Optical waveguide plate for display

An optical waveguide plate for a display including a main optical waveguide plate body for introducing light from a light source thereinto, and surface-smoothing materials formed on both surfaces of the main optical waveguide plate body and having approximately the same optical refractive index as that of an optical waveguide plate. The main optical waveguide plate body is composed of a transparent material such as glass and acrylic resin, because it is necessary to totally reflect the introduced light. The surface-smoothing material is composed of, for example, a liquid having good wettability with respect to the main optical waveguide plate body. The range, in which the optical refractive index of the surface-smoothing material is approximately the same as the optical refractive index of the main optical waveguide plate body, lies in 0.8n.ltoreq.m.ltoreq.1.2n provided that the optical refractive index of the surface-smoothing material is m, and the optical refractive index of the main optical waveguide plate body is n. It is possible to decrease the plane roughness on the surface of the main optical waveguide plate body, substantially eliminate scratches, dirt and the like, and improve the contrast and brightness of the display.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical waveguide plate to be used for
 a display which consumes less electric power, and which has large screen
 brightness. In particular, the present invention relates to a structure of
 the optical waveguide plate to be used for the display for displaying, on
 the optical waveguide plate, a picture image corresponding to an image
 signal by controlling leakage light at a predetermined position on the
 optical waveguide plate by controlling the displacement action of an
 actuator element in a direction to make contact or separation with respect
 to the optical waveguide plate in accordance with an attribute of the
 image signal to be inputted.
 2. Description of the Related Art
 Those hitherto known as the display include, for example, cathode ray tubes
 (CRT) and liquid crystal display devices.
 Those known as the cathode ray tube include, for example, ordinary
 television receivers and monitor units for computers. Although the cathode
 ray tube has a bright screen, it consumes a large amount of electric
 power. Further, the cathode ray tube involves a problem that the depth of
 the entire display device is large as compared with the size of the
 screen.
 On the other hand, the liquid crystal display device is advantageous in
 that the entire device can be miniaturized, and the display device
 consumes a small amount of electric power. However, the liquid crystal
 display device involves problems that it is inferior in brightness of the
 screen, and the field angle of the screen is narrow.
 In the case of the cathode ray tube and the liquid crystal display device,
 it is necessary for a color screen to use a number of picture elements
 (image pixels) which is three times a number of picture elements used in a
 black-and-white screen. For this reason, other problems occur in that the
 device itself is complicated, a great deal of electric power is consumed,
 and it is inevitable that cost is increased.
 In order to solve the problems described above, the present applicant has
 suggested a novel display (see, for example, Japanese Laid-Open Patent
 Publication No. 7-287176). As shown in FIG. 14, this display includes
 actuator elements 100 arranged for respective picture elements. Each of
 the actuator elements 100 comprises a main actuator element 108 including
 a piezoelectric/electrostrictive layer 102 and an upper electrode 104 and
 a lower electrode 106 formed on upper and lower surfaces of the
 piezoelectric/electrostrictive layer 102 respectively, and a substrate 114
 including a vibrating section 110 and a fixed section 112 disposed under
 the main actuator element 108. The lower electrode 106 of the main
 actuator element 108 contacts with the vibrating section 110. The main
 actuator element 108 is supported by the vibrating section 110.
 The substrate 114 is composed of a ceramic in which the vibrating section
 110 and the fixed section 112 are integrated into one unit. A recess 116
 is formed in the substrate 114 so that the vibrating section 110 is
 thin-walled.
 A displacement-transmitting section 120 for obtaining a predetermined size
 of contact area with an optical waveguide plate 118 is connected with the
 upper electrode 104 of the main actuator element 108. In the illustrative
 display shown in FIG. 14, the displacement-transmitting section 120 is
 arranged such that it is located closely near to the optical waveguide
 plate 118 in the ordinary state in which the actuator element 100 stands
 still, while it contacts with the optical waveguide plate 118 in the
 excited state at a distance of not more than the wavelength of the light.
 The light 122 is introduced, for example, from a lateral end of the optical
 waveguide plate 118. In this arrangement, all of the light 122 is totally
 reflected at the inside of the optical waveguide plate 118 without being
 transmitted through front and back surfaces thereof by controlling the
 magnitude of the refractive index of the optical waveguide plate 118. In
 this state, a voltage signal corresponding to an attribute of an image
 signal is selectively applied to the actuator element 100 by the aid of
 the upper electrode 104 and the lower electrode 106 so that the actuator
 element 100 is allowed to make displacement in conformity with the
 ordinary state and the excited state. Thus, the displacement-transmitting
 section 120 is controlled for its contact and separation with respect to
 the optical waveguide plate 118. Accordingly, the scattered light (leakage
 light) 124 is controlled at a predetermined portion of the optical
 waveguide plate 118, and a picture image corresponding to the image signal
 is displayed on the optical waveguide plate 118.
 The display described above is advantageous, for example, in that (1) it is
 possible to decrease the electric power consumption, (2) it is possible to
 increase the screen brightness, and (3) it is unnecessary to increase the
 number of picture elements as compared with the black-and-white screen
 when the display is allowed to have a color screen.
 By the way, as shown in FIGS. 15 and 16, the optical waveguide plate 118 is
 composed of a transparent material such as glass and acrylic resin,
 because it is necessary that the light 122 introduced from a light source
 126 is totally reflected. However, it is feared that light emission (false
 light emission) occurs due to, for example, plane roughness, scratches,
 and dirt on the surface of the transparent material even at portions which
 should not be subjected to light emission, and the contrast of the display
 (ratio between the brightness of the display portion and the brightness of
 the non-display portion) is lowered. FIG. 15 shows an example of
 occurrence of the false light emission due to the scratch "a" formed on
 the surface of the optical waveguide plate 118. FIG. 16 shows an example
 of occurrence of the false light emission due to the dirt "b" adhered to
 the surface of the optical waveguide plate 118.
 Further, the light emission occurs at portions (non-display portions) which
 are not intended to effect light emission, and hence the incident light is
 decreased at portions (display portions) which are intended to effect
 light emission. Therefore, it is feared that the decrease in brightness
 would be caused.
 SUMMARY OF THE INVENTION
 The present invention has been made taking the foregoing problems into
 consideration, an object of which is to provide an optical waveguide plate
 for a display which makes it possible to decrease the plane roughness on
 the surface of a main optical waveguide plate body, substantially
 eliminate scratches, dirt and the like, and improve the contrast and the
 brightness of the display.
 At first, it is premised that an optical waveguide plate for a display
 according to the present invention is used for the display comprising a
 driving section including a number of actuator elements arranged
 corresponding to a large number of picture elements, in particular the
 display for displaying, on the optical waveguide plate, a picture image
 corresponding to an image signal by controlling leakage light at a
 predetermined portion of the optical waveguide plate by controlling
 displacement action of each of the actuator elements in a direction to
 make contact or separation with respect to the optical waveguide plate in
 accordance with an attribute of the image signal to be inputted.
 The optical waveguide plate according to the present invention is
 constructed such that a surface-smoothing material, which has
 substantially the same optical refractive index as that of a main optical
 waveguide plate body, is formed on at least one surface of the main
 optical waveguide plate body into which the light from a light source is
 introduced.
 Accordingly, even when the main optical waveguide plate body involves a
 great deal of plane roughness on the surface, or even when the scratch or
 the dirt exists on the surface, the surface of the main optical waveguide
 plate body is optically smooth owing to the surface-smoothing material.
 That is, the plane roughness on the surface of the main optical waveguide
 plate body is decreased, and the scratch and the dirt are substantially
 eliminated. As a result, it is possible to reduce the occurrence of
 leakage light which would be otherwise caused at portions (non-display
 portions) which are not intended to effect light emission. Thus, it is
 possible to improve the contrast (ratio between the brightness of the
 display portion and the brightness of the non-display portion) of the
 display.
 Moreover, the light component (leakage component), which has been hitherto
 leaked due to, for example, the scratch and the dirt existing on the
 surface of the main optical waveguide plate body, is reduced owing to the
 presence of the surface-smoothing material. Simultaneously, the light
 corresponding to the amount of reduction is utilized for light emission
 effected at the portions (display portions) which are intended to effect
 light emission. Therefore, the brightness of the display is improved as
 well.
 The reason why the optical refractive index of the surface-smoothing
 material is substantially the same as the optical refractive index of the
 optical waveguide plate at the main optical waveguide plate body is that
 it is intended to reduce reflection and scattering of light at the
 interface between the main optical waveguide plate body and the
 surface-smoothing material. In the present invention, it is preferable
 that the optical refractive index m of the surface-smoothing material
 satisfies 0.8n.ltoreq.m.ltoreq.1.2n provided that the optical refractive
 index of the main optical waveguide plate body is represented by n. More
 desirably, there is given 0.9n.ltoreq.m.ltoreq.1.1n.
 The surface-smoothing material may be a liquid having good wettability with
 respect to the optical waveguide plate. Alternatively, the
 surface-smoothing material may be a transparent resin layer such as those
 composed of an adhesive secured or glued to the main optical waveguide
 plate body. The liquid having good wettability is advantageous to form the
 smooth surface.
 A flat plate may be fixed on the surface-smoothing material. In this
 embodiment, it is preferable that the flat plate may be fixed to the main
 optical waveguide plate body by using a joining material with the
 surface-smoothing material interposed therebetween.
 The above and other objects, features, and advantages of the present
 invention will become more apparent from the following description when
 taken in conjunction with the accompanying drawings in which a preferred
 embodiment of th present invention is shown by way of illustrative
 example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Illustrative embodiments of the optical waveguide plate for the display
 according to the present invention (hereinafter simply referred to as
 "optical waveguide plate according to the embodiment") will be explained
 below with reference to FIGS. 1 to 13. Prior thereto, explanation will be
 made with reference to FIG. 1 for the arrangement of the display D to
 which the optical waveguide plate according to the embodiment of the
 present invention is applied.
 As shown in FIG. 1, the display D comprises an optical waveguide plate 12
 according to the embodiment of the present invention for introducing light
 10 thereinto, and a driving section 16 provided opposingly to the back
 surface of the optical waveguide plate 12 and including a large number of
 actuator elements 14 which are arranged in a matrix configuration or in a
 zigzag configuration corresponding to picture elements (image pixels).
 The driving section 16 includes a substrate 18 composed of, for example, a
 ceramic. The actuator elements 14 are arranged at positions corresponding
 to the respective picture elements on the substrate 18. The substrate 18
 has its first principal surface which is arranged to oppose to the back
 surface of the optical waveguide plate 12. The first principal surface is
 a continuous surface (flushed surface). Hollow spaces 20 for forming
 respective vibrating sections, as described later on, are provided at
 positions corresponding to the respective picture elements at the inside
 of the substrate 18. The respective hollow spaces 20 communicate with the
 outside via through-holes 18a each having a small diameter and provided at
 a second principal surface of the substrate 18.
 The portion of the substrate 18, at which the hollow space 20 is formed, is
 thin-walled. The other portion of the substrate 18 is thick-walled. The
 thin-walled portion has a structure which tends to undergo vibration in
 response to external stress, and it functions as a vibrating section 22.
 The portion other than the hollow space 20 is thick-walled, and it
 functions as a fixed section 24 for supporting the vibrating section 22.
 That is, the substrate 18 has a stacked structure comprising a substrate
 layer 18A as a lowermost layer, a spacer layer 18B as an intermediate
 layer, and a thin plate layer 18C as an uppermost layer. The substrate 18
 can be recognized as an integrated structure including the hollow spaces
 20 formed at the positions in the spacer layer 18B corresponding to the
 picture elements. The substrate layer 18A functions as a substrate for
 reinforcement, as well as it functions as a substrate for wiring. The
 substrate 18 may be sintered in an integrated manner, or it may be
 additionally attached.
 As shown in FIG. 1, each of the actuator elements 14 comprises the
 vibrating section 22 and the fixed section 24 described above, as well as
 a main actuator element 30 including a shape-retaining layer 26 composed
 of, for example, a piezoelectric/electrostrictive layer or an
 anti-ferroelectric layer directly formed on the vibrating section 22 and a
 pair of electrodes 28 (a row electrode 28a and a column electrode 28b)
 formed on an upper surface of the shape-retaining layer 26, and a
 displacement-transmitting section 32 connected onto the main actuator
 element 30, for increasing the contact area with respect to the optical
 waveguide plate 12 to obtain an area corresponding to the picture element.
 That is, the display D has the structure in which the main actuator
 elements 30 comprising the shape-retaining layers 26 and the pairs of
 electrodes 28 are formed on the substrate 18. The pair of electrodes 28
 may have a structure in which they are formed on upper and lower sides of
 the shape-retaining layer 26, or they are formed on only one side of the
 shape-retaining layer 26. However, in order to advantageously join the
 substrate 18 and the shape-retaining layer 26, it is preferable that the
 pair of electrodes 28 are formed only on the upper side (the side opposite
 to the substrate 18) of the shape-retaining layer 26 so that the substrate
 18 directly contacts with the shape-retaining layer 26 without any
 difference in height, as in the display D. In the illustrative arrangement
 shown in FIG. 1, for example, the row electrode 28a is led to the back
 surface side of the substrate 18 through the through-hole 34.
 Next, the operation of the display D constructed as described above will be
 briefly described with reference to FIG. 1. At first, the light 10 is
 introduced, for example, from the end portion of the optical waveguide
 plate 12. In this embodiment, all of the light 10 is totally reflected at
 the inside of the optical waveguide plate 12 without being transmitted
 through the front and back surfaces thereof by controlling the magnitude
 of the refractive index of the optical waveguide plate 12. In this
 embodiment, the optical waveguide plate 12 desirably has a refractive
 index n of 1.3 to 1.8, and more desirably 1.4 to 1.7.
 In this state, when a certain actuator element 14 is in the selected state,
 and the displacement-transmitting section 32 corresponding to the actuator
 element 14 contacts, at a distance of not more than the wavelength of
 light, with the back surface of the optical waveguide plate 12, then the
 light 10, which has been subjected to total reflection, is transmitted to
 the surface of the displacement-transmitting section 32 contacting with
 the back surface of the optical waveguide plate 12.
 The displacement-transmitting section 32 is provided to reflect the light
 10 transmitted through the back surface of the optical waveguide plate 12,
 and it is provided to increase the contact area with respect to the
 optical waveguide plate 12 to be not less than a predetermined size. That
 is, the light emission area is determined by the contact area between the
 displacement-transmitting section 32 and the optical waveguide plate 12.
 In the display D described above, the displacement-transmitting section 32
 includes a plate member 32a for determining the substantial light emission
 area, and a displacement-transmitting member 32b for transmitting the
 displacement of the main actuator element 30 to the plate member 32a.
 The contact between the displacement-transmitting section 32 and the
 optical waveguide plate 12 means the displacement-transmitting section 32
 and the optical waveguide plate 12 are positioned at a distance of not
 more than the wavelength of the light 10 (light 10 introduced into the
 optical waveguide plate 12).
 Once the light 10 arrives at the surface of the displacement-transmitting
 section 32, the light 10 is reflected by the surface of the
 displacement-transmitting section 32, and it behaves as scattered light
 36. A part of the scattered light 36 is reflected again in the optical
 waveguide plate 12. However, almost all of the scattered light 36 is not
 reflected by the optical waveguide plate 12, and it is transmitted through
 the front surface of the optical waveguide plate 12. The
 displacement-transmitting section 32 makes contact with the back surface
 of the optical waveguide plate 12 corresponding to the bending
 displacement of the main actuator element 30. When the
 displacement-transmitting section 32 contacts with the back surface of the
 optical waveguide plate 12, for example, the light 10, which has been
 totally reflected at the inside of the optical waveguide plate 12, is
 transmitted through the back surface of the optical waveguide plate 12.
 The light 10 is transmitted to the surface of the
 displacement-transmitting section 32, and it is reflected by the surface
 of the displacement-transmitting section 32. Accordingly, the picture
 element corresponding to the actuator element 14 is in the ON state.
 That is, the presence or absence of light emission (leakage light) at the
 front surface of the optical waveguide plate 12 can be controlled
 depending on the presence or absence of the contact of the
 displacement-transmitting section 32 disposed at the back of the optical
 waveguide plate 12. Especially, in the display device according to the
 embodiment of the present invention, one unit for making the displacement
 action of the displacement-transmitting section 32 in the direction to
 make contact or separation with respect to the optical waveguide plate 12
 is regarded as one picture element. A large number of the picture elements
 are arranged in a matrix configuration or in a zigzag configuration
 concerning the respective rows. Therefore, it is possible to display a
 picture image (characters and graphics) corresponding to the image signal
 on the front surface of the optical waveguide plate 12, i.e., on the
 display screen, in the same manner as the cathode ray tube, the liquid
 crystal display device, and the plasma display, by controlling the
 displacement action in each of the picture elements in accordance with the
 attribute of the inputted image signal.
 As shown in FIG. 2, the optical waveguide plate 12 according to the
 embodiment of the present invention comprises a main optical waveguide
 plate body 42 for introducing the light 10 from a light source 40
 thereinto, and surface-smoothing materials 44 having approximately the
 same optical refractive index as that of the optical waveguide plate 12
 and formed on both surfaces of the main optical waveguide plate body 42.
 The main optical waveguide plate body 42 is composed of a transparent
 material such as glass and acrylic resin, because it is necessary to
 totally reflect the introduced light 10. On the other hand, the
 surface-smoothing material 44 is composed of, for example, a liquid 46
 having good wettability with respect to the main optical waveguide plate
 body 42.
 Accordingly, even when the plane roughness is large, or even when the
 scratch "a" and the dirt "b" exist on the surface of the main optical
 waveguide plate body 42, the surface of the main optical waveguide plate
 body 42 is optically smooth owing to the presence of the surface-smoothing
 material 44. That is, the plane roughness of the surface of the main
 optical waveguide plate body 42 is decreased, and the scratch "a", the
 dirt "b" and the like are substantially eliminated. As a result, the
 occurrence of leakage light can be reduced at portions (non-display
 portions) which are not intended to effect light emission. It is possible
 to improve the contrast (ratio between the brightness of the display
 portion and the brightness of the non-display portion) of the display D.
 Moreover, the light component (leakage component), which has been hitherto
 leaked due to, for example, the scratch "a" and the dirt "b" existing on
 the surface of the main optical waveguide plate body 42, is reduced owing
 to the presence of the surface-smoothing material 44. Simultaneously, the
 light corresponding to the amount of reduction is utilized for light
 emission effected at the portion (display portion) intended to cause light
 emission. Therefore, the brightness of the display D is improved as well.
 The reason why the optical refractive index of the surface-smoothing
 material 44 is substantially the same as the optical refractive index of
 the main optical waveguide plate body 42 will now be explained.
 As shown in FIG. 3A, when the optical refractive index of the
 surface-smoothing material 44 is substantially the same as the optical
 refractive index of the main optical waveguide plate body 42, the light
 10, which has entered the surface-smoothing material 44 through the
 scratch "a" located on the surface of the main optical waveguide plate 42,
 is totally reflected by the interface between the surface-smoothing
 material 44 and the external space (air). That is, the light is not
 recognized as the scattered light 36, and no excessive light emission
 (false light emission) is caused at the non-display portion.
 On the other hand, when the optical refractive index of the
 surface-smoothing material 44 is different from the optical refractive
 index of the main optical waveguide plate body 42, the light 10, which has
 entered the surface-smoothing material 44 through the scratch "a" located
 on the surface of the main optical waveguide plate 42, is radiated as
 leakage light toward the outside through the interface between the
 surface-smoothing material 44 and the external space. That is, the light
 is recognized as the scattered light 36, and false light emission takes
 place.
 The range, in which the optical refractive index of the surface-smoothing
 material 44 is approximately the same as the optical refractive index of
 the main optical waveguide plate body 42, is represented by
 0.8n.sub.1.ltoreq.m.ltoreq.1.2n.sub.1 (more preferably,
 0.9n.sub.1.ltoreq.m.ltoreq.1.1 n.sub.1) provided that the optical
 refractive index of the surface-smoothing material 44 is represented by m,
 and the optical refractive index of the main optical waveguide plate body
 42 is represented by n.sub.1. When this condition is satisfied, the effect
 of the surface-smoothing material 44 as described above can be
 sufficiently exhibited.
 In the embodiment shown in FIG. 2, the liquid 46 for constructing the
 surface-smoothing material 44 is formed on the both surfaces of the main
 optical waveguide plate body 42. However, it is not necessarily
 indispensable to form the liquid 46 on the surface of the driving section.
 Next, modified embodiments of the optical waveguide plate 12 according to
 the embodiment described above will be explained with reference to FIGS. 4
 to 9.
 As shown in FIG. 4, when the main optical waveguide plate body 42 is
 composed of, for example, an acrylic resin, the surface tends to suffer
 scratches "a". Further, the plate made of the acrylic resin is
 insufficient in rigidity as compared with the glass plate, and hence such
 a plate tends to suffer from waviness and warpage.
 Thus, as shown in FIG. 5, an optical waveguide plate 12A according to this
 modified embodiment comprises joining layers (adhesive layers) 50 which
 are composed of, for example, a transparent resin and which are formed on
 both surfaces of a main optical waveguide plate body 42, and rigid
 transparent plates 52 which are thinner than the main optical waveguide
 plate body 42 and which are joined (glued) onto the joining layers 50. In
 this embodiment, the surface-smoothing material 44 is constructed by the
 joining layer 50. Those usable as the rigid transparent plate 52 include,
 for example, white glass plates and inexpensive colored glass plates.
 In this embodiment, the main optical waveguide plate body 42 has a
 thickness t.sub.1 of about 10 mm to 200 mm. The rigid transparent plate
 has a thickness t.sub.2 of about 0.1 mm to 3 mm. On this assumption, it is
 desirable to give 5t.sub.2.ltoreq.t.sub.1.ltoreq.500t.sub.2. When the
 colored glass plate is used as the rigid transparent plate 52, it is
 desirable to set the thicknesses t.sub.1 and t.sub.2 to satisfy
 10t.sub.2.ltoreq.t.sub.1.ltoreq.500t.sub.2 so that the optical path of the
 light passing through the glass plate is short, in order to avoid
 discrepancy of the objective color of the light and to avoid loss due to
 the absorption of light.
 The optical refractive index m of the joining layer 50 is approximately the
 same as the optical refractive index n.sub.1 of the main optical waveguide
 plate body 42. Also in this case, it is desirable to satisfy
 0.8n.sub.1.ltoreq.m.ltoreq.1.2n.sub.1, and more preferably
 0.9n.sub.1.ltoreq.m.ltoreq.1.1n.sub.1. In relation to the optical
 refractive index n.sub.2 of the rigid transparent plate, it is desirable
 to satisfy 0.8n.sub.2.ltoreq.m.ltoreq.1.2n.sub.2, and more preferably
 0.9n.sub.2.ltoreq.m.ltoreq.1.1n.sub.2.
 In this embodiment, for example, the optical path followed in the optical
 waveguide plate 12A is represented by optical paths shown in FIGS. 6 and
 7. FIGS. 6 and 7 are depicted assuming that the optical refractive indices
 of the main optical waveguide plate body 42, the joining layer 50, the
 rigid transparent plate 52, and the external space are n.sub.1, m
 (=n.sub.1), n.sub.2, and no (air=1.0) respectively.
 The embodiment shown in FIG. 6 illustrates an example of the use of a rigid
 transparent plate 52 having its optical refractive index n.sub.2 which is
 larger than the optical refractive index n.sub.1 of the main optical
 waveguide plate body 42. All of the light on the optical path, which
 satisfies the condition described below, is totally reflected by the
 interface between the rigid transparent plate 52 and the external space,
 and the light returns into the main optical waveguide plate body 42.
 The optical path, which satisfies the condition, is the optical path other
 than optical paths along which the light scattered from the light emission
 portion (display portion) follows, i.e., the optical path along which the
 light not intended to effect leakage to the outside follows. Specifically,
 the optical path is represented by an optical path in which the angle of
 incidence .theta..sub.1 into the interface between the joining layer 50
 and the rigid transparent plate 52 is not less than a critical angle
 .theta.c.sub.10, and the angle of outgoing radiation .theta..sub.2 from
 the interface is not less than a critical angle .theta.c.sub.20.
 It is noted that there are given:
EQU critical angle .theta.c.sub.10 =sin.sup.-1 (n.sub.0 /n.sub.1);
EQU critical angle .theta.c.sub.20 =sin.sup.-1 (n.sub.0 /n.sub.2).
 The embodiment shown in FIG. 7 illustrates an example of the use of a rigid
 transparent plate 52 having its optical refractive index n.sub.2 which is
 smaller than the optical refractive index n.sub.1 of the main optical
 waveguide plate body 42. Also in this case, all of the light on the
 optical path, which satisfies the condition described above, i.e., the
 light, which follows the optical path and which is not intended to cause
 leakage to the outside, is totally reflected by the interface between the
 rigid transparent plate 52 and the external space, and the light returns
 into the main optical waveguide plate body 42.
 In the embodiment described above, it has been demonstrated that when the
 optical refractive index m of the joining layer 50 is approximately the
 same as the optical refractive index n.sub.1 of the main optical waveguide
 plate body 42, the light, which is not intended to cause leakage to the
 outside, is totally reflected by the interface between the rigid
 transparent plate 52 and the external space. However, as shown in FIG. 8,
 even when the optical refractive index m of the joining layer 50 is
 different from the optical refractive index n.sub.1 of the main optical
 waveguide plate body 42 (m.noteq.n.sub.1), the light, which is not
 intended to cause leakage to the outside, can be totally reflected by the
 interface between the rigid transparent plate 52 and the external space in
 the same manner as described above provided that the relative magnitude of
 the optical refractive index satisfies n.sub.0 &lt;n.sub.1 &lt;n.sub.2 &lt;m
 (wherein .theta..sub.2 &lt;.theta..sub.3 &lt;.theta..sub.1).
 However, the optical refractive index m of the joining layer 50 is
 excessively large as compared with the optical refractive index n.sub.1 of
 the main optical waveguide plate body 42 and the optical refractive index
 n.sub.2 of the rigid transparent plate 52, the catoptric light is
 generated at the interface between the main optical waveguide plate body
 42 and the joining layer 50 or at the interface between the rigid
 transparent plate 52 and the joining layer 50. For this reason, a problem
 arises in that the display is darkened as a whole.
 Therefore, the optical refractive index m of the joining layer 50 desirably
 satisfies, in relation to the main optical waveguide plate body 42,
 0.8n.sub.1.ltoreq.m.ltoreq.1.2n.sub.1, and preferably
 0.9n.sub.1.ltoreq.m.ltoreq.1.1n.sub.1. The optical refractive index m of
 the joining layer 50 desirably satisfies, in relation to the rigid
 transparent plate 52, 0.8n.sub.2.ltoreq.m.ltoreq.1.2n.sub.2, and
 preferably 0.9n.sub.2.ltoreq.m.ltoreq.1.1n.sub.2.
 Those preferably used for the joining layer 50 include thermosetting resins
 (including ultraviolet curable resins) and thermoplastic resins having
 transparency in the visible light wavelength region, such as those
 composed of acrylic, unsaturated polyester, silicone, phenol,
 polyethylene, and epoxy compounds.
 The relationship between the angle of incidence and the angle of outgoing
 radiation is shown in FIG. 9 when a surface-smoothing material 44 having a
 multiple-layered structure is used on the main optical waveguide plate
 body 42. For example, when a surface-smoothing material 44 having a
 three-layered structure is exemplified, the following expressions are
 given according to the Snell's law, provided that the optical refractive
 indexes of a first layer 44a, a second layer 44b, and a third layer 44c
 are n.sub.1, n.sub.2, and n.sub.3 respectively, the angle of incidence
 into the interface between the first layer 44a and the second layer 44b is
 .theta..sub.1, the angle of outgoing radiation from the interface (=the
 angle of incidence into the interface between the second layer 44b and the
 third layer 44c) is .theta..sub.2, and the angle of outgoing radiation
 from the interface between the second layer 44b and the third layer 44c is
 .theta..sub.3.
EQU n.sub.1 sin .theta..sub.1 =n.sub.2 sin .theta..sub.2
EQU n.sub.2 sin .theta..sub.2 =n.sub.3 sin .theta..sub.3
 Consequently, the following expressions are given.
EQU n.sub.1 sin .theta..sub.1 =n.sub.3 sin .theta..sub.3
 .theta..sub.3 =sin.sup.-1 {(n.sub.1 /n.sub.3)sin .theta..sub.1 }
 That is, the angle of outgoing radiation .theta..sub.3 from the interface
 between the second layer 44b and the third layer 44c depends on only the
 angle of incidence .theta..sub.1 into the interface between the first
 layer 44a and the second layer 44b, and it does not depend on the optical
 refractive index n.sub.2 of the intermediate second layer 44b. This fact
 is true for any surface-smoothing material 44 having three or more layers.
 The angle of outgoing radiation from the interface between the uppermost
 layer and the layer just thereunder depends on only the angle of incidence
 .theta..sub.1 into the interface between the first layer and the second
 layer. However, it is assumed that there is no intermediate portion at
 which total reflection occurs. If there is any intermediate portion at
 which total reflection occurs, the display brightness is darkened.
 Therefore, it is desirable that the optical refractive indexes n.sub.1,
 n.sub.2, and n.sub.3 are allowed to have the same relationship as those
 described above.
 As described above, according to the optical waveguide plate 12A concerning
 the modified embodiment, the rigid transparent plates 52 are joined via
 the joining layers 50 on the both surfaces of the main optical waveguide
 plate body 42. Therefore, it is possible to increase the strength of the
 optical waveguide plate 12A, making it possible to apply the optical
 waveguide plate 12A to the large screen specification. Further, the rigid
 transparent plate 52 scarcely suffers from the scratch "a" as compared
 with the main optical waveguide plate body 42. Therefore, the formation of
 scratch is reduced on the display surface of the optical waveguide plate
 12A. Moreover, for example, warpage and waviness are absorbed by the
 intervening joining layer 50. Thus, the optical waveguide plate 12A having
 high flatness as a whole is provided. Accordingly, it is possible to
 reduce any excessive light emission (false light emission) at the
 non-display portion, and it is possible to improve the contrast and the
 brightness of the display D.
 In the optical waveguide plate 12A concerning the modified embodiment
 described above, the rigid transparent plates 52 are glued through the
 joining layers 50 to the both surfaces of the main optical waveguide plate
 body 42 respectively. Alternatively, an arrangement as shown in FIG. 10
 may be adopted. That is, rigid transparent plates 52 are fixed by using a
 joining material 54 while providing a certain degree of gaps over both
 surfaces of the main optical waveguide plate 42 respectively. A liquid 46
 is injected into the gaps followed by drying and solidification.
 A transparent film, which is composed of, for example, fluoride or oxide
 such as SiO.sub.2, MgF.sub.2, LaF.sub.3, MgO, SiO, and NdF.sub.3, may be
 formed on the rigid transparent plate 52 or the transparent resin layer as
 the surface-smoothing material 44.
 In this embodiment, the film functions to eliminate scratches during the
 use, and it functions as an anti-reflection film, which is preferred.
 The shape-retaining layer 26 of the main actuator element 30 will now be
 briefly described. When the piezoelectric/electrostrictive layer is used
 as the shape-retaining layer 26, those usable as the
 piezoelectric/electrostrictive layer include ceramics containing, for
 example, lead zirconate, lead magnesium niobate, lead nickel niobate, lead
 zinc niobate, lead manganese niobate, lead magnesium tantalate, lead
 nickel tantalate, lead antimony stannate, lead titanate, barium titanate,
 lead magnesium tungstate, and lead cobalt niobate, as well as any
 combination of them. It is needless to say that the major component
 contains the compound as described above in an amount of not less than 50%
 by weight. Among the ceramics described above, the ceramic containing lead
 zirconate is most frequently used as the constitutive material of the
 piezoelectric/electrostrictive layer according to the embodiment of the
 present invention.
 When the piezoelectric/electrostrictive layer is composed of a ceramic, it
 is also preferable to use ceramics obtained by appropriately adding, to
 the ceramics described above, oxide of, for example, lanthanum, calcium,
 strontium, molybdenum, tungsten, barium, niobium, zinc, nickel, and
 manganese, or any combination thereof or another type of compound thereof.
 For example, it is preferable to use a ceramic containing a major
 component composed of lead magnesium niobate, lead zirconate, and lead
 titanate and further containing lanthanum and strontium.
 The piezoelectric/electrostrictive layer may be either dense or porous.
 When the piezoelectric/electrostrictive layer is porous, its porosity is
 preferably not more than 40%.
 When the anti-ferroelectric layer is used as the shape-retaining layer 26,
 it is desirable to use, as the anti-ferroelectric layer, a compound
 containing a major component composed of lead zirconate, a compound
 containing a major component composed of lead zirconate and lead stannate,
 a compound obtained by adding lanthanum to lead zirconate, and a compound
 obtained by adding lead zirconate and lead niobate to a component composed
 of lead zirconate and lead stannate.
 Especially, when an anti-ferroelectric film, which contains a component
 comprising lead zirconate and lead stannate as represented by the
 following composition, is applied as a film-type element such as the
 anti-ferroelectric film-type element, it is possible to perform driving at
 a relatively low voltage. Therefore, application of such an
 anti-ferroelectric film is especially preferred.
 Pb.sub.0.99 Nb.sub.0.02 [(Zr.sub.x Sn.sub.1-x).sub.1-y Ti.sub.y ].sub.0.98
 O.sub.3
 wherein, 0.5&lt;x&lt;0.6, 0.05&lt;y&lt;0.063, 0.01&lt;Nb&lt;0.03
 The anti-ferroelectric layer may be porous. When the anti-ferroelectric
 layer is porous, it is desirable that the porosity is not more than 30%.
 The optical waveguide plate for the display according to the present
 invention has been specifically explained on the basis of the optical
 waveguide plate according to the embodiment of the present invention and
 the optical waveguide plates according to the modified embodiments
 thereof. However, the present invention should not be interpreted as those
 limited by the embodiment and the modified embodiments, to which, for
 example, various modification, correction, and improvement may be added
 without deviating from the scope of the present invention.
 EXAMPLE 1
 As shown in FIGS. 11 and 12, Example 1 and Comparative Example were
 prepared. In Example 1, a liquid 46 having an optical refractive index of
 1.48 was applied to a display surface and an opposite surface of a main
 optical waveguide plate body 42 made of an acrylic resin having a shape of
 200 mm.times.300 mm.times.10 mm and having an optical refractive index of
 1.48 (see FIG. 2). In Comparative Example, nothing was applied to the main
 optical waveguide plate body 42.
 Fifteen scattering elements (for example, aluminum pieces) 60 were arranged
 and glued in a matrix configuration (three ones in the vertical direction
 and five ones in the lateral direction) on the respective back surfaces
 (surfaces opposite to the display surfaces) of Example 1 and Comparative
 Example respectively. A light source 40 was arranged at the side surface
 of the optical waveguide plate 12, and the light 10 was introduced into
 the optical waveguide plate 12 from the light source 40 (see FIG. 12).
 In this arrangement, as shown in FIG. 13, the light 10, which has arrived
 at the surface of the scattering element 60, is reflected by the surface
 of the scattering element 60, and it behaves as scattered light 36. The
 scattered light 36 outgoes from the display surface of the optical
 waveguide plate 12. The portion of light emission effected by the
 scattering element 60 serves as the display portion of the optical
 waveguide plate 12.
 The brightness at the display portion and the brightness at the portion
 different from the display portion (i.e., non-display portion) were
 measured for Example 1 and Comparative Example. As a result, in Example 1,
 the display brightness at the display portion was 7900 cd/m.sup.2, and the
 brightness at the non-display portion was 40 cd/m.sup.2. In Comparative
 Example, the display brightness at the display portion was 7700
 cd/m.sup.2, and the brightness at the non-display portion was 65
 cd/m.sup.2.
 As understood from the result of measurement described above, the
 brightness at the non-display portion was decreased, and the display
 brightness at the display portion was improved in Example 1 as compared
 with Comparative Example. Further, in Example 1, the uniformity of
 brightness was also improved.
 EXAMPLE 2
 Next, Example 2 and Comparative Example were prepared.
 In Example 2, glass plates were glued by using an acrylic adhesive having
 an optical refractive index of 1.48 to a display surface and an opposite
 surface of a main optical waveguide plate body 42 made of an acrylic resin
 having a shape of 200 mm.times.300 mm.times.10 mm and having an optical
 refractive index of 1.48 (see FIG. 5). In Comparative Example, nothing was
 applied to the main optical waveguide plate body 42. Fifteen scattering
 elements 60 were arranged and glued in a matrix configuration in the same
 manner as described above. A light source 40 was arranged at the side
 surface of the optical waveguide plate 12, and the light was introduced
 into the optical waveguide plate 12 from the light source 40 (see FIG.
 12).
 The brightness at the display portion and the brightness at the portion
 different from the display portion (i.e., non-display portion) were
 measured for Example 2 and Comparative Example. As a result, the
 brightness at the non-display portion was decreased, and the display
 brightness at the display portion was improved in Example 2 as compared
 with Comparative Example in the same manner as described above. Further,
 in Example 2, the uniformity of brightness was proved to be improved.
 Especially, in Example 2, it was proved that the display surface of the
 optical waveguide plate 12 scarcely suffered from scratches,
 simultaneously with which the rigidity was improved, and the warpage was
 decreased.
 As explained above, according to the optical waveguide plate for the
 display, it is possible to decrease the plane roughness on the surface of
 the main optical waveguide plate body, it is possible to substantially
 eliminate scratches, dirt and the like, and it is possible to improve the
 contrast and the brightness of the display.