Abstract:
Methods for creating reference images of fiber optic sensor plates for use in electron microscopes. The methods include taking of reference images of stripe or dot patterns. The spatial frequency of the stripe or dot patterns is such that image artifacts of the fiber optic stacks is recorded. The reference images can then be used to correct for these artifacts.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This utility application claims the benefit under 35 U.S.C. §119(e) of Provisional Application Ser. No. 60/974,215 filed on Sep. 21, 2007 and entitled Method for Creating Reference Images in Electron Microscopes, the entire disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to methods and apparatuses for correcting imaging artifacts in an electron microscope. More specifically, the invention relates to an improved method of generating reference images that capture information regarding imaging artifacts that result from distortions in fiber optic bundles that connect a scintillator to a charge coupled device for imaging. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    In an embodiment of the invention, a method for recording distortions in a fiber-optically coupled imaging device having a fiber-optic sensor stack includes the steps of placing an image of a stripe or 2-dimensional dot pattern of uniform spatial frequency in a position to be imaged by the imaging device and recording the image of a stripe or dot pattern. 
         [0004]    In a further embodiment, the uniform spatial frequency of the stripe or dot pattern is between 0.25 and 0.5 of the size of the image pixel resolution of the imaging device. 
         [0005]    In a further embodiment, the image of a stripe or dot pattern is on an opaque material on glass and the method included the additional step of illuminating the image of a stripe or dot pattern with visible light prior to the application of an electron-sensitive phosphor onto the fiber-optic plate. 
         [0006]    In a further embodiment, a 1-D or 2-D periodic image is produced with electrons and imaged onto a sensor after a scintillator has been mounted onto the fused fiber-optic plate. In a further embodiment of producing the periodic image with electrons, the image of a stripe or dot pattern is produced by a perforated metal foil. 
         [0007]    In a further embodiment a uniformly illuminated image is made as a gain reference to remove fixed-pattern gain variations from images of the stripe or dot pattern. 
         [0008]    In a further embodiment the pattern is a stripe pattern wherein two images are recorded of said stripe pattern, and wherein said two images are recorded at an angle between each other, said angle being sufficient to record distortions in two dimensions. 
         [0009]    In a further embodiment the angle between the stripe patterns is 90 degrees. 
         [0010]    In a further embodiment the stripe pattern is tilted away from horizontal and vertical to prevent aliased harmonics of the strip pattern from overlapping a spatial frequency range around the fundamental spatial frequency of the stripe pattern which contains distortion information. 
         [0011]    In a further embodiment the stripe or dot pattern comprises opaque material and said opaque material varies sinusoidally in optical density thereby minimizing the intensity of aliased frequencies. 
         [0012]    In a further embodiment the fiber-optic sensor stack is comprised of fiber-optic plates and said fiber-optic plates are fixed to each other prior to recording of the distortion pattern so that the distortion pattern remains constant throughout the service life of the sensor. 
         [0013]    In a further embodiment, a method of correcting optical artifacts in a fiber-optic stack includes the steps of: placing an image of a stripe or dot pattern of uniform spatial frequency in a position to be imaged through the fiber-optic stack; recording the image of a stripe or dot pattern and using the recorded image as a reference image to correct subsequent images made through the fiber-optic stack. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: 
           [0015]      FIG. 1  is a diagram of a prior art image sensor for an electron microscope; 
           [0016]      FIG. 2  is diagram of a fused fiber optic plate; 
           [0017]      FIG. 3  is an image of a Ronchi stripe pattern as transferred to an image sensor revealing the distortion caused by the fused fiber optic plate. 
           [0018]      FIG. 4  is an illustration of a one-dimensional periodic pattern, also known as a Ronchi stripe pattern;  FIG. 4   a  is an illustration of a two dimensional periodic pattern; 
           [0019]      FIG. 5  is a flow diagram of an exemplary process for making a reference image; 
           [0020]      FIG. 6  is an illustration of an FFT of a reference image made with Ronchi stripes at −22.5 degrees showing sidebands containing distortion information and aliased higher-order frequency components of the square-wave Ronchi pattern; 
           [0021]      FIG. 7  is an illustration of an FFT of a reference image made with Ronchi stripes at 67 degrees; and 
           [0022]      FIG. 8  is an illustration of the distortion in  FIG. 7  reconstructed at least to  4 -pixel resolution by an inverse FFT followed by phase extraction. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    As shown in  FIG. 1 , high-sensitivity prior art digital cameras for transmission electron microscopes use fused fiber-optic plates  20  to transfer an image generated in a scintillator  10  by the electron beam  30  to the image sensor  40  a CCD or CMOS detector. The fiber plate  20  is located between the scintillator  10  and the image sensor  40 . The combination of the scintillator, fiber optic plate and image sensor has many advantages well known in the art, including the fact that the fiber-optic place accepts even high-angle photons  31  generated in the scintillator, which generates many photons  32 ,  33  from each high energy electron that reaches the scintillator. The fiber optic plate also stops the electrons e—before they reach the CCD. Prior art patents describing the use of fiber optic plates include U.S. Pat. Nos. 5,065,029, 6,455,860 and 7,091,492, the disclosures of which are incorporated herein by reference. 
         [0024]    As shown in  FIG. 2 , in a fused-fiber-optic plate, individual fibers  210  are stacked in bundles, drawn thin and then restacked, resulting in some distortion, evident in the gapped area in the center of the figure. 
         [0025]    As shown in  FIG. 3 , an image of a stripe pattern imposed on a fiber optic bundle plate makes distortions  310  visible in a fiber-optic coupled to a CCD. The inventor has discovered that distortions in fiber optic plates can be measured and corrected by the following exemplary methods. 
         [0026]    Distortion in a fiber optic bundle is measured by making a Ronchi ruling image on the device. Ronchi rulings ( FIG. 4 ) are equal bar and space square wave targets, having very high contrast ratio and edge definition. According to an exemplary method, described in flow diagram in  FIG. 5 , a Ronchi ruling with constant spatial frequency (straight stripes with constant even spacing and equal amounts of area covered and uncovered) is selected. At method step  510 , a repeat distance for the Ronchi ruling is chosen that is larger than two pixels and smaller than four pixels. At step  520 , the Ronchi ruling is tilted so that aliasing of the harmonics of the stripe pattern do not fall near the fundamental frequency of the stripe pattern. At step  530 , two images are taken 90 degrees from each other, since the image of a stripe only measures distortion perpendicular to the stripe. At step  540 , a third image is taken with no Ronchi ruling to use to correct sensitivity variations across the imager. At step  550 , the images mage in step  520  are gain compensated using the reference image made in step  540   
         [0027]    The method shown in  FIG. 8  is one of a number of ways to use the Ronchi images to correct distortions. In some cases, this is the best method, in others, it isn&#39;t. For applications of electron microscopes which are interested in deriving the phase of periodic objects, such as electron holography or strain mapping this method or an extension of this method to full resolution are the appropriate method. For applications interested in image distortions in particular, such as membrane segmentation, electron tomography or spectroscopy, distortion correction using these reference images would be built into application processing. 
         [0028]    As seen in  FIGS. 6 and 7 , Information about distortions is contained in the sidebands of the Ronchi fundamental frequency. Information extends to frequencies beyond the Ronchi frequency away from the fundamental because the stripe is a continuum object in the direction of the stripe. Harmonics  610 ,  710  of the Ronchi fundamental are distant from the fundamental due to the tilt of the Ronchi ruling. This allows the information in the sidebands to be extracted separately from the aliased information in the Ronchi harmonics. 
       Partial Phase Reconstruction Example 
       [0029]      FIG. 8  shows how phase, which is proportional to distortion perpendicular to the Ronchi stripes, can be reconstructed at least to 4-pixel resolution by an inverse FFT followed by phase extraction. 
         [0030]    In an alternative embodiment, it is also possible to measure distortion in both directions with a single image if the Ronchi pattern is 2-dimensional.  FIG. 4   a . However, the drawback of the two-dimensional pattern is that there are more harmonics to keep away from the sidebands. In addition, the continuum is lost in the stripe-parallel direction so some high-frequency information is lost from the sideband. However, it is difficult to make an electron-transmitting Ronchi mask which is one-dimensional so the two-dimensional array of perforations is preferred with electron illumination. 
       Making the Ronchi Image 
       [0031]    Once a phosphor has been applied it is not possible to use light to create the Ronchi images. There are two options: 1) Take the images using a light-optical mask prior to phosphor coating. It is then necessary that the fiber-optic stack remains unchanged for the life of the detector. 2) Use a perforated mask or other means to create a Ronchi or  2  dimensional periodic image using electrons. This has the disadvantages noted above but is workable when necessary. 
         [0032]    While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.