Abstract:
A method and apparatus for inspecting the optical quality of a reflective surface providing for the reflecting of a beam of light off the reflective surface, measuring an intensity of the reflected light at a first distance from said reflective surface, measuring an intensity of the reflected light at a second distance from said reflective surface, and comparing the intensity of the light measured at the two distances to determine the distortion of the reflective surface.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/323,539 filed Sep. 20, 2001, which is hereby incorporated by reference. 
     
    
     
       Background  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method and apparatus for measuring the optical quality of a reflective surface, for example, measuring the flatness of a sheet of glass that could contain a defective region where the surface is slightly curved.  
           [0004]    2. Background and Technical Field Of the Invention  
           [0005]    Production of tempered glass consists of heating the material to its softening temperature, then fast chilling, to introduce compressive surface stresses and increase its strength. In this process, the hot material is supported and moved in and out of the heating chamber by a set of rolls. As result of the combined actions of sag between the rolls and roller eccentricity, the glass sheet deforms slightly, acquiring a surface waviness, also called Roller Wave, as shown in FIG. 1. When installed in a building, glass exhibiting this waviness will generate distortion of reflected images and be considered defective.  
           [0006]    Several tools and methods are presently used to inspect tempered glass. With reference to FIG. 1 showing a sheet of glass  10  having a waviness, the simplest measuring tools include a depth gage revealing the depth w of the wave as a difference between peak  12  and valley  13  heights of the glass  10 . The depth w of the wave, however, does not fully describe the optical distortion.  
           [0007]    Other methods use optical means to quantify the optical distortion. With reference to FIG. 1A, devices such as that in U.S. Pat. No. 3,857,637 to Obenreder measures an angle B of a reflected beam of light  16  off of the surface  18  of the glass  10  using a beam-position sensing device  20 . The reflected beam comes from a light source LS providing a beam of light  14  directed at the surface  18 . This approach requires measuring the angle B of reflection at two or more points  22   a ,  22   b , and recording the variation of the reflected angle B and the distance d between the measured points  22   a ,  22   b , to permit the calculation of the optical distortion.  
           [0008]    U.S. Pat. No. 5,251,010 to Maltby discloses a method that eliminates the need of measuring the distances d such as that illustrated in FIG. 1B. Maltby discloses methods whereby two parallel beams of light, which can be split from a single light source LS with partial mirrors  23  as shown, separated by a known distance d, are reflected off the inspected surface  18  and sensed by two position-sensing devices  20 a, 20 b. As result of the curvature due to the roller wave, these two beams  16 a, 16 b diverge or converge, and the change in the angle B between these beams provides the measure of the distortion.  
           [0009]    A nearly identical device is described in U.S. Pat. No. 5,122,672 to Mansour and U.S. Pat. No. 5,210,592 to Betschneider. The two-beam approach requires very accurate beam-position detectors and does not account for the difficulties in measuring the beam position when the beam shape becomes irregular as result of the surface curvature. Another approach, described in Redner &amp; Bhat, “New Distortion Measuring Method Using Digital Analysis of Projection-Moire Patterns” SAE Transactions, 106 (6) 1997, uses the image of a Moire screen projected on a master, forming Moire fringes that reveal changes in magnification due to local curvature of the inspected item. The application of this method is also described in U.S. Pat. No. 5,128,550 to Erbeck. Another method based on measurements of the dimensional size of the reflected beam is described in U.S. Pat. No. 4,585,343 to Schave. In this method, edges of the reflected beam are located using an array of detectors. This method is essentially equivalent in performance to the two-beam method in U.S. Pat. No. 5,251,010 discussed above since the distance between the two edges of the reflected beam is used to measure angular changes. More recently, an approach proposed in U.S. Pat. No. 6,100,990 to Ladewski uses reflected images of gray-scale patterns. Assuming that the roller wave is periodic in nature, the distribution of light intensity analysis permits calculation of the optical power of the inspected surface.  
           [0010]    All of the above methods have serious limitations, conceptual or practical in nature. Common difficulties include the following:  
           [0011]    a) The measured angular deviation B is very small, and the detection of small changes in the reflected beam position cannot be accomplished accurately considering that the glass sheet vibrates as it emerges from the tempering furnace, and  
           [0012]    b) The surface curvature deforms the beam of light, making it difficult to locate its center using a position-sensing device.  
           [0013]    For at least these reasons, a new method and apparatus that overcomes the limitations of the prior methods and apparatuses is desirable.  
           [0014]    One objective of the invention is to provide for measuring optical distortion more accurately, eliminating the reflected beams position-sensing detectors used in the above described devices. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention provides a method for inspecting the optical quality of a reflective surface, such as flat sheet of glass. The method can include the following steps: (a) reflecting a beam of light off of the reflective surface; (b) measuring an intensity of the reflected light at a first distance from said reflective surface; (c) measuring an intensity of the reflected light at a second distance from said reflective surface, where the first distance is different than said second distance; and (d) comparing the light intensity measured in paragraph b with the light intensity measured in paragraph c to determine the distortion of the reflective surface.  
           [0016]    A device for carrying out the method is also provided. The device includes a light source for directing a beam of light towards the reflective surface from which the beam of light is reflected. A first photodetector measures the intensity of the reflected beam of light at a first distance from the reflective surface. A photodetector, preferably a second photodetector, measures the intensity of the reflected beam at a second distance which is further from the reflective surface than the measurement made by the first photodetector. A readout system receives the measurements made by photodetectors and indicates the optical quality. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic diagram showing roller-wave distortion;  
         [0018]    [0018]FIG. 1A is a schematic view of a known optical measuring method;  
         [0019]    [0019]FIG. 1B is a schematic view of another known optical measuring method;  
         [0020]    [0020]FIG. 2A is a schematic view of a reflection of light from a flat surface for purposes of demonstrating the principles of the invention;  
         [0021]    [0021]FIG. 2B is a schematic view of a reflection of light from a curved surface for purposes of demonstrating the principles of the invention;  
         [0022]    [0022]FIG. 2C is a schematic view of a reflection of light from a curved surface being measured for purposes of demonstrating the principles of the invention;  
         [0023]    [0023]FIG. 3 is a schematic view of a preferred embodiment of the invention;  
         [0024]    [0024]FIG. 4 is a schematic view of another preferred embodiment of the invention; and  
         [0025]    [0025]FIG. 5 is a schematic view illustrating the use of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    As discussed above and illustrated in FIG. 1, glass undergoing the heat-tempering process deforms out of its plane, forming a wavy pattern called “Roller Wave”. The reference letters and definitions relevant to FIG. 1 are defined as follows: 
         [0027]    L=wavelength  
         [0028]    w=peak-to-valley depth  
         [0029]    R=radius of curvature (Note: F=R/2)  
         [0030]    D=Optical power (Note: D=1/F)  
         [0031]    The peaks  12  and valleys  13  of the waviness are spaced by a distance L, called wavelength. Light reflection of the convex and concave regions introduce a distortion of reflected images, similar to the action of cylindrical mirrors. The image distortion depends of the local radius of curvature R and of the focal length F, related to R. Typically, a nearly perfect glass sheet has a radius of curvature larger than 100 meters, but a defective item could have locally a radius 10 meters or smaller. The optical effect of the local curvature is best described by the optical power D, related to the focal length and radius of curvature by: 
           D= 1/ F= 2/ R   
         [0032]    The distortion introduced by a wavy surface can be evaluated quantitatively, measuring the optical power D. For example, measuring peak-to-valley depth w and the wavelength L yields the distortion, as shown in the Strainoptic Technologies Inc. Instruction Manual for Maintenance and Use of RWG Roller Wave Gage, and in the U.S. patents discussed above, and which describe the use of a reflected beam of light, or a pair of beams, with position-sensing detectors measuring the angular deviation of these beams. An equivalent result is obtained by the Schave reference, which described a device detecting the position of edges of a reflected beam.  
         [0033]    The present invention eliminates the need for reflected beam position-sensing detectors as used in devices described above and provide means for measuring optical distortion more accurately. Instead, the reflected beam&#39;s divergence or convergence is measured directly, using the energy density principle, which is now described with reference to FIGS.  2 A, 2 B,and  2 C.  
         [0034]    As illustrated in FIG. 2A, when a cylindrical, collimated beam of light  14  from a collimated light source LS reflects off of a perfectly flat surface  18  the reflected beam  16  remains collimated, and neglecting light losses in the air, will illuminate a target placed in its way with the same luminous intensity, regardless of the target distance (e.g., x 1 , and x 2 ) from the glass surface  18 . Thus, with reference to FIG. 2A, the illumination at a target placed in the beam  16  at a distance x 1 , will have the same illumination as at a target placed in the beam  16  at a distance x 2 , or, put another way, the energy of the beam per unit area (the light intensity=I/a which can be expressed in units of watts/area) remains constant since target illumination is independent of the distance x from the surface  18 .  
         [0035]    With reference to FIG. 2B, as a result of surface waviness, the reflected beam  16  will acquire a diverging or converging angle B (diverging being illustrated in the FIG. 2B), covering an increasing or decreasing area a, as the distance x from the inspected surface  18  increases. Since the luminous energy I will be now spread over an increasing or decreasing area a, the light intensity (I/a) becomes a function of the distance x from the inspected surface  18 , and also of the divergence angle B. Thus, with reference to FIG. 2B illustrating a diverging beam  16 , the luminous energy at a distance x 2  is spread over an area a 2  that is larger than the area a 1 , at x 1 , where the luminous energy is spread over a smaller area. Measurement of the light intensity at two points R 1  and R 2 , located at a distance x 1  and x 2  from the inspected surface  18 , provides sufficient information to calculate the optical distortion. A sample calculation is illustrated with reference to FIG. 2C.  
         [0036]    In FIG. 2C, a converging light beam  16  reflected from a cylindrical reflecting surface  18  is shown. The measuring of light intensity using a photodetector detector  28  is performed near the center of the reflected beam  16  at two points, R 1  and R 2 , where the variation due to the vibration and to the motion of the measured item is minimized, making the measuring apparatus more accurate and reproducible than the beam position sensing devices. The term photodetector as used herein is any device capable of converting light intensity into an electric signal, and includes photo electric sensors, photo diodes. As shown in the equations below, derived from the FIG. 2C, the optical distortion can be computed simply from the measured photoelectric currents i 1  and i 2  produced by the photoelectric detectors  28 , i 1 , and i 2  being the current produced by the photoelectric detectors  28  located in areas a, and a 2  respectively.  
             D   =       1   F     =     B     A   o                 (   1   )                               
 
         [0037]    where A 0 =the area of surface  18  being sampled (illuminated);  
             B   =         a   1     -     a   2       d             (   2   )                               
 
         [0038]    where a 1 , and a 2 =the illuminated areas at R1 and R2 respectively, and B is the convergence angle;  
         [0039]    Measured photoelectric currents i 1  and i 2  at points R 1  and R 2  respectively by the photodetectors  28  are proportional to the source intensity I 0 :  
         i   1     =             I   o       a   1       &amp;                     i   2       =       I   o       a   2                               
 
         [0040]    Change in the size of the illuminated areas is related to the change in measured intensities:  
                 a   1     -     a   2       =     Z   =       I   o          (       1     i   1       -     1     i   2         )                 (   3   )                               
 
         [0041]    From the geometry it can also be shown that  
               A   o     =         a   o     +     H   ·   B       =       a   o     +       H   d          (       a   1     -     a   2       )                   (   4   )                               
 
         [0042]    where a 0  is the aperture of the measuring system and H is the distance of the aperture from the surface  18 .  
         [0043]    Combining eq. 1, 2, 3 &amp; 4 yields:  
             D   =           a   1     -     a   2             a   o        d     +     H        (       a   1     -     a   2       )           =     Z         a   o        d     +   HZ                 (   5   )                               
 
         [0044]    For high-sensitivity of detection, H is small and d is large, making H/d negligible. The above equation thus reduces to:  
       D   =     Z       a   o        d                             
 
         [0045]    In essence, the new method permits measuring the optical distortion of a reflecting surface by simply measuring a differential output of two photodetectors.  
         [0046]    Apparatus and Method for Measuring Optical Distortion  
         [0047]    To better illustrate the principle of the new method and apparatus, reference is made to FIGS. 3 and 4. It is understood that these drawings are simplified, to better illustrate the methodology, and that any person skilled in the art can produce a large variety of optical element arrangements to accomplish essentially the same result.  
         [0048]    Shown in FIG. 3 is a preferred embodiment having a light source LS such as an incandescent lamp, a laser beam, a tip of a fiber-optic cable channeling the luminous energy from a remote source, or other suitable source. An incandescent point source of about  20  watts is suitable. The light source is placed in a focal plane of a lens  30 , directing a collimated beam  14  of light toward the inspected region (sample area) of the glass sheet  10 . In practice, as result of the size of the light source, the illuminating beam  14  will be slightly divergent or convergent. A divergence adjustment is provided by a focusing device  32 , such as an adjustable housing, adjusting the distance between the light source LS and the lens  30 . A diffuser  34  can be provided between the light source LS and the lens  30 . A diffused light source can provide equivalent results since the surface of the diffuser functions as an infinite number of point sources located in the same plane, each one behaving as a single point.  
         [0049]    The incidence angle “A” (see FIG. 4) between the incident beam  14  and the normal  36  to the surface  18  of the glass  10  is preferably very small, as shown in the FIG. 3. To obtain a small angle A, a beam splitter  38  is mounted to receive the illuminator beam  14  and direct this beam in a direction towards the surface  18  and perpendicular to it. An alternative design, shown in FIG. 4, incorporating a large angle “A” is equally effective, and offers an equivalent solution. This configuration eliminates light losses due to the presence of the beam splitter  38 .  
         [0050]    The light beam  14  is then reflected from the surface  18 , the reflected beam being illustrated with reference numeral  16 . To permit measurement of the light intensity at two points R 1  and R 2 , distant x 1  and x 2  respectively from the inspected surface  18 , another beam splitter, or a beam-divider cube  40  (a 50-50 divider being preferred) intercepts the reflected beam  16 , dividing the reflected beam into two beams  42  and  44  which reach the R 1  and R 2  points after traveling a distance x 1 , and x 2  respectively, where x 1 =ONR 1 , and x 2 =ONR 2 , as shown in FIGS. 3 and 4 (the distance x 1 , e.g., being the distance from point O to N to R 1 ). Since the sensitivity of detection is proportional to the distance d between the interception points x 1 , and x 2  (see FIG. 2C), it is advantageous to make the distance d as large as practical. To measure distortion between 20 and 150 mdpt (millidiopters) typically encountered when inspecting tempered glass, a 50 mm diameter beam  14  can be used, with the distance d between the paths ONR1 and ONR2 about 400 mm. The distance ON should be kept as small as practical. It is understood by those in the art that the distances from the reflective surface  18  at which the light intensity measurements are made, e.g., x 1 and x 2 , is the distance the light travels from the reflective surface  18  to R 1  and R 2 , not necessarily the actual straight line distance from the reflective surface  8  to R 1  and R 2 . For example, mirrors, in a manner known in the art, can be used to increase the distances x 1  and x 2  without increasing the actual distance of R 1  and R 2  from the surface  18 . The distances x 1  and x 2  are the distances the light travels from the surface  18  to R 1  and R 2 .  
         [0051]    To measure the light intensity at R 1  and R 2 , an aperture mask  46  having an opening of area a 0 , may be incorporated, to control the size of the measured beam, admitting only the central region of the reflected beam  16  where the uniformity of the energy distribution is better. A mask  46  having an aperture a 0  slightly smaller than the original beam  14  is suitable.  
         [0052]    Masks M 1  and M 2 , having apertures a 3  and a 4  respectively, also permit selection of the portion of the beam used for the light intensity measurements, rejecting peripheral regions that are affected by the glass motion. Aperture a 3  is preferably smaller than a 2  since the light beam at the further pont R 2  is spread out more, a suitable a 4  being about 25 mm and a suitable a 3  being about the half that size. Color selective filters F1 and F2 can be used to select a suitable range of wavelengths, especially when coated glasses are inspected. Addition of diffusers  48  and  50  provide an integrating action, further eliminating an undesired sensitivity to small displacement of the beam center due to solid-body motion. Condenser lenses  52  and  54  can be incorporated to improve the light efficiency of these diffusers. The light intensity at R 1  and R 2 , over the area (a, and a 2  in FIG. 2C) limited by the masks M 1  and M 2  is measured using suitable photodetectors PS 1 , and PS 2 , such as silicon photo diodes or any other suitable device. It is seen that each photodetector can be housed in an assembly  51  with the other related components, for example, photodetector PS 1  is in an assembly with Mask M1, filter F1, diffuser  48 , and condenser  52 .  
         [0053]    A readout system  55  in communication with the photodetectors PS 1  and PS 2  through wires  57  is provided to analyze the measurements and display the results in a desired format. For example, the readout system  55  can include a photoelectric amplifier/readout instrument  5  whereby photoelectric currents i 1  and i 2 , proportional to the light intensity at R 1  and R 2  are displayed by the photoelectric amplifier/readout instrument  56 . In addition, the readout system  55  can include a differential amplifier  58 , having an adjustable gain for calibration, which receives the output of the detectors PS 1 and PS2, measuring and displaying the difference Z between the light intensities i 1 , at the point R 1  and i 2  at the point R 2 . The analogue output of the photodetector/amplifier can be furthermore digitized, and connected to a computer  60  as part of the readout system, for data storage, graphic display of information and statistical presentation.  
         [0054]    Using simple relations of geometrical optics, illustrated in FIG. 2C, the measured optical distortion D is related to the measured difference of light intensities Z, by the following equation: 
           D−Z /( H*Z+a   0   *d ) 
         [0055]    Where H, a 0 , and d are geometrical factors defined by distances x 1 , x 2  and by the position of the instrument above the inspected surface, and H is shown in FIGS. 3 and 4. For small values of H, preferably about 50 mm, the above equation reduces to: 
           D=Z /( a   0   *d )= K*Z   
         [0056]    showing a direct proportionality between the optical distortion D and the measured output Z.  
         [0057]    The device of the present invention should preferably be calibrated before use. For example, this can be done by using the device to measure a defect free flat piece of glass and adjusting the device so that the difference between the two currents i 1  and i 2  is zero to indicate an absence of optical distortion, e.g., adjusting the amplifier  56 . The proportionality constant K=1/a 0 *d is measured in a calibration experiment, using a surface with a known radius of curvature, the device then being calibrated, for example, by adjusting the gain of the differential amplifier  58 , or by a suitable software procedure for calibration.  
         [0058]    The present invention also permits measurement of curvature in arbitrarily selected planes of the surface  18  being measured. This selection can be made using masks M 1  and M 2 , shown in FIGS. 3 and 4, with a slit shaped aperture that permits sensing of light intensity variations due to divergence or convergence in a plane parallel to the slit only. For example, when sensing a cylindrical roller wave curvature, the sensitivity to the curvature in the plane perpendicular to the roller wave can be decreased or increased, depending on the test objectives, by placing the slit parallel to the direction of the rolls.  
         [0059]    As shown in FIG. 3, the various components can be mounted within a housing  62  configured to minimize background light from entering the housing and interfering with the measurements. The light source LS can be mounted in an adjacent housing  64 , such as an electrical box, mounted on the outside of the housing  62  to allow convenient access to the light source, such as a bulb, for easy changing without having to open up the main housing  62  An opening in the side of the housings  62  and  64  between the two allows the light to enter the main housing  62 . A window  66 , preferably of flat glass, allows the light beam  14  to leave the housing  62 , reflect off surface  18 , and reenter the housing  62  for measurement. The window  66  is preferably angled slightly as shown to eliminate undesirable reflections of light. Adjacent the beam splitter  38  is an anti-reflecting surface  68  to eliminate stray light that may pass through the beam splitter  38  from the light source LS.  
         [0060]    It is seen that the apparatuses of FIGS. 3 and 4 can inspect the optical quality or distortion of a reflective surface  18  at a single point or region of the sheet  10  (off-line, or can inspect a selected line along the sheet  10  as the sheet  10  is moved relative to the apparatus (on-line). For example, illustrated in FIG. 5 is an apparatus  70  similar to that described with reference to FIG. 3 positioned above a roller bed  72 . The sheet  10  to be inspected moves on the roller bed  72  beneath the device  70  as the device  70  inspects the surface  18  along a line on the surface  18 . Alternatively, the sheet  10  can be placed on a flat table  74  for inspection at specific points or regions on the surface  18 .  
         [0061]    While particular embodiments of the invention are described herein, it is not intended to limit the invention to such disclosure and changes and modifications may be incorporated and embodied within the scope of the appended claims.