Abstract:
Disclosed is an apparatus and method for characterizing attributes of a moving glass sheet comprising complementary mechanical material handling technologies that progressively stabilize, position, capture, flatten, and release the lower portion of glass sheets traveling past the apparatus while posing minimal constraint on the top section of the sheet. The apparatus includes a pressure-vacuum (PV)-type device comprising distinct regions such that the glass sheets experience a non-contact but gradual increase in constraining force until the point where measurements can be performed, then a gradual decrease in constraining force until the glass sheets are released from the inspection station. This graduated force technique is applied along the direction of travel of the sheets and may also be applied vertically upwards along the height of the sheet to restrict the motion of the sheet without constraining it at pinch points near the conveyor.

Description:
This application is a divisional of U.S. patent application Ser. No. 13/591,994 filed on Aug. 22, 2012, now U.S. Pat. No. 8,773,656 which in turn claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/526,860 filed on Aug. 24, 2011 the contents of which applications are relied upon and incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to an apparatus and method for characterizing glass sheets, and in particular to an apparatus adapted to measure one or more selected attributes of a glass sheet while the glass sheet is in motion. 
     2. Technical Background 
     The invention relates to controlling motion and attitude of a glass sheet being conveyed along a predetermined path to enable high resolution online measurements, such as topography inspection (e.g. nanotopography, or topography on a nanometer scale). Accurate online measurements of thin glass sheets, and in particular the measurement of nanometer-scale features, is highly dependent upon consistently presenting the glass sheets in a predetermined plane at a predetermined orientation and eliminating the vast majority of vibrations and oscillations of the glass. Challenges such as high accuracy measurement of thickness, waviness, and cord are highly dependent on presenting the glass surface to measurement gauges with reproducible, high tolerance material handling. 
     Online process control and quality measurements, such as online cord and streak inspection, suffer from a lack of repeatability when performed while simply gripping the glass in a non-quality area. A large part of the material presentation challenge is keeping the sheet of glass moving down a production line while being suspended from a carrier on an overhead conveyor. This constraint often forces measurements to be performed either in a separate inspection portion of the process line, completely offline, or the measurement technology compatible with coarse traditional online material handling is limited in its performance (e.g. resolution). 
     High resolution metrology is performed online in other industries, such as silicon wafers or paper and plastic webs, but in these cases, the product is in direct contact with a supporting plate, as with wafers, or with rollers, as with most webs. The size and contact prohibition on glass sheets suitable for display applications presents a difficult challenge for handling. 
     SUMMARY 
     Measurement of thin glass sheets, generally equal to or less than 1 mm in thickness, may exhibit an amount of curvature or warping that creates difficulty when measuring certain attributes of the glass, particularly if the glass is large (e.g. greater than about 4 m 2 ). To overcome this deficiency, the glass must first be flattened. In the past, flattening and stabilizing the glass sheet has involved removing the sheet from the inline path, transferring the glass sheet to a precision granite base, then vacuuming individual glass sheets to a vacuum table, making the desired measurements, removing the glass sheets, and then performing the same operation with another glass sheet. Such a piecemeal approach adds considerable time and expense to a manufacturing process. The challenge of measuring large thin glass sheets is exacerbated if there is a need to measure the glass sheet while the glass sheet is transported along a manufacturing line. 
     In some manufacturing processes, glass sheets can be conveyed from one location to another location by clamping the glass sheet to a moving member in an overhead conveyor. It would be beneficial if one or more of the aforementioned characterizations could be accomplished while the glass sheet was in motion, without first dismounting the glass sheet and positioning the glass sheet on a measurement table as a stationary object. 
     To that end, an apparatus is disclosed for making precision measurements of moving glass sheets, such as glass sheets suitable for use in a liquid crystal display devices, by constraining the glass sheets while still held by a conveyor carrier. The material handling features of the apparatus include air knives and pressure-vacuum (PV) air bearings, arranged in linear fashion such that a glass sheet entering the apparatus is subjected to a non-contact but gradual increase in constraining force until the point where measurements can be performed. A gradual decrease in constraining force then occurs until the sheet is released from the apparatus. This graduated force technique is applied along the direction of travel of the glass sheets and also vertically upward along the height of the sheets to restrict the motion of the sheets without constraining it at pinch points near the conveyor carriers. 
     Accordingly, an air bearing is disclosed comprising an annular inner porous body portion comprising a circular groove in a surface of the inner porous body portion, and a plurality of radial grooves intersecting the circular groove, the inner porous body portion defining a central passage extending through a thickness of the air bearing; an outer porous body portion disposed about the inner porous body portion, wherein the outer porous body portion comprises a plurality of continuous grooves in a surface of the outer porous body portion; and wherein each continuous groove of the outer porous body portion comprises a plurality of vacuum ports. The circular groove of the inner porous body portion and the radial grooves of the inner porous body portion divide the surface of the inner porous body portion into a plurality of sub-surfaces, and a sub-surface of the plurality of sub-surfaces comprises a vacuum port. Preferably, each sub-surface of the plurality of sub-surfaces comprises a vacuum port. 
     The outer porous body portion preferably comprises an arcuate outer circumference, and preferably the outer porous body portion comprises a circular outer circumference such that the air bearing comprises an annular inner porous body portion and an annular inner porous body portion disposed about and concentric with the inner porous body portion. In some embodiments the air bearing comprises a plurality of inner porous body portions. For example, the plurality of inner porous body portions may be aligned along a horizontal axis. 
     In another embodiment, an apparatus for characterizing glass sheets as the glass sheets move past the apparatus is disclosed comprising: an air bearing comprising an annular inner porous body portion, and an outer porous body portion disposed about the inner porous body portion, the inner porous body portion defining a central passage extending through a thickness of the air bearing; a plurality of stabilizing air knives positioned upstream of the air bearing relative to a direction of travel of the glass sheets; and a measurement device to measure at least one attribute of the glass sheet, the measurement device being aligned with the central passage of the air bearing. The inner porous body portion comprises a circular groove in a surface thereof. The inner porous body portion may further comprise a plurality of radial grooves intersecting the circular groove. The surface of the inner porous body portion comprises a vacuum port. If the inner porous body portion comprises a circular groove and a plurality of radial grooves, the circular groove and the radial grooves define a plurality of sub-surfaces on the inner porous body portion. Preferably, each sub-surface comprises a vacuum port. 
     The outer porous body portion comprises a plurality of continuous (i.e. closed) grooves, each continuous groove comprising a plurality of vacuum ports. For example, each continuous groove can be a circular, oval, elliptical, or any other closed, continuous shape. Preferably, an outer circumference of the outer porous body portion is arcuate. For example, the outer circumference of the outer porous body portion may be circular. The air bearing may in some embodiments comprise a plurality of inner porous body portions. 
     The measurement device preferably measures the at least one attribute through the passage defined by the inner porous body portion. 
     The stabilizing air knives are oriented such that a flow of air from the stabilizing air knives is angled in a downward direction. That is, the flow of air from the stabilizing air knives is preferably directed in a downward direction relative to a horizontal plane so that the flow of air makes an acute angle with the glass sheet. For example, a direction of the flow of air may form an angle in a range from about 15 degrees to about 75 degrees relative to the glass sheet. Preferably, the stabilizing air knives are arcuate in shape. 
     The apparatus according to the present embodiment may further comprise a positioning air knife positioned downstream of the air bearing to force the glass sheet in a direction away from the air bearing. 
     In still another embodiment, a method of characterizing moving glass sheets is described comprising: moving a glass sheet in a first direction along a predetermined path, the glass sheet comprising a pair of opposing major surfaces, a bottom edge, and a leading edge relative to the first direction; dampening movement of the glass sheet in a second direction perpendicular to the first direction by passing the glass sheet between at least two stabilizing air knives as the glass sheet is moving in the first direction; engaging the glass sheet with a circular air bearing, the circular air bearing comprising an inner porous body portion and a outer porous body portion disposed about the inner porous body portion, the inner porous body portion defining a central passage therethrough; and measuring at least one attribute of the glass sheet as the glass sheet moves in the first direction. The method may further comprise guiding a bottom edge of the glass sheet with an edge guiding device comprising guide arms arranged to form a “V”-shaped slot therebetween. 
     A height of the air bearing, and more particularly a height of the outer porous body portion, is less than one half a height of the glass sheet. The air bearing is positioned such that an upper one half of the glass sheet is preferably not adjacent to the air bearing as the glass sheet is measured. The air bearing is preferably capable of maintaining the glass sheet within +/−15 μm of a predetermined distance from the inner porous body portion. 
     The first inner porous body portion comprises a circular groove in a surface thereof. The inner porous body portion preferably also comprises a plurality of radial grooves in the surface of the inner porous body portion, wherein the plurality of radial grooves intersect the circular groove. 
     The outer porous body portion of the air bearing preferably comprises a plurality of concentric grooves, wherein each groove of the plurality of concentric grooves comprising a plurality of vacuum ports. 
     In still another embodiment, a method of making a glass sheet is described comprising: heating a batch material in a melting furnace to form a molten glass material; flowing the molten glass material over converging forming surfaces of a forming body to produce a glass ribbon; cutting a glass sheet from the glass ribbon; suspending the glass sheet vertically from a conveyor, the conveyor moving the glass sheet in a first direction along a predetermined path; dampening movement of the glass sheet in a second direction perpendicular to the first direction by passing the glass sheet between at least two stabilizing air knives as the glass sheet is moving in the first direction; engaging the glass sheet with an air bearing, the air bearing comprising an annular inner porous body portion and a outer porous body portion disposed about the annular inner porous body portion, the annular inner porous body portion defining a central passage therethrough; and measuring at least one attribute of the glass sheet through the central passage as the glass sheet moves in the first direction. The outer porous body portion preferably comprises an arcuate outer circumference, such as a circular outer circumference. 
     The inner porous body portion comprises a circular groove in a surface thereof, and further preferably comprises a plurality of radial grooves intersecting the circular groove. The outer porous body portion comprises a plurality of continuous grooves in a surface thereof. 
     The method may further comprise using a first positioning air knife to move the glass sheet in a direction away from a leading edge of the air bearing. In a further optional step, the method may further comprise using a second positioning air knife to move the glass sheet in a direction toward the air bearing. In still another optional step, the method may further comprise using a third positioning air knife to move the glass sheet away from a trailing edge of the air bearing. 
     Each stabilizing air knife of the at least two stabilizing air knives directs a flow of air in a downward direction. A leading end of a stabilizing air knife of the at least two stabilizing air knives may optionally be pitched or inclined downward relative to a trailing end of the stabilizing air knife. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and constitute a part of this specification. The drawings illustrate various embodiments of the invention and, together with the description, serve to explain the principles and operations of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary fusion glass making system for producing glass sheets; 
         FIG. 2  is a perspective view of an apparatus for characterizing a glass sheet according to an embodiment of the present invention; 
         FIG. 3  is a view of the apparatus of  FIG. 2  seen looking down on the apparatus; 
         FIG. 4  is a top down view of an edge guiding device according to an embodiment of the present invention; 
         FIG. 5  is a top down view of an edge constraining device according to an embodiment of the present invention; 
         FIG. 6  is a side view of a roller of the edge constraining device of  FIG. 5  showing the roller engaged with a rotary encoder; 
         FIG. 7  is a simplified front view of an air bearing according to an embodiment of the present invention illustrating the inner porous body portion and the outer porous body portion; 
         FIG. 8  is a side (edge) view of the air bearing of  FIG. 7 ; 
         FIG. 9  is a front view of an air bearing according to an embodiment of the present invention shown in relation to a position of the air bearing relative to a glass sheet; 
         FIG. 10  is a detailed view of the air bearing of  FIG. 7 ; 
         FIG. 11  is a frontal view of another embodiment of an air bearing wherein the air bearing comprises a plurality of inner piorous body portions; 
         FIG. 12A  is a cross sectional view of a portion of the inner porous body portion of the air bearing of  FIG. 10 ; 
         FIG. 12B  is a cross sectional view of a portion of the outer porous body portion of the air bearing of  FIG. 10 ; 
         FIG. 13  is a perspective view of an exemplary linear stabilizing air bearing according to the present invention, illustrating the flow of air from the elongated nozzle in a planar fashion; 
         FIG. 14  is a front view of the air bearing of  FIG. 7  illustrating downward angle of an exemplary stabilizing air knife; 
         FIG. 15  is a top down view of the air bearing of  FIG. 7  illustrating a sideways angle of an exemplary stabilizing air knife; 
         FIG. 16  is a side (edge) view of the air bearing of  FIG. 7  illustrating a downward of a flow of air issuing from an exemplary stabilizing air knife; 
         FIG. 17  is a front view of the air bearing of  FIG. 7  depicting the leading and trailing edges of the air bearing in relation to the direction of travel of the glass sheet; 
         FIG. 18  is a top down view of the apparatus of  FIG. 2  showing the curvature produced in a glass sheet by the apparatus; 
         FIG. 19  is a cross sectional side view of a portion of the air bearing of  FIG. 17  showing details of the curvature of the glass sheet adjacent to the air bearing; 
         FIG. 20  is a graph of fly height vs. time for two locations on the glass sheet as the glass sheet traveled at a speed of 100 mm/s, and illustrating the stability of the glass sheet position (i.e. fly height consistency). 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of the present invention. Finally, wherever applicable, like reference numerals refer to like elements. 
     In a down draw glass sheet making process, such as a fusion down draw process for example, glass sheets are formed by drawing a viscous glass material vertically downward under suitable conditions of viscosity and draw rate to form a ribbon of glass. The ribbon of glass comprises a viscous liquid at the upper-most extreme position of the ribbon and transitions from a viscous liquid to a solid glass ribbon as the material passes through the glass transition temperature range. When the descending bottom portion of the ribbon has reached a suitable temperature and viscosity, a glass sheet is cut from the ribbon, and so the process continues, with the glass sheets being cut from a continuously descending glass ribbon. 
     An exemplary fusion down draw glass making system  6  is shown in  FIG. 1 . In accordance with  FIG. 1 , batch materials  8  are loaded into melting furnace  10  and heated to form viscous molten glass material  12 . Molten glass material  12  is conveyed through finer  14  where bubbles are removed from the molten glass material, and then stirred in stirring apparatus  16  to homogenize the molten glass material. The stirring operation seeks to eliminate variations in the chemical consistency of the molten glass material, thereby avoiding variations in the physical and optical properties of the final glass. Once the molten glass material has been stirred, it flows through receiving vessel  18  and then to forming body  20 . Receiving vessel  18  functions as an accumulator by dampening minor flow fluctuations. Forming body  20  comprises a ceramic body having an open channel  22  in an upper portion of the body, and a pair of converging exterior forming surfaces  24  that join at a bottom of the forming body. The molten glass material overflows the open channel of the forming body and flows down the converging forming surfaces of the forming body as two separate flows of molten glass material. The separate flows of molten glass material join and form a single flow or ribbon  26  of material where the converging forming surfaces come together. The ribbon cools as it descends through the glass transition temperature region and forms a solid glass ribbon from which glass sheets  28  are cut along cut line  29 . 
     Melting furnace  10  is connected to and in fluid communication with finer  14  through melter-to-finer connecting tube  30 , and finer  14  is connected to and in fluid communication with stirring apparatus  16  through finer-to-stirring apparatus connecting tube  32 . Stirring apparatus  16  is connected to and in fluid communication with receiving vessel  18  through stirrer-to-receiving vessel tube  34 , and receiving vessel  18  is connected to and in fluid communication with forming body  20  through downcomer tube  36  and forming body inlet  38 . While melting furnace  10  is typically formed from a ceramic material, such as ceramic bricks (e.g. alumina or zirconia), those components involved in transporting and processing the molten glass material are typically formed from platinum, or a platinum alloy such as a platinum-rhodium alloy. Thus, melter-to-finer connecting tube  30 , finer  14 , finer-to-stirring apparatus connecting tube  32 , stirring apparatus  16 , stirring apparatus-to-receiving vessel tube  34 , receiving vessel  18 , downcomer tube  36  and forming body inlet  38  typically comprise platinum or a platinum rhodium alloy. 
     Since the glass sheets begin as vertically oriented sheets when they are removed from glass ribbon  26 , reduced handling is possible if the glass sheets can be maintained in a vertical orientation as they are transported through at least a portion of the manufacturing process downstream of the forming process. Thus, in certain manufacturing processes the glass sheet is attached to and supported from a raised conveyor after being cut from the ribbon and moved through at least a portion of the process line in a vertical orientation. In addition, it is more efficient to perform post-forming processing while the glass sheet is traveling rather than dismounting the sheet, placing the sheet in a fixture, processing the sheet, remounting the sheet and transporting it to a subsequent process. To that end, an apparatus is disclosed herein for measuring characteristics of a glass sheet after the sheet has been cut from the ribbon and as the glass sheet is moving. Measured characteristics can include cord, streak or thickness. Cord relates to a compositional inhomogeneity in the bulk glass. This composition inhomogeneity can lead to periodic nanometer-scale topography deviations. In the liquid crystal display (LCD) field, these deviations can lead to periodic cell gap variations in the display panel itself, which in turn lead to contrast streaks to which human perception is finely attuned. Streaks can cause the same distortions in LCD panels but are caused by flow distortions on the body used to form the glass sheet. In accordance with  FIG. 2 , before entering apparatus  40 , the glass sheet is transported by securing the glass sheet only at the upper edge of the glass sheet so that the glass sheet hangs freely from this support. 
     While the preceding brief description is focused on a fusion down draw glass sheet manufacturing process, the present invention is not limited to a fusion down draw process, and could be practiced in other glass sheet manufacturing processes, such as a slot draw process. 
       FIGS. 2 and 3  depict an exemplary embodiment of an apparatus  40  for measuring characteristics of a glass sheet. As shown in both  FIGS. 2 and 3 , apparatus  40  comprises a frame  42  supporting a circular air bearing  44  in an upright, vertical orientation. Air bearing  44  is a pressure-vacuum device designed to maintain a substrate, such as a glass sheet, at a predetermined distance, and within a maximum deviation, from the surface of the air bearing. The predetermined distance is referred to as the fly height. The fly height represents an equilibrium position of the substrate relative to the air bearing. As air is drawn from between the glass sheet and the air bearing through one or more vacuum ports, ambient air pressure forces the substrate toward the air bearing. However, as the substrate moves toward the air bearing, the force against the substrate produced by the air issuing from the porous surface(s) of the air bearing increases, until the substrate reaches a position where the forces are in equilibrium. Thus, the substrate is captured and held by the air bearing. The fly height exhibits some deviation about a given nominal fly height. As used herein, a vacuum port is any opening within a surface of air bearing  44  in fluid communication with a passage, such as a pipe, tube or other structure for the conveyance of a gas, and connected with or intended for connection to a vacuum source, such as a vacuum pump. Vacuum ports may be interconnected, such as through a common plenum disposed within air bearing  44 , through a common plenum external to air bearing  44 , or be individually supplied with a vacuum. 
     Air bearing  44  comprises a major surface  46 , which is the surface closest to the adjacent glass sheet to be measured, comprising channels or grooves, and vacuum ports, as will be discussed in more detail below. For the purpose of clarity, reference to angular locations on major surface  46  will be made in reference to an outer circumference of the circular air bearing, with a position of 0 degrees being located at the top-most point of the air bearing, and increasing angular position relative to a center of the circular air bearing occurs in a clockwise rotation through 360 degrees. 
     Referring to  FIG. 2 , conveyor  48  may be used to transport a glass sheet in a direction of travel  50  along a predetermined path through apparatus  40  so that a measurement of the glass sheet may be made. For example, conveyor  48  may comprise an overhead rail equipped with a clamping mechanism  49  that travels along the rail and which also clamps to a top edge of the glass sheet to be measured. Preferably the clamping mechanism is configured to roll or slide along the rail. Moreover, conveyor  48  is preferably equipped with a drive mechanism that moves the clamping mechanism, and the glass sheet, along the rail assembly. For example, the rail assembly may be fitted with a driven chain or belt connected to the clamping mechanism, wherein a motor or other motive force is used to move the chain or belt, thereby causing the clamping mechanism, and therefore the glass sheet clamped by clamping mechanism  49 , to traverse along the rail assembly and through apparatus  40 . As used herein, the direction of travel  50  represents a forward movement of the glass sheet through apparatus  40 . In addition, the terms upstream and downstream are used relative to direction of travel  50 . That is, upstream is to be construed as a direction generally opposite to direction of travel  50 , whereas downstream is to be construed as being in a direction generally the same as direction  50 . However, it should be noted that upstream and downstream designations do not require that the direction referred to is identical, or exactly opposite the direction of travel  50 . It is only required that the direction referred to has no vector component in the opposite direction. For example, an upstream direction has no downstream vector component. Additionally, upstream and downstream may be used to refer to a stationary position with respect to the moving glass sheet. In this respect, upstream refers to a position that encounters the moving glass sheet first in respect of another stationary position. Thus, the glass sheet traveling in direction of travel  50  may pass one fixed point or object before passing a subsequent point or object. The first-passed point or object is referred to as the upstream point or object relative to the subsequent point, whereas the subsequent point or object is the downstream point or object relative to the first-passed point. 
     Apparatus  40  further comprises a plurality of glass sheet stabilizing air knives comprising a first stabilizing air knife  52   a  positioned such that the first stabilizing air knife will be located opposite a first major surface of the glass sheet (where the first major surface of the glass sheet is the glass surface closest or adjacent to the air bearing), and a second stabilizing air knife  52   b  located opposite a second major surface of the glass sheet. Put more simply, one air knife is positioned adjacent to one side of the glass sheet while the other air knife is positioned adjacent to the other or opposite side of the glass sheet. Additional glass sheet stabilizing air knives  52   a ,  52   b  may be positioned such that they are opposite the first or second major surface of the glass sheet as needed. For example,  FIGS. 2 and 3  depict four pair of stabilizing air knives arrayed in rows and column. Apparatus  40  may comprise additional, positioning air knives positioned upstream of air bearing  44 , downstream of the air bearing and/or opposite the air bearing to assist in positioning the glass sheet relative to the air bearing, as described in more detail further below. 
     Apparatus  40  also comprises an edge guiding device  54  for guiding glass sheets into position for measurement. Edge guiding device  54  is located upstream of the stabilizing air knives relative to the travel direction of the glass sheet and functions to reduce or eliminate side-to-side sway of the glass sheet and to guide the glass sheet between the stabilizing air knives. In the embodiment shown in  FIGS. 2 and 3  and as perhaps best shown in  FIG. 4 , edge guiding device  54  comprises a pair of guide arms  56   a ,  56   b  configured to form guide slot  58  for receiving the lower edge of the glass sheet. Preferably guide slot  58  is wedge or V-shaped, where a distance d between the guide arms at an inlet (upstream) end of guide slot  58  (relative to glass sheet direction of travel  50 ) where a glass sheet enters the guide slot is greater than a distance d′ between guide arms  56   a ,  56   b  at an outlet (downstream) end of the guide slot. More simply put, the distance between the guide arms varies along a length of the guide slot and direction  50  such that the guide slot narrows as a glass sheet progresses through the guide slot, thereby forming a V-shaped slot that narrows in a direction toward air bearing  44 . Preferably, as shown in the embodiment of  FIG. 4 , each guide arm may be rotatably mounted to frame  42  by axle pins or bolts that are inserted into a complimentary hole  60  in each guide arm and fasten to frame  42 . Alternatively, each guide arm may comprise a pin that is fit within a complimentary hole in frame  42 . Thus, the guide arms can be rotated to vary a shape of guide slot  58 . Means for locking the guide arms is also preferably provided, thereby allowing each guide arm to be immobilized when a suitable slot shape is implemented. For example, the guide arms may be fitted with clamps or locking screws. In some embodiments, apparatus  40  may comprise a plurality of edge guiding devices  54 . The width d of guide slot  58  should be sufficient to accommodate the largest anticipated side-to-side movement, or sway, of the glass sheet (where the glass sheet is rotating about clamping mechanism  49 ) to facilitate capture of the glass sheet. For example, if d is not sufficiently wide, a swaying glass sheet may not be captured within guide slot  58  but instead conveyed into contact with elements of apparatus  40 , thereby potentially damaging the glass sheet or the apparatus. Width d will be dependent on the parameters of a particular process configuration. As described, width d′ is smaller than width d, should be sufficiently large to prevent binding of the glass sheet as it travels through guide slot  58 , but should also be sufficiently narrow that side-to-side sway is reduced or eliminated. Width d′ is dependent, for example, on the thickness of the glass sheet and the magnitude of any curvature exhibited by the sheet. Alternatively, edge guiding device  54  may be a block of material comprising a slot machined into an upper surface of the block. 
     Apparatus  40  preferably also comprises an edge constraining device  62 , best seen in  FIG. 5 , to hold the lower edge  63  of the glass sheet as the glass sheet is moving in the first direction  50  adjacent the air bearing. For example, edge constraining device  62  can be a plurality of guide rollers comprising one or more pairs of opposing rollers positioned along the path of the glass sheet as it traverses apparatus  40 . In accordance with the embodiment of  FIG. 5 , each roller pair comprises a fixed position roller  64  and an opposing movable roller  66 . The fixed roller of a roller pair is configured to be rotatable, but not otherwise movable. That is, while fixed position roller  64  can rotate about an axis of rotation of the roller, it is not adapted to translate or swing (describe an arc). On the other hand, the opposing movable roller  66  of the roller pair is configured to be both rotatable and to be movable (e.g. translatable) such that a distance between a fixed position roller  64  and an opposing movable roller  66  can vary. Preferably, movable roller  66  is urged toward fixed position roller  64 , such as with a spring  68 . As shown by  FIG. 5 , movable roller  66  is coupled to a pivot arm  70  that pivots about a pivot point  72 . Spring  68  is compressed between pivot arm  70  and spring stop  74 , whereby movable roller  66  is urged toward fixed position roller  64 . A glass sheet  28  that is inserted between fixed position roller  64  and movable roller  66  causes movable roller  66  to rotate about pivot point  72  and describe a circular arc centered about pivot point  72 . A simultaneous movement of pivot arm  70  against spring  68  further compresses spring  68 . That is, the axis of rotation of movable roller  66  itself rotates about pivot point  72 . Accordingly, movement of movable roller  66  away from fixed position roller  64  is resisted by the force exerted by spring  68  through pivot arm  70 , and movable roller  66  is urged against glass sheet  28  so that glass sheet  28  is pinched between the fixed roller and the movable roller. 
     To track progress of the glass sheet through apparatus  40 , at least one roller of constraining device  62  may include a rotary encoder device to sense the rotational movement of the rotating roller and convert the rotational movement to an electrical signal.  FIG. 6  depicts a side view of a fixed position roller  64  comprising roller axle  76  and rotary encoder  78  coupled to the roller through roller axle  76  and drive belt  80 . Other methods of coupling rotary encoder  78  may be employed, as are known in the art. Rotary encoder  78  rotates as a ratio of the rotation of the roller and develops or modifies an electrical signal  79 . The developed or modified electrical signal  79  from rotary encoder  78  may then be conveyed to a receiving computational device (not shown), where a linear movement of the glass sheet can be calculated using the rotational data from the rotary encoder. As best shown in  FIG. 6 , each fixed position roller  64  and each movable roller  66  comprises a resilient surface  82  to prevent damage resulting from contact between the roller and the glass sheet. 
     Referring now to  FIGS. 7 and 8 , air bearing  44  comprises a porous body  84  comprising generally planar major surface  46 . As used herein porous means a rigid but sponge—like material in that it comprises millions of minute random channels through the thickness of the material resulting in a uniform distribution of holes at an external surface thereof, each hole almost insignificant by itself. However, when the porous material is supplied with a gas under pressure the holes together supply a substantially uniform flow of air from a surface of the material. A suitable porous material in keeping with the present definition is graphite. Other materials, such as sintered metal powders may also be used, but the added risk of scratching the glass surface due to the hard abrasive nature of the metal argues for a softer material, such as graphite. As shown in  FIG. 9 , the overall height D of porous body  84  is typically no more than one half the height H of the glass sheet  28  to be measured (where dashed line  88  represents H/2), and preferably the height of porous body  84  is no greater than one third the height of the glass sheet, or less, where the height of the glass sheet is the dimension of the glass sheet in a vertical direction when the glass sheet is hanging vertically from conveyor  48 . Moreover, as also shown in  FIG. 9 , it is preferable that during operation the air bearing is positioned adjacent only the bottom portion of the glass sheet. That is, the air bearing is preferably positioned so that porous body  84  is adjacent only the lower one half or less of the glass sheet, or a portion thereof. If porous body  84  is positioned high on the glass sheet (e.g. above dashed line  88 ), the glass sheet may be subject to undue stress resulting from the constraint placed on the glass sheet by both the conveyor clamping mechanism and the constraint applied by the air bearing. The resulting stress may break the glass sheet. 
     Returning to  FIG. 7 , porous body  84  is further divided into a first, or inner porous body portion  90  and a second, or outer porous body portion  92  disposed about the inner porous body portion. Accordingly, planar major surface  46  is divided into an inner planar surface  94  comprising inner porous body portion  90 , and an outer planar surface  96  comprising outer porous body portion  92 . Inner planar surface  94  and outer planar surface  96  may be coplanar. 
     Inner porous body portion  90  of air bearing  44  is annular in shape, having a circular inner circumference  98  defined at a radius r 1  from the center of the inner circumference and an outer circumference  100  defined at a radius r 2  from the center of the inner circumference. In addition, inner circumference  98  denotes the outer circumference of a passage  102  extending through air bearing  44 . In a typical embodiment, passage  102  is in a range from about 3 cm to about 8 cm in diameter. However, passage  102  may be larger or smaller, depending on need and the nature of the measurement to be taken. Measurement device  104  (see  FIG. 2 ) is located such that air bearing  44  is positioned between glass sheet  28  and measurement device  104  and so that an optical axis  105  of the measurement device extends through passage  102 . Such a “through” measurement is beneficial that the plane of inspection (fixed by measurement device  104 ) and the plane of the glass are coplanar. Optical axis  105 , may, for example, coincide with the center of inner circumference  98  as shown in  FIG. 7 . In other embodiments, measurement device  104  may be positioned so that glass sheet  28  is between measurement device  104  and air bearing  44 . Nevertheless, measurement device  104  should still be positioned such that optical axis  105  of measurement device is aligned to pass through passage  102 . However, in certain embodiments, passage  102  may be eliminated when the measurement is to be taken from the side of glass sheet  28  facing porous body  84  if reflection of light from porous body  84  does not affect the quality of, or otherwise interfere with, the particular measurement being performed. Optical axis  105  of measurement device  104  may be, for example, a laser beam emitted by the measurement device toward glass sheet  28 . 
     Referring now to  FIG. 10 , inner porous body portion  90  comprises at least one circular groove  106  concentric with inner circumference  98 . Inner porous body portion  90  further comprises a plurality of grooves  108  extending radially on inner planar surface  94  and intersecting with circular groove  106 . Radial grooves  108  are preferably arranged at periodic angular positions in a spoke-like fashion. Circular groove  106  and intersecting radial grooves  108  divide inner planar surface  94  into a plurality of sub-surfaces  87 . Each sub-surface  87  comprises at least one vacuum port  110  in fluid communication with a vacuum source (not shown), as previously described. 
     Like inner porous body portion  90 , outer porous body portion  92  of air bearing  44  is arcuate in shape, but need not possess a circular outer circumference. For example, outer porous body portion may be elliptical or oval in shape. Outer porous body portion  92  is disposed about inner porous body portion  90  and comprises a circular inner circumference  112  defined at a radius r 3  from the center of inner porous body portion  90  described above. In embodiments wherein outer porous body portion  92  comprises a circular outer circumference, i.e. circumference  114  shown in  FIG. 7 , outer circumference  114  is defined at a radius r 4  from the center of inner circumference  98 . In some embodiments, r 2 =r 3  and therefore inner circumference  112  of outer porous body portion  92  is the same as the outer circumference  100  of inner porous body portion  90 . 
     Still in regard to  FIG. 10 , outer porous body portion  92  further comprises a plurality of continuous grooves  116  formed in outer planar surface  96 . Each continuous groove  116  comprises a plurality of vacuum ports  118  extending through the porous body and connected to a vacuum source. Preferably, the plurality of vacuum ports  118  are arrayed periodically within each continuous groove  116  so that the angular displacement between vacuum ports disposed in a given continuous groove is equal. For example, within a given continuous groove  116 , a vacuum port  118  may be positioned every 5 degrees, every 10 degrees or every 15 degrees around the groove. It is not necessary that the vacuum ports of one continuous groove  116  coincide angularly with the vacuum ports of another continuous groove  116 . In some cases, particularly when the outer circumference of outer porous body portion  92  is circular, continuous grooves  116  are preferably circular and concentric. 
     In some embodiments, such as that depicted in  FIG. 11 , air bearing  44  may comprise a plurality of inner porous body portions  90  positioned within outer porous body portion  92 , each inner porous body portion defining a passage  102 . This can be particularly helpful when multiple measurements, for simultaneously determining multiple characteristics of the glass sheet, are to be taken and cannot be incorporated into a single measurement device. 
     The organization of grooves and vacuum ports can be better seen with the aid of  FIGS. 12A and 12B , where  FIG. 12A  depicts a cross sectional view of a portion of inner porous body portion  90 , and  FIG. 12B  depicts a cross sectional view of a portion of outer porous body portion  92 . Both inner porous body portion  90  and outer porous body portion  92  are supplied with a pressurized gas, such as air, that issues from the planar surface of each porous body portion as represented by arrows  117 . Together, the vacuum produced at the vacuum ports, depicted by arrows  119 , and the air pressure produced over the planar surfaces of the porous body portions define two zones: a low precision capture zone adjacent outer planar surface  96  and a high precision capture zone coincident with inner planar surface  94 . In the low precision capture zone the fly height of the glass sheet may be greater than the fly height of the glass sheet adjacent the high precision capture zone. A fly height of the glass sheet adjacent the low pressure capture zone can typically by in the range from about 40 μm to 60 μm, whereas the fly height of the glass sheet adjacent the high precision zone can typically be less than 40 μm. 
     As previously described, and in accordance with the embodiment of  FIGS. 2 and 3 , apparatus  40  comprises a plurality of stabilizing air knives  52   a ,  52   b  positioned upstream of air bearing  44  relative to the direction of travel  50  of glass sheet  28  through apparatus  40 . The plurality of stabilizing air knives comprises a first stabilizing air knife  52   a  positioned such that the first stabilizing air knife will be located opposite first major surface  121  of glass sheet  28  (See  FIG. 18 ), and a second stabilizing air knife  52   b  positioned such that the second stabilizing air knife will be located opposite second major surface  123  of glass sheet  28 . First major surface  121  of glass sheet  28  is the surface of the glass sheet closest to porous body  84  when the glass sheet is adjacent to the air bearing, whereas second major surface  123  is the surface of glass sheet  28  farthest from porous body  84  under the same condition. The flow of air from the at least first and second stabilizing air knives of the plurality of stabilizing air knives stabilizes lateral (side-to-side) motion of the glass sheet in conjunction with the at least one edge guiding device  54  as the glass sheet enters the space between the stabilizing air knives, and helps to flatten the sheet. More simply put, even though the glass sheet may be impeded from lateral movement at the upper and lower edges of the glass sheet by conveyor clamping mechanism  49  at the upper edge of the glass sheet and edge guiding device  54  at the lower edge of the glass sheet, the glass sheet may still deform in a direction perpendicular to the general plane of the glass sheet, much the way a cloth sail can billow in the wind. This is because the glass sheet can be very large, and very thin, giving the glass sheet an increased flexibility when compared to much thicker glass plates. For example, a thickness of the glass sheet can be less than 1 mm. 
     Each stabilizing air knife is oriented such that the flow of air from each stabilizing air knife is directed toward the glass sheet in a downward direction, generally toward the bottom of the glass sheet, to create a more laminar flow of air over the major surfaces of the glass sheet and prevent turbulence and subsequent buffeting of the glass sheet. Preferably, although not necessarily, first and second stabilizing air knives  52   a ,  52   b  are arranged to mirror each other across the glass sheet. For example, in some embodiments, such as the embodiment of  FIGS. 2 and 3 , the plurality of stabilizing air knives are preferably arranged as multiple pairs of partially or substantially opposing air knives. That is, while the air knives may be directly opposing each other, this is not necessary, and there may be some offset between such “pairs” of air knives. However, in some embodiments the offset may be substantial. The number of stabilizing air knives is process dependent, and will depend, for example, on the transport speed of the glass sheet, the size and weight of the glass sheet and the amount of side-to-side sway exhibited by a glass sheet in a particular manufacturing process line. Similarly, the exact placement of an air knife on one side of the glass sheet compared to the placement of another air knife on the other side of the glass sheet will depend on the particular process conditions of the installation. 
       FIG. 13  illustrates an exemplary stabilizing air knife (here designated generally by reference numeral  120 ) comprising a generally elongate body  122  having an elongate orifice  124  from which a flow  126  of air issues. For simplicity, the air knife is represented by a longitudinally extended rectangular block. Each elongate orifice  124  is in fluid communication with a source of pressurized gas that enters the air knife through a coupling. The air knife may include an interior plenum in fluid communication with orifice  124 . As air is quite satisfactory as a gas, being both plentiful and essentially free, the remaining description will assume an air-based air knife. Each elongate body  122  is arranged such that a direction of flow of the air emitted from each elongate orifice  124  is at a downward angle relative to a reference horizontal plane. Each stabilizing air knife, as represented by exemplary stabilizing air knife  120 , includes a forward or leading end L and a rearward or trailing end TR relative to the direction of travel  50  of the glass sheet. That is, the leading end of the air knife is farther upstream than the trailing end of the air knife. When the air knife is supplied with pressurized air, the air issues from elongate orifice  124  at a high velocity. While the air issuing from the elongate orifice  124  may eventually begin to diverge after leaving the stabilizing air knife, for at least a short distance, on the order of several 10s of millimeters, the air issues from the air knife as a substantially laminar flow  126  that can be approximated by a plane. Exemplary stabilizing air knife  120  further comprises a top surface T. 
     In the event that the stabilizing air knives are arranged in a complimentary opposing relationship (i.e. are mirrored across an intervening vertical plane between the stabilizing air knives that is parallel with air bearing major surface  46 ), a distance between the leading ends of an opposing stabilizing air knife pair may be greater than a distance between the trailing ends of the opposing stabilizing air knife pair. That is, the distance between the opposing air knives narrows as the glass sheet progresses between the air knives in a manner similar to the narrowing of guide slot  58 . 
     In still another optional characteristic, each stabilizing air knife of the plurality of stabilizing air knives may be oriented such that the trailing end of each stabilizing air knife is higher (or lower) than the leading end of the stabilizing air knife. In some embodiments each stabilizing air knife can be straight (i.e. rectangular shaped) similar to exemplary air knife  120 . However, preferably each stabilizing air knife is arcuate and may comprises a circular arc. Suitable stabilizing air knives of either the straight (linear) variety, or the arcuate design, can be obtained, for example, through Exair Corporation located in Cincinnati, Ohio, USA. 
     How each stabilizing air knife may be spatially oriented can be visualized in more detail with the following description and aid of  FIGS. 14-16 . The orientation of a body in three-dimensional space requires a frame of reference, and a means of orienting the body in that frame of reference.  FIG. 14  shows a vertical X-Y plane coplanar with major surface  46  of porous body  84 . For the purpose of further discussion, this X-Y plane forms one plane in a three-dimensional Cartesian coordinate reference frame. This X-Y plane lies within the plane of the page on which  FIG. 14  is depicted. A second vertical plane, seen from an edge thereof in  FIG. 14 , forms a Y-Z plane of the Cartesian coordinate system where the Z direction extends perpendicular to and therefore out of the page. The Y-Z plane is perpendicular to the X-Y plane. A third, X-Z plane, also seen from an edge thereof in  FIG. 14 , is arranged to be perpendicular to both the X-Y and Y-Z planes. For the purpose of further discussion, and unless otherwise described, the origin of the Cartesian coordinate system formed by the three planes X-Y, Y-Z and X-Z described above lies at the center of inner porous body portion  90 , and this Cartesian coordinate system will be used to describe the orientation of the air knives in three dimensional space. 
       FIGS. 14-16  depict the three optional orientations of exemplary stabilizing air knife  120 , and by extension therefore the optional spatial orientations of each stabilizing air knife, shown separately to aid in visualizing the orientations.  FIG. 14  depicts an outline of air bearing  44  as seen looking at major surface  46  and indicating the direction of travel  50  of the glass sheet. Exemplary stabilizing air knife  120  exhibits a downward pitch or incline in that the leading end L of the stabilizing air knife is lower than the trailing end TR of the stabilizing air knife relative to the horizontal X-Z plane. To wit, plane  128  representing the flow of air from the exemplary stabilizing air knife makes an angle α with the X-Z plane. 
       FIG. 15  shows a second view looking down on an edge of air bearing  44  and shows an edge of the Y-Z plane and an edge of the X-Y plane. The X-Z plane is perpendicular to both the X-Y plane and the Y-Z plane. Plane  130  is a plane longitudinally bisecting top surface T of exemplary stabilizing air knife  120  and is perpendicular to plane  126  representing the flow of air from the air knife. In accordance with  FIG. 15 , exemplary stabilizing air knife  120  may be angled relative to the vertical X-Y plane such that a non-zero angle β is formed between plane  130  and the X-Y plane. 
       FIG. 16  shows a third view looking down on an edge of air bearing  44  and shows an edge of the X-Z plane and an edge of the X-Y plane. The Y-Z plane is perpendicular to both the X-Y plane and the X-Z plane.  FIG. 16  illustrates exemplary stabilizing air knife  120  from an end thereof oriented such that the flow of air exiting the air knife is directed downward (from a reference horizontal flow, e.g. parallel with the horizontal X-Z plane) and the plane of the air flow makes an acute angle δ with the X-Y plane rather than being directed, for example, perpendicular to the glass sheet. Preferably, δ is in the range from about 15 degrees to about 75 degrees, preferably in the range from about 25 degrees to about 65 degrees, and more preferably in the range from about 35 degrees to about 55 degrees. In one embodiment, the angle of the air flow is about 45 degrees relative to the vertical X-Y plane. It should be noted that the preferred direction for the air flow is downward, since a low positioning of the air bearing relative to the glass sheet gives the lower portion of the glass sheet more stiffness to resist buckling of the glass sheet due to the air flow. However, in some embodiments, an upward air flow may be preferred depending on process conditions and the particular implementation, e.g. positioning, of the air bearing. 
     The preceding description presented three optional orientations of an exemplary stabilizing air knife  120 . Each stabilizing air knife of the plurality of stabilizing air knives may exhibit at least one orientation of the three optional orientations described above in respect of a representative exemplary stabilizing air knife. For example, each stabilizing air knife of the plurality of stabilizing air knives may exhaust air such that the direction of air flow from the air knives is generally downward (i.e. the flow vector comprises a vertical vector component). Thus, for example, two stabilizing air knives located on opposite sides of the glass sheet and wherein the stabilizing air knives are mirror images of each other, will form a generally V-shaped flow of air, with the “V” pointed downward. 
     Similarly, each stabilizing air knife of the plurality of stabilizing air knives may be oriented such that a leading end of each stabilizing air knife is farther from the glass sheet than a trailing end. Thus, for example, two stabilizing air knives located on opposite sides of the glass sheet and wherein the stabilizing air knives are mirror images of each other, will form a generally V-shaped flow of air, with the “V” pointed downstream toward the air bearing. This provides more lateral clearance for a glass sheet exhibiting lateral movement as it enters between the air knives. It also provides for a more gradual application of the curtain of air flowing from the stabilizing air knives, as the pressure on the glass sheet from the flow of air from the leading end of each stabilizing air knife becomes less than the pressure of the air on the glass sheet adjacent the trailing end of a stabilizing air knife. 
     Similarly, each stabilizing air knife of the plurality of stabilizing air knives may be oriented such that a leading end of each stabilizing air knife is lower relative to a horizontal reference plane (for example, the X-Z plane) than a trailing end. It can be said that the stabilizing air knives are pitched or inclined forward to flatten out any shape distortion (e.g. bow) in the sheet. 
     Each stabilizing air knife of the plurality of air knives may exhibit one or more of the orientations described above. In some embodiments, one or more of the stabilizing air knives may simultaneously exhibit all three orientations. 
     In addition to the stabilizing air knives and as seen in  FIG. 3 , for example, a first positioning air knife  132  may be placed between the stabilizing air knives ( 52   a ,  52   b ) and air bearing  44  such that the flow  126  of air from the first positioning air knife impinges on first major surface  121  of the glass sheet adjacent to the leading edge of the air bearing. For example, first positioning air knife  132  may be located at an approximately 270 degree position on air bearing  44 . The pressure produced on the glass sheet as it passes adjacent to first positioning air knife  132  forces the glass sheet away from the air bearing. This prevents contact between the leading or forward edge of the glass sheet as it approaches the air bearing until the glass sheet can be “captured” by the air bearing. 
     A second positioning air knife  134  may be positioned such that air from the second positioning air knife impinges on second major surface  123  of the glass sheet. The effect of the air from the second positioning air knife is to force the glass sheet in a direction toward the air bearing, thus bringing the glass sheet closer to the air bearing and allowing the air bearing to capture the glass sheet. Initial capture of the glass sheet is accomplished by the combination of pressure and vacuum produced by the outer porous body portion. 
     A third positioning air knife  136  may be positioned downstream from the air bearing and positioned such that air emitted by the third positioning air knife is directed against first major surface  121  of the glass sheet. The air pressure produced by third positioning air knife  136  forces the glass sheet away from the air bearing surface near the downstream edge of the air bearing and thereby prevents contact between the glass sheet and the air bearing as the glass sheet moves past and disengages from the air bearing. Each positioning air knife  132 ,  134  and  136  may be similar in design to a stabilizing air knife. For example, each stabilizing air knife and each positioning air knife may be of the arcuate design or of the linear design. Preferably, the air emitted from each of the positioning air knives  132 ,  134  and  136  is directed against the glass sheet such that the curtain of gas from each positioning air knife forms an angle less than 90 degrees but greater than zero with the surface of the glass sheet, for example, greater than 25 degrees and less than 75 degrees, and preferably greater than 35 degrees and less than 65 degrees, preferably greater than 35 degrees and less than 55 degrees. For example, a typical embodiment may orient each positioning air knife so that the flow of air impinges on the glass sheet at an angle of about 45 degrees. An angle of impingement less than 90 degrees produces less turbulence at the surface of the glass sheet than, for example, air flow that is perpendicular to the glass sheet. 
     The overall effect of the various non-contact glass sheet handling components of apparatus  40  is to provide gradually increasing constraint on the glass sheet to prepare the glass sheet for measurement. As previously noted, in some instances the glass sheet is conveyed vertically, secured only at the top of the glass sheet by the conveyor clamp. As the glass sheet may be very thin, equal to or less than 1 mm, and in some cases equal to or less than 0.7 mm, or in other cases equal to or less than 0.3 mm, the glass may easily exhibit lateral movement by swaying side-to-side (i.e. rotate about the fixed carrier contact points), or deform by various bending modes (as used herein, a bending mode is analogous to a vibrational mode). The glass can also be offset due to carrier-to-carrier variations in the clamping and the position of the carrier on the conveyor. As well, the glass can be bowed vertically. 
     Various glass sheet handling components of apparatus  40  serve to reduce or eliminate these motions, and fixed shapes, such as bow. Accordingly, operation of apparatus  40  may proceed along the following steps. 
     A glass sheet  28  is attached to conveyor  48  by one or more clamping mechanisms  49  that grip the glass sheet along a top edge of the glass sheet translate the glass sheet through apparatus  40 . Glass sheet  28  is thereby hanging from the one or more clamping mechanisms, and supported only by the one or more clamping mechanisms clamped to the glass sheet along a top portion of the glass sheet. The lower edge  63  of the glass sheet is unsupported and initially capable of lateral movement, i.e. a swaying movement, before entering apparatus  40 . In addition to lateral movement, the glass sheet may also exhibit flexure or bending. For example, the sheet may bend cylindrically or hyperbolically or be saddle shaped, dome shaped, or exhibit other bending modes, or combinations thereof. 
     As glass sheet  28  nears apparatus  40 , the glass sheet is guided by at least one edge guiding device  54  that engages with lower edge  63  of the glass sheet and guides the leading edge of the glass sheet between stabilizing air knives  52   a ,  52   b . Lower edge  63  forms part of the “non-quality” portion of the glass sheet and may later be removed. The at least one edge guiding device  54  minimizes or eliminates side-to-side swaying. Testing has shown embodiments of edge guiding device  54  as disclosed herein can reduce the lateral motion of swaying from a maximum displacement of +/−75 mm to less than +/−10 mm. However, while the at least one edge guiding device  54  may provide excellent control of lateral movement of the lower edge of the glass sheet, the glass sheet is only substantially constrained at the top and bottom edges, and is still capable of exhibiting various bending modes and fixed shapes within the body of the glass sheet. To minimize or eliminate this additional movement or shape of the glass sheet before moving adjacent to porous body  84 , stabilizing air knives are employed. 
     The flow of air emitted by opposing stabilizing air knives, preferably in a downward direction, may further reduce lateral movement of the glass sheet to eliminate side-to-side swaying of the glass sheet, and in particular, reduces or eliminates bending modes. In effect, the stabilizing air knives help stiffen the glass sheet by at least reducing the magnitude of the bending and in some cases by eliminating one or more bending modes. The number and positioning of the stabilizing air knives is dependent on such factors as the size of the glass sheet, the thickness of the glass sheet, the density of the glass, and the traverse speed of the glass sheet through apparatus  40 . 
     As the glass sheet passes between the stabilizing air knives, lower edge  63  of the glass sheet may be guided by one or more additional edge guiding devices  54  to further guide and stabilize the glass sheet. For example, in some embodiments, multiple edge guiding devices may be employed; with a first edge guiding device employed upstream of the stabilizing air knives and a second edge guiding device positioned just prior to the edge constraining device  62 . 
     As the glass sheet approaches air bearing  44 , optional first positioning air knife  132  may be used to direct a flow of air at first major surface  121  of the glass sheet. The force of the air from first positioning air knife  132  against first major surface  121  of glass sheet  28  pushes the glass sheet away from the leading edge  140  (see  FIG. 17 , and more particularly region A of  FIG. 18 ) of air bearing  44  and prevents contact between leading edge  140  of the air bearing and leading edge  141  of glass sheet  28  as the glass sheet comes under the influence of outer porous body portion  92  of air bearing  44 . Contact between the glass sheet and the air bearing may result in damage, in some instances catastrophic, to the glass sheet. 
     As the glass sheet continues to move forward along direction of travel  50 , the glass sheet passes over a first vacuum port  118  of outer porous body portion  92 . Preferably, air bearing  44  is positioned such that a vacuum port  118  positioned within the outer-most groove of outer porous body portion  92  is positioned so that as the glass sheet advances, it first moves adjacent to this single vacuum port  118 . Referring to  FIG. 10 , this first vacuum port  118  is the vacuum port farthest to the left and lying in the outermost continuous groove  116  in  FIG. 8 , and intersecting with dashed line  119 . The effect of this initial encounter with a first vacuum port  118  is that leading edge  141  of glass sheet  28  is moved closer to outer porous body portion  92 . That is, while the region of the glass sheet adjacent leading edge  141  of the air bearing is being pushed away from the air bearing leading edge, a region of the glass sheet adjacent the first vacuum port  118  is forced in the direction of outer porous body portion  92 . By bringing at least this portion of the glass sheet adjacent the first vacuum port  118  close to the air bearing, that portion of the glass sheet is captured by the outer porous body portion  92  of air bearing  44 . Continued forward movement of the glass sheet brings the glass sheet adjacent to additional vacuum ports  118  of outer porous body portion  92 . Within a short distance of the leading edge of glass sheet  28  passing adjacent to the additional outer porous body portion vacuum ports  118 , sufficient force is exerted on the glass sheet as a result of air flow at the outer porous body portion that a substantial portion of the glass sheet adjacent to air bearing  44  exhibits a substantially uniform fly height relative to the first major surface of the air bearing. 
     Again, as the glass sheet continues to move forward adjacent to inner porous body portion  90 , the flatness and rigidity of the glass sheet increases, particularly those portions of glass sheet  28  directly adjacent to inner porous body portion  90 , such that measurements may be taken by measurement device  104  through passage  102 . 
     It should be recalled that measurements of the glass sheet, such as interferometric measurements for the purpose of determining surface topography of glass sheet  28 , may be taken simultaneously with the forward movement of the glass sheet (i.e. in direction  50 ). As the glass sheet trailing edge passes inner porous body portion  90 , and then outer porous body portion  92 , the constraint to the glass sheet applied by the action of the air bearing  44  decreases, and the air pressure from optional positioning air knife  136  is able to overcome the holding force applied by air bearing  44  such that the trailing edge of the glass sheet is pushed away from the air bearing so that contact between the glass sheet and the air bearing does not occur. The pressurized air supplied to inner porous body portion  90 , and the vacuum, may be adjusted, for example, such that the fly height of the glass sheet is maintained to have a deviation less than about 30 μm (+/−15 μm). 
     From the preceding it can be seen that a region of the glass sheet is opposite passage  102  as the glass sheet moves past air bearing  44 . Thus, passage  102 , while defining a circular measurement zone, “sweeps” a rectangular measurement region  138  of the glass sheet as shown in  FIG. 9 . Measurement device  104  makes continuous measurements of the glass within this rectangular region. For example, measurement device  104  may be an interferometer for making surface topography measurements of the glass sheet within the rectangular measurement zone, or measurement device  104  may make measurement of the thickness of the glass sheet. 
     Eventually, continued forward travel of the glass sheet along direction of travel  50  brings the trailing edge  142  of the glass sheet past air bearing  44 . Air issuing from third positioning air knife  136  and impinging on first major surface  121  of the glass sheet forces the region of the glass sheet proximate the trailing edge  142  of air bearing  44  away from the air bearing, thereby avoiding contact between the glass sheet and the air bearing surface. This becomes particularly beneficial as the surface area of the glass sheet under the influence of the stabilizing air knives and/or the air bearing decreases. 
     The result of the forces supplied by positioning air knives  132 ,  134  and  136  can be seen with the aid of  FIG. 18 , showing apparatus  40  in a top view and depicting glass sheet  28 . The effect of positioning air knives  132  and  136  can be seen in the regions designated by reference numerals A and B and circled by dashed lines. Indeed, it can be seen from  FIG. 18  that the glass sheet overall takes on a non-planar aspect until the glass sheet is fully engaged by the air bearing, at which point the glass sheet adjacent to the air bearing is fully flat, although it should be noted that the only region where planarity is needed is where a measurement is occurring, e.g. in the center of the measurement zone. 
       FIG. 19  depicts an edge view of the glass sheet extending from the center of the inner porous body portion to the leading edge of the air bearing and shows in more detail the shape of the glass sheet over the air bearing. It should be kept in mind that the illustration of  FIG. 19  is greatly exaggerated, as the deflections involved are on the order of tens of microns. As shown, the glass sheet can be divided into several distinct regions separated by equally distinct boundaries, giving the portion of the glass sheet over the air bearing the appearance of a series of relatively flat plateaus  144  separated by S-shaped boundary features  145 , wherein the fly height of the plateaus decrease in a direction toward the center of the inner porous body portion (e.g. the area defined by passage  102 ). 
       FIG. 20  is a graph of measured fly heights for a glass sheet traveling through an embodiment of apparatus  40  with a travel speed of 100 mm/s measured at two different locations for a glass sheet. The vertical “Y” axis represents fly height in microns while the horizontal “X” axis represents time. The measurements were taken at a predetermined frequency (250 sec −1 ) for a given location. Thus, the graph of  FIG. 20  can be used to obtain the fly height at a predetermined position over time as the glass sheet traverses adjacent to the air bearing. The glass sheet had an initial side-to-side sway of +/−75 mm from a nominal centerline position. Each stabilizing air knife directed air against the glass sheet at a downward angle of 45 degrees. The inner porous body portion of the air bearing was supplied with air at a pressure of 20 psi to 60 psi at a flow rate of 0.63+/−0.25 CFM, while the outer porous body portion was supplied with air at a pressure of 40 to 85 psi and a flow rate of 0.96+/−0.35 CFM. The fly height of the glass sheet over the circular measurement zone defined by passage  102  was nominally 28 μm with a variation of less than +/−2.5 μm. Curve  146  of  FIG. 20  shows the fly height of the glass sheet at the outer circumference of inner porous body member  90  at a position of approximately 210 degrees, with the glass sheet traveling at a speed of 100 mm/s, while curve  148  of  FIG. 20  depicts the fly height at the center of the inner porous body portion (i.e. the center of passage  102 ). The curves for travel of the glass sheet are particularly telling for showing the stability of the fly height at both locations of the glass sheet—the measurement location at the outer circumference of the outer porous body portion and the measurement location over the center of the porous body. While the nominal fly height differs between the two regions, being significantly greater for the outer porous body location than for the center of the porous body, the fly height at both locations is surprisingly stable, showing a variation less than about ±2.5 microns. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.