Patent Publication Number: US-6907370-B2

Title: Method and apparatus for calibrating a measurement system

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
   This application claims the benefit of the filing date of provisional patent application No. 60/335,135 entitled “Method and Apparatus for Calibrating A Measurement System,” which has a filing date of Nov. 14, 2001 and which is hereby incorporated by reference. 

   REFERENCE TO COMPUTER PROGRAM SUBMITTED ON COMPACT DISC 
   A computer program listing appendix submitted on compact disc (in duplicate) is included and the material contained on the compact disc(s) is hereby incorporated by reference. 
   
     
       
         
             
             
             
             
           
             
                 
             
             
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               Patent Application 
               74 
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               File Folder 
               Nov. 14, 2002 
             
             
               LightCalDialog.cpp 
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               ASCII text file 
               Nov. 14, 2002 
             
             
               LightCalDialog.h 
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               Nov. 14, 2002 
             
             
               LightSource.cpp 
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               LightSource.h 
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   COPYRIGHT NOTICE 
   A portion of the disclosure of this application, particularly the source code provided on the accompanying Compact Disc contains material that is subject to copyright protection. The copyright owner has no objection to the photographic reproduction of this patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all rights whatsoever. 
   BACKGROUND 
   Technology has advanced to the stage where a variety of objects are constructed with sub-micron features. For example, computer processors are now designed with sub-micron dimensions. And, a new class of medical devices, known as “micro-medical” devices, also includes sub-micron components. 
   Quality control and other inspections must be completed on these miniature features. However, measurement systems capable of sub-micron inspection require their own complex technology. These devices generally include a high-powered microscope and extremely precise positioning equipment. Illumination systems, that include one or more lamps, provide the necessary light. However, variations in lamp intensity can cause measurement differences in otherwise identical parts. These illumination variations might occur when replacing lamps, when a lamp ages, or when measuring otherwise identical parts on different systems. What is true for lamps is also true for cameras and optics. One camera may not yield the same results as another on a given measurement system. Therefore, a need exists for a method and apparatus for calibrating a measurement system that will enable consistent measurements. 
   BRIEF SUMMARY 
   Disclosed herein is a measurement calibration system that compensates not only for lamp variations, but also for camera and optics variations, to allow consistent measurement to be made across various equipment. The calibration system can be readily implemented on a wide variety of precision measurement systems and can be ported from one measurement system to another. One embodiment of the measurement calibration system addresses the issue of variations between light sources by employing an automated lamp calibration system. One method of the system will calibrate existing or new lamps to match an established standard. In a preferred embodiment, the method will generate a reference table, for a measurement system defined as the standard, to act as the reference for all measurement systems. The reference table may be in the form of a data file that may be subsequently copied to other measurement systems. The reference table can then act as a global standard for other measurement systems, including future systems not yet made. These and other features and advantages of the calibration system will be appreciated from review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram depicting several components of one embodiment of a measurement calibration system. 
       FIGS. 2A and 2B  depict a process flow diagram that describes a process in which a calibration file is utilized to make consistent, precision measurements. 
       FIGS. 3A ,  3 B and  3 C depict a process flow diagram that describes one embodiment of a process for generating a revised calibration file. 
   

   DETAILED DESCRIPTION 
   In the following paragraphs, an improved measurement calibration system will be described in detail by way of example with reference to the attached drawings. Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than as limitations on the present invention. As used herein, “the present invention” refers to any one of the embodiments or equivalents of the invention described herein. 
   One embodiment of the calibration system addresses, for example, the issue of variations between light sources by employing an automated lamp calibration system. One method of the calibration system will calibrate existing or new lamps  105  to match an established standard. Preferably, the calibration operation will be capable of functioning within the range of approximately 10% to approximately 80% full-scale illumination of the lamp or lamps  105 . 
   In a preferred embodiment, the method will generate a reference table  140 , for a measurement system  100  defined as the standard, to act as the reference for all measurement systems. The reference table  140  may be in the form of a data file that may be subsequently copied to other measurement systems. The reference table  140  can then act as a global standard for other measurement systems, including future systems not yet made. 
   Once the reference table is created, the measurement system  100  can perform an adjustment operation on any lamps  150 , old or new, to match their output to the reference table  140 . This is done by creating a calibration file  125  specific to each lamp  105 , and adjusting the lamp control signal to match the standard. In one embodiment, the calibration file  125  will match the output of the lamp  105  to the standard for values between approximately 10% and approximately 80% full scale. The values below approximately 10% full scale, and above approximately 80%, are reserved to allow headroom to adjust for variations and aging in individual lamps. 
   For example, after calibration, if an operator selects 44% illumination, the lamp intensity measured by the camera  110  will match the intensity specified in the reference table  140  for that command signal setting within a few A/D counts. If all systems are so calibrated, then for the example setting, the operator&#39;s system will match all systems within a few counts. This calibration feature will also calibrate for variations between cameras  110  or anything else in the optical path, ensuring that measurements will not vary from lamp to lamp, or from camera to camera, or from instrument to instrument. 
   The problem is to ensure that when measuring a given part or object, variations in lamp intensity do not cause measurement differences in otherwise identical parts. These illumination variations might occur when replacing lamps  105 , when a lamp  105  ages, or when measuring otherwise identical parts on different measurement systems  100 . What is true for lamps  105  is also true for cameras  110  and optics. One camera  110  may not yield the same results as another on a given machine. In a preferred embodiment, the calibration process will be semi-automated or automated. 
   The calibration system  100  can also be retrofitted into existing measurement systems to perform the new calibration process. Generally, the existing lamp control and bulb lamphouse will be replaced with a 12 bit DAC board, and two or more commercial, CE certified lamphouses, such as FOSTEC lamps with remote 0-5 VDC analog control inputs, will be installed. One embodiment will provide an operator command control comprising a 0-100% on/off control with a resolution of 1 part in 100, and transparent to the operator, the lamp control will be mapped to 4,095 (12 bit) DAC values. This permits the lamp  105  to be controlled by the operator or software with precise resolution. This gives backward compatibility to users and preserves the existing interface, but it allows the software to “map” new lamps or re-map aging lamps so they will all behave per the desired measurement standard. 
   Another embodiment of the calibration system uses one or more neutral density filters and/or gray reflective standards to assist in calibrating the lamps  105 . The filters will attenuate the lamp  105  output at higher intensities that might otherwise saturate the camera  110 . Intense back-light illumination will be viewed through neutral density filters. Intense top light illumination will be viewed from a calibrated gray reflector. In one embodiment, the exact values and natures of these filters and reflectors will be determined experimentally. 
   One aspect of the improved calibration system is depicted in FIG.  1 .  FIG. 1  is a block diagram depicting some of the relevant components of a measurement system. Some of these components, such as the lamp  105 , camera  110 , stage  115  and monitor  120 , are implemented as hardware devices. Other components, such as the calibration file  125 , the digital analog converter  130 , the image processor  135 , the reference table  140 , the comparator  145  and the updated calibration file  150 , can be implemented in either hardware or software means. 
   For background understanding, the operation of the measurement system will first be described with reference to FIG.  1  and  FIGS. 2A &amp; 2B . An object to be measured will first be placed on the stage  115 . Based upon the needs of the operator or other measurement necessities, the object will be illuminated with a lamp  105  to facilitate precise measurements. It is contemplated that the lamp  105  can be implemented in a variety of ways including, for example, a vertical illumination device, a ring illumination device or a back-light illumination device. To further facilitate precise measurements, the intensity of the lamp may be precisely controlled. According to one embodiment, the intensity of the lamp may be varied among 100 different set points. These set points may correspond to a linear scale, an exponential scale, or a variety of other scales depending upon the needs of the measurement system. Accordingly, the measurement system  100  will be provided with an input command signal corresponding to one of these set points ( 205 ). The command signal is mapped on to a calibration file, which includes a look-up table of input command signals and corresponding digital control values ( 210 ). For example, an input command signal of 10 may correspond to a digital control value of 369. An exemplary calibration file  125  is included in the following Table 1. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Command Setpoint 
               Digital Control Value 
               Step 
             
             
                 
             
           
          
             
                0 
                 0 
               // Step 0 
             
             
               10 
                369 
               // Step 1 
             
             
               20 
                802 
               // Step 2 
             
             
               30 
               1227 
               // Step 3 
             
             
               40 
               1716 
               // Step 4 
             
             
               50 
               2080 
               // Step 5 
             
             
               60 
               2588 
               // Step 6 
             
             
               70 
               3032 
               // Step 7 
             
             
               80 
               3759 
               // Step 8 
             
             
               90 
               4095 
               // Step 9 
             
             
               100  
               4095 
                // Step 10 
             
             
                 
             
          
         
       
     
   
   The digital control value corresponding to the command signal is subsequently provided to a digital analog converter  130  ( 215 ). The digital analog converter  130  converts the digital control value into an actual voltage to be applied to the lamp  105  ( 220 ,  225 ). In this manner, the intensity of the light produced by the lamp  105  will directly correspond to the command signal received by the measurement system  100 . The camera  110  will capture an image of the object resting on the stage  115  at the desired level of illumination ( 230 ). Depending upon the particular system, the image may be further processed by an image processor  135  so that precise measurements can be calculated ( 235 ). A monitor  120  may be utilized to facilitate measurements of the object resting on the stage  115  ( 240 ). Accordingly, it can be seen that the calibration file  125  plays a crucial role in the measurement system by determining the proper intensity of illumination to be provided by the lamp  105 . The process for generating an updated calibration file  150  will be described below. 
   To generate an updated calibration file  150 , many of the same components described above and depicted in  FIG. 1  will be used. In addition, the components connected with the dashed lines in  FIG. 1  will be utilized. Typically, a calibration file  125  comprises a plurality of command set points, as shown in Table 1 above. Therefore, the calibration process will be repeated for each desired command set point. To further illustrate this process, reference will be made to  FIG. 1 , and the process flow diagram depicted in  FIGS. 3A-3C . The process starts ( 300 ) by generating a digital control value corresponding to a desired command signal set point ( 310 ). The desired command signal set point may be provided from an existing calibration file, or it can be generated on-the-fly by a software process. The digital control value will then be provided to a digital analog converter  130  to generate a corresponding analog control value ( 315 ,  320 ). The analog control value will be applied to the lamp  105  to generate illumination at a desired intensity ( 325 ). The illumination provided by the lamp  105  will be applied to a standard that is resting on the stage  115 . The standard resting on the stage  115  can be adapted to the type of lighting that is being applied. For example, a vertical illumination arrangement will require a standard with relatively high reflectivity such as a glass surface. On the other hand, if the lamp  105  is provided as a back-light illumination source, then a reflective standard would not be desired. In fact, one or more neutral density filters may be required to be applied to the camera  110  to prevent saturation of the camera  110 . According to another embodiment, if the lamp  105  is implemented as a ring illumination, a diffuse surface having three-dimensional features may be desired as a standard. 
   An illuminated image of the standard will be captured by the camera  110  so that a corresponding image signal will be generated ( 330 ). As previously described, the signal generated by the camera  110  may be processed by an image processor  135  so as to perform processes such as frame grabbing, digitization, or selection of certain areas within the captured image for measurement. The image processor  135  can also measure the intensity of the captured image ( 335 ). This measured intensity value will be provided to the comparator  145  ( 340 ). At the same time or previous to these measurements, the system will refer to a reference table  140  to determine a reference intensity value corresponding to the current command signal ( 345 ). A representative example of a reference table  140  is depicted below in Table 2. 
   
     
       
         
             
             
             
           
             
               TABLE 2 
             
             
                 
             
             
               Command 
                 
                 
             
             
               Setpoint 
               Reference Intensity Value 
               Filter Instructions 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                0 
               0.0000 
               : 
             
             
               10 
               53.7425 
               :insert 0.5 nd filter 
             
             
               20 
               44.0083 
               : 
             
             
               30 
               150.9708 
               :insert 2.5 nd filter 
             
             
               40 
               57.3172 
               :insert 3.0 + 0.5 nd filters 
             
             
               50 
               92.1128 
               : 
             
             
               60 
               163.8533 
               : 
             
             
               70 
               47.3972 
               :insert 3.0 + 1.5 nd filters 
             
             
               80 
               69.3544 
               : 
             
             
               90 
               97.0108 
               : 
             
             
               100  
               132.7714 
               : 
             
             
                 
             
          
         
       
     
   
   The corresponding reference intensity value will also be provided to the comparator  145  ( 350 ). The comparator  145  will then compare the measured intensity value with the reference intensity value to determine if the current level of illumination provided by the lamp  105  is sufficient ( 355 ). If the measured intensity value is lower than the reference intensity value ( 360 ), then the comparator  145  will incrementally increase the digital control value so that the intensity of the lamp  105  will be incrementally increased ( 365 ). After a brief settling in time, the image intensity measurements will be repeated and another comparison to the reference value will be made ( 315 - 355 ). On the other hand, if the measured intensity value is lower than the reference intensity value ( 370 ), then the digital control value will be incrementally decreased so as to decrease the intensity of the lamp  105  ( 375 ). Similarly, after a brief settling in time, the process of measuring the intensity of image and comparing it with a reference intensity value will be repeated ( 315 - 355 ). Appropriate “centering” algorithms may be utilized to settle the lamp intensity at or near the desired reference intensity value. 
   If, however, the measured intensity value is within acceptable tolerances of the reference intensity value, then the current digital control value being applied to the digital analog converter  130  will be recorded along with its corresponding input command signal in an updated calibration file  150  ( 380 ). After this, the process repeats at the next command signal set point ( 385 ). 
   Although certain embodiments and aspects of the present inventions have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the inventions are not limited to the embodiments disclosed, but are capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims and equivalents thereof. Applicant intends that the claims shall not invoke the application of 35 U.S.C §112, ¶6 unless the claim is explicitly written in means-plus-step or means-plus-function format.