Abstract:
A light source is injected into an optical path of a camera to provide a controlled background to insure optimum uniformity of performance of the camera. An optical system, camera, image background measurement system, source intensity control, and light source comprise a closed loop control around the light source to control an operating point of the camera. Intensity of the light source is decreased as intensity of a background flux is increased.

Description:
FILED OF THE INVENTION  
       [0001]     This invention relates to cameras used in dynamic operating environments where scene average can shift during operation. One application applies to missiles used for interception of other airborne devices which may include missiles and aircraft. Another may be bombs employing sensors for precision strike falling through the atmosphere. During high-speed flight, a window through which an infrared (IR) camera is directed necessarily increases in temperature, thereby producing infrared radiation which is received by the camera. This radiation is a source of dynamic background flux which can interfere with camera calibration.  
       BACKGROUND OF THE INVENTION  
       [0002]     An infrared system may operate in an environment where the system views the scene through a window while flying through the atmosphere. The window will heat throughout the mission resulting in a dynamic background. The camera is calibrated with a two-point fit to a response measurement for each pixel. There is a residual gain error in the calibration due to a calibration error, there are errors due to non-linearity of the pixel response and there is instability in the response over the storage life of the seeker. These gain correction errors when coupled to a highly dynamic background which shift the operating point (background flux level) of the camera can result in significant degradation of uniformity (noise) impacting mission performance.  
         [0003]     Attempts to perform automatic offset correction based on the scene using either sensed or induced motion (dither) are inherently limited in the presence of objects in the field of view. These approaches tend to assume that the displacement of the object space image allows an algorithm to separate the object space from the camera fixed pattern noise that does not shift. The problem with these approaches is that the object space image will change from one frame to the next as it moves with respect to the camera system. Also, the finite spatial sampling of the camera assures that there will be substantial differences in the count distribution of an object image due to displacement of the object relative to the sampling grid.  
         [0004]     The prior art includes a Jones calibration source which is used to calibrate cameras. The Jones calibration source is a small source which is placed near an entrance pupil of a camera system to uniformly flood a focal plane array and is described in  The Infrared Handbook  by William L. Wolfe and George J. Zissis (revised edition 1985, prepared by Infrared Information and Analysis (IRIA) Center, Environmental Research Institute of Michigan for the Office of Naval Research, Department of the Navy, Washington, D.C.). The focal plane array is a charge-coupled device which has an output which is in electronic form.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The invention provides a controlled background in order to ensure optimum uniformity performance of a camera when subjected to dynamic environments. This invention utilizes a Jones calibration source to control and stabilize the background during dynamic operation. A Jones calibration source which is a small light source placed near the entrance pupil of a camera system to uniformly flood a focal plane array is used. Typically, the source is set to two flux levels in order to perform a two-point calibration in a radiometer. The invention introduces a closed loop control around the source to actively control the operating point (flux level) of the camera. The uniformity calibration and subsequent operation of the camera are, therefore, constrained to the calibration point ensuring optimum uniformity.  
         [0006]     A camera having a controlled background comprising: an optical system having at least one pupil; a camera which receives light from said optical system having an electronic image output; an image background measurement system connected to the camera; a light source intensity control connected to the image background measurement system for controlling intensity of the light source; wherein said light source is located near said at least one pupil; and wherein intensity of said light source varies in an inverse relationship to the background flux.  
         [0007]     A camera having a controlled background comprising: an optical system having at least one pupil; a camera which receives light from said optical system having an electronic image output; an image background measurement system connected to the camera; a light source intensity control connected to the image background measurement system for controlling intensity of the light source; wherein the intensity control sets a plurality of flux levels of said Jones calibration source in order to perform a plurality of calibrations in a camera; and wherein intensity of said light source varies in an inverse relationship to background flux.  
         [0008]     A method of controlling background of a camera in order to obtain uniformity of performance of the camera when subjected to a dynamic environment comprising the steps of: placing the camera in a dynamic environment which changes a source of background light flux; receiving the background flux in an optical system; sensing the background flux after passing through the optical system in the camera wherein the camera has an electronic image output; measuring image background light flux; controlling intensity of a light source located near a pupil of said optical system; and controlling intensity of said light source in an inverse relationship to said background flux increases. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows the overall operation system including the closed loop control and placement of the Jones calibration light source.  
         [0010]      FIG. 2  shows an embodiment of the invention which utilizes a periscope to direct light from the Jones calibration light source into an optical path.  
         [0011]      FIG. 3  shows how the Jones source supplies the difference between the calibration point and the background. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  indicates the key concepts of the invention. The system is viewing the scene through a window ( 1 ) acting as a source of background flux. During operation such as high-speed flight, the window heats up over time and introduces background flux into the camera system in the form of infrared radiation. The window may be absent and the object space scene may provide the changing background flux alone. Also, the optical system ( 2 ) introduces background through emission in the wave band of the system due to either self-emission or leakage from other external sources which may vary with time during camera operation.  
         [0013]     The camera ( 3 ) observes the scene ( 7 ) through an optical system ( 2 ). The optical system illustrated here is a single lens defractive system. In general, the optical system can consist of defractive and/or reflective elements and may include multiple elements. The key to the invention is placement of a controlled Jones source ( 4 ) at the entrance pupil of the system. The term Jones calibration source is used in this description because it is a known calibration device, however, the source of light  4 ,  11  may be any source of light which provides light to a camera optical system  2  and camera  3 . The source could also be placed at locations in the optical path where the entrance pupil is reimaged, but the preferred location is at the entrance pupil because it will calibrate the system through the entire optical path.  
         [0014]     The Jones calibration source or light source  4 ,  11  is an in-band source of irradiance, such as a diode, resistive heating wire or other source of radiation. The choice of source is dependent on background dynamics and available power. Ideally, a diode would be employed using a lens  13 ,  8  that will match the emission steradiance of the diode source to the cone defined by the half field angle of the camera. A diode is preferred due to size, power and response time performance but may be limited in terms of output power. A resistive source can provide significantly greater output but with longer response time. This can be compensated for in the control loop. The source should be placed at or very near the entrance pupil in order to ensure uniform illumination of the focal plane. Ideally, to minimize obscuration of the aperture, the source would be placed outside the optical path and its output injected at the entrance pupil as shown in  FIG. 2  and its output injected at the entrance pupil using a periscope  10  attached to the telescope  12  just beyond the aperture stop  9 .  
         [0015]     As shown in  FIG. 1 , the camera  3  observes a summation of the flux from the scene  7 , window background flux  1 , optical path flux  2  and the Jones source flux  4 . The system measures the background from the output image of the camera  3 . The measure of the background is dependent on the complexity of the scene. It may be an average of the image for complex scenes or based on the maximum occurrence of a count value based on a histogram of the scene in less complex scenes.  
         [0016]     The Jones source  4  output level is controlled based on the measure of the image background in order to maintain the operating point through the source intensity controller  6 . The source intensity controller  6  consists of a control of the current, voltage or power driving the source dependent on the measured background.  
         [0017]     At calibration prior to operation, the system establishes operating points at specific background levels. The system during operation selects an operating background flux level from the calibration and dynamically adjusts the Jones source  4  to maintain the background through fluctuations of total system background  
         [0018]     If the system background exceeds the calibration background, the system then can switch to a new operating background. This operation is illustrated in  FIG. 3 . In this figure, the background is increasing with time. Two calibration points have been established in calibration operation prior to observation of the scene. As the background flux, as measured in output counts from the camera, increases, the Jones source output is decreased in order to maintain a constant background. As the scene background approaches calibration point  1 , the camera control selects a new calibration point, again set prior to observation of the scene, at a higher count level. The Jones source supplies the difference between the calibration point and the scene background. As can be seen in  FIG. 3 , as the background increases, the intensity of the Jones source changes in an inverse relationship (decreases) when the cal point is shifted from 1 to 2, the Jones source shifts upward to compensate for the cal point 2, and then again responds in an inverse relationship to the background. Focal planes have a lower bound of operation where they no longer are linear. When operating a system against a low background, operation in the non-linear region regrades performance, as it is difficult to calibrate a system for this region. Incorporation of a controlled background source will elevate the background to ensure linear operation over the entire operational range of the camera, even in low background flux conditions. This is illustrated in  FIG. 3  where the background is below the linearity limit and the Jones source adds background to bring the camera into linear operation.