform spectra of a spectral measurement system in accordance with an embodiment of the present invention;

FIG. 62 illustrates exemplary results of the detector output and transform spectra of a spectral measurement system in accordance with an embodiment of the present invention;

FIG. 63 illustrates exemplary results of the detector output and transform spectra of a spectral measurement system in accordance with an embodiment of the present invention;

FIG. 64 illustrates exemplary results of the detector output from a spectral measurement system in accordance with an embodiment of the present invention in the visible-NIR spectral range;

FIG. 65 illustrates exemplary spectrum results of the transformed data of a spectral measurement system in accordance with embodiment of the present invention operating in the visible-near-infrared spectral region; and

FIG. 66 illustrates an exemplary spectrum results of the transformed data from a spectral measurement system in accordance with an embodiment of the present invention operating in the near-infrared spectral region.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning now to the drawing figures and particularly FIGS. 1A and 1B, a spectrometer assembly 10 constructed in accordance with one embodiment of the invention is illustrated. With reference to FIG. 1A the device broadly includes a source 12 of electromagnetic radiation, a mirror and slit assembly 14, a wavelength dispersing device 16, a spatial light modulator 18, a detector 20, and an analyzing device 22.

In particular, the electromagnetic radiation source 12 is operable to project rays of radiation onto or through a sample 24 that is to be analyzed, such as a sample of body tissue or blood. The radiation source can be any device that generates electromagnetic radiation in a known wavelength spectrum such as a globar, hot wire, or light bulb that produces radiation in the infrared spectrum. To increase the amount of rays that are directed to the sample, a parabolic reflector 26 can be interposed between the source 12 and the sample 24. In a specific embodiment, the source of electromagnetic radiation is selected as to yield a continuous band of spectral energies, and is referred to as the source radiation. It should be apparent that the energies of the radiation source are selected to cover the spectral region of interest for the particular application.

The mirror and slit assembly 14 is positioned to receive the radiation rays from the source 12 after they have passed through the sample 24 and is operable to focus the radiation onto and through an entrance slit 30. The collection mirror 28 focuses the radiation rays through slit 30 and illuminates the wavelength dispersing device 16. As shown in diagram form in FIG. 1B, in different embodiments of the invention radiation rays from the slit can also be collected through a lens 15, before illuminating a wavelength dispersion device 16.

The wavelength dispersing device 16 receives the beams of radiation from the mirror and slit assembly 14 and disperses the radiation into a series of lines of radiation each corresponding to a particular wavelength of the radiation spectrum. The preferred wavelength dispersing device is a concave diffraction grating; however, other wavelength dispersing devices, such as a prism, can be utilized. In a specific embodiment, the wavelengths from the dispersing device 16 are in the near infrared portion of the spectrum and can cover, for example, the range of 1650 1850 nanometers (nm). It should be emphasized, however, that in general this device is not limited to just this or to any spectral region. It is intended that the dispersion device in general is capable of operating in other ranges of electromagnetic radiation, including the ultraviolet, visible, infrared, and microwave spectrum portions, as well as acoustic, electric, magnetic, and other signals, where applicable.

The spatial light modulator (SLM) 18 receives radiation from the wavelength dispersing device 16, individually modulates each spectral line, and reflects the modulated lines of radiation onto the detector 20. As illustrated in FIG. 2, the SLM is implemented in a first preferred embodiment as a micro-mirror array that includes a semi-conductor chip or piezo-electric device 32 having an array of small reflecting surfaces 34 thereon that act as mirrors. One such micro-mirror array is manufactured by Texas Instruments and is described in more detail in U.S. Pat. No. 5,061,049, hereby incorporated into the present application by reference. Those skilled in the art will appreciate that other spatial light modulators, such as a magneto-optic modulator or a liquid crystal device can be used instead of the micro-mirror array. Various embodiments of such devices are discussed in more detail below.

The semi-conductor 32 of the micro-mirror array 18 is operable to individually tilt each mirror along its diagonal between a first position depicted by the letter A and a second position depicted by the letter B in FIG. 3. In preferred forms, the semi-conductor tilts each mirror 10 degrees in each direction from the horizontal. The tilting of the mirrors 34 is preferably controlled by the analyzing device 22, which can communicate with the micro-mirror array 18 through an interface 37.

The micro-mirror array 18 is positioned so that the wavelength dispersing device 16 reflects each of the lines of radiation upon a separate column or row of the array. Each column or row of mirrors is then tilted or wobbled at a specific and separate modulation frequency. For example, the first row of mirrors can be wobbled at a modulation frequency of 100 Hz, the second row at 200 Hz, the third row at 300 Hz, etc.

In a specific embodiment, the mirrors are calibrated and positioned so that they reflect all of the modulated lines of radiation onto a detector 20. Thus, even though each column or row of mirrors modulates its corresponding line of radiation at a different modulation frequency, all of the lines of radiation are focused onto a single detector.

The detector 20, which can be any conventional radiation transducer or similar device, is oriented to receive the combined modulated lines of radiation from the micro-mirror array 18. The detector is operable for converting the radiation signals into a digital output signal that is representative of the combined radiation lines that are reflected from the micro-mirror array. A reflector 36 can be interposed between the micro-mirror array 18 and the detector 20 to receive the combined modulated lines of radiation from the array and to focus the reflected lines onto the detector.

The analyzing device 22 is operably coupled with the detector 20 and is operable to receive and analyze the digital output signal from the detector. The analyzing device uses digital processing techniques to demodulate the signal into separate signals each representative of a separate line of radiation reflected from the micro-mirror array. For example, the analyzing device can use discrete Fourier transform processing to demodulate the signal to determine, in real time, the intensity of each line of radiation reflected onto the detector. Thus, even though all of the lines of radiation from the micro-mirror array are focused onto a single detector, the analyzing device can separately analyze the characteristics of each line of radiation for use in analyzing the composition of the sample.

In accordance with one embodiment of this invention, the analyzing device is preferably a computer that includes spectral analysis software. FIG. 4 illustrates an output signal generated by the analyzing device in accordance with one embodiment. The output signal illustrated in FIG. 4 is a plot of the absorption characteristics of five wavelengths of radiation from a radiation source that has passed through a sample.

In one embodiment of the system of this invention illustrated in FIG. 6, it is used for digital imaging purposes. In particular, when used as an imaging device, an image of a sample 38 is focused onto a micro-mirror array 40 and each micro-mirror in the array is modulated at a different modulation rate. The micro-mirror array geometry is such that some or all of the reflected radiation impinges upon a single detector element 42 and is subsequently demodulated to reconstruct the original image improving the signal-to-noise ratio of the imager. Specifically, an analyzing device 44 digitally processes the combined signal to analyze the magnitude of each individual pixel. FIG. 6A illustrates spatio-spectral distribution of the DMA, where individual elements can be modulated. FIG. 5 is a plot of a three dimensional image showing the magnitude of each individual pixel.

FIG. 7 illustrates the output of a digital micro-mirror array (DMA) filter spectrometer used as a variable band pass filter spectrometer, variable band reject filter spectrometer, variable multiple band pass filter spectrometer or variable multiple band reject filter spectrometer. In this embodiment, the combined measurement of the electromagnetic energy absorbed by sample constituents A and C is of interest. The shaded regions in FIG. 7 illustrate the different regions of the electromagnetic spectrum that will be allowed to pass to the detector by the DMA filter spectrometer. The wavelengths of electromagnetic radiation selected to pass to the detector correspond to the absorption band for compound A and absorption band for compound C in a sample consisting of compounds A, B, and C. The spectral region corresponding to the absorption band of compound B and all other wavelengths of electromagnetic radiation are rejected. Those skilled in the art will appreciate that the DMA filter spectrometer is not limited to the above example and can be used to pass or reject any combination of spectral resolution elements available to the DMA. Various examples and modifications are considered in detail below.

As a DMA filter imager the spatial resolution elemen