Abstract:
Multiwavelength interferometric images have the phase and/or frequency of the illuminating light corrected by statistically analyzing the data, and adjusting the phase and/or frequency until a statistical measure reaches a criterion.

Description:
FIELD OF THE INVENTION  
       [0001]     The field of the invention is the field of measuring surface topography of an object.  
       BACKGROUND OF THE INVENTION  
       [0002]     Interferometry has been used for over a century to measure the surface topography of objects, typically optical components, and distances and small changes in such distances. With the advent of lasers having long coherence lengths and high brightness, the field has expanded greatly. Interferometric imaging, as depicted by  FIG. 1 , has been difficult to implement for objects with surfaces with steps or slopes greater than a half wavelength of light per resolution element of the imaging system, because the phase count is lost, and the height of the surface is known only modulo λ/2, where λ is the wavelength of light used for the interferometer.  
         [0003]     If a series of interferograms are recorded with different wavelengths λ i , the ambiguity in the phase may be resolved, and the heights on the object surface relative to a particular location on the particle surface may be calculated, as is shown in the patents cited below.  
       RELATED PATENTS AND APPLICATIONS  
       [0004]     U.S. Pat. No. 5,907,404 by Marron, et al. entitled “Multiple wavelength image plane interferometry” issued May 25, 1999;  
         [0005]     U.S. Pat. No. 5,926,277 by Marron, et al. entitled “Method and apparatus for three-dimensional imaging using laser illumination interferometry” issued Jul. 20, 1999;  
         [0006]     U.S. patent application Ser. No. 10/893,052 filed Jul. 16, 2004 entitled “Object imaging system using changing frequency interferometry method” by Michael Mater;  
         [0007]     U.S. Patent Application 10/349651 filed Jan. 23, 2003 entitled “Interferometry method based on changing frequency” by Michael Mater;  
         [0008]     U.S. Patent Application filed Jul. 14, 2005 by inventors Jon Nisper, Mike Mater, Alex Klooster, Zhenhua Huang entitled “A method of combining holograms”;  
         [0009]     U.S. Patent Application filed Jul. 29, 2005 by inventor Mike Mater et. al entitled Method for processing multiwavelength interferometric imaging data”.  
         [0010]     The above identified patents and patent applications are assigned to the assignee of the present invention and are incorporated herein by reference in their entirety including incorporated material.  
       OBJECTS OF THE INVENTION  
       [0011]     It is an object of the invention to produce a method of recording interferograms and treating data to increase accuracy.  
       SUMMARY OF THE INVENTION  
       [0012]     Intensity data for m max  interferometric images of an object surface is recorded for a number m max −1 relative phase differences between an object beam and a reference beam, and statistical methods of treating the data from a plurality of pixels of the recordings is used to determine accurate values for the m max −1 phase differences used for recording the interferograms. The pixels used for the data collection are pixels which satisfy a quality criterion. The corrected phase differences are then used to produce a synthetic phase image of the object surface. Intensity data for n max  different wavelengths of light is treated in the same way to determine both the relative phase differences for each wavelength, and wavelengths more accurately than can be determined from the settings used to set wavelengths and phases. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a sketch of an interferometric imaging system.  
         [0014]      FIG. 2  is a plot of intensity measured for a single pixel vs approximate phase.  
         [0015]      FIG. 3  is a plot of a fast fourier transform of frequency vs phase from data like  FIG. 2 .  
         [0016]      FIG. 4  is a block diagram of the method of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     A number of n measurements for synthetic holography at each of a number m of wavelengths λ n  of light are made to determine the phase of light scattered from an object and received at an image receiver such as film, or an electronic CMOS or CCD array detector.  FIG. 1  shows the interferometric imaging diagram. A light source  10 , preferably a laser such as a gas laser, solid state laser, semiconductor injection laser, or a device for parametric generation of coherent light produces light output which is converted into a beam  12  by a lens  11 . The beam  12  is split by a beamsplitter  13  into two parts, one part which illuminates an object  15 , the other part which illuminates a reference surface  14 . The reference surface  14  may be a specularly reflecting surface, a diffusely scattering surface, or any combination of the two. Light scattered from the reference surface  14  and the object  15  is combined at the beamsplitter  13  and propagates to the lens  17 , which images both the surface of the object  15  and the surface of the reference surface  14  on to an image detector  16 . Preferably, an image is exposed, and the reference surface  14  in the reference beam is moved to change the relative phase of the reference beam with respect to the object beam measured at the image receiver. The image recorded for each relative phase difference is called a phase image. Preferably, a number n images are exposed, each most preferably with at least 8 bit intensity resolution for each pixel for an electronic camera. More preferably 10, 12 or 14 bit intensity resolution is used.  FIG. 2  shows a sketch of the intensity measured by a representative pixel as the reference mirror is stepped with approximately 90 degree phase change between each image. In prior art systems, a pixel is chosen as a reference pixel, and the phase data relative to that pixel is then reduced by FFT to a single phase number for each pixel with of bits. In the prior art system, the FFT is very fast, and reduces the data from, say, 84 bits to 4 bits for each pixel for each wavelength λ n . Prior art computer systems could then handle the data fast enough to give reasonable results in a reasonable time. However, there is a lot of information in the bits so discarded. The following disclosure teaches how to treat the normally discarded data to improve the accuracy of the measurements. In essence, the disclosure shows how to replace expensive optical equipment with cheap computer power, which becomes cheaper by a factor 4 every 3 years.  
         [0018]     In prior art systems, light from a laser speckle pattern can spill over and contribute to the signal measured by surrounding pixels, especially if the surface is not in focus. When a pixel is was chosen as the reference pixel, the surface which has the same scatter properties as the surface imaged on to the reference pixel is well measured, but other surfaces, for example, surfaces which are closer and further from the surface than the reference pixel, or surfaces with changes in texture, is less well measured. Another problem with the prior art is that phase changes in the reference arm of the interferometer are not set accurately enough due to time lags in moving mechanical parts and hysteresis in the piezo drivers for moving the reference phase surface. If the wavelengths of the laser used to expose the interferograms is changed, it will not be set accurately enough for the same reason.  
         [0019]     The present invention solves the prior art problems by not using a single reference pixel to give a “zero” of the phase map or phase image. Rather, a set of pixels is identified as “good” pixels by some measure, and the phase and/or frequency relationship among the set of pixels is determined by an iterative or other procedure which minimizes the “error” of the entire set of good pixels. Then, the phases and/or frequencies seem by all pixels in the image are determined with respect to the set.  
         [0020]     For a single wavelength hologram, data is taken as the phase of the reference beam is changed. For each phase chosen, intensity signals are recorded from each pixel. For any particular pixel, the intensity signal will rise and fall as the object beam and the reference beam interfere constructively and destructively at the image receiver. The signal will look like a constant signal with a sine wave displaced from the origin by a phase factor, such as is shown in  FIG. 2 . The amplitude of the variation is preferably used as a measure of the “quality” of the pixel. If the signal at any phase measurement falls below the noise floor or above a saturation level, the pixel is preferably removed from consideration for the “good pixel set”.  
         [0021]     Another measure of quality of a pixel is the quality of pixels surrounding a particular pixel. A weighted average of the quality of a pixel and its surrounding pixels is preferably used to determine whether a particular pixel is to be included in the “good pixel set”.  
         [0022]     As a particular example, the amplitude of the variation of a pixel with a variation in signal between the noise floor of 50 counts and the saturation signal of 500 counts could be as much as 450 counts. Preferably, a criterion for inclusion in the set of “good pixels” is defined to be a “quality” or amplitude variation of more than 300 counts. Consider a square block of 9 pixels each of which have an amplitude count of, say, 400, and none of which has any intensity counts below the noise floor or is saturated. The pixel amplitude count is multiplied by 5 for the central pixel, by 3 for the pixels above, below, and to each side of the central pixel, and 2 for each of the pixels at the corner of the block. These numbers are added to give a “quality” count for the central pixel of 10,000. If the quality count of a pixel is above a determined number, it is added to the set of “good” pixels.  
         [0023]     Generally, there are far more pixels in the “good pixel set” than are necessary for the accuracy required. A subset of, say, 400 pixels out of, say 40,000 good pixels is preferably done by selecting every one-hundredth pixel. More preferably, the data set is segmented and pixels outside the image of the object of interest are excluded. More preferably still, pixels are selected to give the same number of pixels for each unit area of the object. Preferably, pixels are chosen with density proportional to the importance of the particular portion of the surface, where importance may be, for example, areas of the surface where tighter tolerances are required.  
         [0024]     Once the set of 400 pixels is chosen, the Fourier transform of the data for each of the set is taken, and a phase for each pixel is determined by choosing the peak position of the FFT calculated from the n pixel intensities measured at each phase position. In the prior art, for each movement of the reference mirror, the same change of phase determined by the theoretical motion of the reference surface is assigned to each pixel. In the present disclosure, the same change of phase determined by the theoretical motion of the reference mirror is used to make a first approximation of the phases of each pixel. Then a measure of the statistics of the data is taken, such as the ratio of the sum of the peak heights of the 400 pixels to the sum of the sidelobe heights, and a constant is added to the previously used phase difference, the FFT is recalculated, and the ratio is recalculated. The process is continued until the ratio drops below a determined number. In this way, the actual phase difference introduced by the movement of the reference mirror can be measured more accurately than could be predicted by calibration of the reference mirror.  
         [0025]     The statistical calculation of the phases may be done in many ways as is known in the art. The essence of the invention is to use the information heretofore discarded in reducing the data set to match the capability of the computer system used and calculate measures which improve the accuracy of the measurement.  
         [0026]     In the same way that the measured data can be used to improve the accuracy of the phase measurements for a single wavelength phase image, the data may be used to measure the wavelength change of the laser for a multiwavelength synthetic phase image. The wavelength is approximated by, for example, reading the position of a grating which is rotated to tune the illumination laser. Then a measure of the statistics of the data is taken, such as the ratio of the sum of the peak heights of the 400 pixels to the sum of the sidelobe heights, and a constant is added to the previously used frequency, the FFT is recalculated, and the ratio is recalculated. The process is continued until the ratio drops below a determined number.  
         [0027]      FIG. 4  shows a block diagram of an iterative method of determining correct phase differences for a single wavelength synthetic phase image.  
         [0028]     Use of this algorithm gives an 8:1 decrease in projected error of the height of the surface.  
         [0029]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.