Abstract:
A passive skin detection system includes a main body which houses a collection optics system having an image splitting device, a visible light filter mechanism having a plurality of narrow band filters and an image capture system. The image capture system stores visible light data as a plurality of digital images formed from a plurality of pixels. Each of the plurality of digital images is associated with visible light passed through a respective one of the plurality of narrow band filters. An image processing system, operatively connected to the image capture system, compares relative intensities of each of the plurality of digital images to identify one or more of the plurality of pixels having an absorption bandwidth indicating a presence of skin. The processing system determines whether a person, identified by his skin, is present in any of the images captured by the detection system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/756,581 filed Jan. 6, 2006. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     The United States Government has rights in this invention based on Contract/Grant No. FA8650-04-C-5217, Subcontract No. SC5217-02-01. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention pertains to the art of imaging and, more particularly, to an image collection and processing system that passively detects skin through spectral measurement of an acquired image.  
         [0005]     2. Discussion of the Prior Art  
         [0006]     There is a constant need, particularly in the military and security fields, for advanced technologies that provide increased situational awareness in a city, municipality or even a combat zone. Accurate information regarding the number and location of humans in a particular area is an essential component of situational awareness. Automatically detecting humans by passively detecting skin would be extremely beneficial to law enforcement personnel, soldiers and security officers. This knowledge would provide an enhanced picture of a particular area of interest or operating environment.  
         [0007]     At present, several methods employing video surveillance technology are being developed to detect people and/or skin. Typically, these methods exploit either RGB color-matching or spatial object recognition methods to locate humans in a particular scene. Unfortunately, these methods are limited due to a susceptibility to false alarms. RGB or similar color-matching methods rely on low-spectral resolution color bands located in the visible portion of the spectrum and are unable to discriminate between skin and skin-like colored objects, such as can be found in many paints and dyes. In addition, RGB methods are unable to recognize highly pigmented skin.  
         [0008]     Other systems, such as those that employ object recognition methods, rely on an ability to associate a shape of an object in a scene with an individual. One limitation associated with this method is that it is necessary for an individual in the scene to span an extended portion of the field-of-view in order to obtain accurate shape identification. In addition, shape matching methods are susceptible to false negatives, such as may occur if shapes found in a scene are not recognized. For example, shape matching methods cannot reliably detect two individuals walking arm-in-arm, or individuals carrying large objects.  
         [0009]     It is well known that Hyper Spectral Imaging (HSI), which is defined as many tens or hundreds of narrow spectral bands in either the visible or infrared (IR) portion of the spectrum, is capable of adding significantly to information contained in an image as compared to conventional (three color wide band) imaging. In addition, numerous studies and patents have shown that HSI imaging can detect camouflage, crop variations, provide discrimination of various targets, and potentially identify carcinomas. Unfortunately, conventional HSI sensor systems are data transmission intensive, i.e. require data transmission rates to be several orders of magnitude higher than conventional video systems, and/or computationally intensive, i.e., require the processing of tens of thousands of pixels at many wavelengths simultaneously. Data transmission becomes a problem when processing the spectral image is separated from the data gathering, e.g., during use of an unmanned aerial vehicle, due to power or space limitations or simply due to the complexity of the data that must be processed. Even when located with the sensor, current data processing of all pixels in all bands requires teraflop class processing speeds.  
         [0010]     In addition to the above, full HSI sensors are very complex. That is, full HSI sensors must gather data simultaneously in many HSI bands across multiple spatial dimensions. This level of complexity results in a significantly high unit cost. Solutions to this challenge have involved linear scanning arrays which are not well suited to rapidly image large areas, or staring systems which are unable to simultaneously process the hyper-spectral dimension and are thus not well suited to spectrally image moving scenes.  
         [0011]     Based on the above, there exists a need for a low cost, effective imaging system that can accurately detect skin from visual images. More specifically, there exists a need for an imaging system that employs HSI technology and utilizes a very limited number of narrow bands that enables certain features in spectral images to be isolated to detect skin. An imaging system of this type would find a wide range of applications, such as military surveillance and reconnaissance systems, as well as facility security and related video tracking systems.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is directed to a passive skin detection system including a main body which houses a collection optics system having at least one image splitting device, a plurality of narrow band visible light filters and an image capture system. The image capture system stores visible light data as a plurality of digital images formed from a plurality of pixels. Each of the plurality of digital images is associated with visible light which has passed through a respective one of the plurality of narrow band filters.  
         [0013]     The detection system further includes an image processing system operatively connected to the image capture system. The image processing system employs an algorithm that compares relative intensities of each of the plurality of digital images to identify whether one or more of the plurality of pixels possesses an absorption bandwidth indicating a presence of skin. That is, the processing system determines whether a person, as identified through his/her skin, is present in any of the images captured by the detection system.  
         [0014]     In accordance with a preferred form of the invention, the plurality of narrow band filters are constituted by three filters centered around a first predetermined wavelength, while being separated one from another by a second predetermined wavelength. Preferably, the first predetermined wavelength is 577 nm±40 nm and the second predetermined wavelength is separated by approximately 30-50 nm. The first wavelength is associated with a property of skin relating to blood flow. More specifically, it has been determined that an absorption band associated with oxygenated hemoglobin exists centered at approximately 577 nm. The present invention preferably compares information from the three narrow bands to identify the absorption band.  
         [0015]     In further accordance with the invention, the algorithm isolates the identified pixels in further applications to support “feature aided” tracking approaches to video surveillance systems. The detection system is preferably low-cost and compact so as to be used in a wide array of applications, including heads-up displays, fixed site security, vehicle patrol, low-altitude unmanned aerial vehicles and the like.  
         [0016]     Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of preferred embodiments when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram schematically illustrating the passive skin detection system of the present invention;  
         [0018]      FIG. 2  is a top plan view of a common bore multiple image focusing device constructed in accordance with a first embodiment of the invention;  
         [0019]      FIG. 3  illustrates three digital images collected by the passive skin detection system of  FIG. 1 ;  
         [0020]      FIG. 4  is a graph illustrating a reflectance spectrum of human skin over a range extending between 470 and 670 nm;  
         [0021]      FIG. 5  is a top plan view of a common bore, single image focusing device employing the passive skin detection system constructed in accordance with a second embodiment of the present invention; and  
         [0022]      FIG. 6  is a top plan view of a common bore single image focusing device having an integrated image capture system divided into a plurality of image capture zones constructed in accordance with a third embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     With initial reference to  FIG. 1 , a passive skin detection system provided in accordance with the present invention is generally indicated at  2 . As will become evident from the following detailed description of the invention, visible light beams representing an image  5  are received by a collection optics system  7  which splits and diverts the visible light through a plurality of narrow band filters  11 - 13 . The visible light passing through filters  11 - 13  is projected onto an image capture system  17  having a plurality of image capture zones  19 - 21  which are preferably in the form of HSI sensors. Image capture zones  19 - 21  transform the visible light beams passing through respective ones of the plurality of narrow band filters  11 - 13  into a corresponding plurality of digital images which are passed onto an image processing system  27 . Preferably, filters  11 - 13  are centered about a first wavelength, while also being separated one from another by a second wavelength. Most preferably, filters  11 - 13  are centered about 577 nm±20-40 nm and separated by approximately 30-50 nm. In addition, each filter  11 - 13  has a width of approximately 5-10 nm. Image processing system  27  utilizes an algorithm which produces a result  30  reflective of whether image  5  contains any indication of skin which would correlate to a human presence.  
         [0024]     Reference will now be made to  FIG. 2  in describing a first embodiment of the invention in which passive image detection system  2  is configured as a common bore, multiple image focusing device  40 . Multiple image focusing device  40  includes a main housing  42  having a first end  46  which leads to a second end  47  through a hollow, interior portion  48 . Main housing  42  further includes a first branch  49  and a second branch  50 , each having corresponding first and second ends  51 ,  52  and  53 ,  54 . Each second end  47 ,  52  and  54  provides support for a respective one of image capture zones  19 - 21 . In accordance with the embodiment shown, filters  11 - 13  are located adjacent each image capture zone  19 - 21 . With this arrangement, collection optics  7 , shown in the form of a plurality of beam splitting elements  58  and  61 , directs visible light through filters  11 - 13  onto image capture zones  19 - 21 . More specifically, in addition to allowing visible light to pass directly through hollow interior portion  48  onto image capture zone  20 , beam splitting elements or devices  58  and  61  guide the visible light onto image capturing devices  19  and  21 . Towards that end, beam splitting element  58  allows a portion of the visible light to pass onto image capture device  20 , while directing another portion of visible light into first branch  49  which is then projected onto image capture zone  21 . Correspondingly, beam splitting device  61  allows a portion of the visible light to pass onto image capture device  20 , while directing another portion of the visible light into second branch  50  which is then projected onto image capture zone  19 .  
         [0025]     With reference to both  FIGS. 2 and 3 , each image capture zone  19 - 21  transforms the visible light into a respective digital image  69 ,  70  and  71 , each of which is formed from a plurality of pixels ( FIG. 3 ). In order to ensure proper clarity of each digital image, each image capture zone  19 - 21  is provided with a corresponding focusing mechanism  78 - 80 . Focusing mechanisms  78 - 80  can be adjusted manually or automatically to ensure proper clarity of each digital image  69 - 71 . In any event, each digital image  69 - 71  from each image capture zone  19 - 21  is passed to a central control unit  87  through a corresponding communication link  95 - 97 .  
         [0026]     In the most preferred form of the invention, central control unit  87  employs image processing system  27  which, in turn, employs a detection algorithm, as will be discussed more fully below, to produce result  30 . In accordance with the invention, the detection algorithm compares relative intensities of visible light in each digital image  69 - 71 . The detection algorithm compares the relative intensities of the images to isolate and identify any pixels that contain an indication of human skin. More specifically, the detection algorithm exploits a spectral feature found in all human skin, i.e., a response to visible light in a range of between approximately 450-700 nm and, most specifically an absorption band due to oxygenated hemoglobin centered at approximately 577 nm such as illustrated in  FIG. 4 . The detection algorithm is sensitive enough to reveal an indication of skin even if only found in a single one of the plurality of pixels. More specifically, the present invention can detect, on a single pixel level, whether any of digital images  69 - 71  contains visible light in the absorption band, thereby indicating the presence of skin.  
         [0027]     Reference will now be made to  FIG. 5 , wherein like reference numbers represent corresponding parts, in describing a second embodiment of the present invention which is constituted by a common bore single image focusing device  120 . Single image focusing device  120  includes a main housing  132  having first and second ends  136  and  137  separated by a hollow interior portion  138 . In a manner similar to that described above, main housing  132  includes first and second branches  139  and  140  each having corresponding first and second ends  142 ,  143  and  146 ,  147  respectively. In a manner also corresponding to that described above, each second end  137 ,  143  and  147  supports a respective one of image capture zones  19 - 21 . However, unlike the first embodiment wherein each image capture zone includes a corresponding focusing mechanism, single image focusing device  120  includes a single focusing mechanism  150  provided at first end  136  of main housing  132 . Focusing mechanism  150  can be adjusted manually or automatically to set image clarity on each image capture zone  19 - 21 .  
         [0028]     In accordance with the embodiment shown, visible light is passed through first end  136  of main housing  132  and focused by focusing mechanism  150 . Portions of the visible light are redirected by collection optics  7 , constituted by a pair of beam splitters  160  and  162 , into first and second branches  139  and  140 . That is, in a manner similar to that described above, one portion of the visible light passes directly onto first image capture zone  20 , while another portion is redirected by first and second beam splitters  160  and  162  into first and second branches  139  and  140  onto corresponding image capture zones  19  and  21 . Each image capture zone  19 - 21  captures digital images  69 - 71  formed from a plurality of pixels which are then passed to central control  87  and processed by processing system  27 . As discussed above, processing system  27  employs the detection algorithm to produce result  30  indicating whether skin is present within any of the captured images.  
         [0029]     Reference will now be made to  FIG. 6  in describing a third embodiment of the present invention which is defined by a common bore, single image focusing device  190  with an image capture system  17  constituted by an integrated image capture unit  198  separated into image capture zones  19 - 21  as will be discussed more fully below. As shown, single image focusing device  190  includes a main housing  194  having a first end  196  that leads to a second end  197  through a hollow interior portion  199 . In a manner similar to that described with respect to main housing  132 , a focusing mechanism  205  is provided at first end  196  of main housing  194 . However, unlike main housing  132 , main housing  194  is provided with a single branch  207  which houses integrated image capture device  198 . That is, visible light passing through focusing mechanism  205  is redirected through collection optics  7  constituted by three beam splitters  208 - 210  and then passed through corresponding narrow band filters  211 - 213  onto respective ones of image capture zones  19 - 21 . At this point, it should be understood that filters  211 - 213  equate to filters  11 - 13 . In any case, each image capture zone  19 - 21  transforms the visible light passing through narrow band filters  211 - 213  into digital is images  69 - 71  formed from a plurality of pixels. Digital images  69 - 71  are passed through a common communication link  218  to a central control  224 . Central control  224  includes processing system  27  which utilizes the above described detection algorithm to produce result  30  indicating whether human skin is present within any one of the digital images captured by image capturing device  17 .  
         [0030]     At this point, it should be understood that the present invention establishes a simple apparatus for automated detection of human skin employing an HSI sensor system that is not computationally intensive, thereby reducing cost and complexity of the detection system. The system preferably has an effective range of 1-200 m and, most preferably, up to 5 km. In addition, the present invention can be employed as a simple, low cost solution to identify skin in a video image so as to be employed in military surveillance, reconnaissance, facility security and related video tracking systems. Moreover, the reduced complexity enables the system to be employed with unmanned aerial vehicles, heads-up displays, fix base security, vehicle patrol and the like.  
         [0031]     Although described with reference to preferred embodiments of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. In general, the invention is only intended to be limited by the scope of the following claims.