Baseline compensating method and camera used in millimeter wave imaging

Abstract

Scene-independent baseline signals in output value signals from radiometer or receiver channels used in millimeter wave imaging are eliminated or reduced and an improved image is composed. The scene-independent baseline signals are believed to result from a standing wave which is established between an antenna of the channel and a movable scanning element which scans radiant energy from the scene into each channel. The movable scanning element introduces changes in geometry which change the characteristics of the baseline signals depending upon the position of the movable scanning element. The baseline signals are measured by viewing a scene of uniform brightness temperature, and the baseline signal contribution is subtracted from the output value signals from each channel. The baseline compensated output signals are used to compose an image with better contrast.

1 - 23 . (canceled)  
     
     
         24 . A millimeter wave imaging method to reduce a scene-independent baseline signal component of an output signal, the method comprising: 
 determining a magnitude of a baseline signal component of the output signal for a position of a movable scanning element; and    creating a baseline compensated output signal by subtracting the magnitude of a baseline signal component from the output signal.    
     
     
         25 . The method of  claim 24 , wherein the movable scanning element scans radiant energy emanating from a scene.  
     
     
         26 . The method of  claim 25 , wherein the output signal is from a channel.  
     
     
         27 . The method of  claim 26 , wherein the output signal of the channel is derived from the radiant energy emanating from the scene.  
     
     
         28 . The method of  claim 27 , wherein the output signal of the channel is derived from the radiant energy scanned into the channel from the scene of uniform brightness at least one position of the movable scanning element.  
     
     
         29 . The method of  claim 27 , wherein the output signal of the channel is derived from the radiant energy scanned into the channel from the scene of non-uniform brightness at least one position of the movable scanning element.  
     
     
         30 . The method of  claim 24  further comprising determining a magnitude of the baseline signal at a position of the movable scanning element scanning radiant energy emanating from a scene of uniform brightness.  
     
     
         31 . The method of  claim 24 , further comprising weighting each baseline-compensated output signal.  
     
     
         32 . The method of  claim 31 , further comprising weighting each baseline-compensated output signal by a different amount.  
     
     
         33 . The method of  claim 31 , further comprising weighting each baseline-compensated output signal by a predetermined weighting factor related to an amount of noise in each output signal.  
     
     
         34 . The method of  claim 31 , wherein the weighting factor is a reciprocal of a standard deviation of the amount of noise in each output signal.  
     
     
         35 . The method of  claim 34 , further comprising: 
 measuring a magnitude of noise from a channel into which radiant energy is scanned from a scene of uniform brightness; and    computing the standard deviation of the output signal from a channel based on the measured magnitude of noise.    
     
     
         36 . The method of  claim 31 , further comprising normalizing each weighted baseline-compensated output signal.  
     
     
         37 . The method of  claim 36 , further comprising normalizing the weighted baseline-compensated output signal on the basis of a flat fielding response of a channel.  
     
     
         38 . The method of  claim 36 , further comprising normalizing the weighted baseline-compensated output signal by a gain factor related to an amplification capability of a channel.  
     
     
         39 . The method of  claim 38 , further comprising: 
 measuring the amplification capability of the channel in response to scanning radiant energy from two uniform scenes of different and known brightness temperatures into the channel; and    establishing the gain factor of at least one channel in response to the measured amplification capability.    
     
     
         40 . The method of  claim 36 , further comprising normalizing the weighted baseline-compensated output signal by a normalizing factor which is related to a drift in offset of each output signal from a channel.  
     
     
         41 . The method of  claim 40 , further comprising calculating the normalizing factor from information defining an individual gain and offset characteristics of the channel.  
     
     
         42 . The method of  claim 41 , further comprising calculating the normalizing factor with each scan of radiant energy from the entire scene into the plurality of channels.  
     
     
         43 . The method of  claim 42 , further comprising normalizing the weighted baseline-compensated output signal on the basis that each channel observes a different mean scan brightness temperature from the radiant energy scanned from a portion of a scene than a mean brightness temperature of the entire scene.  
     
     
         44 . A detector camera, comprising: 
 a movable scanning element capable of scanning radiant energy emanating from one or more points in a scene;    a plurality of channels into which radiant energy is scanned by the movable scanning element and each channel is capable of converting the radiant energy into an output signal; and    a processor capable of reducing a scene-independent baseline signal component in the output signal by: 
 determining a magnitude of the baseline signal component of the output signal from a position of a movable scanning element; and  
 creating a baseline compensated output signal by subtracting the magnitude of a baseline signal component from the output signal.  
   
     
     
         45 . The camera of  claim 44 , wherein the scene is comprised of non-uniform brightness in at least one position of the movable scanning element.  
     
     
         46 . The camera of  claim 44 , wherein the scene is comprised of uniform brightness in at least one position of the movable scanning element.  
     
     
         47 . The camera of  claim 44 , further comprising weighting each baseline-compensated output signal by a different amount.  
     
     
         48 . The camera of  claim 46 , further comprising weighting each baseline-compensated output signal by a predetermined weighting factor related to an amount of noise in each output signal.  
     
     
         49 . The camera of  claim 44 , further comprising a display effective to provide an image formed on the basis of the output signals from which the baseline signal has been subtracted.  
     
     
         50 . The camera of  claim 44 , wherein the plurality of channels includes a focal plane array of channels.  
     
     
         51 . The camera of  claim 44 , wherein the camera is utilized for one of passive imaging and active imaging.  
     
     
         52 . The camera of  claim 51 , wherein a channel includes a radiometer channel when the camera is utilized in passive imaging.  
     
     
         53 . The camera of  claim 51 , wherein a channel includes a receiver channel when the camera is utilized in active imaging.  
     
     
         54 . The camera of  claim 44 , wherein a channel includes an antenna.  
     
     
         55 . The camera of  claim 54 , wherein the antenna includes an endfire traveling wave slot antenna.  
     
     
         56 . The camera of  claim 44 , wherein the movable scanning element includes a wedge shaped refractive element which rotates about an axis.  
     
     
         57 . The camera of  claim 44 , wherein the movable scanning element includes a rotating mirror arranged in the optical path and retained at a non-orthogonal angle to its rotational axis.  
     
     
         58 . The camera of  claim 44 , further comprising: 
 a position sensor connected to the movable scanning element, wherein the position sensor provides position signals to the processor;    wherein the position signals correspond to positions of the movable scanning element and wherein the processor uses the position signals for further determining the magnitude of the baseline signal.    
     
     
         59 . A system, comprising: 
 a processor;    a movable scanning element;    a plurality of channels;    a computer-readable storing medium storing a set of instructions capable of being executed by the processor to implement a millimeter wave imaging method to reduce a scene-independent baseline signal component an output signal and capable of performing the steps of: 
 determining a magnitude of the baseline signal component of the output signal from a position of a movable scanning element; and  
 creating a baseline compensated output signal by subtracting the magnitude of a baseline signal component from the output signal.  
   
     
     
         60 . A computer-readable storing medium storing a set of instructions capable of being executed by a processor to implement a millimeter wave imaging method to reduce a scene-independent baseline signal component in an output signal and capable of performing the steps of: 
 determining a magnitude of the baseline signal component of the output signal from a position of a movable scanning element; and    creating a baseline compensated output signal by subtracting the magnitude of a baseline signal component from the output signal.