United States Patent
`Nishio et a1.
`
`[19]
`
`[54]
`
`FILM LENS AND A SURFACE LIGHT
`SOURCE USING THE SAME
`
`[75] Inventors: Toshikazu Nishio; Miehiko Takeuchi;
`Nobu Masubuchi, all of Tokyo-to,
`Japan
`
`[73] Assignee: Dai Nippon Printing Co., Ltd., Japan
`
`[21]
`[22]
`[30]
`
`Appl. No.: 215,789
`Filed:
`Mar. 22, 1994
`Foreign Application Priority Data
`
`Mar. 23, 1993
`Nov. 29, 1993
`
`[JP]
`[JP]
`
`Japan .................................. .. 5-086954
`Japan .................................. .. 5-323214
`
`[51] Int. Cl.6 ...................................................... .. G02F 1/13
`[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
`
`3/1988 Ohe.
`5/1990 French et a1. ........................... .. 362/31
`2/1995 Yokota et a1. .......................... .. 362/31
`
`FOREIGN PATENT DOCUMENTS
`
`lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`5,598,280
`Jan. 28, 1997
`
`USOO559828OA
`[11] Patent Number:
`[451 Date of Patent:
`
`l-241590
`2-257188
`4-91905
`5-232466
`5-45474
`5-313004
`
`9/1989
`10/1990
`3/1992
`9/1993
`11/1993
`ll/1993
`
`Japan .
`Japan .
`Japan .
`Japan .
`
`Japan .
`Japan .
`
`Primary Examiner—David C. Nelms
`Assistant Examiner—F. Niranjan
`Attorney, Agent, or F irm—Parkhurst, Wendel & Burr, L.L.P.
`
`ABSTRACT
`
`[57]
`A ?lm 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 pro?le height
`not smaller than the wavelength of a source light and not
`greater than 100 pm. In this arrangement, when the ?lm 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 ?lm 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
`re?ected by the surface of the guide plate. In the surface light
`source using the ?lm 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.
`
`57-44365
`
`9/1982 Japan .
`
`6 Claims, 11 Drawing Sheets
`
`L2t
`
`Lzt
`
`fLgS
`
`LGD_000862
`
`LG Display Ex. 1021
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 1 of 11
`
`5,598,280
`
`L13, Ln‘
`Ln
`y / / IO
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`
`L25"
`/
`
`FIG. I
`
`PRIOR ART
`
`FIG.2 PRIOR ART
`
`LGD_000863
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 2 0f 11
`
`5,598,280
`
`2
`
`If:
`F I G. 3 PRIOR ART
`
`5
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`42
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`3
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`2
`
`FIG. 4 PRIOR ART
`
`LGD_000864
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 3 0f 11
`
`5,598,280
`
`L2t L2t
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`1”
`2 *
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`
`LGD_000865
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 4 0f 11
`
`5,598,280
`
`4
`
`FIG.?
`
`32
`
`4
`
`flOO
`
`LGD_000866
`
`

`
`U.S. Patent
`
`Jan. 28, 1997
`
`Sheet 5 of 11
`
`5,598,280
`
`5
`
`3
`
`200
`6
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`FIG. IO
`
`LGD_000867
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 6 of 11
`
`5,598,280
`
`42
`
`4
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`44\4
`
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`FIG. l2
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`LGD_000868
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 7 0f 11
`
`5,598,280
`
`42
`
`4
`
`42
`
`4
`
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`FIG.I5
`
`LGD_000869
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 8 0f 11
`
`5,598,280
`
`LGD_000870
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 9 0f 11
`
`5,598,280
`
`LGD_000871
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 10 0f 11
`
`5,598,280
`
`4(4—1)
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`REFRACTIVE INDEX‘
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`42
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`REF RACTIVE INDEX :
`OF LENS I n
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`LGD_000872
`
`

`
`US. Patent
`
`Jan. 28, 1997
`
`Sheet 11 of 11
`
`5,598,280
`
`LP»!
`42-1 42-2
`F|G.25A
`
`42-1 42-2 42-3
`FIG.25B
`
`FIG. 25D~
`
`LGD_000873
`
`

`
`5,598,280
`
`1
`FILM LENS AND A SURFACE LIGHT
`SOURCE USING THE SANIE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to a ?lm lens and a surface
`light source using the same, and more particularly, to a
`surface light source adapted for back-lighting for a display
`unit, such as a liquid crystal display unit, illuminated adver
`tising display, traffic-control sign, etc.
`2. Information of the Related Art
`FIG. 1 shows a conventional surface light source of the
`edge-light type, which comprises a light transmitting ?at
`plate as a light guide plate 1, and is used as back-light source
`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 re?ection in the light
`transmitting plate. Part of the propagated light beam is
`re?ected by a light scattering re?ector plate on the reverse
`side of the light guide plate 1, thus forming a diffused
`re?ected light beam with an angle of re?ection narrower
`than the critical angle, and the diffused light beam is emitted
`from the obverse side of the plate 1 (Jpn. UM Appln.
`Laid-Open publication No. 162201/1980).
`In another example of the surface light source for back
`lighting, as shown in FIGS. 2 and 3, a ?lm lens 4, which has
`projections or lentieular lenses, each in the form of a
`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 re?ected light beam
`can be uniformly diffused in an isotropic manner within a
`desired angular range by utilizing the light converging eifect
`of the lens (Jpn. UM Appln. KOKAI Publications Nos.
`4-107201 and 4-107237). When this ?lm lens 4 is used in
`combination with a matte transparent diifuser sheet, the
`optical energy of the light source can be distributed more
`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 diifuser
`sheet is used singly (US. Pat. No. 4,729,067).
`In the aforementioned prior art arrangement (FIG. 1),
`however, the light scattering re?ector 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
`direction of the normal line, along which the brightest light
`is required, is insuf?cient, 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 lentieular ?lm lenses on the light emitting
`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
`reverse side of the ?lm lens and the obverse side of the light
`guide plate are intimately in contact with each other and are
`integrated optically, so that total re?ection cannot occur on
`the obverse side of the light guide plate.
`As regards the luminance distribution within the plane of
`emission, therefore, high luminance is obtained within a
`distance of 2 to 4 cm from the source-side end portion of the
`
`20
`
`25
`
`30
`
`45
`
`50
`
`55
`
`60
`
`65
`
`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
`ing to the end portion on the opposite side to the light source
`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
`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 di?icult 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 ?lm lenses 4-1 and 4-2 in a manner such that
`their respective ridges extend at right angles to each other.
`If the two ?lm lenses 4-1 and 4-2 are stacked in layers,
`however, interference fringes of equal thickness (e.g., New
`ton’s rings) are generated between unit lenses 42 on the
`lower ?lm lens 4-1 and a smooth surface on the reverse side
`of the upper ?lm lens 4-2, thereby lowering the image
`quality.
`
`SUMMARY OF THE INVENTION
`
`An object of the present invention is to provide a ?lm 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 ?lm
`lens and a surface light source using the same, free from
`interference fringes of equal thickness.
`A ?lm 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
`mitting base and having a pro?le height not smaller than the
`wavelength of a source light and not greater than 100 pm.
`According to this arrangement, when the ?lm 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 ?lm 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 re?ected by the surface of the guide plate.
`In the surface light source using the ?lm lens, moreover,
`the lens can provide a uniform luminance in a desired
`
`LGD_000874
`
`

`
`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 ?lm lens according to the present invention, the
`height Ah of each projection is given by
`
`AlgimlzAez
`
`where Km,‘ is the maximum wavelength of the visible
`spectrum of a light source for the observation of the ?lm
`lens, and A6 is the angular radius of the light source as
`viewed through a re?ective surface of the ?lm lens, the
`projections are arranged noncyclically in one- and two
`dimensional modes, and the width Ax of each projection is
`given by AxélOO um and the average distance d between
`each two adjacent projections is given by
`
`where P is the arrangement cycle of the unit lenses.
`In this ?lm lens, the projections with a predetermined size
`distribution are provided on the reverse side (opposite to the
`side on which the unit lenses are formed), so that interfer
`ence fringes of equal thickness can be restrained from being
`formed on the reverse side of the ?lm lens.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a sectional view of a prior art surface light source
`of the edge-light type having no ?lm lens;
`FIG. 2 is a perspective view of a prior art surface light
`source of the edge-light type having a ?lm 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 ?lm lenses stacked
`in layers;
`FIG. 5 is a sectional view of a surface light source of the
`edge—light type according to a ?rst embodiment of the
`present invention, having a group of projections as a sepa
`rate layer;
`FIG. 6 is a sectional view of a surface light source of the
`edge-light type according to a modi?cation of the ?rst
`embodiment of the invention, having a group of projections
`formed directly on a ?lm lens;
`FIG. 7 is a perspective view of a single-layer ?lm lens
`according to a ?rst 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 ?rst embodiment of the invention;
`FIG. 9 is a perspective view of the surface light source
`according to the ?rst embodiment, used as a back-light
`source of a liquid crystal display unit;
`FIG. 10 is a perspective view of a ?lm lens according to
`a modi?cation of the ?rst 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 ?lm lens according to
`another modi?cation of the ?rst embodiment of the inven
`tion, formed on a transparent base sheet;
`FIG. 12 is a sectional view of a ?lm lens according to still
`another modi?cation of the ?rst embodiment of the inven
`tion, having a group of projections formed partially;
`
`15
`
`25
`
`4
`FIG. 13 is a perspective view of a ?lm lens according to
`a further modi?cation of the ?rst embodiment of the inven
`tion, having convex cylindrical lenticular lenses;
`FIG. 14 is a perspective view of a ?lm lens according to
`an additional modi?cation of the ?rst embodiment of the
`invention, having concave cylindrical lenticular lenses;
`FIG. 15 is a perspective view of a ?lm lens according to
`a further modi?cation of the ?rst embodiment of the inven
`tion, formed of an ommateal lens;
`FIG. 16 is a perspective view of a ?lm lens according to
`another modi?cation of the ?rst embodiment of the inven
`tion, in which two cylindrical lenticular lens sheets are
`stacked in layers so that their respective axes extend at right
`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 ?lm lens according to a
`further modi?cation of the ?rst embodiment of the inven
`tion, illustrating the way a part of the light beam advancing
`outward from the light guide plate is totally re?ected and the
`remaining part is transmitted;
`FIG. 20 is a diagram for illustrating a manufacturing
`method according to an example of the ?rst embodiment of
`the present invention;
`FIG. 21 is a sectional view of a ?lm lens according to an
`‘ example of the ?rst embodiment of the invention, having
`cylindroid lenticular lenses;
`FIG. 22 is a perspective view of a ?lm lens according to
`a second embodiment of the present invention;
`FIG. 23 is a diagram for illustrating the principle of the
`?lm lens according to the second embodiment of the inven
`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—
`ing the relationships between projections and unit lenses.
`
`35
`
`40
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`45
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`50
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`55
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`65
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`[First Embodiment]
`For example, a lens sheet 4 according to a ?rst 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 ommateal lens including a number of protuberant unit
`lenses 42, e.g., each in the form of a hemispherical projec
`tion having an independent circumference, which are
`arranged in a two-dimensional manner.
`The pro?le of each unit lens may have the shape of a
`continuous smooth curve, e.g., circular, elliptic, cardioid,
`Rankine’s-egg-shaped, cycloid, or involute, as shown in
`FIGS. 13 and 14. Alternatively, the pro?le 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
`
`LGD_000875
`
`

`
`5,598,280
`
`5
`consideration of ease of design and manufacture, light
`condensing capability, light diffusion properties (half-angle,
`scarcity of side-lobe light (peak of luminance in oblique
`direction), isotropy of in-half-angle luminance, normal-di
`rection luminance, etc.), and the like. An ellipse whose
`major axis extends in the normal direction of a surface light
`source is a particularly preferable con?guration, 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 diifusion angles in two directions
`(vertical and crosswise) by means of the cylindrical lenses.
`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
`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
`acrylate ester or a copolymer thereof such as polymethyl
`methacrylate or polymethyl acrylate, polyester such as poly
`ethylene terephthalate or polybutylene terephthalate, ther
`moplastic resin such as polycarbonate, polyethylene, or
`polymethylpentene, acrylate such as polyfunctional ure
`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
`pm.
`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 ?lm 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 electron-radiation-curing liquid resin so that
`its depressions are ?lled with the resin. Thereafter, ultravio
`let rays or electron rays are applied to the roll intaglio, with
`a transparent base ?lm thereon, through the liquid resin. The
`resulting cured resin and the base ?lm bonded thereto are
`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~
`parent one, depending on the applications.
`A “matte transparent” material is a material which diifu
`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,
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`6
`di?cusive material. More speci?cally, “matte and transpar
`ent” indicates that an angular distribution 10(6) of the
`transmitted light intensity observed when a parallel lumi
`nous ?ux is incident upon the reverse side of the light
`transmitting base (angle of incidence i=0") is a cosine
`distribution given by
`
`6 (—90°§0§90°) is the angle between the obverse side of
`the base and a normal line N, and lomp 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 pro?le height not smaller than the wave
`length of a source light and not greater than 100 um, 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 ?at reverse side of
`the sheet 4, as shown in FIG. 11. In a speci?c 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 ?lm
`surface uneven. According to the aforementioned alternative
`method disclosed in Jpn. Pat. Appln. KOKAI Publication
`No. 3-223883 and U.S. Pat. No. 4,576,850, an ultraviolet- or
`electron- radiation-curing liquid resin is molded on a roll
`intaglio so that the resulting surface is matte, having rrrinute
`indentations.
`The projections 41 are designed so that a gap 9 having a
`width Ax 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 Axis smaller than the wavelength of the
`source light, as mentioned later, satisfactory total re?ection
`of light on the smooth surface 10 of the plate 1 cannot be
`enjoyed. If the gap width is greater than 10 pm, on the other
`hand, the indentations of the projections 41 are improperly
`conspicuous.
`The projections 41 may have any rugged contour pro
`vided that the above object is achieved. Preferably, however,
`irregular indentations (e.g., sand-grain patterns, pear-skin
`patterns, etc.) are formed all over 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
`desired diifusion 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 L1, 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 diiTusive sheet
`8, which is matte and transparent and is formed with a group
`of projections 41 having a pro?le height not smaller than the
`wavelength of the source light on the reverse side and not
`greater than 100 um, may be interposed between the lens
`sheet 4 and the smooth surface 10 of the light guide plate.
`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 ?atting agent) through which light
`diffuses, so that a loss of effective light energy in the vicinity
`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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`LGD_000876
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`7
`patterns, which are distributed within a plate. Since the
`patterns 41 arranged in this manner are too conspicuous,
`however, dispersing the ?atting agent in the lens sheet 4 or
`other measure is required.
`The surface light source according to the ?rst 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 speci?cally, 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 re?ecting 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 re?ecting layer of the
`light guide plate 1 is a level surface which is ?nished so that
`its roughness (measured in terms of the ten-point average
`roughness Rz provided by I IS-B-060l 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 run, so that the surface roughness is
`0.4 pm 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
`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 polycarbonate resin is used for this purpose.
`The thickness of the light guide plate 1 usually ranges
`from about 1 to 10 um.
`Although it is advisable to use a linear light source, such
`as a ?uorescent 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
`outside a side end portion of the light guide plate 1 in a
`separate manner. Alternatively, however, it may be embed
`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
`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 re?ector 5 used may be a conventional
`one which is formed by depositing metal on the inner surface
`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 7» 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.
`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
`length Noverall surface area of light guide plate)><100%, is
`settled depending on the uniformity of the luminance within
`the desired plane, coe?icient 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.
`
`40
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`45
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`50
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`55
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`60
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`5,598,280
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`15
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`25
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`30
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`35
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`8
`According to an experiment, it is indicated that when the
`smooth surface 10 of the light guide plate 1, whose rough
`ness is lower than the level corresponding to the wavelength
`of light, is brought intimately into contact with the surface
`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 re?ected 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
`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)><l00=l0 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 re?ected, respectively. Since there is an approxima
`tion, (totally-re?ected light volume/transmitted light vol
`ume)=(area of region with gap width not smaller than
`wavelength Noverall surface area of light guide plate)=R, it
`was con?rmed 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
`30° to 60° with respect to the normal line.
`The light re?ecting layer 2, which tends to re?ect 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, ?ne indentation pattern is formed on the surface
`of the light guide plate by sandblasting, embossing, etc.,
`and a metal ?lm layer is formed on the patterned surface
`by depositing a metal, such as aluminum, chromium, or
`silver, thereon.
`(3) A metallic ?lm layer is formed on a low-opacity White
`layer which is formed by simply coating a matte surface.
`(4) A meshed white layer is formed, and its area ratio may
`be increased with distance from the light source so that
`attenuation of the light volume of the light source can be
`compensated.
`FIG. 9 shows an arrangement in which a planar light
`source 100 according to the present invention as a back-light
`(back-light source) of a light transmitting display unit 6,
`such as a light transmitting LCD. In this case, it is necessary
`only that the unit 6 be stacked on the lens surface (on which
`the unit lenses 42 are arranged) of the lens sheet of the light
`source 100.
`The