Abstract:
The present invention relates to a method for image calibration and an apparatus for image acquiring. In the method for image calibration, the image formation position for an image acquiring unit of the apparatus is calibrated according to the relative location of the image acquiring unit to a objective lens of the apparatus, wherein the relative location is determined by calculating the focus index of the image acquired by the image acquiring unit so that a clear and sharp interferogram can be obtained for three dimensional surface profile measuring. In addition, it is possible to obtain a clear and sharp image without any interference fringe outside the coherent range by adjusting the image formation position, which is capable of being utilized for two dimensional defect detection and dimension measurement.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to an image acquiring technology and, more particularly, to an apparatus and a method for image calibration and an apparatus for image acquiring. 
         [0003]    2. Description of the Prior Art 
         [0004]    A white-light interferometric system cannot be used for performing lateral measurement and defect detection because interference fringes appear in the image formed at the objective focal point due to the interferometric configuration. 
         [0005]    To overcome such a problem, U.S. Pat. No. 7,068,376 discloses an interferometric system as shown in  FIG. 1 . The optical interferometric system  1  comprises a light source  10 , a beam splitting unit  11 , an interferometric unit  12  and an image acquiring unit  13 . The light source  10  generates a light beam, which becomes a collimated light beam after it passes a collimator lens  14 . The collimated light beam is reflected by the beam splitting unit  11  to the interferometric unit  12  and thus becomes a measurement light beam and a reference light beam. The measurement light beam is incident on an object to be tested  100  and is reflected thereby back to the interferometric unit  12 . Then the measurement light beam is interfered by the reference light beam to form an interfered light beam. The interfered light beam is received by the image acquiring unit  13  after it passes the beam splitting unit  11  so as to form an image with interference fringes. 
         [0006]    In the aforementioned prior art, a phase shift method is used to calculate the three-dimensional profile of an object to be tested. Combined operation is performed on a sequence of images acquired by the phase shift method so as to obtain the direct-current (DC) component of the images. The obtained images are free of interference fringes and are used for two-dimensional lateral detection. However, the two-dimensional fringeless images are obtained after the phase shift process, which causes limited imaging range and wastes longer time. 
       SUMMARY OF THE INVENTION 
       [0007]    In one embodiment, the present invention provides a method for image calibration, comprising steps of: acquiring a sequence of interference images; obtaining a sharpness focusing index curve and a contrast focusing index curve corresponding to the sequence of interference images; obtaining a peak of the sharpness focusing index curve and a peak of the contrast focusing index curve; and performing a calibrating process to calibrate a relative position of the peak of the sharpness focusing index curve to the peak of the contrast focusing index curve so as to obtain a sequence of clear interference images. 
         [0008]    In one embodiment, the present invention provides a method for image calibration, comprising steps of: acquiring an image free of interference fringes; obtaining a sharpness focusing index curve and a contrast focusing index curve corresponding to the image; obtaining a peak of the sharpness focusing index curve and a peak of the contrast focusing index curve; and performing a calibrating process to calibrate the relative position of the peak of the sharpness focusing index curve to the peak of the contrast focusing index curve so as to obtain a clear image free of interference fringes. 
         [0009]    In one embodiment, the present invention provides a method for image calibration, comprising steps of: acquiring an interference image; obtaining a sharpness focusing index and a contrast focusing index corresponding to the interference image; recording the sharpness focusing index and the contrast focusing index; adjusting an image formation position of the interference image; and repeating the aforesaid steps to maximize the sharpness focusing index and the contrast focusing index. 
         [0010]    In one embodiment, the present invention provides an apparatus for image acquiring, comprising: a light beam generator, capable of generating a light beam; a beam splitting unit, capable of reflecting the light beam to generate a reflected light beam; an interferoscope set, capable of transforming the reflected light beam into a reference light beam and a measurement light beam incident on an object to be tested and reflected to the interferoscope set so as to combine with the reference light beam to form an interfered light beam emitted into the beam splitting unit; and an image acquiring unit, capable of receiving the interfered light beam from the beam splitting unit so as to acquire an image, the image acquiring unit being coupled to a first actuator on one side. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The objects, spirits and advantages of the embodiments of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein: 
           [0012]      FIG. 1  is a schematic diagram showing a conventional apparatus for image acquiring; 
           [0013]      FIG. 2  is a flow-chart showing a method for image calibration according to one embodiment of the present invention; 
           [0014]      FIG. 3  is a schematic diagram showing an apparatus for image acquiring according to one embodiment of the present invention; 
           [0015]      FIG. 4A  is a flow-chart showing a method for image calibration on a sequence of interference images according to one embodiment of the present invention; 
           [0016]      FIG. 4B  is a schematic diagram showing an image acquiring unit and an interferoscope set according to one embodiment of the present invention; 
           [0017]      FIG. 4C  is a graph showing a sharpness focusing index curve and a contrast focusing index curve according to  FIG. 4B ; 
           [0018]      FIG. 4D  is a graph wherein a peak of the sharpness focusing index curve and a peak of the contrast focusing index curve are aligned; 
           [0019]      FIG. 5A  is a schematic diagram showing the relation between the image and the image mask; 
           [0020]      FIG. 5B  is a frame histogram showing the relation between the frame number and the intensity; 
           [0021]      FIG. 6A  is a flow-chart showing a method for image calibration on an image according to another embodiment of the present invention; 
           [0022]      FIG. 6B  is a graph showing the change of a sharpness focusing index curve and a contrast focusing index curve according to  FIG. 6A ; 
           [0023]      FIG. 7A  is a flow-chart showing a method for image calibration on an image free of interference fringes according to the present invention; 
           [0024]      FIG. 7B  is a schematic diagram showing an image acquiring unit and an interferoscope set according  FIG. 7A ; and 
           [0025]      FIG. 7C  is a schematic diagram showing a range centered at the peak of the contrast focusing index curve. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    One embodiment of the present invention provides a method for image calibration, in which the focusing index is calculated to determine the relative position of the image acquiring unit to the image pick-up position so that a clear image with/without interference fringes is obtained. 
         [0027]    The present invention provides an apparatus for image acquiring, in which a charge-coupled device (CCD) of the image acquiring unit and the objective lens position in an interferoscope set are moved so as to directly obtain a clear image outside the coherent range for real-time two-dimensional dimension measurement. The clear image is free of interference fringes and thus can be used in auto-focusing without further processing. Therefore, the precision in auto-focusing is enhanced. 
         [0028]    One embodiment of the present invention provides an apparatus for image acquiring, in which a charge-coupled device (CCD) of the image acquiring unit and the objective lens position in an interferoscope set are moved so as to obtain a sequence of high contrast images within the coherent range for three-dimensional surface profile reconstruction using white-light interferometry. 
         [0029]    One embodiment of the present invention provides a method for image calibration and an apparatus for image acquiring, in which the imaging quality by the interferometic system is calibrated by auto-focusing. In this embodiment, a clear image can be obtained inside/outside the coherent range using a look-up table (LUT) for two-dimensional dimension measurement and three-dimensional surface profile measurement without changing the objective lens. Therefore, the interferometic system of this embodiment is two-in-one for both two-dimensional dimension measurement and three-dimensional measurement. 
         [0030]    The present invention can be exemplified by the preferred embodiment as described hereinafter. 
         [0031]    Please refer to  FIG. 2 , which is a flow-chart showing a method for image calibration according to one embodiment of the present invention. The method  2  for image calibration is characterized in that the focusing index is calculated so as to determine whether the relative position of the image acquiring unit to the objective image pick-up position is correct for further calibration. The method  2  comprises steps described hereinafter. 
         [0032]    In Step  20 , a sequence of interference images is acquired. Then in Step  21 , a sharpness focusing index curve and a contrast focusing index curve are obtained corresponding to the sequence of interference images. A peak of the sharpness focusing index curve and a peak of the contrast focusing index curve are obtained, as performed in Step  22 . At last, a calibrating process is performed to calibrate a relative position of the peak of the sharpness focusing index curve to the peak of the contrast focusing index curve so as to obtain a sequence of clear interference images. 
         [0033]    To better understand the method for image calibration according to the present invention, a method for acquiring a clear image with interference fringes by adjusting focusing index is described hereinafter. To begin with, please refer to  FIG. 3 , which is a schematic diagram showing an apparatus for image acquiring according to one embodiment of the present invention. The apparatus  3  for image acquiring comprises a light beam generator  30 , a beam splitting unit  31 , an interferoscope set  32  and an image acquiring unit  33 . The light beam generator  30  is capable of generating a light beam  90 . The beam splitting unit  31  is capable of reflecting the light beam  90  to generate a reflected light beam  91 . The interferoscope set  32  is capable of transforming the reflected light beam  91  into a reference light beam  92  and a measurement light beam  93 . The measurement light beam  93  is incident on an object  100  to be tested and is reflected to the interferoscope set  32  so as to combine with the reference light beam  92  to form an interfered light beam  94 . The interfered light beam  94  is emitted into the beam splitting unit  31 . The image acquiring unit  33  is capable of receiving the interfered light beam  94  after it passes the beam splitting unit  31  to form an image. 
         [0034]    The image acquiring unit  33  is coupled to a first actuator  34  on one side. The image acquiring unit  33  is a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor field-effect transistor (CMOS) device. Alternatively, the image acquiring unit  33  is a detector capable of detecting light intensity. The first actuator  34  is a motor-driven actuator, a piezoelectric actuator or other devices capable of moving the image acquiring unit so as to calibrate the position of the image acquiring unit  33 . Moreover, the interferoscope set  32  is coupled to a second actuator  35  on one side. The second actuator  35  is a motor-driven actuator or a piezoelectric actuator so as to calibrate the relative position of objective lens  320  to the object  100  to be tested in the interferoscope set  32 . 
         [0035]    Please refer to  FIG. 4A  and  FIG. 3 , wherein  FIG. 4A  is a flow-chart showing a method for image calibration on a sequence of interference images according to one embodiment of the present invention. The method  4  for image calibration comprises steps as described hereinafter. 
         [0036]    To begin with, in Step  40 , a sequence of interference images is acquired. The sequence of interference images is acquired by adjusting the position of the image acquiring unit  33  so as to acquire the sequence of interference images in the coherent range. 
         [0037]    In Step  41 , a sharpness focusing index curve and a contrast focusing index curve are obtained corresponding to the sequence of interference images. Please refer to  FIG. 4B  and  FIG. 4C , wherein  FIG. 4B  is a schematic diagram showing an image acquiring unit and an interferoscope set that acquire the sequence of interference images and  FIG. 4C  is a graph showing a sharpness focusing index curve and a contrast focusing index curve according to  FIG. 4B . In  FIG. 4B ,  990  denotes an image pick-up focal point of an objective lens and  991  denotes an image formation position corresponding to  990 .  7  denotes an optical system that comprises the light beam generator  30 , the beam splitting unit  31  and part of the interferoscope set  32  in  FIG. 3 . In one embodiment, the optical system  7  is a microscope set. When the image acquiring unit  33  is not located at a proper image formation position, for example, position  992  in the figure, the image acquiring unit  33  will acquire a sequence of blur images. Meanwhile, the sharpness focusing index curve and the contrast focusing index curve are as shown in  FIG. 4C . 
         [0038]    In the present invention, the focusing index is an index for image evaluation. The sharpness focusing index curve is an index for calculating the difference of content values of adjacent pixels so as to evaluate the sharpness of the sequence of interference images. The contrast focusing index curve is an index for calculating the statistic distribution of content values of pixels so as to evaluate the contrast of the sequence of interference images. Both the sharpness focusing index curve and the contrast focusing index curve can be obtained by using a spatial frequency distribution identification rule such as multi-coefficient correlation, image differentiation, depth of peaks and valleys, image contrast, histogram or frequency-domain analysis. 
         [0039]    As shown in  FIG. 5A , the image sharpness is calculated by performing spatial convolution of a cross-shape mask  20  and an image  21 . The convolution sum is the sharpness value of the image  21 . More particularly, the mask  20  is symmetrical in shape and deploys the weighting values a 1 ˜a 8  and the weighted positions. Convolutions of the weightings on the mask  20  and the pixel values corresponding to weighted positions on the image  21  are performed and the convolution results are summed. Then, the same operation is performed after the mask  20  is moved to a next position where operation is to be performed. The sharpness value of the image  21  is thus obtained by summing all the convolution results. Moreover, the sharpness value of the image  21  can also be obtained by analysis and calculation over the frequency domain. 
         [0040]    As shown in  FIG. 5B , the contrast is the width of intensity distribution of pixels, i.e., the width  16  of the frame histogram, which is the distance between the smallest pixel value  15   a  and the largest pixel value  15   b , filtered by a threshold value Nth to filter out the pixel values with pixel number below the threshold value Nth. The image intensity is obtained by analysis and calculation of the pixel intensity, for example, the sum of the pixel intensities or the average intensity of adjacent pixels in an image region. The image intensity is the average luminance or the maximum luminance of the pixels in an image. 
         [0041]    In Step  42 , a peak of the sharpness focusing index curve and a peak of the contrast focusing index curve are obtained. Please refer to  FIG. 4C , wherein FH denotes the contrast focusing index curve of the sequence of interference images, FT denotes the sharpness focusing index curve of the sequence of interference images and H denotes the peak of the contrast focusing index curve. The peak position indicates the position where the sequence of interference images appears. T denotes the peak of the sharpness focusing index curve. The peak position indicates the position where the sequence of clear images appears. When the interferometic system is correctly configured, H and T appear at the same position where the sequence of clear interference images appears. When the interferometic system is not correctly configured, H and T appear at different positions, as shown in  FIG. 4B . 
         [0042]    To obtain the sequence of clear interference images, Step  43  is performed so as to set up a relation of a distance between an image formation position and an image pick-up position to a distance between the peak of the sharpness focusing index curve and the peak of the contrast focusing index curve. To realize Step  43 , the image acquiring unit  33  in FIG.  3  is moved and the change of the distance between the peak H of the contrast focusing index curve FH and the peak T of the sharpness focusing index curve FT is observed. By repeating the aforementioned steps, the relation can be acquired. 
         [0043]    Finally, Step  44  is performed. The distance between the image formation position and the image pick-up position is adjusted according to the relation so that the peak position of the sharpness focusing index curve is identical to the peak position of the contrast focusing index curve, as shown in  FIG. 4D . Therefore, a sequence of clear interference images can be acquired for three-dimensional profile reconstruction. 
         [0044]    More particularly, to calibrate the image formation position is to calibrate the position of the image acquiring unit  33 . To calibrate the image pick-up position is to calibrate the position of the objective lens  320  of the interferoscope set  32 . In other words, the peak H and the peak T can be aligned so as to acquire the sequence of clear interference images by adjusting the position of the objective lens  320  or the position of the image acquiring unit  33  in the apparatus  3  for image acquiring in  FIG. 3  according to the relation obtained in Step  43 . Furthermore, in Step  43 , a relation table can be set up so that the position of the image acquiring unit  33  and the position of the objective lens  320  of the interferoscope set  32  can be automatically calibrated to align the peak H and the peak T according to the information stored in the relation table. 
         [0045]    Please refer to  FIG. 6A  and  FIG. 3 , wherein  FIG. 6A  is a flow-chart showing a method for image calibration on an image according to another embodiment of the present invention. The method  5  for image calibration comprises steps as described hereinafter. 
         [0046]    To begin with, in Step  50 , an interference image is acquired. The Step  50  is similar to Step  40 , thus detailed description is omitted. Then in Step  51 , a sharpness focusing index and a contrast focusing index are obtained corresponding to the interference image. In Step  52 , the sharpness focusing index and the contrast focusing index are recorded. In Step  53 , an image formation position of the interference image is adjusted. More particularly, in Step  53 , the position of the image acquiring unit  33  is adjusted. Please refer to  FIG. 6B , which is a graph showing the change of a sharpness focusing index curve and a contrast focusing index curve according to  FIG. 6A . The aforesaid steps are repeated to maximize the sharpness focusing index and the contrast focusing index, as performed in Step  54 , so as to acquire a clear interference image for three-dimensional profile reconstruction using white-light interferometry. 
         [0047]    Please refer to  FIG. 7A  and  FIG. 3 , wherein  FIG. 7A  is a flow-chart showing a method for image calibration on an image free of interference fringes according to the present invention. The method  6  for image calibration comprises steps as described hereinafter. 
         [0048]    To begin with, in Step  60 , the position of the image acquiring unit  33  and the position of the interferoscope set  32  are calibrated by moving an image pick-up focal point of the objective lens  320  of the interferoscope set  32  to an image pick-up position  993  outside a coherent range where interference takes place in an image pick-up field of view so as to acquire an image free of interference fringes. Please refer to  FIG. 7B , in which the position of the image acquiring unit  33  is calibrated by adjusting the image acquiring unit  33  to move in a range R 1  around an imaging focal point  991 .  7  denotes an optical system that comprises the light beam generator  30 , the beam splitting unit  31  and part of the interferoscope set  32  in  FIG. 3 . In one embodiment, the optical system  7  is a microscope set. The position of the interferoscope set  32  is adjusted by moving the image pick-up focal point of the objective lens  320  of the interferoscope set  32  outside a coherent range R 0  where interference takes place in an image pick-up field of view so as to acquire a clear image without interference fringes. 
         [0049]    In Step  61 , a sharpness focusing index curve and a contrast focusing index curve corresponding to the image without interference fringes are obtained. Then in Step  62 , a peak of the sharpness focusing index curve and a peak of the contrast focusing index curve are obtained. A relation of a distance between an image formation position and an image pick-up position to a distance between the peak of the sharpness focusing index curve and the peak of the contrast focusing index curve is set up, as performed in Step  63 . Steps  61  to  63  are performed similarly to the previously disclosed, and thus detailed description thereof is omitted. 
         [0050]    After the relation is set up, Step  64  is performed to determine a range corresponding to the peak position of the contrast focusing index curve as a center. Please refer to  FIG. 7C , which is a schematic diagram showing a range D centered at the peak H of the contrast focusing index curve FH. The size of the range D is determined according to the user&#39;s need. 
         [0051]    Finally, Step  65  is performed to adjust the distance between the image formation position and the image pick-up position according to the relation so that the peak T of the sharpness focusing index curve FT is outside the range D so as to exhibit the clear image free of interference fringes for two-dimensional defect detection and dimension measurement. 
         [0052]    According to the above discussion, it is apparent that the present invention discloses a method for image calibration and an apparatus for image acquiring, in which a clear image can be obtained inside/outside the coherent range using a look-up table (LUT) for two-dimensional dimension measurement and three-dimensional surface profile measurement without changing the objective lens. Therefore, the present invention is novel, useful and non-obvious. 
         [0053]    Although this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art. This invention is, therefore, to be limited only as indicated by the scope of the appended claims.