Ulllted States Patent [19]
`Shinohara et al.
`
`US006167182A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,167,182
`*Dec. 26, 2000
`
`[54] SURFACE LIGHT SOURCE DEVICE AND
`
`FOREIGN PATENT DOCUMENTS
`
`lgglggBgggTgpglgNPgNgEmEa INFORMATION TERMINAL EMPLOYING
`
`THE SURFACE LIGHT SOURCE DEVICE
`
`[75] Inventors: Masayuki Shinohara; Shigeru
`Aoyama, both of Kyoto, Japan
`
`[73] Assignee: Omron Corporation, Kyoto, Japan
`
`[*] Notice:
`
`This patent issued on a continued pros-
`ecution application ?led under 37 CFR
`1.53(d), and is subject to the tWenty year
`patent term provisions of 35 USC
`154(a)(2)-
`
`.
`[21] Appl. No..
`[22] PCT Filed:
`
`09/091,429
`Oct. 27, 1997
`
`[86] PCT No.:
`
`PCT/JP97/03892
`
`'
`Jun- 25, 1998
`§ 371 Date-
`§ 102(6) Date: Jun. 25, 1998
`
`[87] PCT pub NO; “logs/19105
`
`PCT Pub. Date:Ma 7 1998
`y ’
`Foreign Application Priority Data
`
`[30]
`
`Japan .................................. .. 8-301073
`[JP]
`Oct. 25, 1996
`[51]
`Int. c1.7 ..................................................... .. 0021?. 6/10
`
`[2522:
`61_240506 10/1986 Japan _
`64-18508
`1/1989 Ja an .
`64-45003
`2/1989 jagan _
`1-152406 10/1989 Japan .
`4-6706
`1/1992 Japan .
`4-61302 5/1992 Japan .
`5-107542 4/1993 Japan .
`5-210014 8/1993 Japan -
`5'21603O 8/1993 Japan:
`[apan -
`apan '
`6-2303” 8
`/1994 Japan .
`6-250025
`9/1994 Japan .
`7-114023
`5/1995 Japan .
`743700 9/1995 Japan '
`Japan .
`
`—
`
`apan .
`
`8-271893 10/1996 Japan .
`8-286037 11/1996 Japan .
`8-2866037 11/1996 Japan .
`8446227 6/1998 Japan"
`
`Primary Examiner—Akm E. Ullah
`Attorney, Agent, or Firm—Foley & Lardner
`
`[57]
`
`ABSTRACT
`
`In a surface light source device using a so-called point light
`Source, the luminance distribution is made uniform, and the
`luminance theréof is increased The direction Of 99d} of a
`
`[52] us CL _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
`
`_ _ _ _ _ _ _ _ n 385/129
`
`plurality of diffuse pattern elements 24/: constituting a
`
`[58] Field of Search
`385
`
`385/129 130
`432 366/109’
`11’0 11’2 44 11_ ’438/6’9 71’_ 359/619’
`’
`’
`'
`’
`’
`’
`-
`References CIted
`
`[56]
`
`U.S. PATENT DOCUMENTS
`
`diffuse pattern 24 formed on the loWer surface of an optical
`Waveguide plate 22 (a direction along the length) is at an
`angle of approximately 90° to a direction along the line
`connecting the diffuse pattern element 24/: and a point light
`source
`.
`e ensIty p o t e 1
`se pattern e ements 1s
`30 Th d
`'
`f h d'ffu
`1
`'
`approximately Zero in the vicinity of the point light source
`30, While linearly increasing as the distance from the point
`
`5,481,515
`
`1/1996 Kando et al. ...................... .. 369/4412
`
`hght Source 30 mcreases'
`
`359/619
`1/1997 Akio ........... ..
`5,593,913
`5,691,548 11/1997 Akio ...................................... .. 257/232
`
`31 Claims, 33 Drawing Sheets
`
`LGE_000522
`
`LG Electronics Ex. 1010
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 1 0f 33
`
`6,167,182
`
`Fig. /
`
`LGE_000523
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 2 0f 33
`
`6,167,182
`
`LGE_000524
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 3 0f 33
`
`6,167,182
`
`L
`I
`52
`5 l
`
`CL
`
`I)
`0
`LL.
`0
`
`L:
`U)
`
`LLI Z
`
`NEAR
`
`F I g. 4
`
`7 ..
`DISTANCE FROM LIGHT INCIDENCE SURFACE
`FAR
`OF OPTICAL GUIDE PLATE
`
`Fig. 5
`
`2
`
`/ '
`
`g
`
`I
`
`._
`7 / /.
`/ If
`III
`
`I'v
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`
`LGE_000525
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`

`
`U.S. Patent
`
`Fig.6
`
`7
`%
`u ]UU-]U[HU;§:UjILE/5
`U
`IIPIUUT" :
`[I I T
`]n U
`ill" [250
`
`Fig.7
`
`I
`
`
`
`INTENSITY OF OUTPUT LIGHT
`
`/\/\/\/\
`
`X - X DIRECTION
`
`LGE_000526
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 5 0f 33
`
`6,167,182
`
`25
`
`25
`
`34
`
`LGE_000527
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 6 0f 33
`
`6,167,182
`
`Fig.9
`
`n? Av
`
`24a
`
`w
`
`Q
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`000
`
`0
`0
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`
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`
`LGE_000528
`
`

`
`U.S. Patent
`
`Dec. 26, 2000
`
`Sheet 7 0f 33
`
`6,167,182
`
`Fig. I0
`
`LGE_000529
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 8 0f 33
`
`6,167,182
`
`80
`
`ANGLE OF INCIDENCE ¢ [deg]
`
`Fig. 12
`
`LGE_000530
`
`

`
`U.S. Patent
`
`Dec. 26, 2000
`
`Sheet 9 0f 33
`
`6,167,182
`
`Fig. /3
`
`LGE_000531
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 10 0f 33
`
`6,167,182
`
`Fig. I50
`
`240
`
`Fig. I51)
`
`I\
`
`LGE_000532
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 11 0f 33
`
`6,167,182
`
`Fig. [6
`
`
`
`CORRELATION STRENGTH
`
`L 2
`L l
`CORRELATION LENGTH
`
`LGE_000533
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 12 0f 33
`
`6,167,182
`
`
`
`IF H _ / \_ A o o
`
`m. m. F F
`m n I /
`
`
`
`w M1,. 10%;,“
`
`4 M D 0/ 0 \ \x d
`
`26, d X % w
`
`
`
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`
`PIG: ..._O >._.C.Z<D.O
`
`LIGHT SOURCE
`
`DISTANCE FROM
`LIGHT SOURCE
`
`D»
`
`0
`
`A
`
`Fig. 170
`E0:
`
`00 E536
`
`Fig. /7d
`
`O
`
`DISTANCE FROM
`LIGHT SOURCE
`
`LGE_000534
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 13 0f 33
`
`6,167,182
`
`F 1g I80
`
`W , R
`Lb il.| __ f o.
`
`Fig. [8b
`
`:6: to EE<=d
`
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`
`. 5m BEE
`
`O
`
`DISTANCE FROM
`POINT LIGHT SOURCE
`
`d
`
`0 P
`
`0
`
`DISTANCE FROM
`POINT LIGHT SOURCE
`
`Fig. [8c
`
`~6EE§0
`
`Em:
`
`Eu: 955w
`
`d 8 l
`m. F
`
`Q 5% SE20
`
`O
`
`DISTANCE FROM
`POINT LIGHT SOURCE
`
`d
`
`LGE_000535
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 14 0f 33
`
`6,167,182
`
`
`
`
`
`DIFFUSE PATTERN DENSITY [%]
`
`|||||| 90 100
`
`80
`
`60 70
`
`|1|:I|||||v|||l1||||||||||||l|||||||4||||||| 50
`
`Fig. [9
`
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`
`30 40
`
`20
`
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`
`LGE_000536
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 15 0f 33
`
`6,167,182
`
`F [57.200
`
`(22
`
`LGE_000537
`
`

`
`U.S. Patent
`
`Dec. 26, 2000
`
`Sheet 16 0f 33
`
`6,167,182
`
`
`
`
`
`:55 Hzpow Em: E66 20% 525%
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`
`LGE_000538
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 17 0f 33
`
`6,167,182
`
`
`
`
`
`T55 8138 Eu: 56¢ 20% 825%
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`
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`
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`
`LGE_000539
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`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 18 0f 33
`
`6,167,182
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`LGE_000540
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 19 0f 33
`
`6,167,182
`
`24
`
`II?
`I: 054351
`
`“a
`
`Fig. 24
`
`LGE_000541
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 20 0f 33
`
`6,167,182
`
`Fig.25
`
`LG E_000542
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`LGE_000542
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 21 of 33
`
`6,167,182
`
`Fig. 26'
`
`LG E_000543
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`LGE_000543
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 22 of 33
`
`6,167,182
`
`Fig. 27
`
`LG E_000544
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`LGE_000544
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 23 0f 33
`
`6,167,182
`
`
`
`
`
`DIFFUSEPATTERNDENSITY
`
`Fig. 280
`
`LG E_000545
`
`DISTANCE FROM POINT LIGHT SOURCE
`
`LGE_000545
`
`

`
`U.S. Patent
`
`Dec. 26, 2000
`
`Sheet 24 of 33
`
`6,167,182
`
`23
`
`22
`
`TI f
`~21:
`
`23
`
`Fig. 29a
`
`Fig. 291;
`
`
`
`
`
`OUTPUTUGHTQUANTITYQ
`
`THICKNESS OF OPTICAL
`
`GUIDE PLATE
`
`t(r)
`
`F/'g.30
`
`LG E_000546
`
`LGE_000546
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 25 0f 33
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`6,167,182
`
`Fig. 3/
`
`-30
`
`26
`
`Fig. 32
`
`30
`
`23
`
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`
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`
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`
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`
`55
`
`23
`
`22
`
`25
`
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`
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`
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`
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`
`30
`
`26
`
`23
`
`22
`
`56
`
`34
`
`24
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`LG E_000547
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`LGE_000547
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`

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`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 26 0f 33
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`6,167,182
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` '
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`55
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`.35H9
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`23
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`56
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`22
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`m/ 55
`
`30
`
`26
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`LG E_000548
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`LGE_000548
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`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 27 0f 33
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`6,167,182
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`Fiq.36‘
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`LG E_000549
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`LGE_000549
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 28 0f 33
`
`6,167,182
`
`Fig. 38
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`30
`
`26
`
`66
`
`66
`
`22
`
`66
`
`66
`
`66a
`
`66a
`
`Fig. 39
`
`LG E_000550
`
`LGE_000550
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 29 0f 33
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`6,167,182
`
`Fig/.40
`
`LGE_000551
`
`LGE_000551
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 30 of 33
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`6,167,182
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`Fig. 4/
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`70
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`LG E_000552
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`LGE_000552
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`
`oooooooo53
`
`LGE_000553
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`

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`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 32 0f 33
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`6,167,182
`
`Fig. 43
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`
`
`LG E_000554
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`LGE_000554
`
`

`
`U.S. Patent
`
`Dec. 26,2000
`
`Sheet 33 of 33
`
`6,167,182
`
`Fig. 44
`
`9|
`
`92
`
`TRANSMITTING
`
`CIRCUIT
`
`DRIVING CIRCUIT LIQUID CRYSTAL
`DISPLAY DEVICE
`
`BUTTON SWITCHES
`
`LIQUID CRYSTAL
`
`8|
`
`93
`
`
`
`
`
`LIQUID CRYSTAL
`
`DISPLAY DEVICE
`
`KEY ENTRY
`
`SECTION
`
`
`
`
`
`j
`LIQUID CRYSTTAL
`'-" DRIVING CIRCUIT
`
`—D
`
` OPERATION &
`
`PROCESSING CIRCUIT
`
`97
`
`LGE_000555
`
`LGE_000555
`
`

`
`6,167,182
`
`1
`SURFACE LIGHT SOURCE DEVICE AND
`LIQUID CRYSTAL DISPLAY DEVICE,
`PORTABLE TELEPHONE AND
`INFORMATION TERMINAL EMPLOYING
`THE SURFACE LIGHT SOURCE DEVICE
`
`TECHNICAL FIELD
`
`The present invention relates to a surface light source
`device used for a liquid crystal display device, a lighting
`device or the like, and a liquid crystal display device, a
`portable telephone and an information terminal using the
`surface light source device.
`BACKGROUND ART
`
`Aconventional surface light source device is illustrated in
`FIGS. 1 and 2. FIG. 1 is an exploded perspective view, and
`FIG. 2 is a cross-sectional view. A surface light source
`device 1 comprises an optical (wave)guide plate 2 for
`confining and propagating light, a light emitting device 3,
`and a reflecting plate 4. The optical guide plate 2 is formed
`of resin which is transparent and has a high index of
`refraction, for example, polycarbonate resin or methacrylic
`resin, and a diffuse pattern 5 is formed by irregular
`processing, dot printing of diffuse reflective ink or the like
`on the lower surface of the optical guide plate 2. The light
`emitting device 3 is one having a plurality of so-called point
`light sources 7, for example, light emitting diodes (LED)
`mounted on a circuit board 6, and is opposite to an end
`surface (a light incidence surface 8) of the optical guide plate
`2. The reflecting plate 4 is formed of a white resin sheet, for
`example, which has a high index of reflection, and is affixed
`to the lower surface of the optical guide plate 2 on both its
`sides by double-sided tapes 9.
`Light emitted from the light emitting device 3 and intro-
`duced into the optical guide plate 2 from the light incidence
`surface 8 is confined in the optical guide plate 2 and travels
`by being totally reflected from the inside of the optical guide
`plate 2. The light inside the optical guide plate 2 is diffuse
`reflected upon impinging on the diffuse pattern 5. A light
`beam fl, incident on a light output surface 10 at a smaller
`angle than the critical angle in the total reflection, of the
`reflected light beams is emitted outward from the light
`output surface 10. Alight beam f2 passing through a portion,
`where the diffuse pattern 5 does not exist, on the lower
`surface of the optical guide plate 2 is returned to the optical
`guide plate 2 again upon being reflected from the reflecting
`plate 4, so that the loss of the quantity of light on the lower
`surface of the optical guide plate 2 is prevented from
`occurring.
`In the conventional surface light source device 1,
`however, a light beam f3, entering a portion between the
`lower surface of the optical guide plate 2 and the reflecting
`plate 4, of the light beams emitted from the light emitting
`device 3 enters the optical guide plate 2 from the lower
`surface of the optical guide plate 2 upon being reflected from
`the reflecting plate 4, and is emitted from the light output
`surface 10 of the optical guide plate 2 without being totally
`reflected, as shown in FIG. 3. Therefore, the intensity of the
`output light is large in the vicinity of the light incidence
`surface 8, as shown in a graph of light output intensity
`characteristics on the light output surface 10 in FIG. 4. As a
`result,
`the luminance of the emitted light is high in the
`vicinity of the light incidence surface 8 (an area where the
`luminance is high is indicated by reference numeral 11), and
`the degree of non-uniformity of the luminance distribution
`on the light output surface 10 of the optical guide plate 2 is
`high.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`In the surface light source device 1, a linear light source
`such as a cold-cathode ray tube or a hot-cathode ray tube is
`replaced with the point light sources 7 such as light emitting
`diodes in order to reduce the power consumption. The point
`light sources 7 such as light emitting diodes are disposed in
`line and brought into a pseudo linear light source. Used is the
`optical guide plate 2 which is fabricated in accordance with
`the same design philosophy as that in a case where the linear
`light source is used. Particularly, the diffuse pattern 5 on the
`optical guide plate 2 is the same as that in the case where the
`linear light source is used. That is, reflecting elements 5a
`constituting the diffuse pattern 5 are disposed in a direction
`parallel to the light incidence surface 8, as shown in FIG. 6.
`As the distance from the light incidence surface 8 increases,
`the density of the reflecting elements gradually increases.
`Since the optical guide plate 2 suitable for the point light
`sources 7 is not used, the luminance distribution is non-
`uniform not only in a direction along the length of the optical
`guide plate 2 (the direction in which light travels in the
`optical guide plate 2 upon being totally reflected) but also a
`direction along the width thereof as shown in FIG. 7 (a
`direction perpendicular to the direction along the length, that
`is, an X—X direction shown in FIG. 6).
`In the conventional surface light source device 1, a light
`beam f4, reaching an end surface opposite to the light
`incidence surface 8 and both side surfaces thereof, of the
`light beams propagating while being confined inside the
`optical guide plate 2 leaks outward from the end surface or
`the side surfaces, so that the utilization efficiency of the light
`is reduced, and the luminance of the surface light source
`device 1 is particularly decreased at edges of the light output
`surface 10.
`
`Furthermore, the diffuse pattern 5 is designed in accor-
`dance with the same philosophy as that in the optical guide
`plate for the linear light source, as described above, in the
`optical guide plate 2 used in the conventional surface light
`source device 1, and all
`the reflecting elements 5a are
`disposed with they being directed in the same direction.
`Since the directionality of the diffuse pattern 5 is not
`designed such that the best light output efficiency is obtained
`with respect to the point light sources 7, so that the output
`efficiency of the optical guide plate 2 is low, and the
`luminance of the surface light source device 1 is decreased.
`DISCLOSURE OF INVENTION
`
`invention is to increase the
`An object of the present
`utilization efficiency of light from a light source in a surface
`light source device using a smaller light source, as compared
`with the width of a light incidence surface of an optical
`guide plate (that is, a point light source or ones similar
`thereto).
`Another object of the present invention is to make it
`possible to make the luminance distribution on a light output
`surface uniform in a surface light source device using a
`smaller light source, as compared with the width of a light
`incidence surface of an optical guide plate.
`Still another object of the present invention is to provide
`a liquid crystal display device, a potable telephone and an
`information terminal using the above-mentioned surface
`light source device.
`A surface light source device according to the present
`invention comprises an optical guide plate for confining and
`propagating light introduced from a light incidence surface
`and emitting the light outward from a light output surface,
`and a light source, smaller than the width (the length of a
`side on which the light output surface and the light incidence
`
`LGE_000556
`
`LGE_000556
`
`

`
`6,167,182
`
`3
`
`surface meet) of the light incidence surface of the optical
`guide plate, disposed on the side of the light incidence
`surface of the optical guide plate. Adiffuse pattern is formed
`almost all over a surface, on the opposite side of the light
`output surface, of the optical guide plate. Each of a plurality
`of diffuse pattern elements constituting the diffuse pattern
`has directionality in its shape. A predetermined angular
`relationship exists between a direction defined by the direc-
`tionality of the diffuse pattern element and a direction along
`the line connecting the diffuse pattern element and the light
`source.
`
`In the angular relationship between the direction defined
`by the directionality of the diffuse pattern element and the
`direction along the line connecting the diffuse pattern ele-
`ment and the light source, a variation or dispersion is
`allowed within a range in which the amount of light emitted
`from the light output surface is not substantially changed.
`It is preferable that the size of the light source (the length,
`along the width of the light incidence surface, of the light
`source) is not more than one-half the width of the light
`incidence surface.
`It
`is more desirably not more than
`approximately one-fifth the width of the light incidence
`surface because the light source can be substantially handled
`as a point light source. In a case where a plurality of light
`sources are disposed, close to one another, the length of the
`whole of a range in which the light sources are disposed can
`be set to the size of the light sources.
`According to the present invention, the direction defined
`by the directionality in the shape of each of the plurality of
`diffuse pattern elements constituting the diffuse pattern has
`a predetermined angular relationship with the direction
`along the line connecting the diffuse pattern element and the
`light source. Therefore, the angular relationship is set to the
`most suitable relationship in consideration of the output
`efficiency of the light emitted from the light output surface
`of the optical guide plate, so that the utilization efficiency of
`the light, that is, the output rate can be maximized within a
`range in which the plurality of diffuse pattern elements are
`provided. In consideration of the density of the diffuse
`pattern elements as described later, it is possible to obtain
`illumination which is uniform and high in luminance almost
`all over the surface of the optical guide plate.
`In one mode of the present invention, the direction defined
`by the directionality of the diffuse pattern element
`is a
`direction along the length of the diffuse pattern element. The
`direction along the length is approximately perpendicular to
`the direction along the line connecting the diffuse pattern
`element and the light source. The approximately perpen-
`dicular direction is a direction which allows a variation or
`
`dispersion of approximately 130° with respect to the per-
`pendicular direction. The direction along the length of the
`diffuse pattern element and the direction along the line
`connecting the diffuse pattern element and the light source
`are made approximately perpendicular to each other, so that
`the output rate of light is the highest, thereby making most
`effective light utilization possible.
`In one mode of the shape of the diffuse pattern element,
`the cross section of the diffuse pattern element
`in the
`direction along the line connecting the diffuse pattern ele-
`ment and the light source is close to an isosceles triangle.
`When the cross section of the diffuse pattern element is
`close to an isosceles triangle, light incident on the diffuse
`pattern from the side of the light source can be emitted from
`the light output surface upon being diffuse reflected, and
`light returned upon being reflected from an end surface, on
`the opposite side of the light source, of the optical guide
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`plate can be emitted from the light output surface upon being
`diffuse reflected. Particularly, the diffuse pattern element in
`such a shape is suitable for a case where a structure for return
`reflecting light is provided on the end surface of the optical
`guide plate.
`In another mode, the cross section of the diffuse pattern
`element in the direction along the line connecting the diffuse
`pattern element and the light source is close to a right angled
`triangle.
`When the cross section of the diffuse pattern element is
`close to a right angled triangle, the density of the diffuse
`pattern elements can be increased, so that the light output
`efficiency can be improved.
`In still another mode,
`the cross section of the diffuse
`pattern element in the direction along the line connecting the
`diffuse pattern element and the light source includes an
`arc-shaped edge.
`Since a peripheral surface of the diffuse pattern element
`includes an arc-shaped surface, light incident on the diffuse
`pattern can be reflected almost uniformly over a wide angle,
`thereby contributing to the uniformity of the distribution of
`emitted light.
`In a further mode of the present invention, the length of
`the diffuse pattern element decreases as the distance from the
`light source decreases.
`Particularly when the diffuse pattern element has a linear
`side,
`the distance between a part of the diffuse pattern
`element and the light source can differ depending on the part
`of the diffuse pattern element. The shortest distance between
`the part of the diffuse pattern element and the light source is
`referred to as the shortest distance, and the longest distance
`there between is referred to as the longest distance. The
`nearer the diffuse pattern element is to the light source, the
`larger the difference between the longest distance and the
`shortest distance becomes. The fact that the difference is
`
`large means that the intensity and the direction of emitted
`light easily vary depending on the part of the diffuse pattern
`element. If the nearer the diffuse pattern element is posi-
`tioned to the light source, the shorter the length thereof is,
`the intensity and the direction of emitted light which are
`caused by the diffuse pattern element can be made uniform,
`thereby contributing to the uniformity of the light intensity
`in the whole of the surface light source device.
`In a still further mode of the present invention, a surface
`of the optical guide plate is divided into a plurality of areas
`by lines extending radially from the light source, a plurality
`of diffuse pattern elements being provided for each of the
`areas.
`
`The quantity of light emitted from the light source has an
`angle distribution. If the surface of the optical guide plate is
`divided into the plurality of areas by the lines extending
`radially from the light source, it is possible to design for each
`of the areas the arrangement or the like of the diffuse pattern
`elements depending on the intensity of the light, which is
`introduced into the area, from the light source. It is possible
`to cope with the light quantity distribution depending on the
`direction of the light emitted from the light source.
`When a plurality of light sources are provided so as to
`face to the light incidence surface of the optical guide plate,
`a surface of the optical guide plate is divided into a plurality
`of areas in correspondence with the light sources. In one
`mode, the directions in which the plurality of diffuse pattern
`elements are arranged in each of the areas have a predeter-
`mined angular relationship with directions along the lines
`connecting the corresponding light source and the diffuse
`pattern elements.
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`When there are provided a plurality of light sources, and
`the light sources are relatively spaced apart
`from one
`another, the above-mentioned diffuse pattern is realized for
`each of the areas corresponding to the light sources, so that
`the utilization efficiency of light can be increased for each of
`the areas, and the luminance distribution in the surface light
`source device can be made uniform in consideration of the
`
`density of the diffuse pattern elements as described later.
`In another mode, the directions of the plurality of diffuse
`pattern elements have a predetermined angular relationship
`with directions along the lines connecting the light sources
`which are taken as one light source and the diffuse pattern
`elements.
`
`When the plurality of light sources are relatively close to
`each other, the whole of the light sources can be taken as one
`light source, so that the diffuse pattern can be simplified.
`A surface light source device according to the present
`invention is defined as follows from another point of view.
`That is, a surface light source device according to the present
`invention comprises an optical guide plate for confining
`light introduced from a light incidence surface and emitting
`the light outward from a light output surface, and a light
`source, smaller than the width of the light incidence surface
`of the optical guide plate, disposed on the side of the light
`incidence surface of the optical guide plate. A diffuse pattern
`is formed almost all over a surface, on the opposite side of
`the light output surface, of the optical guide plate. Each of
`a plurality of diffuse pattern elements constituting the diffuse
`pattern has a surface directed toward the light source, a
`direction normal to the surface being approximately parallel
`to a plane which includes a direction along the line con-
`necting the diffuse pattern element and the light source and
`is perpendicular to the light output surface of the optical
`guide plate. The approximately parallel direction is a direc-
`tion which allows a variation or dispersion of approximately
`130°.
`
`The light incident on the diffuse pattern is greatly changed
`in the travel direction on a surface directed toward the light
`source (the angle of reflection of the light is small, or the
`light passes through the surface), so that the rate of light
`output is increased.
`When the boundary of the diffuse pattern elements is
`unclear or random so that each of the diffuse pattern ele-
`ments is not clear, the present invention can be expressed by
`the correlation length.
`Specifically, a surface light source device according to the
`present invention which is applied to such a case comprises
`an optical guide plate for confining light introduced from a
`light incidence surface and emitting the light outward from
`a light output surface, and a light source, smaller than the
`width of the light incidence surface of the optical guide
`plate, disposed on the side of the light incidence surface of
`the optical guide plate. Adiffuse pattern is formed almost all
`over a surface, on the opposite side of the light output
`surface, of the optical guide plate. A partial area of the
`diffuse pattern has directionality in relation to the shapes of
`a plurality of diffuse pattern elements in the partial area. The
`direction in which the correlation length of the diffuse
`pattern in the partial area is the longest has an approximately
`predetermined angular relationship with a direction along
`the line connecting the partial area and the light source. The
`foregoing is based on the premise that the correlation length
`of the diffuse pattern in the partial area differs in at least two
`directions (which may not be perpendicular to each other).
`In the angular relationship between the direction in which
`the correlation length of the diffuse pattern is the longest and
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`the direction along the line connecting the partial area and
`the light source, a variation or dispersion is allowed within
`a range in which the quantity of light emitted from the light
`output surface is not substantially changed. When there are
`a plurality of directions in which the correlation length is the
`largest, any one of the directions may be selected.
`According to the present invention, the direction in which
`the correlation length of the diffuse pattern in the partial area
`is the largest has a predetermined angular relationship with
`the direction along the line connecting the partial area and
`the light source. Therefore, the angular relationship is made
`the most suitable in consideration of the emission efficiency
`of light from the light output surface of the optical guide
`plate, so that
`it
`is possible to maximize the utilization
`efficiency, that is, the output rate of the light. When the
`density of the diffuse pattern elements as described later is
`considered, it is possible to obtain illumination which is
`uniform and high in luminance almost all over the surface of
`the optical guide plate.
`The present
`invention further provides a structure in
`which the luminance distribution on the light output surface
`can be made uniform.
`
`A surface light source device according to the present
`invention comprises an optical guide plate for confining
`light introduced from a light incidence surface and emitting
`the light outward from a light output surface, and a light
`source, smaller than the width of the light incidence surface
`of the optical guide plate, disposed on the side of the light
`incidence surface of the optical guide plate. Adiffuse pattern
`is formed almost all over a surface, on the opposite side of
`the light output surface, of the optical guide plate. The
`density of the diffuse pattern is approximately zero in the
`vicinity of the light source.
`The diffuse pattern can be not only formed by irregular
`processing but also provided by printing using diffuse reflec-
`tive ink.
`
`In a case where a plurality of light sources are disposed so
`as to face the light incidence surface of the optical guide
`plate, the density of the diffuse pattern is approximately zero
`in the vicinity of each of the light sources.
`In the surface light source device using the smaller light
`source, as compared with the width of the light output
`surface of the optical guide plate, when the density of the
`diffuse pattern which is most suitable for the uniformity of
`the luminance distribution on the light output surface is
`considered, the density of the diffuse pattern reaches zero in
`the vicinity of the light source. The diffuse pattern according
`to the present invention is characterized in that the density
`of the diffuse pattern is approximately zero in the vicinity of
`the light source.
`A surface light source device for making the luminance
`distribution uniform in the present invention comprises an
`optical guide plate for confining light introduced from a light
`incidence surface and emitting the light outward from a light
`output surface, and a light source, smaller than the width of
`the light incidence surface of the optical guide plate, dis-
`posed on the side of the light incidence surface of the optical
`guide plate. A diffuse pattern is formed almost all over a
`surface, on the opposite side of the light output surface, of
`the optical guide plate. The diffuse pattern can be provided
`not only by irregular processing but also printing using
`diffuse reflective ink. In the vicinity of the light source, {(the
`density of the diffuse pattern)/[(the thickness of the optical
`guide plate)><(the distance from the light source)]} is
`approximately constant. As the distance from the light
`source increases, {(the density of the diffuse pattern)/[(the
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`thickness of the optical guide plate)><(the distance from the
`light source)]} increases.
`In either one of a case where the change in the thickness
`of the optical guide plate is considered and a case where it
`is not considered,
`the density of the diffuse pattern is
`determined as described above so that the luminance distri-
`
`bution on the surface light source device can be made
`uniform.
`
`Another surface light source device according to the
`present invention comprises an optical guide plate for con-
`fining light introduced from a light incidence surface and
`emitting the light outward from a light output surface, and
`a light source, smaller than the width of the light incidence
`surface of the optical guide plate, disposed on the side of the
`light incidence surface of the optical guide plate. An inclined
`surface is formed in the optical guide plate within a range
`which is not more than approximately one-half the length of
`the optical guide plate toward the light source from an end,
`which is far from the light source, of the optical guide plate
`such that the optical guide plate gradually thins toward the
`end.
`
`The output rate of light emitted from the optical guide
`plate is controlled by the density of the diffuse pattern. As
`the distance from the light source increases, the density of
`the diffuse pattern increases. Beyond a certain distance, the
`density of the diffuse pattern is saturated, so that
`it
`is
`impossible to control the output rate of light by the diffuse
`pattern. In the surface light source device according to the
`present invention,
`the optical guide plate gradually thins
`toward the end far from the light source. In a thin part of the
`optical guide plate, the frequency at which the light is totally
`reflected between the light output surface of the optical
`guide plate and a surface opposite thereto is increased, so
`that the light is easily emitted from the light output surface.
`The luminance on the side of the end of the optical guide
`plate can be improved by compensating for the limit of the
`control of the output rate of light by the diffuse pattern by the
`inclined surface of the optical guide plate. A range in which
`the inclined surface is provided in the optical guide plate is
`sufficient if it is not more than approximately one-half the
`length of the optical guide plate. Moreover, the optical guide
`plate is not easily warped or cracked, thereby making it ea