Light guide panel and plane illuminator apparatus

A light guide panel includes a front surface portion, a back surface portion located on the opposite side of the front surface portion, and an incidence end surface portion located on one side of the front and the back surface portions. The incidence end surface portion is configured for introducing light from a light source into the light guide panel. A plurality of optical elements are randomly disposed on the front and/or back surface portions. The optical elements typically have a maximum diameter in a range from 10 .mu.m to 150 .mu.m and are configured for emanating light propagating in the light guide panel from the front surface portion and/or back surface portion to the outside of the light guide panel.

This application is based on Patent Application Nos. 367,878/1997 filed on
 Dec. 29, 1997 in Japan and 146,354/1998 filed on May 27, 1998 in Japan,
 the content of which is incorporated hereinto by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a light guide panel in which light
 introduced from a side end surface emanates from a front or back surface,
 the light guide panel being used in a plane illuminator apparatus, and
 more specifically to plane illuminator apparatus for use in illumination
 of a display using a transmission or a reflection type liquid crystal
 device.
 2. Description of the Prior Art
 A plane illuminator apparatus used as the so-called backlight for a liquid
 crystal is adapted such that light from a light source is guided into a
 transparent light guide panel from a side end surface of the same. The
 light uniformly emanates from a front or back entire surface area of the
 light guide panel by making use of the reflection of the light in the
 light guide panel. Considering the characteristics of a liquid crystal
 display for which a plane illuminator apparatus is used, it is
 particularly important that the plane illuminator apparatus emit light
 uniformly over the whole of the apparatus. It is also preferred that the
 apparatus be thin plate-shaped as a whole and that power consumption of a
 corresponding light source be reduced to the utmost.
 To achieve such requirements, a conventional plane illuminator apparatus
 enjoys a structure in which there are superimposed two prism sheets one on
 the other. A light reflection sheet is provided on a back surface side of
 a light guide panel and a plurality of isosceles triangle pillar-like
 shaped prisms are parallely arranged on the front surface side of the
 light guide panel such that these prisms cross perpendicularly to each
 other in their longitudinal directions. More specifically, light emanating
 from the back surface side of the light guide panel is forced to again
 impinge the light guide panel with the aid of the light reflection sheet
 and light emanating from the surface of the light guide panel is converged
 with a pair of the prism sheets to ensure highly bright illumination
 light.
 There is further known a light guide panel in which light incident on the
 light guide panel is intended to be uniformly distributed. For these
 innumerable dots with the size from several hundred micrometers to several
 millimeters, white colored ink is printed on the back surface of the light
 guide panel. For preventing these dots from obstructing the view, a light
 diffusion sheet is interposed between the light guide panel and the prism
 sheet. The light diffusion sheet disperses light emanating from the front
 surface of the light guide panel.
 The light emanating from the front surface of the light guide panel suffers
 ordinarily from uncontrollable directivity which is dependent upon the
 physical properties of the light guide panel. A direction where maximum
 brightness is obtained and a desired direction of a view is obtained are
 practically not coincident with each other. Therefore, simple
 superposition of the prism sheet on the front surface of the light guide
 panel causes the light from the light guide panel to emanate only from one
 slope side of the prism, so that light intensity with uniform distribution
 is not obtained.
 Prior art in which dots formed with white colored ink are printed on the
 back surface of the light guide panel, and light propagating in the light
 guide panel is diffused, suffers from occurrence of absorption loss of the
 light. Further, in order to prevent these dots from becoming conspicuous,
 it is necessary to jointly use the light diffusion sheet so that most of
 the light passing through the light diffusion sheet becomes diffusion
 light to result in sharp reduction of brightness. As a result, a bright
 light source with a greater intensity of radiation must be used. It is
 further quite impossible to control the propagation direction of the
 light.
 The Conventional prism sheet is capable of converging the diffusion light
 from the light guide panel. Light emanating from the light guide panel,
 however, contains components which are not perpendicular to the surface of
 the light guide panel and are generally inclined in the direction where
 they are separated from the light source. Thus, the Conventional prism
 sheet cannot deflect the light emanating from the front surface of the
 light guide panel in a desired direction, i.e., perpendicularly to the
 front surface of the light guide panel.
 The Conventional plane illuminator apparatus uses two prism sheets
 superimposed on the light guide panel in addition to the diffusion sheet.
 This large number of parts makes it impossible to reduce the thickness
 thereof as a whole. Severe optical loss happens owing to the diffusion
 sheet and owing to reflection on an interface between the two prism
 sheets. As such, is necessary to use a bright light source with a greater
 intensity of radiation. This causes a factor of preventing the whole of
 the plane illuminator apparatus from being made compact and the apparatus
 from being made low power consumption.
 A reflection type liquid crystal display has higher image contrast than a
 transmission type liquid crystal display to ensure an excellent quality
 image, but it requires illumination by outdoor daylight and hence it can
 not be used at night or in a room and the like without any illumination.
 Accordingly, a transmission type liquid display in which a backlight
 source can be incorporated is solely used as a display mounted on a
 portable small-sized computer.
 There are situations where effective illumination is required for a limited
 area without use of a large-scaled illuminator instrument without limiting
 to such a reflection type liquid crystal display.
 For such a requirement, a method is used conventionally in which an object
 to be observed is directly illuminated with an LED or a midget lamp.
 Recently, a small-sized illuminator apparatus has been designed for
 reflection type liquid crystal display comparable with the backlight
 source for use in a transmission type liquid crystal display such that a
 reflection type liquid crystal display excellent in view of image quality
 is usable at night or in a room without any illumination.
 However, the conventional illuminator apparatus has the problem that it is
 difficult for the apparatus to achieve uniform illumination because the
 apparatus illuminates an object to be observed from the side thereof or
 obliquely from the front with light from an LED or a midget lamp. It is
 therefore difficult to apply the apparatus to a reflection type liquid
 crystal display.
 Further, there is a possibility that when an illumination light source is
 located between an observation point and an object to be observed, the
 illumination light source interferes with a liquid crystal panel and hence
 the whole of the object fails to be observed.
 SUMMARY OF THE INVENTION
 It is a first object of the present invention to provide a light guide
 panel which is capable of emitting high brightness light with reduced loss
 and with uniform distribution.
 It is a second object of the present invention to provide a plane
 illuminator apparatus in which not only optical loss is reduced but also
 the number of parts is reduced. The plane illuminator apparatus is compact
 and has low power consumption.
 In a first aspect of the present invention a light guide panel comprises a
 front surface portion, a back surface portion located on the opposite side
 of the front surface portion, and an incidence end surface portion located
 on one end sides of the front and back surface portions for introducing
 light from a light source. The light incident from the incidence end
 surface portion is forced to emanate from at least one of the front and
 back surface portions. A plurality of optical elements are provided on at
 least one of the front and back surface portions in a random manner. The
 optical elements emanate light propagating in the light guide panel from
 at least one of the front and back surface portions to the outside of the
 light guide panel.
 In accordance with the present invention, light from a light source
 incident from the incidence end surface portion is changed in its progress
 direction with the optical element provided on at least one of the front
 and back surface portions of the light guide panel. The light is finally
 emanated from the front surface portion and/or the back surface portion of
 the light guide panel to the outside of the light guide panel.
 In the light guide panel according to the first aspect of the present
 invention, the size of the optical element ranges preferably from 10 .mu.m
 to 150 .mu.m.
 The optical element may have an inclined plane where an interval between
 the inclined plane and the front surface portion and/or the back surface
 portion, on which the optical element is provided, is increased as it goes
 to the incidence end surface portion side. Herein, an angle .gamma. formed
 between the inclination plane and the front surface portion and/or the
 back surface portion preferably satisfies:
EQU {(2.pi./9)-(.beta./2)}.ltoreq..gamma..ltoreq.{(11.pi./36)-(.beta./2)},
 where .beta.=sin.sup.-1 (1/n), n is a refractive index of a material
 constructing the light guide panel, .pi. is the circular constant of a
 circle to its diameter.
 Otherwise, the optical element may include part of a convex spherical
 surface having a arc surface of a predetermined radius of curvature in a
 plane perpendicular to the front surface portion and/or the back surface
 portion, on which the optical element is provided, and the incidence end
 surface portion. Herein, a relationship between the radius R of curvature
 of the arc surface and an amount h of protrusion of the arc surface from
 the front surface portion and/or the back surface portion on which the
 optical element is provided satisfies:
EQU h=R(1-cos .epsilon.)
 and
EQU {(2.pi./9)-(.beta./2)}.ltoreq..epsilon..ltoreq.{(11.pi./36)-(.beta./2)}
 where R=r/sin .epsilon., and r is the radius of curvature of the optical
 element.
 The optical element may be adapted such that the contour configuration
 projected perpendicularly onto the front surface portion and/or the back
 surface portion, on which the optical element is provided, comprises a
 triangle with one side thereof set substantially in parallel to the
 incidence end surface portion. In this case, the optical element may be a
 triangular pyramid including a pyramid surface inclined with respect to
 the front surface portion and/or the back surface portion on which the
 optical element is provided. The triangular pyramid includes the one side
 and a pair of pyramid surfaces substantially perpendicular to the front
 surface portion and/or the back surface portion on which the optical
 element is provided. Alternatively, the optical element may be configured
 into a triangular pillar.
 The optical element may be provided exclusively on only one of the front
 and back surface portions, and a light deflection means may be provided on
 the other of the surface portions for deflecting the light in a
 predetermined direction. Herein, the light deflection means may be one
 having a concave or convex surface of the predetermined radius of
 curvature extending perpendicularly to the incidence end surface portion
 and alternately arranged perpendicularly to the extension direction.
 Alternatively, the light deflection means may be one having a plurality of
 triangular pillar-like shaped prism surfaces arranged perpendicularly to
 the extension direction.
 A second aspect of the present invention is a light guide panel which
 includes a front surface portion, a back surface portion located on the
 opposite side of the front surface portion, and an incidence end surface
 portion located on one end sides of these front and back surface portions
 for introducing light from a light source. The light guide panel is
 disposed between an observation position and an object to be observed. A
 plurality of optical elements are provided on at least one of the front
 and back surface portions randomly for emanating light propagating in the
 light guide panel from the back surface portion to the outside of the
 light guide panel. The total area of these plurality of the optical
 elements is set within a range of 1 to 20% of an area of the front surface
 portion or the back surface portion.
 In accordance with the present invention, illumination light from a light
 source impinges into the light guide panel from the incidence end surface
 portion of the light guide panel and propagates with total reflect in the
 light guide panel. Part of the light emanates from the back surface
 portion to the outside of the light guide panel by the optical element and
 illuminates an object to be illuminated. That is, a luminous flux having a
 cross sectional configuration substantially corresponding to that of the
 back surface portion of the light guide panel illuminates the object. The
 object illuminated in such a manner is observed from an observation point
 through the light guide panel.
 In the light guide panel according to the second aspect of the present
 invention, the size of the optical element preferably ranges from 10 .mu.m
 to 150 .mu.m.
 The optical element may be part of a convex spherical surface having a arc
 surface having the predetermined radius of curvature in a plane
 perpendicular to the front surface portion and/or the back surface
 portion, on which the optical element is provided, and the incidence end
 surface portion. Herein, when the refractive index of a material
 constructing the light guide panel is represented by n, the circular
 constant of a circle to its diameter is represented by .pi., the radius of
 curvature of the optical element is represented by r, a relationship
 between the radius R of curvature of the arc surface and the amount h of
 protrusion of the arc surface from the front surface portion and/or the
 back surface portion on which the optical element is provided satisfies:
EQU h=R(1-cos .epsilon.)
 and
EQU {(2.pi./9)-(.beta./2)}.ltoreq..epsilon..ltoreq.{(11.pi./36)-(.beta./2)},
 where .beta.=sin.sup.-1 (1/n), R=r/sin .epsilon..
 The optical element may be adapted such that a contour configuration
 thereof projected perpendicularly to the front surface portion and/or the
 back surface portion on which the optical element is provided is a
 triangle with its side being substantially parallel to the incidence end
 surface portion. Herein, the optical element is preferably a triangular
 pyramid having a pyramid surface inclined to the front surface portion
 and/or the back surface portion on which the optical element is provided.
 The triangular pyramid includes the one side and a pair of pyramid
 surfaces substantially perpendicular to the front surface portion and/or
 the back surface portion.
 A third aspect of the present invention is a plane illuminator apparatus
 wherein it includes a light guide panel including a front surface portion,
 a back surface portion located on the opposite side of the front surface
 portion, and an incidence end surface portion located on one end sides of
 these front and back surface portions. A light source projects light into
 the light guide panel from the incidence end surface portion of the light
 guide panel. A light reflection sheet covers the light guide panel
 excepting the front surface portion and the incidence end surface portion
 of the same. A plurality of optical elements are provided randomly on at
 least one of the front and back surface portions for emanating light
 propagating in the light guide panel from at least one of the front and
 back surface portions to the outside of the light guide panel.
 In accordance with the present invention, light incident into the light
 guide panel from the incidence end surface portion is changed in its
 progress direction with an optical element provided on at least one of the
 front and back surface portions of the light guide panel. The light is
 emanated from the front surface portion and/or the back surface portion of
 the light guide panel to the outside of the light guide panel. The light
 emanating from the light guide panel excepting the front surface and the
 incidence end surface is again introduced into the light guide panel with
 the light reflection sheet. The light is finally emanated from the front
 surface portion of the light guide panel.
 In the plane illuminator apparatus according to the third aspect of the
 present invention, the optical elements may be provided exclusively on
 only one of the front and back surface portions, and light deflection
 means may further be provided on the other of the front and back surface
 portions for deflecting the light in a predetermined direction.
 The optical element in the present invention is desirably set such that a
 rate of the optical element occupied by the front surface portion and/or
 the back surface portion per unit area on which the optical element is
 provided is larger as it goes away from the incidence end surface portion.
 A prismatic-shaped surface of the light deflection plate alternately has a
 first inclined surface where an interval with a plane part is increased as
 it goes to an incidence end surface portion side. A second inclined
 surface is disposed next to the first inclination surface. An angle
 between the plane part and the first inclined surface is effectively
 smaller than an angle between the plane part and the second inclined
 surface.
 In accordance with the light guide panel and the plane illuminator
 apparatus of the present invention, there is provided the plurality of the
 optical elements on at least one of the front and back surface portions of
 the light guide panel for emanating light incident from the incidence end
 surface portion of the light guide panel from at least one of the front
 and back surface portions. The light from the light source incident into
 the light guide panel from the incidence end surface portion is changed in
 its progress direction with the optical elements provided on the light
 guide panel. The light can be finally emanated from at least one of the
 front and back surface portions to the outside of the light guide panel.
 Light deflection means for deflecting the light in a predetermined
 direction is provided on a side of the front and back surface portions of
 the light guide panel where no optical element is provided. It is thus
 possible to deflect light emanating from the front surface portion of the
 light guide panel to the outside of the same in a predetermined direction.
 It is also possible to deflect light emanating from the back surface
 portion of the light guide panel to the outside of the light guide panel
 or deflect light incident into the light guide panel from the back surface
 portion with the aid of the light reflection sheet in a predetermined
 direction.
 When the light deflection means is provided in a united manner on a side of
 the light guide panel where the optical elements on the front surface
 portion and the back surface portion of the light guide panel are not
 provided, it is possible to eliminate the prism sheet used conventionally.
 As a result, a further thinner plane illuminator apparatus with reduced
 optical loss and power consumption is obtained.
 The above and other objects, effects, features and advantages of the
 present invention will become more apparent from the following description
 of embodiments thereof taken in conjunction with the accompanying
 drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 In what follows, preferred embodiments of the present invention will be
 described in detail with reference to FIGS. 1 to 29. The present invention
 is not limited to these embodiments and is applicable to techniques in
 other fields where the preferred embodiments are combined and/or the same
 subjects as those of the present application are included.
 FIGS. 1 to 15 illustrate embodiments of a light guide panel where the
 present invention is applied to illuminate a reflection type liquid
 crystal display. The Figures also disclose embodiments of a plane
 illuminator apparatus using the light guide panel.
 Illustrated in FIG. 1 is a cross sectional structure of a plane illuminator
 apparatus 11 according to this embodiment. Illustrated in FIG. 2 is an
 external appearance of the plane illuminator apparatus 11 in a decomposed
 state. The plane illuminator apparatus 11 in this embodiment is disposed
 so as to be superimposed on an object O to be observed, such as a
 reflection type liquid crystal display. The object O is observed through
 the plane illuminator apparatus 11 from an observation position (above the
 plane illuminator apparatus 11 in FIG. 1) opposing to the object O.
 The plane illuminator apparatus 11 includes a rectangular plate-shaped
 light guide panel 12, a linear light source 14 disposed along an incidence
 end surface portion 13 of the light guide panel 12, and a light reflection
 sheet 17. The light reflection sheet 17 covers a portion of the light
 guide panel 12 excepting the incidence end surface portion 13, a front
 surface portion 15, and a back surface portion 16. It is herein noted that
 the light source 14 constituted with a fluorescent lamp is surrounded with
 a reflector 18 having a reflection surface, a cross section of which is
 parabolic-shaped. Reflected light from the reflector 18 impinges into the
 light guide panel 12 from the incidence end surface portion 13 of the
 light guide panel 12 substantially parallel to the back surface portion
 16.
 In this embodiment, although there was used the light source 14 using a
 fluorescent lamp as a light source, the light source may be constructed
 with an LED array arranged on a straight line.
 In this embodiment, the light guide panel 12 is formed with a transparent
 acrylic resin (PMMA) and polycarbonate (PC) and the like. The light guide
 panel 12 includes an incidence end surface portion 13 for introducing
 light from the light source 14, a reflection end surface portion 19
 located on the opposite side of the incidence end surface portion 13, and
 a pair of side end surface portions 20 connected with both side ends of
 the incidence end surface portion 13 and the reflection end surface
 portion 19. The front surface portion 15 is surrounded by the incidence
 end surface portion 13, the reflection end surface portion 19, and the
 pair of the side end surface portions 20. The front surface portion 15 is
 directed to an observation position side. The back surface portion 16 is
 located on the opposite side of the front surface portion 15 and is
 positioned just above an observation area of the object O.
 Schematically illustrated in FIG. 3 is the front surface portion 15 of the
 light guide panel 12. FIG. 4 is an extracted and enlarged shape of the
 front surface portion 15 taken along a cross section shown by an arrow
 IV--IV of FIG. 3. Optical elements 22 as depicted in FIG. 4 are formed
 with an arc surface 21 of a predetermined radius R of curvature. Optical
 elements 22 are disposed randomly such that there is prevented a moire
 fringes pattern between the optical elements 22 and individual liquid
 crystal cells constituting the object O, e.g., a reflection type liquid
 crystal display.
 The optical elements 22 are to effectively totally reflect light L
 impinging into the light guide panel 12 from the incidence end surface
 portion 13 and propagating in the same and guide it to the side of the
 back surface portion 16. Each optical element 22 has its diameter 2r set
 less than 150 .mu.m such that the individual optical elements 22 are not
 discriminated with the naked eye.
 In consideration of the occurrence of optical diffusion caused by a fact
 that the diameter is too small and to facilitate ease of fabrication of
 the optical element, the diameter is desirable to be 10 .mu.m or more.
 Hence, it is necessary to properly set the radius of curvature R and a
 height h from the front surface portion 15 such that the diameter falls
 within a range of from 10 .mu.m to 150 .mu.m. It is generally desirable
 that the height h of the optical element 22 from the front surface portion
 15 falls within a range of 1 to 50 .mu.m.
 Herein, the light guide panel 12 is designed such that when the refractive
 index of a material constructing the light guide panel 12 is indicated by
 n, the circular constant of a circle to its diameter by .pi., the radius
 of the optical element 22 by r, a relationship between the radius R of
 curvature of the arc surface 21 and the amount h of protrusion of the arc
 surface 21 from the front surface portion 15 satisfies:
EQU h=R(1-cos .epsilon.)
 and
EQU {(2.pi./9)-(.beta./2)}.ltoreq..epsilon..ltoreq.{(11.pi./36)-(.beta./2)},
 where .beta.=sin.sup.-1 (1/n), R=r/sin .epsilon..
 Light incident on the light guide panel 12 is reduced in its energy as it
 progresses in the light guide panel 12. As such, it is necessary to
 gradually change the ratio of the light guide panel 12 to the optical
 elements 22 protruded on the front surface portion 15 of the same. To be
 concrete, the ratio of the optical elements 22 occupied per unit area of
 the front surface portion 15 (hereinafter described as an occupation
 ratio) is set larger as it goes to the side of the reflection end surface
 portion 19. Illustrated in FIG. 5, a relationship is provided between the
 position of the front surface portion 15 in the direction of progress
 (right in FIG. 1) of light from the light source 14 and the occupation
 ratio of the optical elements 22. The occupation ratio is set such that
 reflected light emanating from the back surface portion 16 provides
 uniform brightness over the entire back surface portion 16.
 The front surface portion 15 located in close vicinity to the incidence end
 surface portion 13 of the light guide panel 12 is liable to have high
 brightness because light from the light source 14 directly transmits
 therethrough. As such, the occupation ratio of the optical elements 22 on
 the front surface portion 15 located in close vicinity to the incidence
 end surface portion 13 is set smaller than part thereof succeeding those
 optical elements 22. Likewise, the front surface portion 15 located in
 close vicinity to the reflection end surface portion 19 of the light guide
 panel 12 is liable to have higher brightness because reflected light from
 the reflection end surface portion 19 transmits therethrough. As such, the
 occupation ratio of the optical elements 22 on the front surface portion
 15 located in close vicinity to the reflection end surface portion 19 is
 set slightly smaller than part thereof succeeding those optical elements
 22.
 In this embodiment, although the back surface portion 16 is set parallel to
 the front surface portion 15, the light guide panel may be formed into a
 taper in which the front surface portion 15 is slightly inclined (for
 example, from about 0.5 degree to 1 degree) with respect to the back
 surface portion 16. As such, an interval between the front surface portion
 15 and the back surface portion 16 is narrower on the side of the
 reflection end surface portion 19 than on the side of the incidence end
 surface portion 13.
 When the object O is a reflection type liquid crystal display, as
 illustrated in FIG. 6 where a position relation between individual liquid
 crystal cells constituting the reflection type liquid crystal display and
 the optical elements 22 is schematically shown, it is effective to
 position the optical elements 22 at the center and four corners of the
 liquid crystal cell O.sub.c. (Actually, the optical elements 22 are needed
 to be arranged randomly to prevent a moire fringes pattern from being
 produced.) Herein, the ratio (%) of the area of the optical element 22 to
 that of the liquid crystal cell O.sub.c can be represented by
 (100.multidot.2.pi.r.sup.2)/xy, where x, y are longitudinal and lateral
 sizes of the rectangular liquid crystal cell, respectively.
 When the ratio of the optical elements 22 occupying the entire front
 surface portion 15 is increased, the entire front surface portion is
 converted into a state of frost glass and is deteriorated in its
 transmission property. Observation through the light guide panel 12 is
 thus difficult. When the ratio of the optical elements 22 is too small,
 the amount of illumination of the illumination light to the side of the
 object O is reduced and hence the object becomes dark. Since the
 preferable size of the optical element 22 ranges from 10 to 150 .mu.m, the
 ratio of the area of the optical element 22 to that of the liquid crystal
 cell O.sub.c, i.e., the total area of the optical element 22 to that of
 the front surface portion 15, is desirably set to fall within the range of
 from 1 to 20% of the area of the front surface portion 15.
 In the aforementioned embodiment, the optical element 22 is formed with
 part of a spherical surface having the arc surface 21. Illustrated in FIG.
 7 is an external appearance of another embodiment of an optical element 50
 according to the present invention. The optical element 50 has a columnar
 surface 23 which takes as its center an axial line parallel to the
 incidence end surface portion 13. Alternatively, optical element 50 may be
 formed into a substantially trapezoidal configuration by forming a planar
 part 24 indicated by a two dot chain line on the top of the columnar
 surface 23. In other words, provided the optical element 50 has a surface
 configured to totally reflect the light propagating in the light guide
 panel 12 toward the side of the back surface portion 16 when the optical
 element 50 is at an angle where the light emanates to the observation
 position without being totally reflected on the front surface portion 15,
 any other configuration thereof may be used.
 In these embodiments, the optical elements 22 and 50 are protruded from the
 front surface portion 15 to the outside of the light guide panel 12. In
 alternative embodiments, the same effect is ensured by forming intactly
 the optical element, having the aforementioned contour configuration, such
 that the optical element is concaved from the front surface portion 15 or
 the back surface portion 16 to the inside of the light guide panel 12.
 When the optical element 22,50 is formed on the side of the front surface
 portion 15, as described above, the optical element 22,50 is basically
 needed to have a total reflection function. When the optical elements are
 protruded on the side of the back surface portion 16, provided the light
 propagating in the light guide panel 12 is at an angle where it is totally
 reflected on the back surface portion 16, the optical elements are needed
 to have a function where the light emanates to the side of the object O
 without totally reflecting the light on the back surface portion 15.
 FIG. 8 illustrates a schematic structure of another embodiment of a plane
 illuminator apparatus 52 according to the present invention. FIG. 9
 illustrates an extracted arrow IX part in FIG. 8 and FIG. 10 illustrates
 an external appearance of the optical element in FIG. 9. Identical symbols
 will be applied to like members having the identical functions to those in
 the foregoing embodiments, and overlapped descriptions will be omitted.
 Optical elements 54 are randomly disposed on the back surface portion 16
 of the light guide panel 12 a triangular contour projects perpendicularly
 to the back surface portion 16. Optical element 54 includes a pair of
 symmetrical vertical pyramid surface 25 and an inclined pyramid surface
 26, and is protruded into an isosceles triangle pyramid from the back
 surface portion 16. Herein, a bottom side 27 of the inclined pyramid
 surface 26 is set substantially parallel to the incidence end surface
 portion 13, and a top 28 of the inclined pyramid surface 26 is positioned
 on the side of the reflection end surface portion 19.
 The optical element 54 in this embodiment forms a triangular pyramid in
 which the width of a diagonal line in the direction of progress of light L
 incident from the incident end surface portion 13, i.e., the length of the
 bottom side 27 of the inclined pyramid surface 26 is w, the length of a
 bottom side 29 of the vertical pyramid surface 25 is s, and the height
 from the back surface portion 16 is h, and illumination light emanates
 chiefly from the vertical pyramid surface 25 to illuminate the object O.
 The ratio of the aforementioned w and s, and the ratio of the
 aforementioned s and h are properly changeable. The length w of the bottom
 side 27 of the inclined pyramid surface 26 and the length s of the bottom
 side 29 of the vertical pyramid surface 25 preferably range within a range
 of from 1 to 150 .mu.m. The height h from the back surface portion 16
 preferably ranges within a range of from 1 to 50 .mu.m.
 The pair of the vertical pyramid surfaces 25 emanate, to the outside of the
 light guide panel 12, part of the light propagating in the light guide
 panel 12 toward the side of the reflection end surface 19 and emanate the
 same light toward the surface of the object O in a focused state. Thus
 optical elements 54 have a light focusing and emanating function.
 Illustrated in FIG. 11 is a plane configuration of the optical element 54
 viewed from the side of the back surface portion 16. In order to emanate
 the light L incident into the optical element 54 without totally
 reflecting the light on the pair of the vertical pyramid surfaces 25, an
 incidence angle .theta. of the light L to the vertical pyramid surface 25
 is needed to satisfy:
EQU .theta..ltoreq.sin.sup.-1 (1/n),
 when there is considered the light L advancing in a plane parallel to the
 back surface portion 16. Herein, an angle .alpha.1 between the pair of the
 vertical pyramid surfaces 25 is needed to satisfy:
EQU .alpha.1.gtoreq..pi.-2 sin.sup.-1 (1/n),
 using a relation;
EQU .alpha.1=2.multidot.{(.pi./2)-.theta.},
 where .pi. is the circular constant of a circle to its diameter. However,
 practically, a plane including an optical path of the light L is inclined
 to the back surface portion 16, and it is found that an angle .alpha.
 between the pair of the vertical pyramid surfaces 25 in the plane may
 satisfy:
EQU .alpha.1.gtoreq.tan.sup.-1 [cos .beta..multidot.tan {.pi.-2 sin.sup.-1
 (1/n)}],
 using a relation:
EQU tan .alpha.=cos .beta..multidot.tan .alpha.1,
 when .beta. is assumed to be an angle between the light L and the back
 surface portion 16.
 To be concrete, in this embodiment where acrylic resin with the refractive
 index n of 1.49 is adopted as the light guide panel 12, .alpha. is needed
 to be about 95 degrees or more. As a practical problem, the plane
 including the optical path for the light L is inclined with respect to the
 back surface portion 16. Accordingly, in order for the light L to emanate
 from the optical element without being totally reflected on the vertical
 pyramid surface 25 a relation:
 .beta..ltoreq.(3/2).multidot.sin.sup.-1 (1/n)
 is needed to be satisfied. Herein, with .beta.=(3/2).multidot.sin.sup.-1
 (1/n), the aforementioned .alpha. in this embodiment is about 135 degrees.
 It is thus preferable to form the optical element 22 such that .alpha.
 falls within a range of from 95 to 135 degrees.
 Accordingly, part of the light L incident into the light guide panel 12
 from the incidence end surface portion 13 and totally reflected on the
 back surface portion 16 impinges into the optical element 54. The light
 emanates in a focused state from the optical element 54 to the outside of
 the light guide panel 12. The light illuminates the object O to ensure
 observation of the bright object O through the light guide panel 12.
 When the triangular pyramid-shaped optical element 54 is formed on the side
 of the front surface portion 15, the optical element 54 is set in an
 opposite direction so as to permit the top 28 to be located closer to the
 side of the incidence end surface portion 13 than the bottom side 27.
 Hence, light incident on the optical element is totally reflected on the
 inclined pyramid surface.
 Further, the optical element 54 in this embodiment is formed into the
 triangular pyramid. Illustrated in FIG. 12 is an alternative optical
 element 56 formed into a trapezoid having a plane part 58 parallel to the
 back surface portion 16. A vertical wall surface 30 having a bottom side
 60 is directed toward the side of the incidence end surface portion 13. A
 pair of vertical wall surfaces 31 each have a bottom side 62 and are
 disposed symmetrically to the vertical wall surface 30. Vertical wall
 surfaces 30 may be inclined such that an angle .phi. formed between the
 back surface portion 16 and a vertical surface ranges from 0 to about 60
 degrees. This ensures a drawing slope for a mold upon molding of the light
 guide panel 12.
 FIG. 13 illustrates an external appearance of another embodiment of an
 optical element 64 usable for the light guide panel and the plane
 illuminator apparatus, extracted and enlarged. Hatched areas in the figure
 show a junction area between the back surface portion 16 and the optical
 element 64. An arrow indicates the direction of progress of the light L,
 i.e., a direction perpendicular to the incidence end surface portion 13.
 The optical element 64 in this embodiment is triangular prismatic-shaped
 corresponding to a rectangular parallelepiped shown by a two dot chain
 line. The optical element 64 has an inclined face 66 that extends between
 a bottom side 67 and a vertical wall surface 68. The lateral width of
 optical element 64 perpendicular to the direction of progress of the light
 L is w, length in the direction of progress of the light L is s, height
 from the back surface portion 16 is h, and the light L emanates from the
 vertical wall surface 30 mainly directed toward the reflection end surface
 portion 19 to illuminate the object O. Accordingly, the optical element 64
 is constructed into a rectangular parallelepiped shown by a two dot chain
 line in FIG. 13 substantially without losing its function.
 Illustrated in FIG. 14 is another embodiment of an optical element 72. In
 order to provide a drawing gradient for a mold, the optical element 72 may
 be constructed into a trapezoid. A ratio of the lateral width w and the
 length s, and the length s and the height h from the back surface portion
 16 are properly changeable, and identical symbols will be applied to like
 members with the same functions as those of the previous embodiments.
 FIG. 15 illustrates an external appearance of another embodiment of the
 present invention including a second optical element corresponding to the
 optical element 22 shown in FIGS. 8 to 11. Identical symbols will be
 simply applied to like members having the same functions as those of the
 members of the previous embodiment, and overlapped descriptions will be
 omitted.
 An optical element 32 in this embodiment (hereinafter, called a first
 optical element 32 for brevity) and a second optical element 33 are formed
 so as to protrude outwardly from the back surface portion 16 (refer to
 FIG. 8.) of the light guide panel 12. The first optical element 32
 corresponds to the optical element 54 of the embodiment illustrated in
 FIGS. 8 to 11.
 The second optical element 33 is combined with the first optical element 32
 such that it makes contact with and faces the bottom side 27 of the
 inclined pyramid surface 26 of the first optical element 32. These
 elements form a rhombus as a whole in their projection configuration
 projected perpendicularly to the back surface portion 16. The second
 optical element 33 is adapted such that the top 35 with respect to the
 bottom side 27 of the inclined pyramid surface 26 of the first optical
 element 32 is set substantially parallel to the incidence end surface
 portion 13 (refer to FIG. 8.). That is, the top 35 with respect to the
 bottom side 27 of the inclined pyramid surface 34 of the second optical
 element 33 is located on the side of the incidence end surface portion 13
 from the bottom side 27. This ensures an isosceles triangular pyramid
 further having a pair of symmetrical vertical pyramid surfaces 36.
 In this embodiment, the first optical element 32 and the second optical
 element 33 are set in a mirror image relation with the bottom side 27
 taken as a symmetrical axis. Herein, the length of the bottom surface 37
 of the other vertical pyramid surface 36 may be different from the length
 of the bottom side 29 of the vertical pyramid surface 25 of the first
 optical element 32. They are desirably symmetrical with respect to a
 diagonal line C extending perpendicularly to the bottom side 27 of the
 inclined pyramid surfaces 26, 36.
 The second optical element 33 chiefly serves to totally reflect to the side
 of the back surface portion 16 the light returning in the light guide
 panel 12 from the side of the refection end surface portion 19 (refer to
 FIG. 1.) to the side of the incidence end surface portion 13.
 In the aforementioned embodiment, although the first optical element 32 and
 the second optical element 33 are set into a combined configuration, the
 second optical elements 33 are separately disposed from the first optical
 elements 32 with different distribution states. For example, the second
 optical elements 33 are disposed more densely as they go to the side of
 the incidence end surface portion 13. Hereby, also the light returned from
 the side of the reflection end surface portion 19 is more uniformly
 totally reflected to the side of the back surface portion 16 of the light
 guide panel 12 to effectively illuminate the object O.
 It is herein noted that the aforementioned optical elements can be formed
 on both the front surface portion 15 and the back surface portion 16. It
 is desired, however, that the total area of these optical elements be set
 within a range of from 1 to 20% with respect to any one area of the front
 surface portion 15 and the back surface portion 16.
 In one embodiment, a plurality of the optical elements 22, 32, 33, 50, 54,
 56, 64, 72 are formed randomly on at least one of the front surface
 portion 15 and the back surface portion 16 of the light guide panel 12.
 The optical elements emanate the light L propagating in the light guide
 panel 12 from the incidence end surface portion 13 toward the object O.
 The total area of these optical elements 22, 32, 33, 50, 54, 56, 64, 72 is
 set within a range of from 1 to 20% of the area of the front surface
 portion 15 or the back surface portion 16 as described above. As a result,
 illumination luminous flux emanating from the back surface portion 16 of
 the light guide panel 12 is illuminated to the object O and an illuminated
 portion of the object O is observed from an observation position through
 the light guide panel 12. Henceforth, this embodiment is also usable as a
 compact thin type illuminator apparatus for reflection type liquid crystal
 display.
 When a ratio of the optical elements 22, 32, 33, 50, 54, 56, 64, 72 per
 unit area occupying the front surface portion 15 and/or the back surface
 portion 16 is set larger as the optical elements move away from the
 incidence end surface portion 13 or when a ratio of the optical elements
 22, 32, 33, 50, 54, 56, 64, 72 per unit area occupying the front surface
 portion 15 and/or the back surface portion 16 is set such that opposite
 ends in a width direction of the front surface portion 15 and/or the back
 surface portion 16 in a longitudinal direction of the incidence end
 surface portion 13 become relatively large, the quantity of light
 illuminated from the front surface portion and/or the back surface portion
 16 is made uniform over the entire area of the front surface portion 15
 and the back surface portion 16 of the light guide panel 12 for ensuring
 of illumination light without even illumination.
 When the size of the optical elements 22, 32, 33, 50, 54, 56, 64, 72 is set
 at 150 .mu.m or less, upon observation of the object O through the light
 guide panel 12 from the observation position, the object O can be observed
 without noticing the existence of the optical elements 22, 32, 33, 50, 54,
 56, 64, 72.
 In the aforementioned embodiment, the present invention is used for
 illumination for a reflection type liquid crystal display. In FIGS. 16 to
 28, embodiments are illustrated of a light guide panel applied for
 illumination of a transmission type liquid crystal display and of a plane
 illuminator apparatus using the light guide panel.
 FIG. 16 illustrates a cross sectional structure of a plane illuminator
 apparatus 74 according to this embodiment. FIG. 17 illustrates an external
 appearance of the plane illuminator apparatus 74 in a decomposed state.
 Identical symbols will be simply applied to like members having the
 identical functions to those of the previous embodiments, and overlapped
 descriptions will be omitted. Plane illuminator apparatus 74 includes a
 light guide panel 76, a light source 14, and a light deflection plate 38
 superimposed on the front surface portion 15 of the light guide panel 76.
 A light reflection sheet 78 covers a portion of the light guide panel
 excepting the incidence end surface portion 13 and the front surface
 portion 15 of the same.
 The light guide panel 76 in this embodiment is tapered such that the back
 surface portion 16 is inclined by about 0.5 to 1 degrees with respect to
 the front surface portion 15. As such, an interval between the front
 surface portion 15 and the back surface portion 16 is narrower as it goes
 to the side of the reflection end surface portion 19 with respect to the
 side of the incidence end surface portion 13.
 The foregoing light reflection sheet 78 covers the reflection end surface
 portion 19, a pair of the side end surface portions 20, and the back
 surface portion 16, all of the light guide panel 12. Light reflection
 sheet 78 reflects light emanating from these parts into the light guide
 panel 76 such that the light emanates from the front surface portion 15 of
 the light guide panel 76. Light reflection sheet 78 is formed by folding a
 white colored paper, etc.
 Illustrated in FIG. 18 are enlarged side configurations of the front
 surface portion 15 and the light deflection plate 38. On the front surface
 portion 15 of the light guide panel 76 there are randomly arranged the
 optical elements 22 configured into an isosceles triangular pyramid. The
 pyramid has a contour configuration projected perpendicularly to the front
 surface portion 15 being a triangle and includes a pair of symmetrical
 vertical pyramid surface 25 and an inclined pyramid surface 26. Care is
 taken that a moire fringes pattern is prevented from being produced
 between the optical elements 22 and the light deflection plate 38 and
 between cells of a liquid crystal panel and the optical elements 22 when
 the plane illuminator apparatus 74 is used as a backlight source of a
 liquid crystal display. The optical element 22 serves to deflect the
 vertically directed light L emanating from the front surface portion 15
 and has the same 27 configuration as that of the optical element 22 in the
 aforementioned embodiment illustrated in FIGS. 10 and 11.
 The vertical pyramid surface 25 of the optical element 22 is preferably
 perpendicular to the front surface portion 15. To set a proper drawing
 gradient for a mold to facilitate fabrication of the light guide panel 76,
 the vertical pyramid surface 25 may be set such that an angle of the
 vertical pyramid surface 25 with respect to the front surface portion 15
 exceeds 90 degrees. The bottom side 27 of the inclined pyramid surface 26
 of the optical element 22 is set substantially parallel to the incidence
 end surface portion 13.
 The light L incident at an incidence angle with respect to the incidence
 end surface portion 13 of the light guide panel 76, i.e., at an angle
 .beta. with respect to the front surface portion 15, progress in the light
 guide panel 76 in the range of the incidence angle .beta. satisfying
EQU 0.ltoreq./.beta./.ltoreq.sin.sup.-1 (1/n)
 in response to the refractive index n (n=1.49 in the case of acrylic resin
 in this embodiment) of a material constituting the light guide panel 76.
 Part of the light propagating to the side of the front surface portion 15
 enters the optical element 22. Another part of the light emanates from the
 front surface portion 15 intactly to the outside of the light guide panel
 76. A remaining part of the light is totally reflected on the front
 surface portion 15 and is transmitted to the side of the back surface
 portion 16.
 An occupation ratio of the optical elements 22 protruded on the front
 surface portion 15 of the light guide panel 76 is randomly set such that
 the side of the reflection end surface portion 19 provides a large
 occupation ratio. FIG. 19 shows a relation between the position of the
 front surface portion 15 along the direction of progress of the light from
 the light source 14 (right direction in FIG. 16) and the occupation ratio
 of the optical elements 22. The occupation ratio is preferably set at a
 density at about 3.5 times lighter than the previous embodiment
 corresponding to the reflection type liquid crystal display.
 When an emission region of the light source 14 is shorter than the size of
 the width of the incidence end surface portion 13 of the light guide panel
 76, the quantity of light incident on the width direction opposite side
 ends of the light guide panel 76 is liable to be insufficient. It is
 therefore desirable to set the occupation ratio of the optical elements 22
 at the width direction opposite side ends of the front surface portion 15
 of the light guide panel 76 relatively larger than the other portion. In
 any case, although in this embodiment a maximum of the occupation ratio of
 the optical elements 22 is set to about 70%, it is of course possible to
 set the same to a larger value than the former.
 The light deflection plate 38 in this embodiment includes a smooth plane
 part 39 formed with transparent acrylic resin. Smooth plane part 39
 opposes the front surface portion 15 of the light guide panel 12. Light
 deflection plate 38 also includes a triangular prismatic-shaped surface 40
 extending parallel to the incidence end surface portion 13 of the light
 guide panel 12 and arranged perpendicularly to the incidence end surface
 portion 13. The prismatic-shaped surface 40 includes a first inclination
 surface 41 where an interval between it and the plane part 39 increases as
 it goes to the side of the incidence end surface portion 13 of the light
 guide panel 76. Prismatic-shaped surface 40 also includes a second
 inclination surface 42 succeeding the first inclination surface 41. An
 angle .delta.1 formed between the plane part 39 and the first inclination
 surface 41 is set smaller than an angle .delta.2 between the plane part 39
 and the second inclination surface 42. For example, .delta.1 is set
 (28.+-.3) degrees and .delta.2 is set (62.+-.3) degrees.
 The prismatic-shaped surface 40 in this embodiment has fine unevenness with
 proper surface roughness for diffusing light emanating from the
 prismatic-shaped surface 40 to some degree, and the fine unevenness may be
 formed on the plane part 39. Although the optical element 22 is set to be
 a triangular pyramid, it may be formed into a triangular prism as in the
 previous embodiment shown in FIG. 12. In this case there is ensured a
 light guide panel having the same efficiency as that of the previous
 embodiment.
 FIG. 20 illustrates a cross sectional structure of another embodiment of a
 plane illuminator apparatus 80 according to the present invention. FIG. 21
 illustrates a cross sectional structure taken along an arrow XXI--XXI.
 Identical symbols are applied to like members of the same function as
 those of the previous embodiment. On the front surface portion 15 of the
 light guide panel 12 there are formed a first optical element 32 and a
 second optical element 33 each having a contour configuration projected
 perpendicularly to the front surface portion 15. The contour
 configurations are triangular and are in a state where they face each
 other. Optical elements 32, 33 have the same configuration as that of the
 previous embodiment shown in FIG. 15.
 The second optical element 33 chiefly serves to transfer the light
 returning in the light guide panel 12 from the side of the reflection end
 surface portion 19 to the side of the incidence end surface portion 13 to
 the outside of the front surface portion 15. This embodiment eliminates
 the need of inclining the back surface portion 16 into a tapered shape
 with respect to the front surface portion 15, as in the previous
 embodiment. It is thus possible to set the side of the incidence end
 surface portion 13 and the side of the reflection end surface portion 19
 to uniform plate thicknesses.
 On the back surface portion 16 of the light guide panel 12 there is formed
 a triangular prismatic-shaped surface 43. Surface 43 is arranged parallel
 to the incidence end surface portion 13 and extends perpendicularly to the
 incidence end surface portion 13 to provide a well known optical
 reflection function. Direction of emanating light is controlled
 substantially perpendicularly from the front surface portion 15 of the
 light guide panel 12 with the aid of these first and second optical
 elements 32, 33 and the prismatic-shaped surface 43.
 The prismatic-shaped surface 44 of the light deflection plate 38 in this
 embodiment is arranged such that it is an acute-angle isosceles triangle
 prism with a vertical angle being less than 90 degrees. Prismatic-shaped
 surface 44 is superimposed on the front surface portion 15 of the light
 guide panel 12 such that it opposes the front surface portion 15 of the
 light guide panel 12.
 In this embodiment, the light returning in the light guide panel 12 from
 the side of the reflection end surface portion 19 to the side of the
 incidence end surface portion 13 is actively intended to be derived from
 the front surface portion 15 so that highly bright light can be emitted in
 the direction opposite to the front surface portion 15 of the light guide
 panel 12.
 In the aforementioned embodiment, the first optical element 32 and the
 second optical element 33 are set into a combined configuration. In an
 alternative embodiment, they may be configured into a quadrangular pillar
 by setting the inclined pyramid surfaces 26, 34 (refer to FIG. 15.)
 parallel to the front surface portion 15. This configuration ensures a
 light guide panel with the same excellent efficiency as that of the
 previous embodiment. Further, the second optical elements 33 may be
 disposed, separated from the first optical elements 32 in a distribution
 state different from that of the first optical elements 32. For example,
 in one embodiment the second optical elements are more distributed as they
 go to the side of the incidence end surface portion 13. As a result, light
 returned from the side of the reflection end surface portion 19 can be
 derived more uniformly from the front surface portion 15 of the light
 guide panel 12.
 A plurality of the optical elements 22, 32, 33 which have a light
 collection property and a direction control property and which serve to
 emit light incident from the incidence end surface portion 13 of the light
 guide panel 12 are formed on the front surface portion 15 and/or the back
 surface portion 16 of the light guide panel 12. As a result, part of the
 light incident from the light source 14 incident into the light guide
 panel 12 from the incidence end surface portion 13 is totally reflected on
 the front surface portion 15 and/or the back surface portion 16 of the
 light guide panel 12 and is emanated from the light guide panel 12 to the
 outside thereof without being lost. For this, there is ensured a thin
 plane illuminator apparatus 11 with reduced light loss and power
 consumption.
 The optical elements 22, 32, 33 protrude on the front surface portion 15
 and/or the back surface portion 16 of the light guide panel 12 and are set
 at a ratio of the optical elements per unit area occupying the front
 surface portion 15 and/or the back surface portion 16 as they go away from
 the incidence end surface portion 13. The ratio is configured such that
 brightness distribution of the emanating light can be made uniform. The
 sizes of the optical elements 22, 32, 33 are set to be 10 to 150 .mu.m.
 This configuration ensures a light guide panel 12 where the optical
 elements 22, 32, 33 are inconspicuous and there is eliminated the need of
 the joint use of an optical diffusion plate.
 Further, there is ensured a light guide panel 12 having desired brightness
 distribution by controlling the occupation ratio of the optical elements
 22, 32, 33.
 FIG. 22 illustrates a cross sectional structure of another embodiment of
 the plane illuminator apparatus 84 according to the present invention.
 FIG. 23 illustrates a cross sectional structure taken along an arrow
 XXIII--XXIII. Identical symbols will be applied to like members having the
 same functions of those of the previous embodiment and overlapped
 descriptions will be omitted. On the back surface portion 16 of the light
 guide panel 76 in this embodiment there are disposed randomly the optical
 elements 56 each configured into an isosceles triangular prism including a
 vertical wall surface 30 (refer to FIG. 12.). Optical elements 56 have a
 triangular contour configuration projected perpendicularly to the back
 surface portion 16. The vertical wall surface 30 is directed to the side
 of the incidence end surface portion 13. A pair of vertical wall surfaces
 31 are disposed symmetrically with respect to the vertical wall surface
 30, and a triangular plane part 24 is disposed substantially parallel to
 the back surface portion 16. The optical element 56 is basically the same
 as that in the embodiment shown in FIG. 12.
 The pair of the vertical wall surfaces 31 of these optical elements 56 have
 a light focusing/emanating function. Specifically, vertical wall surfaces
 31 emanate light reflected from the side of the front surface portion 15
 and incident on the optical elements 56 to the outside of the light guide
 panel 76, i.e., to the surface of the light reflection sheet 17 in a
 focused state. The vertical wall surface 30 and the plane part 24 of the
 optical element 56 serve to again impinge light emanating from the side of
 the back surface portion 16 of the light guide panel 76 and reflected in a
 scattered state from the light reflection sheet 17 into the optical
 element 56. The optical element 56 emanates part of the light propagating
 in the light guide panel 76 toward the light reflection sheet 17 in a
 focused state. The emanating light is strongly reflected on the light
 reflection sheet 17. As a result, the emanating light is introduced into
 the light guide panel 76 from the back surface portion 16 of the light
 guide panel 76, boundaries of the optical elements 22, and the plane part
 24.
 On the front surface portion 15 of the light guide panel 76 there is formed
 an isosceles triangular prismatic-shaped surface 43 which extends
 perpendicularly to the incidence end surface portion 13 (right direction
 in FIG. 22) and is arranged perpendicularly (left and right directions in
 FIG. 23) to the extension direction. The prismatic-shaped surface 43 in
 this embodiment adopts a vertical angle ranging from about 80 to 110
 degrees. The same effect is ensured even when a convex spherical lens
 array with the predetermined radius of curvature is protruded on the front
 surface portion 15 of the light guide panel 76 instead of the
 prismatic-shaped surface 43.
 A plurality of the optical elements 56 are formed on the front surface
 portion 15 and/or the back surface portion 16 of the light guide panel 76
 for emanating the light to the outside in a focused state, as described
 above. The light from the light source 14 entering the light guide panel
 76 from the incidence end surface portion 13 can be emanated to the
 outside of the light guide panel 76 with uniform brightness distribution
 without loss.
 A ratio of the optical elements 56 per unit area occupying the front
 surface portion 15 and/or the back surface portion 16 of the light guide
 panel 76 is set to be larger as they go away from the incidence end
 surface portion 13. As a result, brightness distribution of the emanating
 light can be made uniform. The size of the optical element 56 is set to
 range from 10 to 150 .mu.m. This ensures an excellent light guide panel 76
 where the optical elements are inconspicuous and there is eliminated the
 need of the joint use of many light deflection sheets.
 When the prismatic-shaped surface 43 is provided on the front surface
 portion 15 and/or the back surface portion 16 of the light guide panel 76
 in a united manner, the conventional prism sheet can be further omitted.
 This ensures a thinner plane illuminator apparatus with reduced optical
 loss and power consumption.
 FIG. 24 illustrates a cross sectional structure of another embodiment of
 the plane illuminator apparatus 88 according to the present invention.
 FIG. 25 illustrates an enlarged cross sectional structure of the light
 guide panel 76 taken along an arrow XXV--XXV. FIG. 26 illustrates an
 enlarged cross sectional structure taken along an arrow XXVI--XXVI.
 Identical symbols will be applied to like members with the same functions
 as those of the previous embodiment and overlapped descriptions will be
 omitted. The plane illuminator apparatus 88 in this embodiment includes a
 light guide panel 76, a light source 14, a light deflection plate 38, and
 a light reflection sheet 17.
 On the back surface portion 16 of the light guide panel 76 there are
 distributed randomly rectangular optical elements 64 each configured into
 a triangular prism having an inclined surface 45 where an interval between
 it and the front surface portion 15 is increased as it goes toward the
 side of the incidence end surface portion 13. The optical element 64 is
 basically the same configuration as that in the embodiment shown in FIG.
 13, but its direction is reversed by 180 degrees. Optical element 64 has a
 function for effectively totally reflecting the light entering from the
 incidence end surface portion 13 and propagating in the light guide panel
 76 to guide it to the side of the front surface portion 15.
 In order to totally reflect light L entering the light guide panel 76 at an
 incidence angle .beta., with respect to the front surface portion 15, on
 the inclination surface 45 having an inclination angle .gamma., with
 respect to the front surface portion 15, a relation
EQU .gamma..ltoreq.(.pi.r/2)-.beta.-.beta..sub.0,
 is needed, where sin .beta..sub.0 =1/n, n is a refractive index of the
 light guide panel 76 (n=1.49), .beta..sub.0 is a critical angle of the
 light guide panel 76, and .pi. is the circular constant of a circle to its
 diameter. In the case of the light guide panel 76 using acrylic resin, as
 in this embodiment, the critical angle .beta..sub.0 is about 42 degrees.
 In order to emanate reflection light totally reflected on the back surface
 portion 16 to the outside from the front surface portion 15 of the light
 guide panel 76, a relation
EQU .gamma..gtoreq.{(.pi./2)-.beta.-.beta..sub.0 }/2
 must be satisfied. In order to effectively take out the light propagating
 in the light guide panel 76, the inclination angle .gamma. of the
 inclination surface 45 of the optical element 64 must satisfy a relation:
EQU -.pi./36.ltoreq..gamma.-(.pi./4)+(.beta./2).ltoreq..pi./18.
 As illustrated in FIG. 27, energy of the reflection light emanating to the
 outside of the light guide panel 76 when the light reflection sheet 17 is
 not used becomes maximum when the incidence angle .beta. is 0 degree. The
 energy of reflection light gradually decreases as the incidence angle
 .beta. is increased, and becomes substantially 0 when above about 42
 degrees. The reflection light is eliminated completely from the front
 surface portion 15 to the outside of the light guide panel 76 owing to
 existence of the light reflection sheet 17. Some loss may be due to
 boundary reflection and absorption at the inclination surface 45.
 In order to totally reflect incident light L.sub.1 on the inclination
 surface 45 of the optical element 64 and emanate reflected light L.sub.0
 to the outside of the light guide panel 76 from the front surface portion
 15 without totally reflecting the reflected light L.sub.0 on the front
 surface portion 15, it is necessary that the incidence angle .beta. and
 the inclination angle .gamma. are existent in the hatched region of FIG.
 27. The inclination angle .gamma. satisfies such conditions in this
 embodiment where acrylic resin with the refractive index of 1.49 is used,
 when inclination angle .gamma. falls within a range of from about 24 to
 about 48 degrees and the incidence angle .beta. in this case falls within
 a range of from 0 to about 24 degrees.
 The incident light with the incidence angle .beta. less than 24 degrees is
 completely totally reflected on the inclination surface 45 of the optical
 element 64 and is adapted to propagate to the side of the front surface
 portion 15. Most of the incident light L.sub.1 with the incidence angle
 .beta. beyond 24 degrees emanates to the outside of the light guide panel
 76 from the optical element 64. The incident light again enters the light
 guide panel 76 with the aid of the light reflection sheet 17, and finally
 emanates to the outside of the light guide panel 76 from the front surface
 portion 15. Part of the incident light with the incidence angle .beta.
 beyond 24 degrees is rendered to interfacial reflection on the inclination
 surface 45 of the optical element 64. The light is propagated to the side
 of the front surface portion 15 and is emanated to the outside of the
 light guide panel 76.
 In the front surface portion 15 of the light guide panel 76 there are
 formed uneven corrugated surfaces 46, 47 with the predetermined radius of
 curvature extending perpendicularly to the incidence end surface portion
 13 (right and left direction in FIG. 24) and arranged perpendicularly to
 the extension direction (right and left direction in FIG. 25). The concave
 surface 46 diffuses light emanating from the front surface portion 15
 while the convex surface 47 converges the light emanating from the front
 surface portion 15 whereby uniform brightness distribution is ensured.
 Intervals between the adjacent concave surfaces 46 and between the convex
 surfaces 47 are desirably set to about 30 to 100 .mu.m. A difference
 between heights of the concave surface 46 and the convex surface 47 is
 desirably about 10 to 45 .mu.m.
 In the aforementioned embodiment, although the corrugated uneven surfaces
 46,47 with the predetermined radius of curvature are formed in the front
 surface portion 15 of the light guide panel 12, an isosceles triangle
 prismatic-shaped surface with the vertical angle of about 95 to 105
 degrees may be continuously formed. Although there is adopted as the
 optical element 64 one configured into an isosceles triangle, it is also
 possible to adopt a configuration having a circular-arc surface with the
 predetermined radius of curvature.
 FIG. 28 illustrates a schematic structure of another embodiment of the
 light guide panel 76 according to the present invention. Identical symbols
 will be simply applied to like parts of the same functions as those of the
 previous embodiment, and overlapped descriptions will be omitted. On the
 back surface portion 16 of the light guide panel 76 there are disposed
 randomly the optical elements 22 each formed with a circular-arc surface
 21 with the predetermined radius of curvature. Care is taken that a moire
 fringes pattern is prevented from being produced among the optical
 elements 22, the uneven surfaces 46, 47 formed in the front surface
 portion 15, and the light deflection plate 41. The present optical element
 22 has basically the same configuration as that of the optical element 22
 in the embodiment shown in FIG. 4.
 A plurality of the optical elements 22 are formed on the front surface
 portion 15 and/or the back surface portion 16 of the light guide panel 76
 for totally reflecting light incident from the incidence end surface
 portion 13 of the light guide panel 76. Part of the light form the light
 source 14 entering the light guide panel 76 from the incidence end surface
 portion 13 is totally reflected on the optical elements 22 protruded on
 the front surface portion 15 and/or the back surface portion 16 of the
 light guide panel 76 and is emanated to the outside of the light guide
 panel 76 without any loss.
 The optical elements 22 protruded on the front surface portion 15 and/or
 the back surface portion 16 of the light guide panel 76 are set such that
 a ratio of the optical elements 22 per unit area occupying the front
 surface portion 15 and/or the back surface portion 16 is increased as they
 go away from the incidence end surface portion 13. As a result, brightness
 distribution of the emanating light can be made uniform. The size of the
 optical element 22 is set to be 10 to 150 .mu.m. This ensures an excellent
 light guide panel 76 where the optical elements 22 are inconspicuous, and
 hence there is eliminated the need of the joint use of the light diffusion
 sheet.
 Provided the light deflection means, such as the uneven surfaces 46, 47, is
 provided in the front surface portion 15 and/or the back surface portion
 16 of the light guide panel 76 in a united manner, a conventionally used
 prism sheet can also be eliminated. This ensures a thin plane illuminator
 apparatus with reduced optical loss and power consumption.
 The present invention has been described in detail with respect to
 preferred embodiments, and it will now be apparent from the foregoing to
 those skilled in the art that changes and modifications may be made
 without departing from the invention in its broader aspects. The
 invention, therefore, in the appended claims is intended to cover all such
 changes and modifications as fall within the true spirit of the invention.
 For example, the embodiment illustrated in FIG. 1 and the embodiment
 illustrated in FIG. 8 can be combined to also utilize them as a backlight
 source of a transmission type liquid crystal display. The optical elements
 22 are formed respectively on the front surface portion 15 and the back
 surface portion 16 of the light guide panel 12 as illustrated in FIG. 29
 to emanate most of the light L propagating in the light guide panel 12
 from the side of the back surface portion 16. The light L illuminates the
 back surface portion 16 of the light guide panel 12 and the light
 reflection sheet (not shown) facing the back surface portion 16. Resulting
 diffusion and reflection light is again introduced into the light guide
 panel 12. This eliminates the need of the use of the aforementioned light
 deflection means and there is ensured low loss illumination light where a
 moire fringes pattern is prevented from happening.
 Identical symbols will be applied to like members with the same functions
 as those in the previous embodiment in symbols in FIG. 29.
 The present invention has been described in detail with respect to
 preferred embodiments, and it will now be apparent from the foregoing to
 those skilled in the art that changes and modifications may be made
 without departing from the invention in its broader aspect, and it is the
 intention, therefore, in the appended claims to cover all such changes and
 modifications as fall within the true sprit of the invention.