Abstract:
The disclosure is directed to systems and methods for precisely measuring birefringence properties of large-format samples of optical elements. A gantry-like configuration is employed for precise movement of birefringence measurement system components relative to the sample. There is also provided an effective large-format sample holder that adequately supports the sample to prevent induced birefringence therein while still presenting a large area of the sample to the unhindered passage of light.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/419,685 filed Oct. 16, 2002. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This application relates to measurement of birefringence properties of optical elements, and primarily to large-format elements, such as large sheets of material used for liquid crystal displays (LCDs).  
         BACKGROUND  
         [0003]    Many important optical materials exhibit birefringence. Birefringence means that different linear polarizations of light travel at different speeds through the material. These different polarizations are most often considered as two components of the polarized light, one component being orthogonal to the other. Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces applied to the material.  
           [0004]    Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam traversing the sample. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient, however, to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm). An “average” birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample.  
           [0005]    Oftentimes, the term “birefringence” is interchangeably used with and carries the same meaning as the term “retardance.” Thus, unless stated otherwise, those terms are also interchangeably used below.  
           [0006]    The two orthogonal polarization components described above are parallel to two orthogonal axes, which referred to as the “fast axis” and the “slow axis” of the optical material. The fast axis is the axis of the material that aligns with the faster moving component of the polarized light through the sample. Therefore, a complete description of the retardance of a sample along a given optical path requires specifying both the magnitude of the retardance and its relative angular orientation of the fast (or slow) axis of the sample.  
           [0007]    The need for precise measurement of birefringence properties has become increasingly important in a number of technical applications. For instance, it is important to specify linear birefringence in optical elements that are used in high-precision instruments employed in semiconductor and other industries.  
           [0008]    Moreover, some applications require that the retardation measurements be made across the surface of large-format optical elements or samples. For example, a manufacturer may wish to examine the retardance across the area of a large sheet of such material, thereby to determine whether the material is satisfactory (from a birefringence standpoint) before incurring further expense in processing the panel into a plurality of units.  
           [0009]    The measurement of the birefringence across such large-format samples raises problems relating to the precise handling of the sample and instrumentation that is employed for such measurement. For example, it is impractical to move such large-format samples relative to the birefringence measurement instrument. Instead, the necessary optical components of the system can be moved relative to a stationary sample. One problem that arises with such a system is the need to ensure that components of the birefringence measurement system move precisely relative to one another and relative to the sample, thereby to provide consistently accurate birefringence measurement data irrespective of the amount the system components need to be moved in traversing large-format samples.  
           [0010]    As noted above, external forces acting on the optical element or sample can induce birefringence. Such forces arise, for example, when a sample is bent or otherwise stressed while being held. The mass of the sample can induce some birefringence as a result of gravitational force, especially in instances where the sample is oriented with a significant amount of its mass vertically aligned. Thus, accurate measurement of the intrinsic birefringence of large-format samples requires that the optical element or sample of concern be held or supported in a manner that does not induce birefringence in the sample, which would produce an erroneous measure of the intrinsic birefringence. Specifically, such support requires that a flat sample be substantially uniformly supported in a plane without stress applied to the sample.  
           [0011]    In addition to the need for adequately supporting the sample in a plane, the mechanism for supporting the sample must permit the passage of a light beam through the sample without interfering with that beam. The unhindered passage of a light beam through the sample and to an associated detection assembly is a critical aspect of accurate birefringence measurement. Moreover, it is most often desirable to measure the birefringence of a sample at closely spaced locations across the area of the sample. The design for a large-format sample holder, therefore, must strike a balance between adequately supporting the sample to prevent stress-induced birefringence, while still presenting a large area of the sample to the unhindered passage of light for birefringence measurement.  
           [0012]    Of course, the ease and cost of manufacture, as well as the requirements for shipping and assembling a birefringence measurement system that includes a large-format sample holder are also important design considerations.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention is directed to systems and methods for precisely measuring birefringence properties of large-format samples of optical elements.  
           [0014]    In one preferred embodiment, a gantry-like configuration is employed for precise Y-direction movement of birefringence measurement system components relative to the sample. The components are mounted for precise X-direction movement. Accordingly, the entire area of the sample is traversed by the birefringence measurement components.  
           [0015]    There is also provided an effective large-format sample holder that adequately supports the sample to prevent induced birefringence therein while still presenting a large area of the sample to the unhindered passage of the light beam of the birefringence measurement system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a diagram of one embodiment showing a preferred arrangement of the optical components of a birefringence measurement system that is used for measuring large-format optical elements in accordance with the present invention.  
         [0017]    [0017]FIG. 2 is a block diagram of the signal processing components of the system depicted in FIG. 1.  
         [0018]    [0018]FIG. 3 illustrates one preferred apparatus for holding a large-format optical element (sample) and for securing and moving certain of the components of the system of FIGS. 1 and 2 for measuring the birefringence at locations across the area of the sample.  
         [0019]    [0019]FIGS. 4 and 5 are enlarged, detailed sectional views of one part of the sample holder portion of the apparatus of FIG. 3.  
         [0020]    [0020]FIG. 6 is an enlarged, detailed section view showing an alternative embodiment of part of the sample holder of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]    One embodiment of a system for measuring birefringence is described with reference to FIGS. 1 and 2. The system uses a dual photoelastic modulator (PEM) setup to measure low-level linear birefringence in optical elements. This embodiment determines the birefringence magnitude and angular orientation and has specifically designed signal processing, a data collection scheme, and an algorithm for measuring low-level linear birefringence at very high sensitivity.  
         [0022]    As shown in FIG. 1, the dual-PEM setup  20  of this embodiment contains two modules. The source module comprises a light source  22 , a polarizer  24  oriented at 45 degrees, and a PEM  26  oriented at 0 degrees. The light source  22  is a polarized He—Ne laser that produces a beam having 632.8 nm wavelength and a spot size (diameter) of about 1 mm.  
         [0023]    The detector module includes a second PEM  28  that is set to a modulation frequency that is different from the modulation frequency of the first PEM  26 . The second PEM  28  is oriented at 45 degrees. The detector module also includes an analyzer  30  at 0 degrees and a detector  32 .  
         [0024]    Between the source and detector modules is a sample holder  34  (shown schematically in FIG. 1) that supports an optical element or sample  36  and is described more fully below. The vertically aligned arrows in FIG. 1 represent the path of a light beam emanating from the source  22  to pass through the sample  36  (as well as the other optical elements of the system) and into the detector  32 .  
         [0025]    With continued reference to FIG. 1, the polarizer  24  and analyzer  30  are each a Glan-Thompson-type. A Si-photodiode detector  32  is used in this embodiment. Both PEMs  26 ,  28  are bar-shaped, fused silica models having two transducers. The transducers are attached to the fused silica optical element with soft bonding material. To minimize birefringence induced in the optical element, only the transducers are mounted to the PEM housing. The two PEMs  26 ,  28  have nominal resonant frequencies of 50 and 55 KHz, respectively.  
         [0026]    With reference to FIG. 2, the electronic signals generated at the detector  32  contain both “AC” and “DC” signals and are processed differently. The AC signals are applied to two lock-in amplifiers  40 ,  42 . Each lock-in amplifier, referenced at a PEM&#39;s fundamental modulation frequency (IF), demodulates the IF signal provided by the detector  32 . In a preferred embodiment, the lock-in amplifier is an EG&amp;G Model 7265.  
         [0027]    The DC signal is recorded after the detector signal passes through an analog-to-digital converter  44  and a low-pass electronic filter  46 . The DC signal represents the average light intensity reaching the detector  32 . The DC and AC signals are recorded at different PEM retardation settings.  
         [0028]    The theoretical analysis underlying the measurement of the birefringence properties of the sample  36  in this embodiment is based on a Mueller matrix analysis and associated light-intensity signal processing to provide data representing the magnitude and angular orientation of the birefringence. Such processing does not form part of the present invention.  
         [0029]    With reference to FIG. 3, the particulars of the large-format birefringence measurement system of the present invention are now described. The birefringence measurement system includes a cabinet  49  that has a top  51 . The sample  36  is supported on the top  51  of the cabinet by the holder  34 . The sample  36  is in a large format and may be, for example, a 1250 mm×1100 mm sheet of LCD material having a thickness of about 0.5 mm. The thickness of the sample is greatly exaggerated in FIG. 3.  
         [0030]    The sample  36  remains stationary, supported by the holder  34 . In one preferred embodiment, the holder comprises a plurality of spaced-apart, taut wires  37  strung between two support beam assemblies  39 ,  41 , one beam assembly on either side of an opening  63  in the top surface of the cabinet. The particulars of the holder are described more fully below.  
         [0031]    An optical path “P” is provided between a source module  50  and a detector module  52  (FIG. 3). The source module  50  is an encasement of the components that make up that module as described above, and the detector module  52 , is an encasement of the above-described components that make up that module.  
         [0032]    The source module  50  is mounted to an upper beam member  56  that spans, in an X-direction, the width of the sample holder  34  (hence, the sample  36 ). That upper beam member is supported at its opposite ends by vertical gantry columns  58 . The beam member  56  is fastened to move with the columns in the Y-direction. Each column extends through an elongated clearance slot  60  formed near the side edges of the cabinet top  51 .  
         [0033]    The detector module  52  is mounted to a lower beam member  62  that is beneath the sample holder  34  and connected between (to move with) the gantry columns  58 .  
         [0034]    The slots  60  permit the gantry columns  58  to move in the Y-direction to span the length of the sample  36 . To this end, the lower ends of the gantry columns are mounted to a matched pair of actuators  64  (only one seen in FIG. 3) such as a ballscrew linear actuator of sufficient length to traverse the length of the sample. Suitable position sensors and processor-controlled motors are also provided for ensuring synchronous movement of the gantry columns; hence uniform movement of the source and detector modules in the Y-direction.  
         [0035]    The upper beam member  56  and lower beam member  62  are both configured to carry a servo motion control unit  66 , to which each module  50 ,  52  is connected. The units  66  include suitable encoders, and associated motion controllers for ensuring that, as respects the X-direction motion, both modules  50 ,  52  move in unison.  
         [0036]    It will be appreciated that the precisely controlled X-Y movement of the source and detector modules as described above ensures repeatable birefringence measurements. For example, such movement ensures that the optical path “P” will not change relative to the detector aperture, which change might otherwise introduce systematic errors into the birefringence measurement results.  
         [0037]    With reference to FIGS.  3 - 5 , the holder  34  includes a fixed beam assembly  39  that includes a flat base plate  70  that is attached to the top  51  of the cabinet  49 . The base plate  70  is attached near an edge of the opening  63  in the top  51 . A number of spacer plates  72  (see FIG. 3) are fixed to the upper surface of the base plate  70  to extend therefrom and support an anchor plate  74  above the base plate  70 . The anchor plate  74  is generally “L” shaped in cross section with a flat leg  76  and an up upwardly projecting flange  78 . The underside of the leg  76  is fixed to the tops of the spacer plates  72 . The uppermost edge  77  of the flange  78  is rounded.  
         [0038]    One end of each of the wires  37  mentioned above is fixed to the anchor plate  74 . In particular, the wire ends (only a single wire end appearing in FIGS. 4 and 5) pass through an aperture  80  made in the leg  76  and through a hollow, cylindrical stop sleeve  82 . The sleeve  82  is crimped to fix the sleeve to the wire end and, since the sleeve diameter exceeds that of the aperture  80 , the wire  37  can thereafter be tensed with the sleeve abutting the leg  76  of the anchor plate  74  to anchor the end of the wire. The wire  37  is drawn by the tension over the rounded edge  77  to the other beam assembly  41  described below.  
         [0039]    In a preferred embodiment the wire  37  is stainless steel wire rope that may or may not be coated with low-friction coatings such as Teflon. Nylon-coated wire rope and a number of other materials may also be used for the wires.  
         [0040]    Preferably, the diameter of the wire  37  is selected to be small enough (for example 1 or 2 mm) to minimize the amount of space across the window  63  that is occupied by the wires (and that will interfere with the light beam path “P,” FIG. 3). The wire material and the uniform spacing between each wire is selected so that, depending on the weight of the sample, sufficient tension can be placed on each wire (as described more below) to ensure that the sample is held in a plane without any bending stress, which might be introduced if the sample were permitted to sag.  
         [0041]    The spacing between individual wires  37  in the holder is as large as possible (depending upon the unit weight and flexibility of the sample) so that, as just mentioned, space across the window  63  that is occupied by the wires is minimized. The spacing between wires may be a few millimeters to several centimeters, depending, as mentioned, on the physical characteristics of the sample. Preferably, a minimum spacing (for example, 5 mm) is maintained to ensure that there remains between each wire a sufficiently large gap so that contaminants (glass particles, coatings debris etc.) that could interfere with the light beam do not become trapped between the wires.  
         [0042]    In FIGS. 4 and 5 the thickness of the sample  36  is depicted in a scale that, unlike the relatively thick sample  36  shown in FIG. 1 for illustrative purposes, reflects the relatively thin nature of at least some types of samples that are used with the present holder  34 , such as the 0.5 mm-thick LCD material mentioned above.  
         [0043]    As shown in FIG. 5, the other end of each wire  37  is connected to the tension beam assembly  41  that permits the wire tension to be established and maintained. The tension beam assembly  41  includes a flat base plate  90  that is attached to the top  51  of the cabinet  49 . The base plate  90  is attached near the edge of the opening  63  in the top  51 . A number of cylindrical spacer posts  92  are fixed at spaced-apart intervals to the upper surface of the base plate  90  to extend therefrom and support an anchor plate  94  above the base plate  90 . The anchor plate  94  is generally “L” shaped with a flat leg  96  and an up upwardly projecting flange  98 . The underside of the leg  96  is fixed to the tops of the spacer posts  92 . The uppermost edge  97  of the flange  98  is rounded.  
         [0044]    The end of each of the wires  37  is pulled over the rounded edge  97  and connected to the leg  96  of the anchor plate  94  in a manner that both anchors the end and that permits the application of tension to the wire. One way for making this connection is to employ a conventional wire end fitting, such as a stud end fitting  100  shown in FIG. 5. The stud end fitting  100  captures the end of the wire in an externally threaded sleeve  102  that threads into a hex-ended stud  104 . The threaded shaft  106  of the stud passes through an aperture in the leg  96  and through a lock nut  108  that bears against the underside of the leg. The nut is tightened once sufficient tension is placed on the wire  37 .  
         [0045]    The beam assemblies  39 ,  41  are configured and arranged so that the uppermost parts of the respective rounded edges  77 ,  97  (FIGS. 4 and 5) are in a common plane such that the taut wires  37  extending between those assemblies will hold the sample flat, without bending stress, thereby ensuring that the light beam passing through the sample is unaffected by birefringence that would otherwise be induced in the sample by such bending.  
         [0046]    It will be appreciated that in the course of manufacturing the present holder, it is only necessary to ensure that the top edges  77 ,  97  of the beam assemblies are in a common plane and that suitable tension is placed on the wires to precisely maintain the flatness of the sample that the holder supports. This can be compared to the complexities of, for example, manufacturing a large, rigid, precisely flat support plate with openings machined therethrough for permitting the passage of light.  
         [0047]    It is contemplated that, as an alternative to the taut wires  37 , other thin elongated members may be employed. For example, as depicted in FIG. 5, small-diameter cylindrical rods  110  can span the window  63 . In one such embodiment, the rods are rotatably mounted, as at bearings  112 , between members like the above discussed anchors  74 ,  94  that are mounted to opposing edges of the window  63 . The rotatable rods minimize the contact between the holder and the sample and also provide a way for easily rolling a sample onto and off the holder.  
         [0048]    It is also contemplated that the sample holder could be constructed in a manner that permits a relatively rapid application of tension to the wires and a correspondingly rapid release, thereby to facilitate assembly and disassembly of the holder as may be desired for shipping. One embodiment directed to this aspect of the invention is illustrated in FIG. 6.  
         [0049]    [0049]FIG. 6 depicts a way of anchoring the ends of the support wires  37  so that the entire set of wires can be tensioned and released by adjusting a movable tension plate  190  to which the ends are fastened. In this embodiment, the beam assembly  139  comprises a base plate  170  that is attached close to an edge of the opening  63  in the top  51 . That plate may be attached by attachment bolts  171 , for example, that can be removed to permit the detachment of the entire assembly  139  from the cabinet  49 . In this regard, a beam assembly substantially identical to the fixed beam assembly  39  of FIG. 4, or like the assembly  41  of FIG. 5, may be used on the opposite edge of the window  63  to fasten the other ends of the wires.  
         [0050]    A number of spacer plates  172  are fixed to the upper surface of the base plate  170  to extend therefrom and support an anchor plate  174  above the base plate  170 . The anchor plate  174  is generally “L” shaped with a flat leg  176  that extends inwardly beyond the spacers  172  and terminates in an upwardly projecting flange  178 .  
         [0051]    The uppermost edge  177  of the flange  178  is rounded. One end of each of the wires  37  mentioned above is passes through an aperture  180  made in the inwardly projecting section of the leg  176  and then through a hole in the center of a rigid tension plate  190  that is located between the top  51  of the cabinet and the inwardly extending part of the anchor plate  174 . The ends of the wire are captured in stop sleeves  182 , which, like sleeves  82  in the earlier described embodiment are crimped to fix the sleeve to the wire end. Similarly, since the sleeve diameter exceeds that of the aperture in the tension plate, the wire  37  can thereafter be tensioned with the sleeve abutting the underside of that plate  190 .  
         [0052]    It is contemplated that grooves, such as shown at  179  in FIG. 6, may be formed in the top edge  177  of the beam assembly (as well as in the earlier discussed edges  77 ,  97 ) and sized to receive the wires  37  thereby to permit and maintain proper spacing of the wires.  
         [0053]    A few spaced-apart tension-adjusting, shoulder-type bolts  192  are passed through clear holes in the tension plate and threaded into the base plate  170 . It will be appreciated, therefore, that the threading and unthreading of these few bolts  192  will respectively increase and decrease the tension in all of the wires  37 . It will also be understood that with the ends of the wires captured as a single set in a single rigid bar member or the like, any of a number of quick release clamping mechanisms could be used for tensioning and releasing the set of wires. Moreover, any of a number of mechanisms can be employed for securing the anchor plate  174  to the cabinet while permitting motion of the tension plate. For example, one can do away with the bolts  192  and connect, via a hinge, a long edge of the plate  190  to the cabinet or to the base plate  170 . A handle can be attached to the plate for moving the plate about the hinge to simultaneously tighten and loosen all of the wires. A toggle or latch mechanism could be included to secure the plate in the wires-tightened position.  
         [0054]    Although preferred and alternative embodiments of the present invention have been described, it will be appreciated that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents. For example, although the sample holder was discussed above in the context of a birefringence measurement system, it will be understood that the holder can be adapted for use in any of a variety of optical setups or systems.  
         [0055]    Moreover, although the focus here was on a large-format sample, it will be appreciated that the holder of the present invention will also be useable with samples of any size, including quite small ones, without the need for modifying the holder.