Abstract:
A tunable imaging sensor includes a housing with four lenses mounted on a front side. A removable or rotatable filter plate/wheel fits inside the enclosure adjacent the lenses, with a camera plate holding four CMOS or CCD imagers fitting inside the enclosure adjacent the filter plate/wheel. The filter plate includes four filters, one for each lens, while the filter wheel includes sixteen filters which are rotated into position so that four filters are always aligned with the lenses and imagers. Rotating the filter wheel provides sixteen different filter combinations for the sensor. The images from each of the imagers are processed to form a composite image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application Ser. No. 60/463,750 filed Apr. 16, 2003 and entitled TUNABLE IMAGING SENSOR, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of imaging sensors, and more particularly to an imaging sensor that can be tuned via various filters. 
     BACKGROUND OF THE INVENTION 
     Multispectral imaging systems record spatial pictures of an image scene in many different spectral wavelength bands, for example, a scene image at red wavelength, green wavelength and at blue wavelength. Differences are also detected when using polarized light of different polarization angles. Differences in the observed spatial image at different wavelengths are useful for finding hidden targets, assaying agricultural conditions, and detecting other subtle features that would not be noticed in a video picture of the scene. 
     SUMMARY OF THE INVENTION 
     Briefly stated, a tunable imaging sensor includes a housing with four lenses mounted on a front side. A removable or rotatable filter plate/wheel fits inside the enclosure adjacent the lenses, with a camera plate holding four CMOS or CCD imagers fitting inside the enclosure adjacent the filter plate/wheel. The filter plate includes four filters, one for each lens, while the filter wheel includes sixteen filters which are rotated into position so that four filters are always aligned with the lenses and imagers. Rotating the filter wheel provides sixteen different filter combinations for the sensor. The images from each of the imagers are processed to form a composite image. 
     According to an embodiment of the invention, a tunable imaging sensor includes a housing; a lens plate attached to a front of said housing; a filter plate inside said housing adjacent said lens plate; a camera plate inside said housing adjacent said filter plate; a plurality of imagers mounted on said camera plate; a plurality of lenses mounted on said lens plate; a plurality of filters mounted on said filter plate; said lens plate, said filter plate, and said camera plate being aligned such that radiation passing through one of said lenses passes through one of said plurality of filters onto one of said imagers. 
     According to an embodiment of the invention, a tunable imaging sensor system includes a housing; first means for mounting a plurality of lenses in said housing; second means for mounting a plurality of filters adjacent said plurality of lenses in said housing; third means for mounting a plurality of imagers adjacent said plurality of filters in said housing; wherein said first means, said second means, and said third means are aligned such that radiation passing through one of said lenses passes through one of said plurality of filters onto one of said imagers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of a sensor assembly according to a first embodiment of the invention; 
         FIG. 2  shows a sensor housing for the first embodiment of the invention; 
         FIG. 3A  shows a camera plate according to the first embodiment of the invention; 
         FIG. 3B  shows the camera plate of  FIG. 3A  with the cameras attached; 
         FIG. 4  shows a filter plate according to the first embodiment of the invention; 
         FIG. 5  shows a perspective view of a sensor assembly according to a second embodiment of the invention; 
         FIG. 6  shows a shell of the sensor assembly of  FIG. 5 ; 
         FIG. 7  shows a lens plate according to the second embodiment of the invention; 
         FIG. 8  shows a filter plate according to the second embodiment of the invention; 
         FIG. 9  shows a camera plate according to the second embodiment of the invention; 
         FIG. 10  shows a front elevation view of the sensor assembly of  FIG. 5  with the lens plate removed; 
         FIG. 11  shows a block diagram of the sensor system according to both embodiments of the invention; and 
         FIGS. 12A and 12B  show schematics of synchronization and communication circuits used in both embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1-2 , the first embodiment of the present invention is a small medium scale integration (MSI) sensor  10 . The first embodiment is an extremely low-cost, low power, light weight temporally simultaneous spectral imager for a variety of applications. The first embodiment sensor is a four aperture four CMOS camera system, in which the CMOS cameras are synchronized to produce four temporally simultaneous channels of analog RS-1 70 data. The first embodiment weighs less than 5 pounds and requires 5 volt DC power. 
     Sensor  10  includes a housing  12  which is generally rectangular in shape, with a lid  14 . Lid  14  is removably fastened to housing  12  preferably by screws, but any fastener capable of repeated removals will suffice. A front of housing  12  contains four holes  18  for attaching four standard C-Mount lenses  38 . An alignment hole  20  is generally centered in the front of housing  12 . 
     Referring to  FIGS. 3A-3B , a camera plate  22  preferably contains two cutout portions  24 , with each cutout portion  24  including a notch  25 . Each notch  25  permits an imager  28  to be mounted therein. Each notch  25  is aligned with each lens  38 . Referring to  FIG. 4 , a filter plate  30  includes four holes  32  and an alignment hole  34 . Each hole  32  receives a filter  36 . When assembled, filter plate  30  is inserted into housing  12  between the front of housing  12  and camera plate  22 . Light enters sensor  10  through lenses  38 , passes through filters  36 , and is received by imagers  28 . Housing  12  includes space for the electronics necessary to synchronize and trigger imagers  28 . Sensor housing  12  also provides all external connections to imagers  28  and associated electronics. 
     Imagers  28  are preferably CMOS cameras such as the M3 1 88A manufactured by COMedia, Ltd. in Hong Kong. The clock crystals and optics of the M3 1 88A are removed before installation into sensor  10 . The M3188A is a ⅓″ B/W camera module with digital output, using Omni Vision&#39;s CMOS image sensor 0V7 120. The digital video port supplies a continuous 8-bit wide image data stream. All camera functions such as exposure, gamma, gain, and windowing, are programmable through the 1 2 C (InterIntegrated Circuit) interface. 
     The first embodiment sensor is spectrally selectable by the user, simply by choosing the appropriate filters  36  for filter plate  30 . Filters  36  are preferably user selectable spectral or polarization filters. CMOS imagers  28  support spectral ranges of visible to near infrared, that is, sensor  10  can be used to image spectra from 400 nm to 1000 nm. Sensor  10  can also be used to collect polarimetric data without the need of determining the sensor Mueller matrix prior to collecting polarimetric imagery. This is done by simply inserting polarizers appropriately into filter plate  30 . Finally, the field of view of sensor  10  is determined by the selection of lenses  38  employed on the front of sensor  12 . 
     Sensor  10  produces both interlaced and progressive scan RS-170 analog data. Digital data can be supplied with the appropriate selection of CMOS and/or CCD cameras (imagers). Sensor  10  also has the ability to easily establish sensor gain, manually or automatically, and select the camera operational mode. 
     Sensor  10  is not a common aperture imager. Therefore, due to the spatial positioning of the individual imagers  28 , a registration solution at infinity produces registration errors in the near-field. This near field registration effect may be used to generate range information. A key advantage to sensor  10 , besides being low-cost, lightweight, low-power, and spectrally adaptable, is the ability of CMOS imagers  28  to individually address pixels independently, making it possible for basic spectral processes to be implemented prior to digitization. A fast analog process easily competes with a digital process in terms of speed and this avoids quantization error in the analog to digital conversion for limited bit systems. Creative electronic design combines these basic functions into just about any complex process. In addition, since CMOS imagers  28  use the same semiconductor substrate as processing chips, the sensor and the processor, either analog or digital, can be implemented on a single piece of silicon. 
     Referring to  FIGS. 5-10 , a second embodiment of the invention is an enhanced version of the first embodiment. As in the first embodiment, the second embodiment includes three main components; i.e., a sensor  50  includes a camera plate  64 , a filter plate shown as a filter wheel  58 , and a sensor housing shown as a shell  52 . Lens plate  54 , either integral with shell  52  or one-piece with it. supports four C-Mount lenses  70  in four holes  56 , which are positioned to cooperate with filter wheel  58  in a unique fashion. The original four-slot filter plate  30  of the first embodiment is replaced with a 16-position filter wheel  58 . Filter wheel  58 , in conjunction with the sensor layout, provides sixteen unique spectral and/or polarization band sets. Note that the lenses are positioned so that each lens  70  always lines up with a filter element  60  in filter wheel  58  and its associated imager  68 . A radius  72  describing a corresponding arc  74  which defines the shape of the sides of shell  52  permits lenses  70  mounted in holes  56  to be aligned with filter elements  60 . The asymmetry in the location of lenses  70  permits 16 unique filter sets. 
     Camera plate  64  hosts four CMOS or CCD cameras, shown as imagers  68 , in a unique configuration to enhance the sensor capability. A cutout portion  66  accommodates wires to connect imagers  68  with the processing electronics. The second embodiment maintains the benefits of its predecessor and provides greater capabilities in a small, light weight package with similar power requirements. 
     A shaft hole  62  in filter wheel  58  accommodates a shaft (not shown) that is preferably connected to a motor such as a DC stepper motor (not shown) which is interfaced to a computer  80  so that filter wheel  58  can be moved to align different sets of filters  60  without opening sensor  50 . The initial positioning and alignment, i.e., calibration, of filter wheel  58  is preferably done at the factory, with the sets of filters chosen for whatever particular application the user has in mind. Four sets of four filter sets would available on one filter wheel  58 , or a combination of full filter sets and partial filter sets. For instance, if sensor  50  is to be used in an aircraft overflying an area performing a search and rescue operation, the filter sets would include color filters chosen to obtain data optimizing finding a person in the midst of foliage, sand, dirt, and/or water. For polarization studies, a complete set of polarizing filters would use four filters, with two filters being linear filters orthogonal to each other, and with the other two filters being circularly polarized, one clockwise and the other counterclockwise. For some applications, only linearly polarized filters would be used, with the filters being polarized 45 degrees apart. The flexibility of the filter wheel supports many options. 
     There is significant benefit in data collected by the systems. Sensors  10  and  50  provide (1) narrow bandwidth spectral data for spectral target detection capabilities, (2) polarization data to enhance the ability to detect targets in shadows and increase the detection capabilities of man-made objects, and (3) since the apertures are not coincident, the systems provide stereographic imaging capabilities inherently determining relative distances from the collected imagery through the use of software known in the imaging art. Finally, the systems are extendable to house small LWIR (long wave infrared) microbolometer sensors to further increase sensor capabilities. The first and second embodiments also support gathering the image data necessary for multispectral imaging, such as is disclosed in U.S. Pat. No. 6,539,126 (Socolinsky et al.) issued on Mar. 25, 2003 and entitled VISUALIZATION OF LOCAL CONTRAST FOR N-DIMENSIONAL IMAGE DATA, incorporated herein by reference. 
     Referring to  FIG. 11 , a block diagram of a complete sensor system is shown in which we use the second embodiment as the example, although the system applies to both embodiments. Sensor  50  includes two sections, camera optics  82  which includes lenses  70  and filter wheel  58  with its associated filters, and camera electronics  84  which includes imagers  69  and a synchronization /communications circuit  86 . An embodiment of synchronization /communications circuit  86  is shown in more detail in  FIG. 12 . A computer  80  includes an 12C PCI communications adapter  88  such as the Calibre 12C Communications Adapter, Model PC 190 manufactured by Calibre UK Ltd,. Cornwall House, Cornwall Terrace, Bradford, West Yorkshire in the United Kingdom. Communication between computer  80  and synchronization/communications circuit  86  is carried out over an 12C bus  90 . GUI-based software in GUI-based camera communications software module  92 , developed by the applicants, accommodates user control of imagers  68  in a Microsoft Windows® environment using Windows® software libraries, source code, and object code from Calibre UK Ltd. 
     Computer  80  also preferably includes a frame grabber  94  which captures simultaneously the outputs from the RS-170 outputs of imagers  68  at a 30 Hz rate using a 4-channel bitflow Raven™ PCI Frame Grabber, Model no. RAV-PCI-440-VNS, manufactured by BitFlow, Inc. of Woburn, Mass. The separate camera outputs form imagers  68  are essentially treated as though they are separate channels or taps from a single virtual camera, in this case, sensor  50 . GUI-based software in GUI-based acquisition /display /recording software module  96 , developed by the applicants, accommodates the real-time acquisition, display, and recording of the sensor  50  image data in a Microsoft Windows® environment using frame grabber  94 . The data is stored in a data storage medium  98 . The developed software in module  96  includes the use of Windows® software libraries, source code, and object code from the Bitflow, Inc. Software Development Kit version 3.0. The software in module  96  manipulates the images, including forming the composite image using techniques well known in the art. 
     The schematic of  FIG. 12  actually shows two circuits. Synchronization/communications circuit  86  includes a synchronization circuit  100  which preferably includes a 27 MHz clock  102  which feeds a clock signal into a clock buffer/splitter  104  such as the CDC329A chip manufactured by Texas Instruments, Dallas, Tex. The CDC329A chip takes the clock signal and splits it up into six identical signals. Only four of the signals are used, which are fed to a group of resistor capacitor (RC) filters  106 . The output signal from RC filters  106  then goes into a connector  108  which eventually goes to the cameras (imagers  68 ) and keeps them all on the same clock cycle. 
     Synchronization/communications circuit  86  also includes a communication circuit  110  which preferably uses the ADG729 chip, manufactured by Analog Devices, Norwood, Mass. The ADG729 is a CMOS analog matrix switch with a serially controlled two-wire interface. It is a dual four-channel matrix switch. ON resistance is closely matched between switches and very flat over the full signal range. This part operates equally well as a multiplexer, demultiplexer or switch array and the input signal range extends to the supplies. Each channel is controlled by one bit of the data byte, which means that these multiplexers can be used in a number of different configurations with all, any, or none of the channels on at any one time. On power up of the device, all switches will be in the OFF condition and the internal shift register will contain all zeros. All channels exhibit break before make switching action preventing momentary shorting when switching channels. For the first and second embodiments, the ADG729 chip essentially works like a switch. When the computer gives it a certain address it opens either one, two, three, or all four switches in the circuit allowing the communication to the cameras to control multiple options. 
     While the present invention had been described with reference to a particular preferred embodiment and the accompany drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.