Abstract:
A method of mapping pixel locations of a detector array includes measuring a location on the detector array, initiating a frame readout of the detector array, measuring a location of one or more metrology targets on the detector array, analyzing the frame readout to identify a pixel at the location on the detector array, and defining a location of the identified pixel with respect to the location of the one or more metrology targets. Subsequent measurement of the metrology targets alone by another metrology system allows one to infer the six degree of freedom alignment of the detector array in space.

Description:
ORIGIN OF THE INVENTION 
     Invention by Government Employee(s) Only 
       [0001]    The invention described herein was made by one or more employees of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     BACKGROUND 
       [0002]    The disclosed embodiments generally relate to mapping detector pixel locations, and to aligning a detector accurately in an optical system such that a projected image is in plane with pixels of the detector (i.e., aligned in focus, tip/tilt, and boresight/field space). 
         [0003]    Optical imaging systems are required to produce images with adequate resolution located on a range of pixels in a large format, active pixel array detector, The construction of optical systems thus requires accurate alignment of the active area of the detector in multiple degrees of freedom, depending on the sensitivity of optical image quality to detector orientation. This must be accomplished in a non-contact manner, because detectors are highly sensitive. A three dimensional, optical metrology sensor, such as a laser radar or Light Detection and Ranging (LiDAR) system, produces target location information in three dimensions and supplies its own illumination independent of external lighting sources or the optical system under construction. If the detector can image the LiDAR beam, then non-contact metrology of the detector&#39;s surface is possible. The ability to locate a particular pixel in a large format, active pixel array detector would be beneficial. Furthermore, having an ability to accurately align a large format active pixel array detector into an optical system such that locations of particular pixels or ranges of pixels may be established in a working coordinate system of the optical system would be advantageous. 
       SUMMARY 
       [0004]    A method has been developed to directly measure and map the physical location and orientation of large format detector active pixel arrays in a given coordinate system in six degrees of freedom in a non-contact manner. This method can be used to aid in the optical alignment of various systems and instruments in both terrestrial and extraterrestrial applications, for example, in the aerospace and other industries. In particular, the method may be used for pupil alignment reference measurements made on various sensors, and sensors comprising large format detector active pixel arrays at the focus of an optical system. The non-contact technique involves using a LiDAR system to measure the physical location and orientation of detector pixels on a large format detector with respect to an array of metrology targets attached to the detector bench or housing. Subsequent measurement of the metrology targets alone by another metrology system allows one to infer the six degree of freedom alignment of the detector array during construction. This information can be used to align a detector in an optical system or interpolate image data from the detector and correlate image features with physical locations in real space. 
         [0005]    This technique was developed during metrology testing of the Fine Guidance Sensor engineering test unit for the James Webb Space Telescope (JWST) project. One of the objectives of the metrology test was to directly measure the image location of the Fine Guidance Sensor&#39;s pupil alignment target in the telescope&#39;s coordinate system. To accomplish this, a large format detector was placed at the nominal location of the JWST telescope&#39;s exit pupil location, as defined in the coordinate system. This technique solved two technical challenges. The first was to place a detector accurately such that the telescope pupil image is in plane with the detector pixels. The second was that, once the detector alignment is accomplished, to establish the location of key features (which correspond to a unique pixel or range of pixels on the detector) in the image in the working coordinate system. 
         [0006]    In at least one aspect of the disclosed embodiments, a method of mapping pixel locations of a detector array includes measuring a location on the detector array with a laser radar system, initiating a frame readout of the detector array, measuring, using the laser radar system to measure the location of one or more metrology targets on the detector array, analyzing the frame readout to identify a pixel at the location on the detector array measured by the laser radar system, and defining a location of the identified pixel with respect to the location of the one or more metrology targets. 
         [0007]    The method may include translating the detector array and repeating the elements of the method for a plurality of locations on the detector array. Alternatively or in addition, the method may include re-pointing the LiDAR beam to another location on the detector array and subsequent detector readout to establish a pattern of multiple “spots” between repeat measurements of the external metrology targets. 
         [0008]    The method may include storing a plurality of locations of identified pixels with respect to the location of the one or more metrology targets in a table. 
         [0009]    Measuring a location on the detector array may include positioning the detector array approximately normal to a measurement beam emitted by the laser radar system, and using the laser radar system to record X, Y, and Z coordinates of the location on the detector. 
         [0010]    Analyzing the frame readout to identify a pixel at the location on the detector array may include reading an output of each pixel in the frame readout and identifying a pixel at a center of a measurement beam emitted by the laser radar system. Various fitting or centroid algorithms may be used to find the center pixel or sub-pixel location of the LiDAR spot. 
         [0011]    In at least one other aspect of the disclosed embodiments, an apparatus for mapping pixel locations of a detector array includes a laser radar system operable to measure a location on the detector array and operable to measure a location of one or more metrology targets on the detector array, and a computer operable to perform a frame readout of the detector array, analyze the frame readout to identify a pixel at the location on the detector array measured by the laser radar system, and define a location of the identified pixel with respect to the location of the one or more metrology targets. 
         [0012]    The apparatus may include a stage for translating the detector array, and the laser radar system and computer may be operable to cause the stage to move the detector array to a plurality of positions, measure a location on the detector array while the detector array is at each of the plurality of positions, perform a frame readout of the detector array while each location on the detector array is being measured, measure a location of one or more metrology targets on the detector array, analyze each frame readout to identify a pixel at the location on the detector array measured by the laser radar system, and define a location of each identified pixel with respect to the location of the one or more metrology targets. 
         [0013]    The apparatus may include a table in a memory of the computer, and the table may store a plurality of locations of identified pixels with respect to the location of the one or more metrology targets. 
         [0014]    The computer may cause the stage to position the detector array normal to a measurement beam emitted by the laser radar system, and the laser radar system may measure a location on the detector array by recording X, Y, and Z coordinates of the location on the detector array. 
         [0015]    The computer may analyze the frame readout to identify a pixel at the location on the detector array by reading an output of each pixel in the frame readout and identifying a pixel at a center of a measurement beam emitted by the laser radar system using a centroid or fitting routine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing aspects and other features of the embodiments are explained in the following description, taken in connection with the accompanying drawings, wherein: 
           [0017]      FIG. 1  illustrates an exemplary detector characterization system according to the disclosed embodiments; 
           [0018]      FIG. 2  shows a front view of a pixel array detector according to the disclosed embodiments; and 
           [0019]      FIG. 3  shows an exemplary measurement and analysis process according to the disclosed embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates one embodiment of an exemplary detector characterization system  100  in which aspects of the disclosed embodiments can be applied. Although the disclosed embodiments will be described with reference to the embodiments shown in the drawings and described below, it should be understood that these could be embodied in many alternate forms. In addition, any suitable size, shape or type of elements or materials could be used. 
         [0021]    The disclosed embodiments are directed to obtaining detector pixel location knowledge in 6 degrees of freedom. This knowledge can be applied to results derived from image analysis. The X, Y pixel value results for image analysis can be transformed into a three dimensional coordinate system. Using laser radar (i.e., focused LiDAR), the detector pixels may be physically mapped and then related to external metrology targets on the detector housing. The detector can next be aligned into a system, such as an optical instrument, using the metrology targets. For the purposes of the disclosed embodiments, the terms laser radar and LiDAR are used interchangeably. 
         [0022]    The system  100  may also include a LiDAR system  105  with a laser  110  for illumination and a LiDAR detector array  115  for measuring return intensity and range. The laser  110  may produce ultraviolet, visible, or near infrared light with a beam divergence selected to illuminate a desired scene. For example, a near infrared laser may be used having a 20 degree or any suitable beam divergence. The laser beam divergence and gain may be adjustable in order to obtain a desired field of view, return intensity and range. The LiDAR detector array  115  may comprise, for example, a 256×256 PIN diode array or any detector array appropriate for use in a LiDAR system. The LiDAR detector array  115  may also have an adjustable gain and field of view in order to optimize the return intensity and range detection. The LiDAR system  105  generally provides output data in the form of range data in terms of X, Y, and Z values. 
         [0023]    In at least one aspect of the disclosed embodiments, the detector characterization system  100  includes at least one computer  120  under the control of one or more programs in the form of computer readable program code  125  stored on at least one computer readable medium  130 . The computer readable program code  125  stored on the at least one computer readable medium  130  may be implemented for carrying out and executing the operations of the detector characterization system  100  described herein. The computer readable medium  130  may be a memory of the computer  120 . It should be understood that the computer readable medium  130  may store one or more operations for controlling the LiDAR system  105 , for reading pixels, groups of pixels and full frames of a large format pixel array detector  145 , for determining the locations of pixels of the large format pixel array detector  145 , for determining the locations of metrology targets, for processing frames read from the pixel array detector  145 , for storing pixel locations, metrology target locations, frames and any other data, for processing the data and mapping pixel locations, and for any other suitable processes for implementing the disclosed embodiments and techniques. Computer  120  may also include a microprocessor  135  for executing the computer readable program code  125  stored on the at least one computer readable medium  130 . In at least one aspect, computer  120  may include one or more input or output devices, generally referred to as a user interface  140  which may operate to allow input to the computer  120  or to provide output from the computer  120 . 
         [0024]    The LiDAR system  105  emits a measurement beam  150  directed to the large format pixel array detector  145 . The large format pixel array detector  145  may be mounted on or within a detector housing  155 . The housing  155  may include a number of metrology targets  160 . The metrology targets  160  may be specular spheres, spherically mounted retro-reflectors, or any other suitable metrology targets, permanently or temporarily mounted to the detector housing  155 . The large format pixel array detector  145  and the detector housing  155  may be mounted on an actuator stage  170  capable of single or multi-axis movement under control of the computer  120 . In at least one embodiment, the stage  170  may operate to translate the pixel array detector  145  in a direction perpendicular to the measurement beam  150 . 
         [0025]    Using the LiDAR system  105 , pixels of the large format pixel array detector  145  are physically mapped and then related to the metrology targets  160  on the detector housing  155 . A relationship is then established among the pixel locations and the metrology targets  160 , enabling the pixel array detector  145  to be aligned into another system, such as an optical instrument. 
         [0026]      FIG. 2  shows a front view of the pixel array detector  145 . To accomplish the mapping, a measurement process and an analysis process may be initiated. The measurement process may be initiated by pointing and focusing the LiDAR system  105  on a location, for example, location  205 A on the pixel array detector&#39;s active area  210 . A surface point measurement of the location  205 A may be made with the LiDAR system  105 , while a full frame readout of the pixel array detector  145  may be captured. The surface point measurement may record the X, Y, Z coordinates of the exemplary location  205 A on the pixel array detector surface as measured by the LiDAR system  105 . The LiDAR system  105  may then be used to measure the location of one or more of the metrology targets  160 . The surface point measurement, the full frame readout of the pixel array detector, and the metrology target location measurements, made with respect to the exemplary location  205 A, may be stored for further processing. The measurement process may be repeated for more of locations  205 B- 205   n.    
         [0027]    Once the measurement process is complete, the analysis process may include an image analysis of each full frame readout to identify a pixel at each exemplary location  205 A- 205   n  in terms of X, Y values of the pixel array detector  145 . Each X, Y pixel array detector location may be mapped to the corresponding LiDAR system X, Y, Z coordinates with respect to the metrology target locations. Thus, using the map, the location of each identified pixel may be defined with respect to the metrology target locations. The locations of remaining pixels that are not measured directly may be interpolated using the locations of the measured pixels, because the pixel pitch is tightly controlled during the pixel array detector&#39;s manufacturing processes. Once the pixel mapping is complete, the pose of each pixel and the pose of the pixel array detector  145  itself can be established in space using the metrology targets  160  on the detector housing  155 . 
         [0028]    Alternately, the analysis process may be combined with the measurement process such that after each surface point measurement, full frame readout of the pixel array detector  145 , and related metrology target location measurements, for a particular pixel location  205 A- 205   n , the corresponding X, Y pixel array detector location may be mapped to the corresponding LiDAR system X, Y, Z coordinates with respect to the metrology target locations, and incorporated into a table. Thus, in the table, the location of each identified pixel may be defined with respect to the metrology target locations. This process may be repeated until each X, Y pixel array detector location is mapped to the corresponding LiDAR system X, Y, Z coordinates with respect to the metrology target locations. 
         [0029]    The measurement and analysis process may generally be described in more detail as follows with respect to  FIGS. 1 through 3 . As shown in block  305 , the LiDAR measurement beam  150  may be set to an elevation angle of approximately 90 degrees which may place the measurement beam  150  approximately horizontal to a ground or floor surface. The pixel array detector  145  may then be placed such that the measurement beam  150  at 90 degrees strikes an approximate center  165  of the pixel array detector  145 , as displayed in block  310 . The stage  170  under control of the computer  120  may be used to adjust the position of the pixel array detector  145 . Referring to block  315 , the pixel array detector  145  may be aligned so that it is normal to the 90 degree measurement beam  150 . For example, the LiDAR system  105  may be used to measure a number of points on the surface of the pixel array detector  145  in polar coordinates and the pixel array detector position may be adjusted until the distance or radial polar coordinate of all the points is the same. 
         [0030]    Using the LiDAR system  105 , a surface point measurement may be made on the surface of the pixel array detector  145 , for example at location  205 A, as presented in block  320 . Without moving the LiDAR system  105  or pixel array detector  145 , a frame may be read out from the pixel array detector  145  with an exposure time set such that the pixel location  205 A on which the LiDAR is focused is not saturated, as shown in block  325 . The LiDAR system  105  may then measure the detector housing metrology targets  160 , as displayed in block  330  using, for example, a tooling ball mode of measurement. Referring to block  335 , the measurement beam  150  may be returned to its original horizontal position. The pixel array detector  145  may then be translated, by the stage  170  under control of the computer  120 , so that the measurement beam  150  strikes a different location of the pixel array detector  145 , for example, location  205 B, as displayed in block  340 . This process may be repeated until at least a predetermined number of pixel locations, for example, three to five, or as another example, locations  205 B- 205   n , are measured on the pixel array detector  145 , as presented in block  345 . 
         [0031]    The computer  120  may be utilized to perform image analysis on the captured frames to find a center of the measurement beam  150  for each measured point in pixel space in terms of X, Y coordinates on the pixel array detector  145 , as shown in block  345 . The X, Y coordinates on the pixel array detector  145  from the image analysis may be correlated with the LiDAR system X, Y, Z measurements, as shown in block  350 , via a best-fit process, which links the detector X, Y location values to the LiDAR system X, Y, Z measured values for the pixel location and the metrology targets  160 . As mentioned above, the locations of remaining pixels that are not measured directly may be interpolated using the locations of the measured pixels, because the pixel pitch is tightly controlled during the pixel array detector&#39;s manufacturing processes. Once the measurement and analysis process is complete, the pose of each pixel and the pose of the pixel array detector  145  itself can be established in space using the metrology targets  160  on the detector housing  155 . 
         [0032]    This technique is unique in that it provides a non-contact method for the mapping of pixels on the pixel array detector  145 . The non-contact nature of this technique is advantageous, especially when the arrays are sensitive to low-force contact and electrostatic discharge. This technique may also be faster than measuring a detector using a coordinate measuring machine. This technique uses the laser radar and the detector&#39;s readout to locate the actual active pixel itself. In addition to large format pixel detectors, the disclosed structures and techniques may easily be extended to CMOS arrays, IR-sensitive detectors, and other detector technologies. 
         [0033]    Using the structures and techniques disclosed herein, detector alignment can be precisely achieved, for example, to a level of tens of microns of absolute uncertainty in a mechanical coordinate system. The disclosed structures and techniques may be used in a wide range of applications involving detectors. For example, they may be implemented in the assembly and alignment verification of large focal plane arrays for telescopes or instruments with very large field of view requirements, such as imaging systems for commercial imagery satellites. Other exemplary applications may include the location and calibration in an absolute, mechanical coordinate system of the optical focus and other optical alignment indicators for an as-built optical system (e.g., pupil). The disclosed structures and techniques may also be utilized to improve field stop alignment and knowledge for telescope systems, for camera construction, and for the construction of medical instruments that use detectors, such as medical imaging scanners. 
         [0034]    It is noted that the embodiments described herein can be used individually or in any combination thereof. It should be understood that the foregoing description is only illustrative of the embodiments. Various alternatives and modifications can be devised by those skilled in the art without departing from the embodiments. Accordingly, the present embodiments are intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.