Abstract:
There is provided a planar light source apparatus, including: a light source configured to emit light in a spot-like pattern or a bar-like pattern; a reflective plate configured to reflect the light from said light source; a light guide plate disposed on the opposite side to said reflective plate with respect to the light source; and a lens array sheet disposed between the light source and the light guide plate; the lens array sheet having a plurality of convex-shaped lenses disposed in a predetermined state on a face thereof adjacent the light guide plate.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2006-352605 filed in the Japan Patent Office on Dec. 27, 2006, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a planar light source apparatus suitable for use for illuminating, for example, a liquid crystal display panel, a display apparatus which incorporates a planar light source apparatus, and a planar illumination method applied to a planar light source apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    A planar light source apparatus for use as a backlight for a liquid crystal panel which is used in a television receiver or a like apparatus which includes a screen of a comparatively large size typically has such a configuration as shown in  FIGS. 20 and 21 . It is to be noted that  FIG. 21  is a sectional view taken along line II-II of  FIG. 20 . Referring to  FIGS. 20 and 21 , the planar light source apparatus shown is of the type called direct type wherein it is disposed directly on the rear face of a display panel. In particular, a plurality of lamps  11  are disposed uniformly in a case formed from a reflective plate  10 . The front face of the reflective plate  10  is painted in white so that light from the lamps  11  is reflected efficiently. For the lamps  11 , for example, a CCFL tube (Cold Cathode Fluorescent Lamp) is used. 
         [0006]    A diffusion plate  20  is disposed on the reflective plate  10  on which the lamps  11  are disposed such that light incident directly to the rear face of the diffusion plate  20  from the lamps  11  side, or light reflected by the reflective plate  10  and incident to the diffusion plate  20 , is radiated from the front face of the diffusion plate  20 . The rear face of the liquid crystal display panel is illuminated with the light radiated from the surface of the diffusion plate  20 . The diffusion plate  20  is formed by mixing light dispersing material in a transparent resin plate or by adhering a sheet having a light diffusing action to the front face of the diffusion plate  20  so that the brightness on the front face of the diffusion plate  20  may be uniformized. To control the light emitting state of a planar light source apparatus so as to obtain uniform brightness is very significant in order to uniformize the luminance of the display screen. 
         [0007]    An example of a backlight apparatus of the type described is disclosed in Japanese Patent Laid-Open No. Hei 8-221013. 
       SUMMARY OF THE INVENTION 
       [0008]    Incidentally, it is generally demanded to reduce the thickness of an entire liquid crystal display panel. Therefore, it is desirable to form also a direct type planar light source apparatus having the configuration described above with reference to  FIG. 20  with a reduced thickness. In order to reduce the thickness of the direct type planar light source apparatus of the configuration shown in  FIG. 20 , the diffusion plate  20  is disposed closer to the lamps  11 . 
         [0009]    However, where the planar light source apparatus is configured such that the diffusion plate  20  is disposed closer to the lamps  11 , it becomes difficult to sufficiently diffuse light only by means of the diffusion plate  20 . This gives rise to a problem that the position just above each lamp  11  becomes brighter and the uniformity in luminance within the light emitting plane is damaged. As a countermeasure against this problem, measures for reducing the light immediately above the lamps using a light reflecting member only at the position of the diffusion plate just above each lamp are sometimes taken. However, this gives rise to another problem that the luminance of the overall area drops, and therefore, such measures as described above are not adopted frequently. 
         [0010]    Therefore, it is desirable to provide a planar light source apparatus, a display apparatus and a planar illumination method wherein both of reduction in thickness and uniformization of the light emission luminance can be achieved. 
         [0011]    According to an embodiment of the present invention, there is provided a planar light source apparatus including a light source configured to emit light in a spot-like pattern or a bar-like pattern, a reflective plate configured to reflect the light from the light source, a light guide plate disposed on the opposite side to the reflective plate with respect to the light source, and a lens array sheet disposed between the light source and the light guide plate, the lens array sheet having a plurality of convex-shaped lenses disposed in a predetermined state on a face thereof adjacent the light guide plate. 
         [0012]    In the planar light source apparatus, light from the light source is diffused by an action of the convex-shaped lenses of the lens array sheet and the light guide plate disposed on the lens array sheet. Consequently, the light from the light source can be diffused and radiated favorably from the light guide plate. Accordingly, even if the distance from the reflective plate in which the light source is accommodated to the light guide plate is reduced, a light diffusion characteristic similar to that obtained by existing planar light source apparatus can be obtained. Consequently, reduction in thickness of the light source apparatus and reduction in thickness of a display apparatus in which the light source apparatus is built can be anticipated. 
         [0013]    The above and other features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference characters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view showing an example of a general configuration of a planar light source apparatus to which the present invention is applied; 
           [0015]      FIG. 2  is a sectional view taken along line I-I of  FIG. 1 ; 
           [0016]      FIG. 3  is an enlarged exploded perspective view of part of the planar light source apparatus; 
           [0017]      FIG. 4  is a schematic view generally showing a path of light through the planar light source apparatus; 
           [0018]      FIG. 5  is a schematic view showing an example of a path of light incident at an angle of 0° to a lens array sheet in the planar light source apparatus; 
           [0019]      FIG. 6  is a schematic view showing part of  FIG. 5  in an enlarged scale; 
           [0020]      FIG. 7  is a schematic view showing an example of a path of light incident at another angle of 10° to the lens array sheet in the planar light source apparatus; 
           [0021]      FIG. 8  is a schematic view showing part of  FIG. 7  in an enlarged scale; 
           [0022]      FIG. 9  is a schematic view showing an example of a path of light incident at a further angle of 20° to the lens array sheet in the planar light source apparatus; 
           [0023]      FIG. 10  is a schematic view showing part of  FIG. 9  in an enlarged scale; 
           [0024]      FIG. 11  is a schematic view showing an example of a path of light incident at a still further angle of 45° to the lens array sheet in the planar light source apparatus; 
           [0025]      FIG. 12  is a schematic view showing part of  FIG. 11  in an enlarged scale; 
           [0026]      FIG. 13  is a characteristic diagram illustrating an example of a luminance distribution by the planar light source apparatus; 
           [0027]      FIG. 14  is a partial sectional view of another planar light source apparatus to which the present invention is applied showing an example of a configuration wherein the surface of a light guide plate is worked; 
           [0028]      FIG. 15  is a sectional view of a further planar light source apparatus to which the present invention is applied showing an example of a configuration wherein lens projections are disposed non-uniformly; 
           [0029]      FIG. 16  is a perspective view of a still further planar light source apparatus to which the present invention is applied showing an example of a configuration wherein a point light source is used; 
           [0030]      FIG. 17  is an exploded perspective view of the light source apparatus shown in  FIG. 16 ; 
           [0031]      FIG. 18  is a perspective view of a yet further planar light source apparatus to which the present invention is applied showing an example of a configuration wherein a circular lens is used; 
           [0032]      FIG. 19  is a perspective view of a yet further planar light source apparatus to which the present invention is applied showing another example of a configuration wherein a circular lens is used; 
           [0033]      FIG. 20  is a perspective view showing an example of a configuration of a conventional direct type planar light source apparatus; and 
           [0034]      FIG. 21  is a sectional view taken along line II-II of  FIG. 20 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Referring first to  FIGS. 1 and 2 , there is shown a planar light source apparatus  100  to which an embodiment of the present invention is applied. The planar light source apparatus  100  includes a reflective plate  110  disposed on the bottom thereof, and a plurality of, five in the arrangement shown, bar-shaped lamps  101 ,  102 ,  103 ,  104  and  105  disposed in parallel on a flat face  111  of the reflective plate  110 . It is to be noted here that, while, in the arrangement shown, a CCFL tube (cold cathode fluorescent lamp) is used for the lamps  101  to  105 , the lamp to be used for the lamps  101  to  105  is not limited to a CCFL tube but may be, for example, a hot cathode tube. 
         [0036]    The inner face of the reflective plate  110  is painted in white so that light from the lamps  101  to  105  can be reflected efficiently. The reflective plate  110  has a pair of inclined portions  112  at opposite end portions thereof such that upper end portions  113  of the inclined portions  112  contact with a lens array sheet  120 . It is to be noted that, although the opposite end portions of the reflective plate  110  in the longitudinal direction of the lamps  101  to  105  are shown open in  FIG. 1  and so forth so that an internal configuration can be observed, actually they are covered with some members. The reflective plate  110  may be formed, for example, from foamed polyethylene terephthalate. 
         [0037]    The lens array sheet  120  disposed above the reflective plate  110  has a flat face  124  (rear face; refer to  FIG. 3 ) adjacent the reflective plate  110  (on the lower side in  FIG. 1 ) and has a plurality of lens projections  121 , which each serves as a convex lens, disposed on the front face thereof. Further, a light guide plate  130  is disposed on a front face of the lens array sheet  120 . The light guide plate  130  is formed from a transparent member having a uniform thickness and having flat front and rear faces. In  FIG. 2 , a display panel  190  disposed on the front face of the light guide plate  130  is indicated by an imaginary line. The rear face of the display panel  190  is illuminated with light from the front face of the light guide plate  130  in this manner thereby to perform a process of illuminating an image displayed on the display panel  190 . The display panel  190  may be formed, for example, from a liquid crystal display panel. 
         [0038]    The lens array sheet  120  and the light guide plate  130  may be formed from a transparent material, and any material may be used if it is transparent in the range of visible radiation such as, for example, polycarbonate, glass and acrylic resin. 
         [0039]      FIG. 3  is an enlarged exploded view of part of the lens array sheet  120  and the light guide plate  130 . Referring to  FIG. 3 , each of the lens projections  121  disposed on the lens array sheet  120  is formed as a linear rib having a convex cross section such as a substantially triangular cross section. The lens projections  121  are disposed continuously such that they extend in parallel to each other. 
         [0040]    Each of the lens projections  121  has curved face portions  122  on the opposite sides thereof and has a flat face portion  123  at an upper portion thereof between the curved face portions  122 . The curved face portions  122  have a parabolic curved face such that light reflected from the parabolic curved faces thereof is focused at a focal position F ( FIG. 4 ) in the proximity of the flat face portion  123 . It is to be noted, however, that the shape of the cross section of the curved face portions  122  need not have a completely parabolic shape but can have various other shapes such as a curved shape similar to a parabola, a curved shape represented by a hyperbola, a mere elliptic shape and a shape of a combination of a straight line and a curved line. Where a parabolic curved face is applied, the focal position of light therefrom is defined. However, depending upon the shape, no focal point may be provided. 
         [0041]    The flat face portions  123  of the lens projections  121  of the lens array sheet  120  optically closely contact with the rear face of the light guide plate  130 . Although the lens array sheet  120  and the light guide plate  130  may be held in close contact with each other by any means, for example, a transparent adhesive or an ultraviolet curing resin material may be used. However, whatever means is applied, it is significant that the lens array sheet  120  and the light guide plate  130  optically closely contact with each other. 
         [0042]    Referring to  FIG. 4 , where the arranged distance (pitch) of the lens projections  121  is represented by W 1  and the width of the flat face portion  123  at the upper portion of the lens projections  121  by W 2 , the distance W 1  is set, for example, to approximately 0.2 to 0.3 mm while the width W 2  is set, for example, to approximately 0.005 to 0.02 mm. The lens projections  121  of the lens array sheet  120  shown in the drawings are shown with a size greater than that given above for clarified illustration. It is to be noted that the arranged distance of the lamps  101  to  105  is, for example, approximately 20 to 30 mm. Preferably, the focal position F ( FIG. 4 ) formed by the parabolic curved face portions  122  is within a distance of the [distance W 1 ×0.1) from the flat face portion  123 . The rear face  124  of the lens array sheet  120  remote from the upper face on which the lens projections  121  are disposed is flattened. 
         [0043]    Now, an outline of a path of light incoming from the rear face (lower face) side of the lens array sheet  120  configured in such a manner as described above is described with reference to  FIG. 4 . Light A emitted from any lamp and incoming orthogonally to the lens array sheet  120  comes at a high ratio to the lens surface of the lens array. Thereupon, reflection of the light arises from a difference in refractive index between the air and the material of the lens array. The reflected light of the light A passes the focal position F of the lens projection  121  and comes to an upper face  132  of the light guide plate  130 . Here, if the angle of the light is set suitably, then the light is totally reflected by the upper face  132  of the light guide plate  130 . The light reflected by the upper face  132  of the light guide plate  130  is totally reflected also by a lower face  131  because the upper face  132  and the lower face  131  of the light guide plate  130  extend in parallel to each other. As a result, the light propagates in the light guide plate  130 . The light propagating in the light guide plate  130  is radiated to the outside of the light guide plate  130  at a position at which the total reflection condition is lost by an optical mutual action of the light with the adhesion plane of the lens array sheet  120 , the light guide plate  130  and the lens array surface and so forth. Where the radiated light to the outside of the light guide plate  130  goes out from the upper face  132  of the light guide plate  130 , it makes illuminating light for illuminating the display panel. However, some light goes out from the lower face  131  of the light guide plate  130 . The light going out from the lower face  131  of the light guide plate  130  is radiated to the reflective plate  110  side through the lens array sheet  120  and reflected by the reflective plate  110  so that it is introduced into the lens array sheet  120 . Consequently, all light finally goes out from the upper face  132  of the light guide plate  130 . 
         [0044]    It is to be noted that light is reflected by the upper face  132  and the lower face  131  of the light guide plate  130  where the incident angle of light to the upper face  132  or the lower face  131  is equal to or smaller than some particular angle (for example, equal to or smaller than 43°). However, where the angle is greater than the particular angle but equal to or smaller than 90°, the light goes out as it is without being reflected. The particular angle for reflection depends upon the material of the light guide plate  130 . Meanwhile, end faces  133  ( FIG. 1 ) of the light guide plate  130  are processed so that incident light is reflected as it is irrespective of the incident angle so that radiation of light from the end faces  133  to the outside may be prevented. 
         [0045]    Since the lens array sheet  120  acts in such a manner as described above, according to the configuration of the present embodiment, part of light emitted directly upwardly from the lamps is propagated in the light guide plate  130  such that it can be radiated from different positions of the light guide plate  130 . The average distance of light in the light guide plate  130  and the ratio of light which makes a guided light component from within light incoming perpendicularly to the light guide plate  130  depend upon the size of pertaining portions of the planar light source apparatus  100  and the lamp arrangement. Therefore, the particular shape of the lens projections  121 , the adhesion area between the flat face portions  123  of the lens projection  121  and the light guide plate  130  and so forth are examined suitably. 
         [0046]    On the other hand, if light B emitted obliquely from the lamps comes to the surface of a lens projection  121  of the lens array sheet  120 , then it is deflected to various directions depending upon the incident angle, the position of the lens and so forth. As a result, the light guide plate  130  functions so as to diffuse light emitted obliquely from the lamps. 
         [0047]    Examples of a simulation of a state wherein light is introduced at various angles to an adhered block of the lens array sheet  120  and the light guide plate  130  are shown in  FIGS. 5 to 12 . It is to be noted that the angles shown in  FIGS. 5 to 12  are represented as angular differences from the angle of 0° defined by light incident perpendicularly to the lens array sheet  120  which is in the form of a flat plate. Further, incident light (L 0 , L 10 , L 20  and L 45  hereinafter described) illustrated in the figures is light incident with a width of the pitch of one lens projection  121 . 
         [0048]      FIG. 5  illustrates a reflection state of light L 0  incident at the angle of 0° to the lens array sheet  120 .  FIG. 6  shows a central portion of  FIG. 5  in an enlarged scale. As recognized from  FIGS. 5 and 6 , light advancing straightforwardly to the flat face portion  123  of a lens projection  121  advances straightforwardly as it is in the light guide plate  130  and goes out from the upper face  132  of the light guide plate  130 . Meanwhile, part of the light coming to the curved face portions  122  of the lens projection  121  is reflected by the curved face portions  122  and enters the light guide plate  130  past the focal position (or the proximity of the focal position). 
         [0049]      FIG. 7  and  FIG. 8  which is an enlarged view of  FIG. 7  illustrate a reflection state of light L 10  incident at an angle of 10° to the lens array sheet  120 .  FIG. 9  and  FIG. 10  which is an enlarged view of  FIG. 9  illustrate a reflection state of light L 20  incident at an angle of 20° to the lens array sheet  120 .  FIG. 11  and  FIG. 12  which is an enlarged view of  FIG. 11  illustrate a reflection state of light L 45  incident at an angle of 45° to the lens array sheet  120 . As can be seen from the figures mentioned, as the incident angle increases, the reflection state varies such that the light radiated from the upper face  132  of the light guide plate  130  increases. 
         [0050]    Since the combination of the lens array sheet  120  and the light guide plate  130  functions as a diffusion plate in this manner, the brightness at bright portions immediately above the lamps can be moderated. Accordingly, even if the thickness from the reflective plate  110  to the light guide plate  130  is reduced from that in existing planar light source apparatus, a luminance distribution similar to that of existing planar light source apparatus can be obtained. 
         [0051]      FIG. 13  illustrates the distribution of the relative luminance of the planar light source apparatus  100  produced in the configuration according to the present embodiment in comparison with the distribution of the relative luminance of a planar light source apparatus of an existing configuration shown in  FIG. 20 . A curve indicated by a broken line in  FIG. 13  represents the luminance distribution of the planar light source apparatus of the existing configuration. The luminance distribution of the existing configuration indicated by the broken line exhibits very small variation and is substantially uniform. If the planar light source apparatus of the existing configuration is formed with a reduced thickness while the configuration is maintained, then the luminance exhibits a great variation depending upon the position as seen from an imaginary line (alternate long and two short dashes line) in  FIG. 13 . Therefore, the planar light source apparatus of the existing configuration having a reduced thickness is not suitable for a backlight for a display panel. 
         [0052]    Here, if the planar light source is formed with a reduced thickness similarly to that of the planar light source whose luminance is indicated by the imaginary line and the lens array sheet  120  and the light guide plate  130  having the configuration according to the present embodiment described above are incorporated, then a luminance distribution indicated by a solid line in  FIG. 13  is obtained. The luminance distribution indicated by the solid line is substantially equivalent to the luminance distribution of the existing planar light source apparatus whose thickness is not reduced in such a manner as described above. Thus, the luminance distribution indicates a good characteristic suitable to a backlight for a display panel. Accordingly, even if the planar light source is reduced in thickness from that of the existing planar light source, a luminance distribution similar to that of the existing planer light source can be achieved. 
         [0053]    It is to be noted that, while, in the embodiment described above, the light guide plate  130  is formed as a flattened transparent plate, it may otherwise be formed as a member having a light diffusing action or formed, for example, as a diffusing action portion  134  on the surface of a light guide plate  130 ′ as seen in  FIG. 14 . The lower face  131  remains in the form of a flattened face. The diffusing action portion  134  may be formed, for example, by forming vary small concave and convex portions on the surface. Or, the light guide plate  130  itself may not have a diffusion function, but alternatively a separate resin sheet having a diffusion action may be adhered to the surface of the light guide plate  130 . 
         [0054]    Further, while, in the embodiment described above, the lens projections  121  on the lens array sheet  120  are disposed continuously without leaving a gap therebetween on the lens array sheet  120  as seen in  FIG. 3 , they may otherwise be disposed with some gap left therebetween. Further, some difference may be provided to such gaps depending upon the position. 
         [0055]      FIG. 15  shows the configuration just mentioned in cross section. Referring to  FIG. 15 , in the lens array sheet  120 ′ shown, the lens projections  121  are disposed continuously without a gap left therebetween at positions directly above lamps  101  to  104 , but the lens projections  121  are disposed such that the arrangement distance therebetween increases as the distance from the position just above each lamp increases. By the arrangement of the lens projections  121 , at the position just above each lamp, a diffusion action by the lens projections  121  acts strongly, but at a position displaced from the position just above each lamp, the diffusion action is weak. Thus, the lens projections  121  contribute much to uniformization of the luminance distribution. 
         [0056]    Further, while, in the embodiment described above, a CCFL tube which emits light in a bar-like distribution is used as a light source, a light source of a different shape may be used alternatively. For example, even in the case of a bar-like light source, a fluorescent lamp curved in a U shape may be used. Or, a spot-like light source which emits light from a location similar to a point such as a light emitting diode (LED) may be used. The spot-like light source here is used in contrast with a bar-like light source but does not point to a point light source. 
         [0057]    Where a spot-like light source such as a light emitting diode is adopted, preferably the shape and the arrangement state of the lens projections to be disposed on the lay array sheet are set so as to be suitable for the spot-like light source. 
         [0058]      FIGS. 16 and 17  show an example of a planar light source apparatus wherein a light emitting diode is used as a light source, and particularly  FIG. 17  shows the apparatus of  FIG. 16  in an exploded fashion. Referring to  FIGS. 16 and 17 , in the planar light source apparatus shown, four light sources  211  each formed from a light emitting diode are disposed at different locations on the reflective plate  110 . A lens array sheet  220  is formed such that a plurality of continuous and circular lens projections  221  are disposed concentrically around the position just above each of the light sources  211 . The lens projections  221  may have a cross section similar to that of the lens projections  121  of the lens array sheet  120  shown in  FIG. 4 , that is, a cross section having the curved face portions  122  and the flat face portion  123 . The light guide plate  130  disposed on the lens array sheet  220  may have a configuration same as that of the light guide plate  130  described hereinabove. 
         [0059]    Where the planar light source apparatus has the configuration described above with reference to  FIGS. 16 and 17 , strong diffusion of light occurs at a position proximate to the position just above each light source  211 , and consequently, the luminance distribution of light emitted from the light guide plate  130  can be uniformized favorably. Therefore, reduction in thickness of a planar light source apparatus which uses the spot-like light source such as a light emitting diode can be anticipated. Also in the arrangement of  FIG. 16 , the distance of arrangement of the circular lens projections  221  may be provided and increased as the distance from each light source increases. Further, while, in the arrangement of  FIG. 16 , the lens projections  221  are not provided at positions spaced away from the light sources  211 , the circular lens projections  221  may otherwise be disposed over the overall surface of the lens array sheet  220 . However, where the lens projections  221  are disposed over the overall surface, the lens projections  221  at peripheral portions cannot have an annular shape. 
         [0060]    Further, a plurality of lens projections disposed on the lens array sheet may have a conically projecting shape. 
         [0061]      FIG. 18  shows an example of a configuration in this instance. Also in the example of  FIG. 18 , four light sources  211  each formed from a light emitting diode are disposed at different positions on the reflective plate  110 . A lens array sheet  320  is shaped such that conical lens projections  321  are disposed continuously longitudinally and transversely in a matrix. The lens projections  321  may have a sectional shape same as that of the lens projections  121  of the lens array sheet  120  shown in  FIG. 4 , that is, a shape having the curved face portions  122  and the flat face portion  123 . It is to be noted that, since the lens projections  321  have a conical shape, same cross sectional shapes are exhibited in whichever direction the section is taken, and the face corresponding to the curved face portions  122  has an annular shape and also the flat face portion  123  has a circular shape. The light guide plate  130  disposed on the lens array sheet  220  has a configuration same as that of the light guide plate  130  described hereinabove. 
         [0062]    Also where the configuration described above with reference to  FIG. 18  is adopted, incident light can be diffused efficiently by an action of the lens array sheet  320  and the light guide plate  130 , and reduction in thickness of a planar light source apparatus can be anticipated. 
         [0063]    It is to be noted that, also where the configuration shown in  FIG. 18  wherein the lens projections  321  are disposed is adopted, the lens projections  321  may be disposed continuously without a gap left therebetween at positions just above the light sources  211  while, at positions displaced from the positions mentioned, the lens projections  321  are disposed in a spaced relationship by some distance from each other. 
         [0064]      FIG. 19  shows an example of a configuration in this instance. Referring to  FIG. 19 , on a lens array sheet  320 ′ in the planar light source apparatus shown, the lens projections  321  of a conical shape are disposed such that, at positions just above the light sources  211 , the lens projections  321  are disposed continuously without leaving a gap therebetween. However, at positions displaced from the positions just above the light sources  211 , the lens projections  321  are disposed in a spaced relationship from each other. By varying the arrangement distance of the lens projections  321  as seen in  FIG. 19 , the diffusion state of light can be further uniformized. 
         [0065]    While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.