Optical distortion calibration for electro-optical sensors

Abstract

Optical distortion calibration for an Electro-Optical sensor in a chamber eliminates calibration of the mirror controller and allows for calibration while the target is in motion across the FOV thus providing a more efficient and accurate calibration. A target pattern is projected through sensor optics with line of sight motion across the sensor FOV to generate a sequence of frames. Knowing that the true distances between the same targets remain constant with line of sight motion across the sensor's FOV, coefficients of a function F representative of the non-linear distortion in the sensor optics are fit from observed target positions in a subset of frames to true line of sight so that distances between targets are preserved as the pattern moves across the FOV. The coefficients are stored as calibration terms with the sensor.

1 . A method of performing distortion calibration for an electro-optical sensor, comprising:
 projecting and scanning a physical target pattern having a plurality of physical targets through sensor optics that introduce non-linear distortion to a projected target pattern with a line of sight motion across a sensor FOV;   observing multiple target positions in the sensor FOV in a plurality of frames; and   subject to a constraint that the true angular distances between the same targets in the target pattern remain constant with line of sight motion, fitting coefficients for a function F representative of the distortion in the sensor optics between observed target positions and true target positions so that distances between true target positions are approximately preserved as the pattern moves; and   storing the coefficients as calibration terms in a tangible medium.   
     
     
         2 . The method of  claim 1 , wherein the function F is representative of the distortion in the sensor optics from observed target positions to true line of sight. 
     
     
         3 . The method of  claim 1 , wherein the function F is representative of the distortion in the sensor optics from the true line of sight for a plurality of targets to the observed positions of those targets in a plurality of input frames. 
     
     
         4 . The method of  claim 1 , further comprising:
 observing a position of a reference in each of said plurality of frames; and   using the position of the reference in each frame to establish a unique match of the observed target positions to the true target positions.   
     
     
         5 . The method of  claim 1 , wherein the coefficients are fit by:
 observing a position of a reference in said frames;   applying the function F to the observed target and reference positions to provide corrected target and reference positions;   representing a corrected difference positions for a plurality of the targets as the a difference between the corrected target positions and a the corrected reference position; and   fitting coefficients for the function F using the observed target and reference positions over a subset of the targets and frames to minimize the scale-normalized variability of the corrected difference positions as the targets move across the FOV.   
     
     
         6 . The method of  claim 1 , wherein a plurality of said targets are less than one pixel wide in all axes. 
     
     
         7 . The method of  claim 1 , wherein at least a plurality of said targets are greater than one pixel wide in one or more axes. 
     
     
         8 . The method of  claim 1 , wherein said reference constitutes a subset of said plurality of targets. 
     
     
         9 . The method of  claim 1 , wherein said reference varies over the frames. 
     
     
         10 . A method of performing distortion calibration for an electro-optical sensor, comprising:
 projecting and scanning a target pattern having a plurality of targets through sensor optics with a line of sight motion across a sensor FOV;   observing multiple target positions in the sensor FOV in a plurality of frames; and   subject to a constraint that the true angular distances between the same targets in the target pattern remain constant with line of sight motion, fitting coefficients for a function F representative of the distortion in the sensor optics from observed target positions to true line of sight so that distances between targets are approximately preserved as the pattern moves; and   storing the coefficients as calibration terms in a tangible medium.   
     
     
         11 . The method of  claim 8 , wherein the coefficients are fit by:
 observing a position of a reference in said frames;   applying a function F representative of the distortion the sensor optics to the observed target and reference positions to provide corrected target and reference positions;   representing a corrected difference position for a plurality of the targets as the difference between the corrected target position and a corrected reference position; and   fitting coefficients for the function F using the observed target and reference positions over a subset of the targets and frames to minimize the scale-normalized variability of the corrected difference position as the targets move across the FOV.   
     
     
         12 . A test system for performing distortion calibration for an electro-optical sensor, comprising:
 a vacuum test chamber including,
 a physical target pattern having a plurality of physical targets; 
 a flight sensor including an electro-optical sensor, a tangible medium and sensor optics; 
 a collimating lens that projects the target pattern so that the pattern may be shifted in a field-of-view (FOV) measured by the sensor while preserving the relative positions of the targets; and 
 a scanning mirror that directs the target pattern through the sensor optics with a line of sight motion across the sensor's FOV over a plurality of frames, said optics' introducing to the projected target pattern producing apparent differences in the relative positions of the targets as the target pattern moves; and 
   a computer configured to observe multiple target positions in the sensor FOV in each of a plurality of frames, and subject to a constraint that the true angular distances between the same targets in the target pattern remain constant with line of sight motion, fit coefficients for a function F representative of the distortion in the sensor optics between observed target positions and true target positions so that distances between true target positions are approximately preserved as the pattern moves, and store the coefficients in the tangible medium as calibration terms for the sensor.   
     
     
         13 . The test system of  claim 12 , wherein the function F is representative of the distortion in the sensor optics from observed target positions to true line of sight. 
     
     
         14 . The test system of  claim 12 , wherein the function F is representative of the distortion in the sensor optics from the true line of sight for a plurality of targets to the observed positions of those targets in a plurality of input frames. 
     
     
         15 . The test system of  claim 12 , wherein the physical target pattern includes a reference, wherein said computer observes a position of a reference in each of said plurality of frames and uses the position of the reference in each frame to establish a unique match of the observed target positions to the true target positions.