Abstract:
A camera comprises an image plane for capturing shortwave infrared wavelengths. The image plane captures only a portion of a shortwave infrared wavelength band, and excludes other wavelengths. A method of designing a camera is also disclosed.

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/911,116, filed Dec. 3, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates to a camera for capturing images in a shortwave infrared bandwidth wherein modifications are made to eliminate capture of certain portions of the shortwave infrared bandwidth. 
     Various camera systems are known and utilized for any number of applications. One type of camera has an image detecting device which may be a photo-voltaic photo diode. These cameras are known to capture images in what is known as the shortwave infrared (SWIR) bandwidth. This may also be known in some applications as “near infrared.” A capture bandwidth for such cameras may be between about 0.9 and 1.7 microns. 
     Such cameras have particularly good application in capturing images in situations where normal visual light images would be compromised. As an example, night vision cameras may operate across this bandwidth. When such cameras are used for night vision systems on commercial land vehicles such as cars, buses and trucks, the full range of such SWIR bandwidth is not needed. 
     SUMMARY OF THE INVENTION 
     A camera comprises an image plane for capturing shortwave infrared wavelengths. The image plane captures only a portion of a shortwave infrared wavelength band, and excludes other wavelengths. A method of designing a camera is also disclosed. 
     These and other features may be best understood from the following drawings and specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment. 
         FIG. 2  shows a feature of  FIG. 1  embodiment. 
         FIG. 3  shows another embodiment. 
         FIG. 4  shows yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a camera  20  having an outer window  22  received in a housing  24  which may be formed of an appropriate metal such as a nickel alloy. An image plane  28  and a silicone readout inlet circuit  30 , along with a printed wiring board  32  together form a focal plane array of photo-voltaic photo diodes. In one embodiment, the image plane  28  may be formed of a semiconductor compound including both indium arsenide and gallium arsenide. Cameras that use such image planes may be known as InGaAs cameras. A thermal electric cooling sink  34  is positioned to capture heat. Input/output pins  90  communicate with a controller  91  which can evaluate and store images captured by camera  20 , as known. 
       FIG. 2  shows a standard bandwidth of capture  40  for a shortwave infrared camera. 
     The cameras have typically been designed to capture the entire bandwidth range of about 0.9-1.7 microns. 
     The camera of  FIG. 1  is provided with a filter  26  which acts to limit the bandwidth of the shortwave infrared bandwidth which will reach the image plane  28 . As shown in this embodiment, the filter  26  is placed directly on the image plane  28 . 
     The filter  26  may be any interference or dichroic bandwidth filter or any other appropriate filter capable of filtering the bandwidth of shortwave infrared light that reaches the image plane  28 . The filter  26  may be a simple filter coating applied directly to the surface of the image plane  28  or may be a separate filter which is attached to the plane. 
       FIG. 3  shows yet another embodiment wherein a filter  98  is attached directly to the window  96  which may operate much like the window  22  in the camera  20 . 
     Dichroic filters and how they can be applied to various surfaces are known and within the skill of a worker in this art given the goals of this application as follows. 
     Returning to  FIG. 2 , the filter may limit the shortwave infrared bandwidth which reaches the image plane  28  such as shown at  42  in  FIG. 2 . Now, a shorter end  44  of the capture bandwidth  42  is approximately 1.1 microns while a longer end  46  is just above about 1.5 microns. 
     When such cameras are used for night vision systems on commercial land vehicles such as cars, buses and trucks, the full range of the SWIR bandwidth is not needed. Instead, only the range  42  between about 0.9 and 1.5 microns is needed. 
       FIG. 4  shows an embodiment  120  wherein there may not be a filter. Rather, the image plane  128  is modified such that it is only sensitive to the bandwidths such as bandwidth  42 . 
     As an example, it is known that the long wave cut-off point for a standard photo detector is associated with the ratio of indium arsenide to gallium arsenide. A worker of ordinary skill in the art given the teachings of this application would know how to modify that ratio such that the bandwidth  42  is all that is captured and not the entire bandwidth  40 . 
     This alteration may create a crystal lattice mismatch of an absorbing layer to a substrate which could result in non-uniformities. Thus, it may be preferred to add phosphide to an absorbing layer making it a compound that alters a bandwidth to a shorter bandwidth while also maintaining a lattice matched to the substrate. Aluminum may also be used rather than phosphide. 
     A first layer of the detecting structure in the image plane  130  passes light through before hitting a detection layer. The first layer can be modified to remove its normal absorbing cut-out from the approximately 0.9 current prior art of the bandwidth  40  closer to the 1.1 micron wavelength. One can do this by increasing the thickness, such that the combination prevents photons short of 1.1 microns from reaching the detecting layer. These first layer changes can be combined with the changes mentioned above to a detector bandwidth such that the bandwidth  42  will be achieved. 
     Detectors can be designed for front illumination where the light reaches the detection layer through the top surface or back illumination where the light travels through the backside or substrate layer. The embodiments mentioned above can be applied either design. 
     Finally, returning to  FIG. 1 , the modifications to the image plane  128 , as mentioned with regard to  FIG. 4 , can also be utilized in combination with the filter structure of  FIG. 1 . For this reason, an optical filter  126  is shown in  FIG. 4 . 
     The use of both of these features will result in a camera that is unlikely to actually capture a wavelength in the excluded bands between the bandwidth  42  and the bandwidth  40 . 
     While an example of the excluded wavelengths is illustrated outside of band  42  in  FIG. 2 , it should be understood that lesser wavelengths may be excluded, or more could be excluded. In one embodiment, the camera excludes wavelengths that are shorter than 1.0 microns. In a narrower embodiment, the excluded wavelengths include wavelengths shorter than approximately 1.1 microns. In embodiments, the excluded wavelengths also exclude wavelengths longer than 1.6 microns, and more narrowly wavelengths that are longer than approximately 1.5 microns. 
     A filter such as described above, can also be manufactured to have one or more notch filters layered together to exclude specific wavelengths. These could be assembled in any of the locations described above. 
     Although embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.