This invention pertains to spectrometers, and more particularly to imaging spectrometers that operate according to hybrid scanning methods.
Imaging spectrometers have been applied to a variety of disciplines, such as the detection of defects in industrial processes, satellite imaging, and laboratory research. These instruments detect radiation from a sample and process the resulting signal to obtain and present an image of the sample that includes spectral and chemical information about the sample. A few imaging spectrometers have been proposed that employ a variable-bandwidth filter. Such spectrometers generally include dispersive elements to limit the spectral information received by the array, or slits, apertures, or shutters to limit the spatial information received by the array.
Several aspects of the invention are presented in this application. These are applicable to a number of different endeavors, such as laboratory investigations, microscopic imaging, infrared, near-infrared, visible absorption, Raman and fluorescence spectroscopy and imaging, satellite imaging, quality control, industrial process monitoring, combinatorial chemistry, genomics, biological imaging, pathology, drug discovery, and pharmaceutical formulation and testing.
Systems according to the invention are advantageous in that they can perform precise spectral imaging and computation with a robust and simple instrument. By acquiring a scanned series of mixed spectral images and then deriving pure spectral images from them, systems according to the invention can be made with few moving parts or more robust mechanisms than prior art systems. This is because they can be made using a simple variable optical filter in place of more costly interferometers, or active variable filters such as liquid crystal tunable filters (LCTF). The resulting systems can therefore be less expensive and more reliable.
Systems according to the invention can also acquire images with more efficiency because their detector arrays have a field of view that is not obstructed by slits or shutters and the average optical throughput of the filter is greater than other active tunable filter approaches. As a result, systems according to the invention need not suffer from the problems that tend to result from high levels of illumination, such as excessive heating of the sample, and the cost and fragility of high intensity illumination sources.
In the figures, like reference numbers represent like elements.
Referring to
Where multiple sub-areas are used, the image sensor is preferably oriented with one or both of its dimensions generally along an axis that is parallel to the spatial distribution of sample elements. Note that the instrument need not rely on a predetermined shape for the elements, but instead relies on the fact that the actuator motion and acquisition are synchronized by the instrument.
The filter 12 has a narrow pass-band with a center wavelength that varies along one direction. The leading edge A of the filter passes shorter wavelengths, and as the distance from the leading edge along the process flow direction increases, the filter passes successively longer wavelengths. At the trailing edge N of the filter, the filter passes a narrow range of the longest wavelengths. The orientation of the filter can also be reversed, so that the pass-band center wavelength decreases along the process flow direction. Although the filter has been illustrated as a series of strips located perpendicular to the process flow direction, it can be manufactured in practice by continuously varying the dielectric thickness in an interference filter. Preferably, the filter should have a range of pass-bands that matches the range of the camera. Suitable filters are available, for example, from Optical Coatings Laboratory, Inc. of Santa Rosa, Calif. The variable filter can be located between the sample and the detector or between the source and sample. In a microscopic application, for example, the actuator can move the variable filter between the source and the sample, before light interacts with the sample. Alternatively, with the same optical configuration, the sample could be moved to achieve the same effect.
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The system also includes an image acquisition interface 22 having an input port responsive to an output port of the image sensor 10. The image acquisition interface receives and/or formats image signals from the image sensor. It can include an off-the shelf frame grabber/buffer card with a 12-16 bit dynamic range, such as are available from Matrox Electronic Systems Ltd. of Montreal, Canada, and Dipix Technologies, of Ottawa, Canada.
A spectral processor 26 has an input responsive to the image acquisition interface 22. This spectral processor has a control output provided to a source control interface 20, which can power and control an illumination source 14, which can be placed to reflect light off the sample or transmit light through the sample. The illumination source for near-infrared measurements is preferably a Quartz-Tungsten-Halogen lamp. For Raman measurements, the source may be a coherent narrow band excitation source such as a laser. Other sources can of course also be used for measurements made in other wavelength ranges.
The spectral processor 26 is also operatively connected to a standard input/output (IO) interface 30 and may also be connected to a local spectral library 24. The local spectral library includes locally-stored spectral signatures for substances, such as known process components. These components can include commonly detected substances or substances expected to be detected, such as ingredients, process products, or results of process defects or contamination. The IO interface can also operatively connect the spectral processor to a remote spectral library 28.
The spectral processor 26 is operatively connected to an image processor 32 as well. The image processor can be an off-the-shelf programmable industrial image processor, that includes special-purpose image processing hardware and image evaluation routines that are operative to evaluate shapes and colors of manufactured objects in industrial environments. Such systems are available from, for example, Cognex, Inc.
An actuator 15 can be provided to move the filter 12 using a motive element, such as a motor, and a mechanism, such as a linkage, a lead screw, or a belt. The actuator is preferably positioned to move the filter linearly in the same direction along which its characteristics vary, or at least in such a way as to provide for at least a component of motion in this direction. In a related embodiment, the actuator moves the sample, such as by moving a sample platform. It may even be possible in some embodiments to move the camera or another element of the instrument, such as an intermediate mirror, if the arrangement allows for radiation from one sample point to pass through parts of the filter that have different characteristics before reaching the detector. In the present embodiment, the actuator includes a computer controlled motorized translation stage such as is available from National Aperture, of Salem, N.H.
The actuator can be a precise open-loop actuator, or can provide for feedback. Open loop actuators, such as precise stepper motors, allow the system to precisely advance the filter during acquisition. Feedback-based systems provide for a position or velocity sensor that indicates to the system the position of the filter. This signal can be used by the system to determine the position or velocity of the filter, and may allow the system to correct the filter scanning by providing additional signals to the actuator. The actuator can be designed to move the filter in a stepped or continuous manner.
In one embodiment, the system is based on the so-called IBM-PC architecture. The image acquisition interface 22, IO interface 30, and image processor 32 each occupy expansion slots on the system bus. The spectral processor is implemented using special-purpose spectral processing routines loaded on the host processor, and the local spectral library is stored in local mass storage, such as disk storage. Of course, other structures can be used to implement systems according to the invention, including various combinations of dedicated hardware and special-purpose software running on general-purpose hardware. In addition, the various elements and steps described can be reorganized, divided, and combined in different ways without departing from the scope and spirit of the invention. For example, many of the separate operations described above can be performed simultaneously according to well-known pipelining and parallel processing principles.
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One or more of the sample areas can include a reference sample. These sample areas can be located at fixed positions with respect to the other sample areas, or they can be located in such a way that they move with the scanning element of the instrument. This implementation can allow for the removal of transfer of calibration requirements between systems by simultaneously collecting reference spectra for spectral comparison. Referring to
In the second stage image data are extracted from the mixed spectral data set and processed. In the embodiment described, pure spectral images are extracted in the form of a series of line images acquired at different relative positions (steps 46 and 48). Part or all of the data from the extracted line image data sets can then be assembled to obtain two-dimensional spectral images for all or part of the sample area and pure spectra for each pixel in the image
The conversion can take place in a variety of different ways. In one approach, a whole data set can be acquired before processing begins. This set can then be processed to obtain spectral images at selected wavelengths. The instrument may also allow a user to interact with an exploratory mode, in which he or she can look at representations of any subset of the data. This can allow the user to zoom in to specific parts of the sample and look at wavelengths or wavelength combinations that may not have been contemplated before the scan.
Data can also be processed as scanning of the filter occurs. In this approach, data may be processed or discarded as it is acquired, or simply not retrieved from the detector to create an abbreviated data set. For example, the instrument may only acquire data for a certain subset of wavelengths or areas, it may begin spectral manipulations for data as they are acquired, or it may perform image processing functions, such as spatial low-pass filtering, on data as they are acquired. Adaptive scanning modes may also be possible in which the instrument changes its behavior based on detected signals. For example, the instrument can abort its scan and alert an operator if certain wavelength characteristics are not detected in a reference sample.
In one example, the data can be accumulated into a series of single-wavelength bit planes for the whole image, with data from these bit planes being combined to derive spectral images. Data can also be acquired, processed, and displayed in one fully interleaved process, instead of in the two-stage approach discussed above. And data from the unprocessed data set can even be accessed directly on demand, such as in response to a user command to examine a particular part of the sample area, without reformatting the data as a whole.
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The examples presented above assume that the filter is advanced by increments that each correspond to one row of pixels in the array. Other progressions are also possible, such as systems that move in sub-row (or multi-row) increments. And continuous systems may deviate significantly from their ideal paths, especially at the end of a scan. The specific nature of a particular instrument must therefore be taken into consideration in the designing of an acquisition protocol for a particular system.
Once the spectral images are assembled, the spectral processor 26 evaluates the acquired spectral image cube. This evaluation can include a variety of univariate and multivariate spectral manipulations. These can include comparing received spectral information with spectral signatures stored in the library, comparing received spectral information attributable to an unknown sample with information attributable to one or more reference samples, or evaluating simplified test functions, such as looking for the absence of a particular wavelength or combination of wavelengths. Multivariate spectral manipulations are discussed in more detail in “Multivariate Image Analysis,” by Paul Geladi and Hans, Grahn, available from John Wiley, ISBN No. 0-471-93001-6, which is herein incorporated by reference.
As a result of its evaluation, the spectral processor 26 may detect known components and/or unknown components, or perform other spectral operations. If an unknown component is detected, the system can record a spectral signature entry for the new component type in the local spectral library 24. The system can also attempt to identify the newly detected component in an extended or remote library 28, such as by accessing it through a telephone line or computer network. The system then flags the detection of the new component to the system operator, and reports any retrieved candidate identities.
Once component identification is complete, the system can map the different detected components into a color (such as grayscale) line image. This image can then be transferred to the image processor, which can evaluate shape and color of the sample or sample areas, issue rejection signals for rejected sample areas, and compile operation logs.
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While the system can operate in real time to detect other spectral features, its results can also be analyzed further off-line. For example, some or all of the spectral data sets, or running averages derived from these data sets can be stored and periodically compared with extensive off-line databases of spectral signatures to detect possible new contaminants. Relative spectral intensities arising from relative amounts of reagents or ingredients cat also be computed to determine if the process is optimally adjusted.
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In one embodiment, the spectrometer can be equipped with an additional magnifying optic that can be used to focus further in to specific points of interest within the instrument's field of view. This lens can even be such that it causes light from a single point on the sample to be incident across the entire filter and array, resulting in a single point “point-and-shoot” spectrometer in which the filter or sample do not need to be moved.
The present invention has now been described in connection with a number of specific embodiments thereof. However, numerous modifications which are contemplated as falling within the scope of the present invention should now be apparent to those skilled in the art. Therefore, it is intended that the scope of the present invention be limited only by the scope of the claims appended hereto. In addition, the order of presentation of the claims should not be construed to limit the scope of any particular term in the claims.
This application is a continuation of an application entitled “Hybrid-Imaging Spectrometer,” Ser. No. 09/828,281, filed on Apr. 6, 2001, which is a continuation-in-part of an application entitled “Hybrid-Scanning Spectrometer,” Ser. No. 09/817,785, filed on Mar. 26, 2001. Both of these applications are herein incorporated by reference.
Number | Date | Country | |
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Parent | 09828281 | Apr 2001 | US |
Child | 10996189 | US |
Number | Date | Country | |
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Parent | 09817785 | Mar 2001 | US |
Child | 09828281 | US |