Abstract:
A multi-mode detector and detection method that utilize shared optical components to detect multiple different incoming wavelengths of energy. Partial de-focusing of incoming light enables imaging and non-imaging outputs from the same detector. Analog and digital pixels enable mixed-mode output. Selective filtering at the detector enables the simultaneous detection of two or more wavelengths of light. Signal processing hardware and algorithms enable the system to simultaneously provide integrated and multi-channel analog outputs to guidance and display systems.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the detection of disparate-wavelength signals with a single detector. Specifically, the present invention relates to the detection of semi-active laser (SAL) and infra-red signals using a single focal plane array (FPA). The invention further relates to providing simultaneous mixed read-out methods on a single ROIC. 
     BACKGROUND 
     Multi-mode sensors have become an important component in precision-guided munitions. To ensure accuracy and proper target acquisition, it has become increasing more useful to have multiple ways of detecting and tracking a potential target. 
     One common example of a multi-mode device is a missile seeker that combines mid-wave infra-red imaging capability and semi-active laser detection and tracking. The infra-red allows for tracking based on emitted heat or expected heat signature of a target, while a semi-active laser (SAL) detector allows the munition to home in on a target being ‘painted’ with a laser spot. The combination of the two creates a more robust and fault-tolerant seeker system that is less likely to be confused by counter-measures and capable of proving greater accuracy and operating in a wider range of environments. 
     With all of these advantages, there come several downsides to a multi-mode detector device. Generally, such devices require two disparate sensor systems whose outputs are combined and analyzed to give a true multi-mode detection capability. This causes such systems to generally be costly and complicated, limiting their use in the field. A multi-mode detection system that could take advantage of common components to provide multi-mode detection data from a single set of optics and detection components would offer the combined benefits of simplicity and cost reduction. A simpler device with fewer components is less likely to break or malfunction, and a lower-cost device can be more broadly applied. This is especially true in the realm of guided munitions, where the entire device is meant for single use in a high-impact operating environment. The advantages of a cheaper, simpler multi-mode detector for such applications should be readily apparent. 
     SUMMARY 
     The present invention seeks to solve the aforementioned problems by providing a multi-mode detector that utilizes shared optical components to detect multiple different incoming wavelengths of energy. Selective filtering at the detector enables the present invention to simultaneously detect two or more wavebands of light. Signal processing hardware and algorithms enable the system to simultaneously provide integrated and multi-channel analog outputs to guidance and display systems. 
     One embodiment of the present invention is as an imaging device for multiple-waveband detection, where the device consists of a focal plane array (FPA) and a waveband filter array situated directly on the focal plane array. The waveband filter of this embodiment has a ‘checkerboard’ pattern with each ‘square’ of the pattern covering a pixel of the focal plane array. Each of these squares is designed to permit a one of a set of specific wavelength ranges, with each square covering one or more pixels. In one particular embodiment, the specific waveband ranges are mid-wave infra-red (MWIR) wavelengths and semi-active laser (SAL) wavelengths. Other embodiments may use different wavebands, different patterns, and may employ more than two wavebands. 
     The FPA in this embodiment may comprise an InSb staring focal plane detector divided into four quadrants. The four-quadrant approach may be useful in tracking devices or missile seekers where the direction of incoming light may be ascertained based on its distribution and intensity over the quadrants. The present embodiment may also employ a beam spoiler to de-focus incoming light at SAL wavelengths so that it will cover a wider area on the FPA, allowing for more proportional control over wide angles of incidence. 
     Other embodiments of the present invention may include multi-mode threat detectors such as devices intended to detect if someone is using one of several active imaging methods to sweep a target area. Other embodiments of the invention may employ PIN photodiodes or APDs (avalanche photodiodes) instead of, or in addition to, the FPA, or use a different form of CCD (charge-coupled device), or may use different filtering techniques, such as magneto-optical filters. 
     Regardless of the underlying hardware, the fundamental inventive concepts are enabling the simultaneous detection of two or more wavelengths with a shared set of optical components and simultaneously providing both analog and integrated output. In the case of an FPA detector with a ‘checkerboard’ waveband filter, this may be accomplished by filtering the incoming light before it is detected by the FPA, capturing the output signal from each quadrant on a separate output channel, and filling gaps in the output signal of each quadrant with an extended-range image processing technique embodied on a high-speed readout integrated circuit. 
     Other inventive embodiments may require different signal capture and processing techniques due to different filtering, detection, or data output hardware. The imaging resolution may also be affected by different detector hardware, potentially allowing for effective visual-image capture, or for the elimination of post-detection image processing intended to gap-fill or otherwise compensate for resolution loss or image under-sampling as a result of waveband filtering. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       FIGURES 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawing which is given by way of illustration only, and thus is not limitative of the present invention, and wherein: 
         FIG. 1   a  shows an embodiment of an FPA-type detector with a ‘checkerboard’ waveband filter and a de-focused image signal; 
         FIG. 1   b  shows a side-view of an embodiment of an FPA-type detector with a ‘checkerboard’ waveband filter; 
         FIG. 1   c  shows a prior art FPA-type detector for infrared detection; 
         FIG. 2   a  shows an embodiment of a four-quadrant FPA detector according to the present invention; 
         FIG. 2   b  shows an embodiment of an image signal detection portion of a multi-mode detector according to the present invention; 
         FIG. 2   c  shows an embodiment of an analog channel signal detection portion of a multi-mode detector according to the present invention; 
         FIG. 3  shows an embodiment of a beam spoiler to de-focus portions of an incoming light signal according to the present invention; and 
         FIG. 4   a  shows an embodiment of the imaging components of a multi-mode target acquisition and imaging device according to the present invention; and 
         FIG. 4   b  shows an embodiment of the analog signal detection and control components of a multi-mode target acquisition and imaging device according to the present invention. 
     
    
    
     The drawings will be described in detail in the course of the detailed description of the invention. 
     DETAILED DESCRIPTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. 
     In order to reduce the cost of detector components while preserving functionality, the present invention seeks to provide multiple-wavelength light detection and imaging capabilities using a single set of detector hardware. 
     In a missile-seeker embodiment of the inventive concept, redundant and duplicate components in a multi-mode missile seeker are eliminated through the use of a four-quadrant focal plane detector overlaid with a ‘checkerboard’ waveband filter, with each quadrant having a separate analog output channel. This focal plane array (FPA) detector design, shown in  FIG. 1  a, uses a patterned filter  130  that covers each pixel of the FPA with a waveband filter area. Half of the pixels in this embodiment are filtered to detect only wavelengths associated with mid-wave infra-red light (MWIR) and are used for imaging, and the other half are filtered to detect only the wavelengths associated with semi-active laser (SAL) targeting systems and are used to produce analog output. 
     Other embodiments of the inventive concept may be filtered to detect visible light and ultra-violet light, and may have different types of waveband filter masks such as ones using larger filter block sizes or different filter area patterns (e.g. triangles or a honeycomb), or may use an entirely different form of filtering altogether—such as magneto-optical filters. Other embodiments of the inventive concept may employ different forms of detectors, such as PIN photodiodes or APDs. Other embodiments of the inventive concept that use an FPA may dispense with the four quadrant system and either use one output channel for the entire FPA or divide the FPA some other way—such as into eighths or fifths. Yet other embodiments may use polarization filtering on groups of pixels, either instead of or in addition to spectral filtering. In such embodiments it may be advantageous to break the FPA up into many pixel groups, with each group of anywhere from 4 to 400 pixels having a separate output channel. Still further embodiments of the invention may dispense with spectral filtering altogether and instead rely on red spike and blue spike detection to differentiate between things like mid-wave and long-wave infra-red signals. 
     For the purposes of this specification and the attendant claims, “analog pixels” refer to those pixels that are used to produce analog output and “imaging pixels” refer to those pixels that are used for imaging. No structural or material difference between the two pixel types is required as far as the detector portion of the FPA is concerned, but certain embodiments may employ different pixel shapes or materials for the two pixel types. 
     In a missile-seeker embodiment, one of the concerns is cost control. One embodiment of a multi-mode missile seeker may give a munition the capability of augmenting a directed target acquisition approach through SAL by allowing for autonomous target acquisition through electro-optical (EO) imaging with mid-wave infra-red (MWIR). While making the missile more effective and enabling it to better locate and retain targets, the need for two detection systems—one for MWIR and one for SAL—increases the cost of the detector and therefore of the missile. By allowing the SAL and EO detection wavelengths to share a single detector, the component cost of the missile-seeker decreases, decreasing the overall cost of the missile while still allowing for multi-mode target acquisition. 
     As shown in  FIG. 1   a , a patterned waveband filter  130  may be applied over the pixels in the detector array  120 . This limits the wavelengths of light that are allowed to contact the detector array  120  and cause it to pass an electrical signal to the read-out integrated circuit (ROIC)  110  mounted on the substrate  101 . Embodiments of waveband filters may be patterned to cover individual pixels with different filter areas, or may spread a filter area across groups of pixels. Also, the pixels in the detector are not all required to be of identical shape and size, so an associated patterned waveband filter may further reflect any variation in pixel size of its underlying detector. 
     A side-view of a particular embodiment of an FPA detector with a patterned waveband filter, shown in  FIG. 1   b , shows an embodiment of a checkerboard-type filtering pattern. As in a conventional FPA, the silicon ROC  140  is connected to the detector array  160  with a combination of epoxy and indium interconnects  150 . The patterned waveband filter rests on top of the detector array  160 . The filter is composed of a transparent substrate  180  (by transparent, it is meant that the substrate is transparent to those wavelengths admitted by the filter—visible-spectrum transparency is not necessarily essential or desirable) which, in this embodiment, is sandwiched by top and bottom anti-reflective coatings  170   190 . The waveband filter portion,  195  is patterned on top of the transparent substrate  180  and, in some embodiments, the top anti-reflective coating  190  may have additional or inherent waveband filtering capabilities as well. The areas with the waveband filter  195  only admit one particular waveband of photons, whereas the areas with only the anti-reflective coating  190  may admit a broader waveband. The waveband filters may each have a different waveband of light that they will admit, and the areas depicted as only covered with an anti-reflected coating  190  may, in other embodiments, also be covered with different types of waveband filters. 
     Before proceeding further, it may be helpful to review how an FPA detector operates so that the distinguishing features of the present invention can be better understood. Referring to  FIG. 1   c , an FPA-type detector typically comprises an extrinsic or intrinsic detector array  010 . This detector array may be made of a wide range of materials, including GaAs (gallium-arsenide) and InSb (indium-antimonide). The detector array may be a continuous layer of material, or it may be composed of pixels of the detector material, with each pixel on the detector array corresponding to one pixel of image resolution. A 600×800 detector array, therefore, has 48,000 pixels and can produce an image of up to 600×800 screen resolution. Each pixel in the array is a ‘collection hole’ that absorbs incident photons and produces an electric charge relative to the amount and intensity of the photons absorbed. The detector material may be covered with an anti-reflective coating to improve its detection characteristics. The particular embodiment of an FPA detector depicted here is meant to detect incoming infrared radiation  001 . The detector array  010  is connected to an ROIC (read-out integrated circuit) array  030 , which is printed on a substrate—typically made of silicon  020 . The detector array  010  is connected to the ROIC  030  through interconnects  040  that match each pixel of the detector array  010  to a point on the ROIC array  030 . In this embodiments, the interconnects are made of indium  040  because it is suitable for cold-welding and cold-temperature operation. The input to the ROIC array elements  030  is then multiplexed before being output  050  for further signal processing and image display. This integrated, multiplexed output  050  combines the individual inputs to the ROIC array  030  into a single, cohesive image signal suitable for subsequent image processing and analysis. 
     An FPA detector of this type produces integrated, multiplexed image output  050  by combining all of the pixel-level signals in a given column, and then combining the columns. An example of how this operation is carried out is shown in  FIG. 2   b.    
     In the detector  401 , an incident photon creates a charge in a pixel  405 . This charge is transferred to the ROIC  420  and integrated in the integration capacitor  427  over a set period of time. The Reset switch  429  is then opened to stop integration and the integrated current moves to a holding capacitor  423 . The Hold switch  421  is then opened to stop the flow of current between the capacitors, and the integration capacitor charge  427  is cleared and the Reset switch  429  closed to begin integrating charge from the pixel again. The hold capacitors  423  of all pixels in a given row feed into a column bus  410  when the Multiplexing switch is closed  425 , and the column buses are then further multiplexed together (not shown) when the Multiplexing switch  425  to stop charge transfer from the hold capacitors. This set of operations repeats every set time period, which determines what the frame rate of an FPA is. The higher the frame rate, the more quickly the FPA is able to move charge from a pixel to the ROIC, integrate the pixel outputs, and produce an image signal. 
     The present invention has discrete areas of non-imaging, analog pixels on the FPA that are connected to additional components in the ROIC for the purpose of generating non-sampling, analog ouput. In a missile-seeker embodiment of the present invention, this would be the SAL data, and it may be used for target tracking, range-finding, and trajectory adjustments. An embodiment of a four-quadrant SAL-detecting FPA detector according to the present invention is depicted in  FIG. 2   a , and non-sampling analog output generation from an FPA is depicted in  FIG. 2   c    
     As shown in  FIG. 2   a , the FPA detector array  480  is divided into four quadrants  4801 ,  4805 ,  4811 ,  4821  that each have a separate analog output channel. In this SAL/MWIR missile seeker embodiment, each channel of non-sampling, analog detector output  401  is converted into a digital signal  450  and then processed in an FPGA logic circuit  460  to determine first pulse or last pulse logic. 
     Alternative embodiments of the present invention may have more or fewer channels. One embodiment may have a separate channel for each analog pixel such that each analog output channel is eventually combined into a single overall analog output signal that can be used for subsequent tracking, control, or ranging operations. 
     The present embodiment of thethe conversion process for an output channel begins with a two step filtering and amplification process by a pair of A/C coupling detectors  4505 ,  4515 . The filtering and amplification process strips out background and ambient noise in the signal, which may be caused by sunlight and ambient light. Other embodiments of the present invention may employ a single-step filtering and amplification process, additional filtering and amplification processes, or may dispense with this step entirely. Yet further embodiments of the present invention may accomplish signal filtering and amplification using different types of components. Still other embodiments of the preset invention may dispense with signal filtering and conversion process altogether, working instead with the pure analog output of the FPA channels. The amplification, filtering, A/D conversion, and other signal processing may be performed in the ROIC unit cell, may be performed by external circuitry, or may be a combination of ROIC and external circuits. 
     In the present embodiment, after filtering and amplification, the signal passes into an Analog to Digital converter  4520  to convert the analog channel output into a digital signal suitable for further processing by the FPGA  460 . The FPGA encompasses the logic to acquire the and track the laser pulse in the presence of noise and clutter, sense PIM codes, reject countermeasures, perform M out of N pulse correlation to improve detection in poor SNR conditions, compute target centroid based on signals from each quadrant, and compute guidance and control commands to maneuver the airframe. 
     In the embodiment described, the separate channel signals are combined after FPGA processing (not shown) and then translated into actuator commands that direct the canards of a missile based on the relative centroid location of a de-focused laser sport detected by the multi-mode FPA. Alternative embodiments of the present invention may multiplex the analog output channels and have one set of amplification, filtering, and AID conversion hardware  450  for the multiplexed signal. In such embodiments, the FPGA  460  would have to demultiplex the separate channel signals in order to properly process and threshold them. Alternative embodiments of the FPGA may also entail a range-finder aspect where the laser pulses are analyzed to determine distance from the target. 
     Alternate embodiments of the present invention may dispense with FPGA processing. This may be accomplished in embodiments that use pure analog signal processing, or that employ wavelengths other than SAL for analog signal detection aspects. Alternative wavelengths include radio-frequency and millimeter-wave radiation bands, and analog signal processing may include techniques such as marking each channel output with an identifying characteristic so that simply comparing signal intensities across channels and over time provides centroid and range information. 
     In an embodiment of a detector according to the present invention where the desired wavebands are in the SAL (semi-active laser) and MWIR (mid-wave infrared) ranges, the two incoming light signals serve different purposes and are processed differently. In a missile-seeker embodiment of an SAL/MWIR multi-mode detector according to the present invention, it may be advantageous to de-focus part of the incoming light signal before it is detected by the FPA. This de-focusing may be accomplished by a beam spoiler device of the type depicted in  FIG. 3 . In the embodiment shown, incoming light in both MWIR  205  and SAL  211  wavelengths enters a sensor through a window  201  and passes into a cassegrain-type reflector arrangement. The main reflector  225  directs the MWIR  205  and SAL  211  light to a secondary cassegrain reflector  221  made of a dichroic material that reflects the MWIR  205  through refractive optics  235  that direct the light signal onto the detector array of the FPA  231 . The SAL  211  light passes through the dichroic  221  and reflects off an SAL spoiler  215  that de-focuses the SAL  211  light, so that the SAL signal will appear on the FPA detector array as a large, de-focused spot  140 . 
     De-focusing an SAL beam serves two purposes in a multi-mode missile seeker embodiment of the present invention. First, a tightly-focused SAL beam may be blocked if it is concentrated on an imaging pixel, causing no detection at all. De-focusing the beam ensures that it will be detected by multiple analog pixels. Second, in a multi-quadrant FPA embodiment, a highly focused SAL beam may only fall in one quadrant, making it difficult to determine the direction of the incoming beam with any precision. De-focusing the beam so that it crosses quadrants  490  allows for a more precise determination of beam direction because rationing the signal amplitude in each quadrant to the others provides a signal proportional to the signal centroid location on the FPA. Embodiments of a multi-mode missile-seeker according to the present invention may also slightly de-focus the MWIR signal before sampling in order to eliminate issues such as aliasing during image detection. For a more comprehensive treatment of processing under-sampled images, please see published U.S. patent application Ser. No. 11/429,958, the contents of which are hereby incorporated by reference. 
     Alternative embodiments of the inventive concept may use a different kind of beam spoiler, such as holographic surfaces, diffractive optics, curved mirrors, or reflect-arrays or may instead seek to more tightly focus portions of the beam through devices such as microlens arrays. Such an approach may be useful in variants of the MWIR/SAL multi-mode missile seeker embodiment of the present invention where a highly focused MWIR image may be desirable. Yet other embodiments may not employ any form of beam focus or de-focus, merely using the minimum set of optics required for light capture and transmission to the FPA. Yet further embodiments of the invention may employ a refractive holographic element instead of a beam spoiler device, such as a hologram of a prism. This may accomplish beam spreading and signal uniformity without requiring a larger and more mechanically complex beam spoiler device. 
     In a missile-seeker embodiment having a beam spoiler, the de-focused light beam may then be detected by a staring FPA with a patterned waveband filter of a type described above. Alternative embodiments may employ further signal processing techniques, such as wavelength shifting, magneto-optical filtering, or light intensification before allowing incoming light to register on a detector. Such techniques may be useful in embodiments meant to operate in low-light environments for things such as simultaneous visible and ultra-violet detection, embodiments seeking to detect light wavelengths associated with the combustion of specific elements or compounds (such as magnesium or thermite) as well as provide general MWIR detection. 
     Alternative embodiments of the present invention may also employ various shapes and sizes for the analog channel areas. An embodiment meant to provide range-finding and imaging capability on the same FPA may have a large area in the center of the FPA set as a ‘super pixel’ with a single analog channel output so that it can detect incoming laser signals and determine how far it is from the target. This may be accomplished, for instance, by having four separate imaging quadrants on an FPA with a ‘plus-sign’ shaped area between them acting as a large laser detection analog pixel for range-finding. An embodiment intended for simultaneous, multi-mode detection of MWIR and SAL light may have alternating waveband filter squares designed to pass only wavelengths of between 3 and 5 μm and 1.06 and 1.57 μm, respectively. Such an embodiment of a waveband filter accomplishes waveband filtering by reflecting or absorbing radiation outside of the desired wavebands. This can be achieved through a selection of filter materials that only pass the bands of interest, or through prisms, diffraction gratings, or anti-reflective coatings. 
     The MWIR portion of incoming light is also detected by this FPA embodiment, and the gaps created through the omission of every other pixel (due to a combination of the waveband filter mask and ROIC circuitry) are filled in by an extended-range image processing technique of a type disclosed in U.S. patent application Ser. No. 11/429,958—the contents of which are hereby incorporated by reference. The MWIR signal is detected through a standard FPA integration/multiplexing technique depicted in  FIG. 2   b.    
     Alternative embodiments of the inventive concept may employ different methods for separately processing the multiple types of input detected by a multi-mode detector. Possible alternatives include charge injection devices, digital ROICs, any other variations of detection devices capable of providing multi-channel analog output in addition to integrated image signal output. 
       FIG. 4   a  shows the imaging portion of an embodiment of a multi-mode missile seeker device that provides both target imaging and target tracking capabilities using a sampled, imaging output, and a non-sampled, analog channel output. The FPA  315 , which contains both the detector with a patterned waveband filter and the ROIC with the added analog channel outputs, is operatively connected to a controller  330  that governs the operating mode of the FPA  310 , feeds in a power signal  305 , and feeds in a timing signal  301 . The controller also monitors FPA temperature and controls cooling of the FPA unit  320 . The signal output of the FPA  325  is fed through the controller  330  to a calibration and processing unit  340 . The signal coming from the controller  335  contains the multiplexed imaging output of the FPA. The calibration and processing unit  340  may direct the imaging signal  355  to an image processing unit  350 , and subsequently to a display unit  365 . The signal  355  may also be directed to a tracking and target recognition system  345  which analyzes the imaging portion of the signal for known or readily ascertained targets and performs further processing and subsequent target tracking, or missile course correction based on the detected location of the target in the imaging plane. 
       FIG. 4   b  shows the analog signal detection portion of an embodiment of a multi-mode missile seeker device that provides both target imaging and target tracking capabilities using a sampled imaging output and a non-sampled analog channel output. This embodiment of the FPA  370  contains four analog output channels that correspond to four quadrants of analog detection on the FPA. Alternative embodiments may contain more or fewer analog channels, up to a separate channel for each analog pixel on the FPA. Each analog channel outputs a signal to a set of filters and amplifiers  375 - 1 ,  375 - 2 ,  375 - 3 ,  375 - 4 . Each of these filter/amplifier sets may contain some or all of the components in the digital signal conversion aspect  450  of  FIG. 2   c . As shown there, each filter/amplifier portion may have one or more AC couplings to amplify the detected signal and filter out background noise, and may also have an AID converter to generate digital output. 
     Alternate embodiments of the present invention may dispense with the A/D converter aspect entirely and employ a purely analog signal output. Yet further alternative embodiments may embed the filter/amplifier aspect into the ROIC of the FPA device. In one possible embodiment, each analog pixel may be operatively connected to a separate AC coupling or set of AC couplings for pixel-level analog signal filtering. 
     After filtering and amplification, the analog signal or signals may be sent for further signal processing  380 . This processing may include combining disparate analog channel outputs into a single signal, de-multiplexing a cross-channel multiplexed analog output signal, or processing either combined or separate signals without combining or dividing them. In the present invention, this additional signal processing  380  may include AID conversion and FPGA analysis, or may simply include basic cross-channel signal comparisons to determine either a range or centroid location of the analog signal source. This processed signal is then passed on to any number of subsequent devices  390  including controllers to adjust things like steering/guidance fins to make missile heading corrections, timing and detonation control devices to determine range-based warhead detonation parameters, or, in some embodiments, into a target tracking system that may dynamically adjust the integration time of the imaging aspect based on properties of the analog signals. Such an embodiment would enable variable frame-rate imaging that could adaptively increase integration time for improved resolution or decrease integration time for improved frame rate based on things like relative target speed and proximity and heading. 
     The present invention is not limited to MWIR/SAL multi mode missile seekers, and is not limited strictly to two-mode detector devices. Multiple foul&#39;s of analog channel output and sampled imaging output may be maintained on the same FPA device. Spectral filtering may be eliminated altogether or replaced with polarization filtering and possible alternative embodiments may include an MWIR/SAL/visible spectrum multi-mode detector that provides MWIR and visible spectrum imaging as well as SAL range-finding and orientation on the same FPA. Yet other alternative embodiments may employ variants such as eye-safe and non eye-safe laser detection on different sets of FPA sections and associated channels. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.