United States Patent [19]
`Nishio et al.
`
`[54] FILM LENS AND A SURFACE LIGHT
`SOURCE USING THE SAME
`
`[75]
`
`Inventors: Toshikazu Nishio; Michiko Takeuchi;
`Nobu Masubuchi, all of Tokyo-to,
`Japan
`
`[73] Assignee: Dai Nippon Printing Co., Ltd., Japan
`
`[21] Appl. No.: 215,789
`
`[22] Filed:
`
`Mar. 22, 1994
`
`[30]
`
`Foreign Application Priority Data
`
`Mar. 23, 1993
`Nov. 29, 1993
`
`[JP]
`[JP]
`
`Japan .................................... 5-086954
`Japan .................................... 5-323214
`
`Int. CI.6
`••••••..••.....•......•.............••.••.•.••...•••.•.. G02F 1/13
`[51]
`[52] U.S. Cl. ................................. 349/57; 362/31; 362/32;
`385/147; 349/62
`[58] Field of Search ..................................... 359/261,263,
`359/291,572,589,366, 642, 36,40,49,
`69,51,619; 362/31, 32, 26, 27, 330, 329;
`385/147
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,729,067
`4,924,356
`5,394,255
`
`311988 Ohe.
`511990 French et al .............................. 362/31
`211995 Yokota et al ............................. 362/31
`
`FOREIGN PATENT DOCUMENTS
`
`57-44365
`
`911982
`
`Japan.
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US005598280A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,598,280
`Jan. 28, 1997
`
`9/1989
`1-241590
`2-257188 10/1990
`4-91905
`3/1992
`5-232466
`9/1993
`5-45474 1111993
`5-3l3004 11/1993
`
`Japan.
`Japan.
`Japan.
`Japan.
`Japan.
`Japan.
`
`Primary Examiner-David C. Nelms
`Assistant Examiner-F. Niranjan
`Attorney, Agent, or Firm-Parkhurst, Wendel & Burr, L.L.P.
`
`[57]
`
`ABSTRACT
`
`A film lens comprises a light transmitting base having one
`side and an opposite side, a plurality of concave or convex
`unit lenses formed on the one side of the light transmitting
`base, and a plurality of projections formed on the opposite
`side of the light transmitting base and having a profile height
`not smaller than the wavelength of a source light and not
`greater than 100 11m. In this arrangement, when the film lens
`is placed on a smooth surface of a light guide plate of a
`surface light source of the edge-light type, the projections on
`the reverse side of the light transmitting base can secure a
`gap with a width not smaller than the wavelength of the
`source light between the film lens and the light guide plate.
`Thus, the source light can be uniformly distributed without
`hindrance throughout the light guide plate as it is totally
`reflected by the surface of the guide plate. In the surface light
`source using the film lens, moreover, the lens can provide a
`uniform luminance in a desired angular range, and a uniform
`luminance distribution can be obtained for the whole surface
`without concentration in the vicinity of the light source.
`
`6 Claims, 11 Drawing Sheets
`
`6X>A
`-.-L
`____ T 9rr-7t---~----j--....,........--l-----.
`
`3
`
`Sony Corp. Exhibit 1011
`
`SONY_000562
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 1 of 11
`
`5,598,280
`
`C
`
`F
`
`FIG. I PRIOR ART
`
`3
`
`4
`
`/100
`
`FIG.2 PRIOR ART
`
`SONY_000563
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 2 of 11
`
`5,598,280
`
`3
`
`2
`
`FIG. 3 PRIOR ART
`
`4-2 100
`/
`
`2
`
`42
`
`FIG. 4 PRIOR ART
`
`SONY_000564
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 3 of 11
`
`5,598,280
`
`llX>A
`L
`r
`
`3
`
`FIG. 5
`
`FIG.6
`
`SONY_000565
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 4 of 11
`
`5,598,280
`
`42
`
`41
`
`FIG.7
`
`9
`
`2
`
`FIG.8
`
`SONY_000566
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 5 of 11
`
`5,598,280
`
`6
`
`200
`/
`
`5
`
`FIG. 9
`
`42
`
`41
`
`43
`
`FIG.IO
`
`SONY_000567
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 6 of 11
`
`5,598,280
`
`42
`
`44
`
`41
`
`43
`
`FIG. I I
`
`42
`
`41
`
`FIG. 12
`
`SONY_000568
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 7 of 11
`
`5,598,280
`
`FIG. 13
`
`FIG. 14
`
`FIG. 15
`
`FIG. 16
`
`SONY_000569
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 8 of 11
`
`5,598,280
`
`10
`=~
`.~~--------+-~~------~----~
`
`4
`
`AX<A
`
`n2 > n1
`8 >8c
`
`I
`
`FIG. 17
`
`FIG. 18
`
`9
`
`FIG. 19
`
`SONY_000570
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 9 of 11
`
`5,598,280
`
`23
`
`b
`
`23
`~
`
`t
`
`16
`
`II
`
`22
`
`21
`
`FIG.20
`
`42~~
`
`41
`
`FIG.21
`
`SONY_000571
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 10 of 11
`
`5,598,280
`
`4 (4-1 )
`
`41
`
`42
`
`4
`(4-2)
`
`FIG.22
`
`4-f~~
`REFRACTIVE INDEX
`OF LENS : n
`
`i
`'----..L.-l:i-h~
`-
`--..>-l'"---=-
`- ~ -
`
`42
`
`AIR LAYER----
`( REFRACTIVE
`INDEX = no )
`4-2~
`REFRACTIVE INDEX:
`OF LENS: n
`
`:
`
`FIG. 23
`
`7
`
`~X
`
`SONY_000572
`
`

`
`u.s. Patent
`
`Jan. 28, 1997
`
`Sheet 11 of 11
`
`5,598,280
`
`100
`~
`
`4-2
`
`2
`FIG. 24
`
`3
`
`y
`
`42-i 42-2 ~pJ
`FIG.25A
`
`41
`
`y
`
`l
`
`42
`
`42-i 42-2 42-3
`FIG. 25 B
`
`/
`
`/
`
`1-- * --'-J/
`~
`FIG. 25 C
`
`/
`
`I I ' n, It\ 11\ II \
`il> -}it:-
`~ ~ -f/,-
`-?- + + +. ~-?-...
`~ .:u-
`
`,
`
`I I
`
`\ I I
`
`\ I I
`I I
`\ I I
`.:tJ- ~ .:ff.-
`
`\
`
`42
`
`FIG. 25 0
`
`,
`/~ r- * --
`, /
`/
`--7\ I- - - .:p::
`~
`,
`,
`,
`,
`/
`,/
`~ -- ~ -- ~
`\..
`
`/
`
`\
`
`/
`
`/
`
`\
`
`\
`
`SONY_000573
`
`

`
`5,598,280
`
`1
`FILM LENS AND A SURFACE LIGHT
`SOURCE USING THE SAME
`
`BACKGROUND OF THE INVENTION
`
`2
`light guide plate. The luminance gradually lowers with
`distance from the light source, and darkness is conspicuous
`in the region most remote from the light source (correspond(cid:173)
`ing to the end portion on the opposite side to the light source
`5 or the central portion of the surface light source).
`In order to eliminate these drawbacks, an attempt has been
`made to correct and equalize the luminance distribution
`within the plane of the light guide plate (Jpn. Pat. Appln.
`KOKAI Publication No. 1-245220). According to this
`10 arrangement, a light scattering layer on the reverse side of
`the light guide plate is formed in mesh patterns, and the area
`of the patterns is increased with distance from the light
`source.
`In order to obviate the aforesaid drawbacks, moreover,
`another attempt has been made to correct and equalize the
`luminance distribution within the plane of the light guide
`plate (Jpn. Pat. Appln. KOKAI Publication No. 3-9306). In
`this case, two or more light sources are arranged around the
`side end portions of the light guide plate.
`In either case, however, it is difficult to equalize the
`luminance perfectly. In the case of Jpn. Pat. Appln. KOKAI
`Publication No. 1-245220, the mesh patterns of the light
`scattering layer are inevitably conspicuous. In the case of
`Jpn. Pat. Apptn. KOKAI Publication No. 3-9306, on the
`other hand, the necessary space and power consumption of
`the light sources are doubled at the least.
`As shown in FIG. 4, moreover, the diffusion angle may be
`controlled in two directions (vertical and horizontal) by
`combining two film lenses 4-1 and 4-2 in a manner such that
`their respective ridges extend at right angles to each other.·
`If the two film lenses 4-1 and 4-2 are stacked in layers,
`however, interference fringes of equal thickness (e.g., New(cid:173)
`ton's rings) are generated between unit lenses 42 on the
`lower film lens 4-1 and a smooth surface on the reverse side
`of the upper film lens 4-2, thereby lowering the image
`quality.
`
`1. Field of the Invention
`The present invention relates to a film lens and a surface
`light source using the samc, and more particularly, to a
`surfacc light source adapted for back-lighting for a display
`unit, such as a liquid crystal display unit, illuminated adver(cid:173)
`tising display, traffic-control sign, etc.
`2. Information of the Related Art
`FIG. 1 shows a conventional surface light source of the
`cdgc-light type, which comprises a light transmitting flat
`plate as a light guide plate 1, and is used as back-light source 15
`for a liquid crystal display unit (LCD). In this surface light
`source, a light beam is applied from both or one of the side
`end faces of the light guide plate 1 so that it is propagated
`throughout the plate 1 by utilizing total reflection in the light
`transmitting plate. Part of the propagated light beam is 20
`reflected by a light scattering reflector plate on the reverse
`side of the light guide plate 1, thus forming a diffused
`reflected light beam with an angle of reflection narrower
`than the critical angle, and the diffused light beam is emitted
`from the obverse side of the plate 1 (Jpn. UM Appln. 25
`Laid-Open publication No. 16220111980).
`In another example of the surface light source for back(cid:173)
`lighting, as shown in FIGS. 2 and 3, a film lens 4, which has
`projections or lenticular lenses, each in the form of a 30
`triangular prism, on one side and a smooth surface on the
`other side, is stacked on the obverse side of a light guide
`plate 1 of the surface light source, with the projections
`upward. In this arrangement, a diffused reflected light beam
`can be uniformly diffused in an isotropic manner within a 35
`desired angular range by utilizing the light converging effect
`of the lens (Jpn. UM Appln. KOKAI Publications Nos.
`4-107201 and 4-107237). When this film lens 4 is used in
`combination with a matte transparent diffuser sheet, the
`optical energy of the light source can be distributed more 40
`intensively within a desired limited angular range so that a
`diffused light beam with higher isotropy can be obtained in
`this range than in the case where a matte transparent diffuser
`sheet is used singly (U.S. Pat. No. 4,729,067).
`In the aforementioned prior art arrangement (FIG. 1),
`however, the light scattering reflector plate is only provided
`on the reverse side of the light guide plate 1, so that the
`emitted light beam has a relatively sharp distribution with a
`peak angle of 60° to a line normal to the obverse side of the
`light guide plate. Thus, the luminance with respect to the 50
`direction of the normal line, along which the brightest light
`is required, is insufficient, and the optical energy is dispersed
`in oblique directions in which less light is demanded.
`According to the alternative prior art arrangement (FIG. 2),
`the triangular lenticular film lenses on the light emitting 55
`surface of the light guide plate refractively converge the
`emitted light beam, so that the ratio of the optical energy of
`the light beam emitted within the angular range of 30° to 60°
`increases with its peak in the normal direction of the light
`emitting surface. As shown in FIG. 3, smooth surfaces on the 60
`rcverse side of the film lens and the obverse side of the light
`guide plate are intimately in contact with each other and are
`integrated optically, so that total reflection cannot occur on
`the obverse side of the light guide plate.
`As regards the luminance distribution within the plane of 65
`emission, therefore, high luminance is obtained within a
`distance of 2 to 4 cm from the source-side end portion of the
`
`45
`
`SUMMARY OF THE INVENTION
`
`An object of the present invention is to provide a film lens
`and a surface light source using the same, capable of
`emitting uniform, high-luminance light only within a desired
`angular range without increasing power consumption or heat
`release value, and free from variation in luminance depend-
`ing on the in-plane position.
`Another object of the present invention is to provide a film
`lens and a surface light source using the same, free from
`interference fringes of equal thickness.
`A film lens according to the present invention comprises
`a light transmitting base having one side and an opposite
`side, a plurality of concave or convex unit lenses formed on
`the one side of the light transmitting base, and a plurality of
`projections formed on the opposite side of the light trans(cid:173)
`mitting base and having a profile height not smaller than the
`wavelength of a source light and not greater than 100 1lIIl.
`According to this arrangement, when the film lens is
`placed on a smooth surface of a light guide plate of a surface
`light source of the edge-light type, the projections on the
`opposite side (reverse side) of the light transmitting base can
`secure a gap with a width not smaller than the wavelength
`of the source light between the film lens and the light guide
`plate. Thus, the source light can be uniformly distributed
`without hindrance throughout the light guide plate as it is
`totally reflected by the surface of the guide plate.
`In the surface light source using the film lens, moreover,
`the lens can provide a uniform luminance in a desired
`
`SONY_000574
`
`

`
`5,598,280
`
`3
`angular range, and a uniform luminance distribution can be
`obtained for the whole surface without concentration in the
`vicinity of the light source.
`In another film lens according to the present invention, the
`height i1h of each projection is given by
`
`5
`
`where Amax is the maximum wavelength of the visible
`spectrum of a light source for the observation of the film
`lens, and i18 is the angular radius of the light source as 10
`viewed through a reftective surface of the film lens, the
`projections arc arranged noncyclically in one- and two(cid:173)
`dimensional modes, and the width i1x of each projection is
`given by i1x:2i 100 11m and the average distance d between
`each two adjacent projections is given by
`
`d<2P
`
`where P is the arrangement cycle of the unit lenses.
`In this film lens, the projections with a predetermined size 20
`distribution are provided on the reverse side (opposite to the
`side on which the unit lenses are formed), so that interfer(cid:173)
`ence fringes of equal thickness can be restrained from being
`formed on the reverse side of the film lens.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`30
`
`4
`FIG. 13 is a perspective view of a film lens according to
`a further modification of the first embodiment of the inven(cid:173)
`tion, having convex cylindrical lenticular lenses;
`FIG. 14 is a perspective view of a film lens according to
`an additional modification of the first embodiment of the
`invention, having concave cylindrical lenticular lenses;
`FIG. 15 is a perspective view of a film lens according to
`a further modification of the first embodiment of the inven(cid:173)
`tion, formed of an ommateal lens;
`FIG. 16 is a perspective view of a film lens according to
`another modification of the first embodiment of the inven(cid:173)
`tion, in which two cylindrical lenticular lens sheets are
`stacked in layers so that their respecti ve axes extend at right
`15 angles;
`FIG. 17 is a sectional view illustrating the behavior of a
`light beam advancing outward from inside a light guide
`plate;
`FIG. 18 is a sectional view illustrating the way a light
`beam coming from the light guide plate by the tunnel effect
`becomes traveling waves again;
`FIG. 19 is a sectional view of a film lens according to a
`further modification of the first embodiment of the inven(cid:173)
`tion, illustrating the way a part of the light beam advancing
`25 outward from the light guide plate is totally reftected and the
`remaining part is transmitted;
`FIG. 20 is a diagram for illustrating a manufacturing
`method according to an example of the first embodiment of
`the present invention;
`FIG. 21 is a sectional view of a film lens according to an
`example of the first embodiment of the invention, having
`cylindroid lenticular lenses;
`FIG. 22 is a perspective view of a film lens according to
`a second embodiment of the present invention;
`FIG. 23 is a diagram for illustrating the principle of the
`film lens according to the second embodiment of the inven(cid:173)
`tion;
`FIG. 24 is a perspective view of a surface light source of
`the edge-light type according to a second embodiment of the
`invention; and
`FIGS. 25A, 25B, 25C and 25D are diagrams for illustrat(cid:173)
`ing the relationships between projections and unit lenses.
`
`FIG. 1 is a sectional view of a prior art surface light source
`of the edge-light type having no film lens;
`FIG. 2 is a perspective view of a prior art surface light
`source of the edge-light type having a film lens with a
`smooth reverse-side surface;
`FIG. 3 is a sectional view of the surface light source
`shown in FIG. 2;
`FIG. 4 is a perspective view of a prior art surface light
`source of the edge-light type having two film lenses stacked 35
`in layers;
`FIG. 5 is a sectional view of a surface light source of the
`edge-light type according to a first embodiment of the
`present invention, having a group of projections as a sepa- 40
`rate layer;
`FIG. 6 is a sectional view of a surface light source of the
`edge-light type according to a modification of the first
`embodiment of the invention, having a group of projections
`formed directly on a film lens;
`FIG. 7 is a perspective view of a single-layer film lens
`according to a first embodiment of the invention, having
`lenticular lenses each in the form of a triangular prism and
`a group of projections formed directly on the reverse side
`thereof;
`FIG. 8 is a perspective view of the surface light source
`according to the first embodiment of the invention;
`FIG. 9 is a perspective view of the surface light source
`according to the first embodiment, used as a back-light
`source of a liquid crystal display unit;
`FIG. 10 is a perspective view of a film lens according to
`a modification of the first embodiment of the invention,
`having lenticular lenses each in the form of a triangular
`prism and a group of projections formed as a separate layer
`on the reverse side thereof;
`FIG. 11 is a perspective view of a film lens according to
`another modification of the first embodiment of the inven-
`tion, formed on a transparent base sheet;
`FIG. 12 is a sectional view of a film lens according to still 65
`another modification of the first embodiment of the inven(cid:173)
`tion, having a group of projections formed partially;
`
`50
`
`45
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`55
`
`[First Embodiment]
`For example, a lens sheet 4 according to a first embodi-
`ment of the present invention may be composed of a group
`of cylindrical unit lenses 42 (lenticular lenses in a broad
`sense) adjacently arranged with their respective longitudinal
`axes (ridges) parallel to one another, as shown in FIG. 7, or
`of an ommateallens including a number of protuberant unit
`lenses 42, e.g., each in the form of a hemispherical projec(cid:173)
`tion having an independent circumference, which are
`arranged in a two-dimensional manner.
`The profile of each unit lens may have the shape of a
`60 continuous smooth curve, e.g., circular, elliptic, cardioid,
`Rankine's-egg-shaped, cycloid, or involute, as shown in
`FIGS. 13 and 14. Alternatively, the profile may be formed of
`part or whole of a polygon, such as a triangle or hexagon, as
`shown in FIG. 7.
`Each unit lens may be convex, as shown in FIG. 13, or
`concave, as shown in FIG. 14. Preferably, the unit lens
`should be shaped like a circular or elliptic cylinder in
`
`SONY_000575
`
`

`
`10
`
`5
`considcration of ease of design and manufacture, light
`condcnsing capability, light diffusion properties (half-angle,
`scarcity of sidc-lobe light (peak of luminance in oblique
`dircction), isotropy of in-half-angle luminance, normal-di(cid:173)
`rection luminance, etc.), and the like. An ellipse whose 5
`major axis extends in the normal direction of a surface light
`source is a particularly preferable configuration, since it
`ensures high luminance. Preferably, the ratio of the major
`axis length to the minor axis length ranges from 1.27 to 1.56,
`in particular.
`Although the lens sheet may be used singly, two lens
`sheets may be arranged in layers such that their respective
`longitudinal axes cross at right angles, as shown in FIG. 16,
`in order to control light diffusion angles in two directions
`(vertical and crosswise) by means of the cylindrical lenses. 15
`In this case, the best optical transmission can be obtained if
`the respective lens surfaces of the two sheets face in the
`same direction, as shown in FIG. 16. Naturally, however, the
`lens sheets may be arranged so that their lens surfaces face
`each other. Alternatively, moreover, the lens sheet may be 20
`obtained by integrally molding a light transmitting base
`material, as shown in FIG. 7, or by forming unit lenses 42
`on a light transmitting plate (or sheet) 44, as shown in FIG.
`11.
`The lens sheet 4 is formed of a light transmitting base. The
`base material used may be a simple acrylic ester or meth(cid:173)
`acrylate ester or a copolymer thereof such as polymethyl
`methacrylate or polymethyl acrylate, polyester such as poly(cid:173)
`ethylene terephthalate or polybutylene terephthalate, ther(cid:173)
`moplastic resin such as polycarbonate, polyethylene, or
`polymethylpentene, acrylate such as polyfunctional ure(cid:173)
`thane acrylate, crosslinked by means of ultraviolet rays or
`electron rays, or polyester acrylate, transparent resin such as
`unsaturated polyester, transparent glass, or transparent
`ceramics.
`If the light transmitting base is used for the lens sheet, its
`overall thickness normally ranges from about 20 to 1,000
`f.lm.
`There are some methods for shaping the lenses. Among
`these methods, a conventional heat-press method
`is
`described in Jpn. Pat. Appln. KOKAI Publication No.
`56-157310, for example. In another method described in
`Jpn. Pat. Appln. KOKAI Publication No. 61-156273, an
`ultraviolet-curing thermoplastic resin film is embossed by
`means of a roll-embossing plate, and is then exposed to
`ultraviolet rays to be cured. According to an alternative
`method disclosed in Jpn. Pat. Appln. KOKAI Publication
`No. 3-223883, U.S. Pat. No. 4,576,850, etc., moreover, a roll
`intaglio engraved with a lens shape pattern is coated with an
`ultraviolet- or eleetron-radiation-curing liquid resin so that
`its depressions are filled with the resin. Thereafter, ultravio-
`let rays or electron rays are applied to the roll intaglio, with
`a transparent base film thereon, through the liquid resin. The
`resulting cured resin and the base film bonded thereto are 55
`released from the intaglio. Thus, the cured resin layer is
`shaped to the lens shape pattern of the roll intaglio.
`The light transmitting base should have light transmission
`properties such that it can transmit a minimum volume of
`diffused light without a hindrance to applications. Although
`it is most advisable to use a colorless, transparent base, the
`base may be a colored, transparent one or matte semitrans(cid:173)
`parent one, depending on the applications.
`A "matte transparent" material is a material which diffu(cid:173)
`sively transmits light in a substantially uniform, isotropic
`manner with respect to every direction within a semisolid
`angle, and is used as a synonym for an optically isotropic,
`
`50
`
`5,598,280
`
`6
`diffusive material. More specifically, "matte and transpar(cid:173)
`ent" indicates that an angUlar distribution 1°(8) of the
`transmitted light intensity observed when a parallel lumi(cid:173)
`nous flux is incident upon the reverse side of the light
`transmitting base (angle of incidence i=OO) is a cosine
`distribution given by
`
`/"(8)=1" mpcos8,
`
`8 (-900~8~900) is the angle between the obverse side of
`the base and a normal line N, and 1° mp is the transmission
`strength in the normal direction, or a similar distribution.
`A group of projections 41 on the reverse side of the lens
`sheet, having a profile height not smaller than the wave(cid:173)
`length of a source light and not greater than 100 f.lm, may be
`formed directly on the reverse side of the integrally molded
`lens sheet 4 by heat-press embossing, sandblasting, etc., as
`shown in FIG. 7, or obtained by forming a light transmitting
`material layer having projections on the flat reverse side of
`the sheet 4, as shown in FIG. 11. In a specific available
`method, the lens sheet is coated with a paint which is
`composed of transparent minute particles of calcium car-
`bonate, silica, or acrylic resin dispersed in a transparent
`binder, so that the minute particles make the coating film
`surface uneven. According to the aforementioned alternative
`method disclosed in Jpn. Pat. Appln. KOKAI Publication
`25 No. 3-223883 and U.S. Pat. No. 4,576,850, an u1traviolet- or
`eleetron- radiation-curing liquid resin is molded on a roll
`intaglio so that the resulting surface is matte, having minute
`indentations.
`The projections 41 are designed so that a gap 9 having a
`30 width ~ not smaller than the wavelength of the source light
`is formed at least partially between a smooth surface 10 of
`a light guide plate 1 and the lens sheet 4, as shown in FIG.
`6. If the gap width ~ is smaller than the wavelength of the
`source light, as mentioned later, satisfactory total reflection
`35 of light on the smooth surface 10 of the plate 1 cannot be
`enj oyed. If the gap width is greater than 10 f.1ID, on the other
`hand, the indentations of the projections 41 are improperly
`conspicuous.
`The projections 41 may have any rugged contour pro-
`40 vided that the above object is achieved. Preferably, however,
`irregular indentations (e.g., sand-grain patterns, pear-skin
`patterns, etc.) are formed allover the reverse side of the lens
`sheet 4, as shown in FIGS. 6, 7, 10 and 11, in order to obtain
`an angular distribution for uniform luminance within a
`45 desired diffusion angle and a uniform luminance distribution
`within the light source plane.
`In this arrangement, the projection group 41 also serves as
`a light diffusion layer which diffuses light beams Ll, L2S,
`etc. incident upon the reverse side of the lens sheet 4 in an
`isotropic manner. Thus, a uniform angular distribution can
`be obtained without separately using a polished transparent
`sheet, and the mesh patterns are unobtrusive, as shown in
`FIG. 6.
`Naturally, moreover, an optically isotropic diffusive sheet
`8, which is matte and transparent and is formed with a group
`of projections 41 having a profile height not smaller than the
`wavelength of the source light on the reverse side and not
`greater than 100 f.1ID, may be interposed between the lens
`sheet 4 and the smooth surface 10 of the light guide plate.
`60 In this case, however, there are a plurality of interfaces
`(smooth surface 10, projection group 41, isotropic diffusive
`sheet 8, reverse side of lens sheet 4, and inside of the sheet
`8 in the presence of a flatting agent) through which light
`diffuses, so that a loss of effective light energy in the vicinity
`65 of the normal line increases.
`As shown in FIG. 12, furthermore, the projection group
`41 may be composed of spaced dotted patterns, such as mesh
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`20
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`7
`patterns, which are distributed within a plate. Since thc
`patterns 41 arranged in this manncr arc too conspicuous,
`however, dispersing the flatting agent in the lens sheet 4 or
`other measure is required.
`The surface light souree according to the first embodiment
`of the present invention is constructed in the manner shown
`in the sectional view of FIG. 6 and the perspective view of
`FIG. 8. More specifically, the light source comprises the
`light guide plate 1, a linear or point light source 3 located
`adjacent to at least one spot of the terminal portion of the
`plate 1, a light reflecting layer 2 on the reverse side of the
`plate 1, the lens sheet 4 on the opposite side of the plate 1
`to the layer 2, at the least. Usually, the surface light source
`additionally comprises a source light reflector 5, a housing
`(not shown) containing all the elements and having a light
`emitting surface in the form of a window, a power source
`(not shown), etc.
`The opposite surface 10 of the light reflecting layer of the
`light guide plate 1 is a level surface which is finished so that
`its roughness (measured in terms of the ten-point average
`roughness Rz provided by JIS-B-0601 or the like) is reduced
`to the level of the wavelength of the source light or below.
`Usually, the source light is a visible light whose wavelength
`ranges from 0.4 to 0.8 /illl, so that the surface roughness is
`0.4 /illl or less.
`Finishing to this level of roughness may be effected by a
`conventional method, such as heat-pressing using a mirror
`plate, injection molding using a specular mold, cast mold(cid:173)
`ing, or precision polishing applicable to optical lenses and
`the like.
`The material of the light guide plate 1 is selected from the
`light transmitting materials for the lens sheet. Usually,
`acrylic or poly carbonate resin is used for this purpose.
`The thickness of the light guide plate 1 usually ranges
`from about 1 to 10 /illl.
`Although it is advisable to use a linear light source, such
`as a fluorescent lamp, as the light source 3, in order to obtain
`a uniform luminance throughout the surface, a point light
`source, such as a incandescent lamp, may be used instead.
`As shown in FIGS. 6 and 8, the light source 3 is located 40
`outside a side end portion of the light guide plate 1 in a
`separate manner. Alternatively, however, it may be embed(cid:173)
`ded partially or entirely in a recess which is cut in the side
`end portion the light guide plate 1.
`In order to increase the luminance and improve the 45
`in-plane uniformity of the luminance, another light source
`may be provided at the other side end portion of the light
`guide plate 1.
`The source light reflector 5 used may be a conventional
`one which is formed by depositing metal on the inner surface 50
`of a plate in the shape of, for example, a paraboloidal,
`hyperboloidal, or elliptic cylinder.
`The lens sheet 4 is stacked on the smooth surface 10 of the
`light guide plate 1. In doing this, the gap 9 having the width
`not smaller than the wavelength A of the source light is
`formed at least partially between the lens sheet 4 and the
`smooth surface 10 of the plate 1 by putting the sheet 4 on the
`plate surface 10 with its lens surface outward (or on the side
`remote from the surface 10) and its projection group 41
`inward (or on the side of the surface 10), as shown in FIG. 60
`6. The area ratio R of the gap portion 9, which is given by
`R=(area of region with gap width not smaller than wave(cid:173)
`length Aloverall surface area of light guide plate)x100%, is
`settled depending on the uniformity of the luminance within
`the desired plane, coefficient of utilization of light energy,
`light guide plate size, etc. Usually, the ratio R should be
`adjusted to 80% or more, preferably 90% or more.
`
`8
`According to an experiment, it is indicated that when the
`smooth surface 10 of the light guide plate 1, whose rough(cid:173)
`ness is lower than the level corresponding to the wavelength
`of light, is brought intimately into contact with the surface
`5 of the lens sheet 4, as shown in FIG. 3, most of input light
`beams from the linear light source 3 are emitted without
`being totally reflected in the region within a distance y from
`the source-side end portion of the guide plate, and the
`luminance is considerably lowered in the region beyond the
`10 distance y.
`Also, it was ascertained that the ratio of the length y of the
`light emitting portion to the overall length Y of the light
`guide plate with respect to the light propagation direction is
`(y/Y)x100=1O to 20%. In order to distribute equally the
`energy of light from the light source incident upon the light
`guide plate surface 10 throughout the overall length Y,
`therefore, 10 to 20% of the light incident upon the surface
`10 and the remaining 90 to 80% should be transmitted and
`totally reflected, respectively. Since there is an approxima(cid:173)
`tion, (totally-reflected light volumeltransmitted light vol(cid:173)
`ume)=(area of region with gap width not smaller than
`wavelength Aloverall surface area of light guide plate)=R, it
`was confirmed that R should be 80 to 90% or more.
`In forming the gap with the width not smaller than the
`wavelength of the source light between the lens sheet 4 and
`the light guide plate 1, the sheet 4 may be oriented (not
`shown) so that its lens surface and projections 41 face
`oppositely to the arrangement of FIG. 6.
`In this case, however, the light converged once within the
`desired angle by the lens surface emanates again in an
`isotropic manner, so that it is difficult to control the light
`diffusion angle for an optimum value, which ranges from
`35 30° to 60° with respect to the normal line.
`The light reflecting layer 2, which tends to reflect light
`diffusively, may be arranged as follows.
`(1) A white layer, containing a high-opacity, high-whiteness
`pigment, e.g., titanium dioxide or aluminum powder,
`dispersed therein, is formed on one side of the light guide
`plate layer by coating or the like.
`(2) A matte, fine indentation pattern is formed on the surface
`of the light guide plate by sandblasting, embossing, etc.,
`and a metal film layer is formed on the patterned surface
`by depositing a metal, such as aluminum, chromium, or
`sil ver, thereon.
`(3) A metallic film layer is formed on a low-opacity white
`layer which is formed by simply