Ulllted States Patent [19]
`Takeuchi et al.
`
`US005944405A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,944,405
`*Aug. 31, 1999
`
`[54] FLAT LIGHT SOURCE USING LIGHT.
`DIFFUSING SHEET WITH PROJECTIONS
`
`5,521,725
`5,521,797
`
`5/1996 Beeson et a1. .......................... .. 362/26
`5/1996 Kashima et al.
`362/26
`
`[75] Inventors: Michiko Takeuchi; Toshikazu Nishio;
`Taiji Ishii, all of Tokyo-To, Japan
`
`[73] Assignee: Dai Nippon Printing Co., Ltd., 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
`pgtiznt 2term provisions of 35 U.S.C.
`(ax )~
`08/619,623
`
`.
`
`.
`
`[21] Appl- NOJ
`-
`22 PCTFl d:
`A . 11 1995
`1 6
`[
`1
`ug
`’
`[86] PCT No.:
`PCT/JP95/01608
`§ 371 Date:
`May 30, 1996
`§ 102(e) Date: May 30, 1996
`
`[87] PCT Pub. No.: WO96/05466
`
`PCT Pub. Date: Feb. 22, 1996
`Foreign Application Priority Data
`
`[30]
`
`Aug. 12, 1994
`
`[JP]
`
`Japan .................................. .. 6210694
`
`[51] Int. Cl.6 ...................................................... .. F21V 7/04
`[52] US. Cl. ............................................. .. 362/31; 362/551
`[58] Field of Search ............................... .. 362/26, 31, 551
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,729,067
`4,729,068
`
`3/1988 Ohe ......................................... .. 362/26
`3/1988 Ohe
`.... .. 362/26
`
`4,775,222 10/1988 Ohe . . . . . . .
`
`. . . . .. 362/26
`
`4,998,804
`
`3/1991 Horiuchi . . . . . . .
`
`. . . . .. 362/26
`
`362/26
`5,262,928 11/1993 Kashima et al. ..
`362/26
`5,289,351
`2/1994 Kashima et al. ..
`362/26
`5,408,388
`4/1995 Kobayashi et al.
`362/26
`5,442,523
`8/1995 Kashima et al. ..
`362/26
`5,467,208 11/1995 Kokawa et al. ..
`5,467,417 11/1995 Nakamura et al. ..................... .. 362/26
`
`362/31
`1/1997 Nishio et a1.
`5,598,280
`6 1997 M' b ................................... .. 362 26
`/
`‘Z0 6
`/
`5’641’219
`FOREIGN PATENT DOCUMENTS
`
`_
`
`[2522:
`1/1987 121 an.
`62-3226
`1-112202 4/1989 Jagan .
`1-245220 9/1989 Japan.
`4
`iapan -
`5496940 8/1993 Japan '
`6-15008 2/1994 Japan .
`Primary Examiner—Sandra O’Shea
`Assistant Examiner—Marshall Honeyman
`Attorney, Agent, or Firm—Parkhurst & Wendel LLP
`[57]
`ABSTRACT
`
`apan .
`
`A ?at light source used as back-lighting for a liquid crystal
`display device includes an optically conductive plate (1)
`having a light re?ecting layer (2), and a light source
`A
`light-diffusing sheet (8) is provided outside the optically
`conductive plate (1) opposite to the light re?ecting layer
`A lens sheet (4) is provided outside the light-diffusing sheet
`(8), With a transmission type of display element (6) provided
`on the outer side of the lens sheet
`The light-diffusing
`sheet (8) is formed of a transparent material free from
`light-diffusing agent particles and has minute irregularities
`(41) formed at random on the front surface thereof. The
`irregularities are of a surface roughness that is at least the
`Wavelength of the light from the light source, but is no more
`than 100 pm. When the percentage of the cumulative fre
`quency distribution of the heights of the minute irregularities
`are plotted along the Y-aXis and the heights of these minute
`irregularities are plotted along the X-aXis, the cumulative
`frequency distribution curve has a conveX portion oriented
`toWards the loWer side of the coordinates and the average
`value of the heights of the minute irregularities is greater
`than the median value thereof. Thus, a ?at light source is
`provided Which emits highly luminant light uniformly
`Within a predetermined angular range, and Which has no
`local differences in intensity over the surface thereof.
`
`11 Claims, 20 Drawing Sheets
`
`i
`x
`
`wemmmm x m0 v 100. z 500
`
`VIZIO EX. 1024
`
`K.J. Pretech Ex. 1024
`
`Pretech_000898
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 1 0f 20
`
`5,944,405
`
`200\
`
`f9
`
`Pretech_000899
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 2 0f 20
`
`5,944,405
`
`FIG.3
`
`42
`
`4
`
`L4|
`FIG. 4
`
`Pretech_000900
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 3 0f 20
`
`5,944,405
`
`42
`
`42
`
`FIG.6
`
`FIG. 7
`
`FIG.8
`
`42\/
`F
`
`Pretech_000901
`
`

`
`U.S. Patent
`
`Aug. 31, 1999
`
`Sheet 4 0f 20
`
`5,944,405
`
`Pretech_000902
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 5 0f 20
`
`5,944,405
`
`FIG. l3
`
`Pretech_000903
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 6 0f 20
`
`5,944,405
`
`1
`
`' ':
`
`FIG. 14 PRIOR ART
`
`FIG. 16 PRlOR ART
`
`Pretech_000904
`
`

`
`U.S. Patent
`
`Aug. 31, 1999
`
`Sheet 7 of 20
`
`5,944,405
`
`(°/o)
`
`E S
`
`fil-
`
`________________ __
`U>J§ '00 CURVE fA(R)
`§%
`g2; 50
`0%
`:3
`E8
`"0:
`~+-LL
`
`
`
`
`
`CURVE fB (R)
`
`0
`
`SURFACE ROUGHNESS
`
`RMAX
`
`F I G.
`
`I7
`
`F I G.
`
`I8
`
`Fl G.
`
`I9
`
`Fl G. 20
`
`M
`
`M
`
`M
`
`50
`
`Pretech_000905
`
`Pretech_000905
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 8 0f 20
`
`5,944,405
`
`CUMULATIVE
`FREQUENCY
`DISTRIBUTION (%)
`0
`
`HEIGHT OF
`IRREGULARITIES
`(ABSOLUTE VALUE)
`57.7 Wm)
`
`28.8
`
`I00
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`AVERAGE
`VALUE 50
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`0 mm
`MINIMU
`50
`I00 (0/0)
`VALUE M
`MEDIAN
`HEIGHT OF
`vALUE
`IRREGULARITIES
`(ABSOLUTE VALUE )
`
`FIG. 2|
`
`Pretech_000906
`
`

`
`U.S. Patent
`
`Aug. 31, 1999
`
`Sheet 9 0f 20
`
`5,944,405
`
`oomHN
`
`.09 H > .09 H X
`
`mm .0_n_
`
`Pretech_000907
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 10 0f 20
`
`5,944,405
`
`CUMULATIVE
`FREQUENCY
`DISTRIBUTION (%)
`0
`
`HEIGHT OF
`IRREGULARITIES
`(ABSOLUTE VALUE)
`6.88 I Pm)
`
`3.44
`
`I00
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`,1
`
`v
`QAELTJ’QGE so
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`I
`
`MINIMUM
`VALUE
`
`50
`MEDIAN
`VALUE
`
`IOO (% )
`HEIGHT OF
`IRREGULARITIES
`( ABSOLUTE VALUE I
`
`FIG. 23
`
`Pretech_000908
`
`

`
`Pretech_000909
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 12 0f 20
`
`5,944,405
`
`CUMULATIVE
`FREQUENCY
`DISTRIBUTION (%)
`0
`
`I00
`
`I
`
`I
`
`IO.6
`
`I
`
`I
`
`I
`
`I
`
`HEIGHT OF
`IRREGULARITIES
`(ABSOLUTE vALUE)
`3
`2|.
`( ,um)
`
`v R
`QAELU’QGE so
`
`I
`
`I
`
`I
`
`I
`
`/
`
`I
`
`I
`
`I
`
`I
`
`O W
`50
`I00 ( °/o )
`MEDIAN
`HEIGHT OF
`VALUE
`IRREGULARITIES
`( ABSOLUTE vALUE )
`
`MINIMUM
`VALUE
`
`FIG. 25
`
`Pretech_000910
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 13 0f 20
`
`5,944,405
`
`MAGNIFICATION x : IOO, Y: I00, 2 : 500
`
`FI G. 26
`
`.
`
`‘is .i
`
`s
`
`Pretech_000911
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 14 0f 20
`
`5,944,405
`
`CUMULATIVE
`FREQUENCY
`DISTRIBUTION (%)
`0
`IOO
`
`HEIGHT OF
`IRREGULARITIES
`(ABSOLUTE VALUE)
`20.3 (Am)
`
`lo.‘
`
`AVERAGE
`vALUE 50
`
`O
`
`MINIMUM
`vALUE
`
`50
`MEDIAN
`vALUE
`FIG. 27
`
`I00 (°/°)
`HEIGHT OF
`IRREGULARITIES
`( ABSOLUTE VALUE )
`
`Pretech_000912
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 15 0f 20
`
`5,944,405
`
`CUMULATIVE
`FREQUENCY
`DISTRIBUTION (%)
`0
`
`I00
`
`I
`
`I
`
`I
`
`l
`
`2.02
`
`AVERAGE
`
`VALUE 50
`
`I
`
`I
`
`I
`
`I
`
`/
`
`HEIGHT OF
`IRREGULARITIES
`(ABSOLUTE VALUE)
`4.'°(#m)
`
`I
`
`I
`
`I
`
`I
`
`l
`
`I
`
`O
`
`MINIMUM
`VALUE
`
`50
`MEDIAN
`VALUE
`
`I00 (°/o)
`HEIGHT OF
`IRREGULARITIES
`( ABSOLUTE VALUE )
`
`FIG. 29
`
`Pretech_000913
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 16 0f 20
`
`5,944,405
`
`MAGNIFICATION X 2 I00, Y 3 I00, Z 3 I000
`
`FIG. 30
`
`'P
`
`‘
`
`)
`g’
`9.
`BE
`\ 13»
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`l,“
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`“
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`u\-. a; Qfkiwv \A
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`U 1 in
`
`I ..
`
`t '
`
`ii,
`
`_‘
`
`Pretech_000914
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 17 0f 20
`
`5,944,405
`
`CUMULATIVE
`FREQUENCY
`DISTRIBUTION (%)
`O
`I00
`
`I
`
`I
`
`HEIGHT OF
`IRREGULARITIES
`( ABSOLUTE VALUE )
`306
`
`I 53
`
`l
`
`I
`
`I
`
`I
`
`T
`
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`
`AVERAGE
`
`[
`|
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`l
`l
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`,
`I
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`l
`
`0
`
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`VALUE
`
`50
`MEDIAN
`VALUE
`
`I00 (% )
`HEIGHT OF
`IRREGULARITIES
`(ABSOLUTE VALUE )
`
`FIG. 3|
`
`Pretech_000915
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 18 0f 20
`
`5,944,405
`
`*3 % Z 2500
`
`FIG. 32
`
`
`xzso, Y:250.
`MAGNIFICATION
`
`9 X
`
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`v1
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`
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`
`Pretech_000916
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 19 0f 20
`
`5,944,405
`
`oomr . . _
`
`
`8: _ 29555 232
`\% Go?mzw E 132102 v
`
`Pretech_000917
`
`

`
`U.S. Patent
`
`Aug.31, 1999
`
`Sheet 20 of 20
`
`5,944,405
`
`1,O\mm._oz<
`
`432EZ_
`
`V p
`
`zmzmmam
`
`C_>_mm__>_mz
`
`Pretech_000918
`
`Pretech_000918
`
`

`
`5,944,405
`
`1
`FLAT LIGHT SOURCE USING LIGHT-
`DIFFUSING SHEET WITH PROJECTIONS
`THEREON
`
`TECHNICAL FIELD
`
`The present invention relates to a flat light source that can
`be used in applications such as back-lighting for a light
`transmissive type of display device such as a liquid crystal
`display device, an illuminated advertisement, or a traffic
`sign. In particular, it relates to a flat light source that uses a
`light-diffusing sheet.
`
`BACKGROUND OF THE INVENTION
`
`light sources that are used for back-lighting in
`Flat
`devices such as liquid crystal displays (LCDs) are known in
`the art, as described below.
`Afirst known type of flat light source is an edge-lit system
`that uses a flat optically transmissive plate as an optically
`conductive member. The flat light source used in this system
`causes light to be incident on one or both side edge surfaces
`of the optically conductive member which is formed of a
`transparent flat plate. Total reflection within the flat optically
`transmissive plate is utilized to propagate the light through-
`out the entire optically conductive plate. Part of the thus
`propagated light becomes diffused reflected light of less than
`the critical angle from a light-scattering reflective plate on
`the rear surface of the optically conductive plate, and thus
`diffused light
`is emitted from the outer surface of the
`optically conductive plate (refer to Japanese Utility Model
`Laid Open No. 55-162,201).
`A second known type of flat light source has a lens sheet
`wherein one surface has projections and the other surface is
`smooth, which is placed with the projection side thereof on
`the outer surface of the optically conductive plate of the flat
`light source of the above first type. The light-focusing action
`of this lens is utilized to ensure that the diffused, reflected
`light is diffused uniformly and isotropically within a prede-
`termined angular range (refer to Japanese Utility Model Laid
`Open No. 4-107,201).
`The above described lens sheet could be used in combi-
`
`nation with a frosted transparent diffusion plate (a frosted
`transparent sheet) formed by dispersing particles of a light-
`diffusing agent such as TiO2 within a transparent plastic. In
`such a case,
`the optical energy of the light source is
`distributed in a more concentrated manner within a prede-
`termined limited angular range, than when a frosted trans-
`parent diffusion plate alone is placed over the optically
`conductive plate (refer to U.S. Pat. No. 4,729,067 and
`Japanese Patent Laid Open No. 61-55,684). Moreover, a
`uniform and highly isotropic diffused light can be obtained
`within this angular range.
`However, both of the above prior-art techniques have
`problems. The first one simply places a light-scattering
`reflective plate on the rear surface of the optically conduc-
`tive plate so that the emitted light has a comparatively sharp
`distribution that peaks at an angle of 60 degrees to the
`normal of the surface of the optically conductive plate.
`Therefore a phenomenon is observed in which the degree of
`luminance is insufficient in the normal direction (the forward
`direction) where brightness is most required, while optical
`energy is wasted in the lateral directions where it is com-
`pletely unnecessary.
`The second prior-art technique has a problem in that,
`when a lenticular sheet that comprises an array of a large
`number of individual triangular prismatic lenses is super-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`imposed on the light-emitting surface of the optically con-
`ductive plate as the lens sheet, the ratio of optical energy
`emitted within angles between 30 and 60 degrees from the
`normal to the light-emitting surface is comparatively high,
`but even if the portion within 2 to 4 cm of the side edge
`portion of the optically conductive plate is very bright, the
`luminance drops gradually further away from this portion so
`that the edge at the opposite side from the light source is
`noticeably darker.
`If a frosted transparent scattering diffusion plate is used,
`a further problem arises in that the particles of light-diffusing
`agent within the optically conductive plate absorb some of
`the light, so that the optical energy thereof is lost.
`In addition, interference patterns such as Newton’s rings
`could be generated by the optical seal between the lens sheet
`and the surface of the optically conductive plate.
`Techniques that have been tried to solve these problems
`include:
`
`1. An attempt to correct and make uniform the luminance
`distribution within the surface of the optically conductive
`plate by creating a pattern in a light-scattering reflective
`layer on the rear surface of the optically conductive plate,
`such as a dot pattern, in such a manner that the surface area
`of the pattern is decreased closer to the light source and
`increased further away therefrom, as disclosed in Japanese
`Patent Laid-Open No. 1-245,220 and Japanese Utility Model
`Laid-Open No. 6-15,008.
`2. An attempt to correct and make uniform the luminance
`distribution within the surface of the optically conductive
`plate by disposing two or more light sources at the side edge
`portions of the optically conductive plate, as disclosed in
`Japanese Patent Laid-Open No. 3-9306.
`3. An attempt to obtain a directed output light that has a
`substantially uniform luminance from the entire surface of
`the optically conductive plate, by providing a linear pris-
`matic array (an array of prismatic lenses) that partially
`reflects and partially passes light on either the front or rear
`surface of the optically conductive plate, and varying the
`angle of inclination of the surfaces of these prisms and
`locally varying the thickness of the optically conductive
`plate, as disclosed in Japanese Patent Laid-Open No.
`62-3226.
`
`All of the above measures, and others, have problems in
`that it is difficult to provide a completely uniform luminance
`thereby. In addition, technique 1 has a further problem in
`that the dot pattern of the light-scattering reflective layer is
`visible from the side from which light is emitted. Technique
`2 has a further problem in that the space required for the
`entire light source and the power consumption thereof are
`more than doubled.
`
`Technique 3 has problems in that the form of the optically
`conductive plate is complicated,
`the fabrication of this
`design is extremely difficult, and it is also difficult to make
`the dot pattern of the light-scattering reflective layer invis-
`ible.
`
`An objective of the present invention is therefore to solve
`the above problems with the prior art and provide a flat light
`source that implements a uniform and very bright light that
`is limited to a predetermined angular range, and that has no
`variations in luminance due to position within the light
`surface, without increasing the power consumption, amount
`of heat generated, or the size of the entire apparatus.
`DISCLOSURE OF THE INVENTION
`
`The present invention achieves the above objective by
`providing a flat light source that uses a light-diffusing sheet
`
`Pretech_000919
`
`Pretech_000919
`
`

`
`5,944,405
`
`3
`formed of a transparent material containing no particles of a
`light-diffusing agent. The front surface of this light-diffusing
`sheet has minute irregularities formed at random;
`these
`minute irregularities have a surface roughness of at least the
`wavelength of the light from the light source, but no more
`than 100 gm; and, when a cumulative frequency distribution
`curve of the heights of the minute irregularities is plotted,
`with the percentage of the cumulative frequency distribution
`of the heights of these irregularities along the Y-axis and the
`heights of the minute irregularities along the X-axis,
`the
`cumulative frequency distribution curve has a convex por-
`tion oriented towards a lower side of coordinates and the
`
`average value of the heights of the minute irregularities is
`greater than a median value thereof.
`A light source is disposed along at least one side edge
`surface of this light-diffusing sheet, and the light-diffusing
`sheet can be provided superimposed on a light-emitting
`surface of an optically conductive member that is a flat
`optically transmissive plate, or an optically conductive
`member that has a rectangular cavity therein, where the
`optically conductive member has a light-reflecting layer on
`the rear surface thereof.
`
`The light-diffusing sheet may also be provided in such a
`manner as to cover a window in a lamp housing. This lamp
`housing is configured to contain a light source,
`light-
`reflecting wall surfaces of the lamp housing cover the rear
`and side surfaces of the light source, and the window is
`formed in the front surface of the light source.
`When the above described optically conductive member
`is formed of a fiat, optically transmissive plate, the front
`surface of the optically conductive member may be a fiat
`surface having a surface roughness less than the wavelength
`of the light of the light source.
`A sheet that is a one-dimensional or two-dimensional
`
`array of convex or concave lenses may be superimposed
`over the light-diffusing sheet. Similarly, another, identical
`light-diffusing sheet may be superimposed over the first
`light-diffusing sheet.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a perspective view of an example of a trans-
`missive type of display device using an edge-lit type of flat
`light source in accordance with this invention;
`FIG. 2 is a perspective view of an example of a trans-
`missive type of display device using a back-lit type of flat
`light source in accordance with this invention;
`FIG. 3 is a cross-sectional illustrative view through an
`example of an edge-lit flat light source, showing groups of
`projections formed on both surfaces of the light-diffusing
`sheet;
`FIG. 4 is a perspective view of an example of a lens sheet
`used by the present invention;
`FIG. 5 is a perspective view of another example of a lens
`sheet used by the present invention;
`FIG. 6 is a perspective view of a further example of a lens
`sheet used by the present invention;
`FIG. 7 is a perspective view of yet another example of a
`lens sheet used by the present invention;
`FIG. 8 is a perspective view of an example of two
`superimposed lens sheets used by the present invention;
`FIG. 9 is a sectional view showing the paths of light rays
`travelling from the interior of the optically conductive plate
`to the outside thereof;
`FIG. 10 is a sectional view showing light rays that have
`been emitted by the tunnel effect from the optically conduc-
`tive plate as they proceed into the lens sheet;
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`FIG. 11 is a sectional view showing how some of the light
`rays proceeding towards the exterior of the optically con-
`ductive plate are totally reflected while others are
`transmitted, when the light-diffusing sheet of the present
`invention is used;
`FIG. 12 shows an example of the method of fabricating
`the light-diffusing sheet of the present invention;
`FIG. 13 is a perspective view of an example of the
`light-diffusing sheet of the present invention, fabricated by
`the fabrication method shown in FIG. 12;
`FIG. 14 is a cross-sectional view through an edge-lit flat
`light source of the prior art, showing the lack of a lens sheet;
`FIG. 15 is a perspective view of the edge-lit fiat light
`source of the prior art, showing a lens sheet having a flat rear
`surface;
`FIG. 16 is a sectional view of the configuration of FIG.
`15;
`FIG. 17 is a graph of cumulative frequency distribution
`f(R), with surface roughness (that
`is,
`the percentage of
`cumulative frequency distribution of depths R of concavities
`in the mold) plotted along the X-axis and with surface
`roughness (that is, the depths R of the concavities in the
`mold) plotted along the Y-axis, in a fabrication mold for
`imprinting projections of the light-diffusing sheet in accor-
`dance with the present invention;
`FIG. 18 is a sectional view of a fabrication mold having
`the characteristic of the cumulative frequency distribution
`curve fA(R) in FIG. 17;
`FIG. 19 is a is a sectional view of a fabrication mold
`
`having the characteristic of the cumulative frequency dis-
`tribution curve fB(R) in FIG. 17;
`FIG. 20 is a sectional view illustrating undercutting
`formed by aggregations of minute spherical particles of the
`metal chrome, during the matte-plating of chrome onto the
`indented surface of the roll mold used for fabrication;
`FIG. 21 shows a graph relating to the heights of projec-
`tions of the light-diffusing sheet used in this invention (that
`is, the surface roughness of the light-diffusing sheet) and the
`distribution thereof as a cumulative frequency distribution
`curve f(R), with surface roughness (that is, the percentage of
`cumulative frequency distribution of heights R of the peaks)
`plotted along the X-axis and surface roughness (that is, the
`heights R of the peaks) along the Y-axis, in a case in which
`the cumulative frequency distribution curve has a downward
`convexity and a relationship (average valueémedian value)
`is satisfied;
`FIG. 22 is a fragmentary expanded three-dimensional
`visualization of measurements showing the projections of a
`light-diffusing sheet used in this invention having the cumu-
`lative frequency distribution characteristics of FIG. 21;
`FIG. 23 shows a graph relating to the heights of projec-
`tions of the light-diffusing sheet used in this invention (that
`is, the surface roughness of the light-diffusing sheet) and the
`distribution thereof as a cumulative frequency distribution
`curve f(R), with surface roughness (that is, the percentage of
`cumulative frequency distribution of heights R of the peaks)
`plotted along the X-axis and surface roughness (that is, the
`heights R of the peaks) plotted along the Y-axis, in a case in
`which the cumulative frequency distribution curve has a
`downward convexity and a
`relationship (average
`valueémedian value) is satisfied;
`FIG. 24 is a partial expanded three-dimensional visual-
`ization of measurements showing the projections of a light-
`diffusing sheet used in this invention having the cumulative
`frequency distribution characteristics of FIG. 23;
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`FIG. 25 is a graph showing the heights of projections of
`the light-diffusing sheet used in this invention (that is, the
`surface roughness of the light-diffusing sheet) and the dis-
`tribution thereof as a cumulative frequency distribution
`curve f(R), with surface roughness (that is, the percentage of
`cumulative frequency distribution of heights R of the peaks)
`plotted along the X-axis and surface roughness (that is, the
`heights R of the peaks) plotted along the Y-axis, in a case in
`which the average value of R has a maximum at 52% and the
`cumulative frequency distribution curve has a downwardly
`convex portion and an upwardly convex portion;
`FIG. 26 is a partial expanded three-dimensional visual-
`ization of measurements showing the projections of a light-
`diffusing sheet used in this invention having the cumulative
`frequency distribution characteristics of FIG. 25;
`FIG. 27 is a graph showing the heights of projections of
`the light-diffusing sheet used in this invention (that is, the
`surface roughness of the light-diffusing sheet) and the dis-
`tribution thereof as a cumulative frequency distribution
`curve f(R), with surface roughness (that is, the percentage of
`cumulative frequency distribution of heights R of the peaks)
`plotted along the X-axis and surface roughness (that is, the
`heights R of the peaks) plotted along the Y-axis, in a case in
`which the cumulative frequency distribution curve is linear;
`FIG. 28 is a partial enlarged perspective view of projec-
`tions of a light-diffusing sheet formed of ridges of a rect-
`angular equilateral triangular sectional shape and having the
`cumulative frequency distribution curve of FIG. 27;
`FIG. 29 is a graph showing the heights of projections of
`the light-diffusing sheet used in this invention (that is, the
`surface roughness of the light-diffusing sheet) and the dis-
`tribution thereof as a cumulative frequency distribution
`curve f(R), with surface roughness (that is, the percentage of
`cumulative frequency distribution of heights R of the peaks)
`plotted along the X-axis and surface roughness (that is, the
`heights R of the peaks) plotted along the Y-axis, in a case in
`which the cumulative frequency distribution curve has a
`partial upwardly convex portion and a partial downwardly
`convex portion, and the average value is less than the
`median value.
`
`FIG. 30 is a partial expanded three-dimensional visual-
`ization of measurements showing the projections of a light-
`diffusing sheet used in this invention having the cumulative
`frequency distribution characteristics of FIG. 29;
`FIG. 31 is a graph showing the heights of projections of
`the light-diffusing sheet used in this invention (that is, the
`surface roughness of the light-diffusing sheet) and the dis-
`tribution thereof as a cumulative frequency distribution
`curve f(R), with surface roughness (that is, the percentage of
`cumulative frequency distribution of heights R of the peaks)
`plotted along the X-axis and surface roughness (that is, the
`heights R of the peaks) plotted along the Y-axis, in a case in
`which the cumulative frequency distribution curve has a
`downward convexity over the entire region thereof;
`FIG. 32 is a partial expanded three-dimensional visual-
`ization of measurements showing the projections of a light-
`diffusing sheet used in this invention having the cumulative
`frequency distribution characteristics of FIG. 32;
`FIG. 33 shows the angular distribution of luminance of a
`light-emitting surface of a fiat light source (Example 1 and
`Comparative Example 1); and
`FIG. 34 shows the angular distribution of luminance of a
`light-emitting surface of another fiat light source (Example
`8) of this invention.
`BEST MODES FOR CARRYING OUT THE
`INVENTION
`
`Embodiments of a flat light source in accordance with the
`present invention and a display device using this flat light
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`source will be described below with reference to the accom-
`
`panying drawings.
`An edge-lit type of fiat light source is shown in FIG. 1. In
`this figure, reference numeral 1 denotes an optically con-
`ductive plate, where this optically conductive plate 1 is
`configured of a solid optically transmissive fiat plate. A light
`source such as a linear light source 3 is provided along one
`edge surface of this optically conductive plate 1. A reflective
`mirror 5 is provided behind the light source 3. Light emitted
`from the light source 3 enters the interior of the optically
`conductive plate 1 either directly or after being reflected by
`the reflective mirror (also a lamp housing with reflective
`surfaces) 5. It is reflected internally as shown for example in
`FIG. 3 or it
`is emitted directly outside from within the
`optically conductive plate 1. A light reflecting layer 2 is
`provided on the rear surface of the optically conductive plate
`1. The surface of the optically conductive plate 1 opposite to
`the light reflecting layer 2 forms a light-emitting surface. A
`light-diffusing sheet 8 is provided facing this light-emitting
`surface, and light emitted through the light-emitting surface
`passes through the light-diffusing sheet 8. Light that has
`passed through the light-diffusing sheet 8 passes through a
`lens sheet 4 and then reaches a transmission type display
`device 6. A gap 9 is formed between the light-diffusing sheet
`8 and the optically conductive plate 1, and between the
`light-diffusing sheet 8 and the lens sheet 4. Projections 41
`that will be described in more detail later are formed on the
`
`surfaces of the light-diffusing sheet 8 facing these gaps 9.
`Instead of being solid, the optically conductive plate 1
`could have an empty structure. In such a case, the light-
`emitting surface of the solid optically conductive plate 1 and
`the surface thereof in contact with the light reflecting layer
`2 would each be in the form of a flat plate, with the space
`therebetween forming a rectangular cavity.
`With a back-lit light source as shown in FIG. 2, the light
`source 3 is a linear or point light source and accommodated
`within a lamp housing 5. The lamp housing 5 extends over
`the rear and sides of the light source 3 and acts as reflective
`surfaces for reflecting the light from the light source 3 in the
`direction of the light-diffusing sheet 8.
`In FIGS. 1 and 2, reference number 100 denotes a flat
`light source in its entirety and reference number 200 denotes
`a display device in its entirety.
`In accordance with this present invention, projections 41
`are obtained by forming minute irregularities at random
`(such as in a sharkskin pattern or a pear skin pattern) over
`the entire surface of the light-diffusing sheet 8. These minute
`irregularities have a surface roughness that is at least the
`wavelength of the light emitted from the light source but is
`no more than 100 gm. When the percentages of the cumu-
`lative frequency distribution of the heights of the minute
`irregularities are plotted along the Y-axis and the heights of
`these minute irregularities along the X-axis, the cumulative
`frequency distribution curve should typically have a convex
`portion oriented towards the lower side of the coordinates
`and the average value of the height of the minute irregu-
`larities should be greater than the median thereof.
`The light-diffusing sheet 8 used by the present invention
`is formed from a transparent material. In this case,
`the
`transparent material may be a (meth)acrylic ester such as
`polymethyl (meth)acrylate or polyethyl (meth)acrylate
`[where “(meth)acrylate” means either methacrylate or acry-
`late throughout the specification] or a copolymer thereof; a
`polyester such as polyethylene terephthalate or polybutylene
`terephthalate; a thermoplastic resin such as a polycarbonate,
`polystyrene or polymethylpentene resin; an ultraviolet light-
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`or electron beam-curable, transparent resin which has been
`crosslinked and cured by ultraviolet rays or electron beams;
`a transparent glass; or a transparent ceramic material.
`As ultraviolet light curable resins or electron beam cur-
`able resins, compositions comprising a prepolymer, oligo-
`mer and/or monomer having, in its molecule, a polymeriz-
`able unsaturated bond such as a (meth)acryloyl group or a
`(meth)acryloyloxy group or an epoxy group.
`In this
`connection,
`the term “(meth)acryloyl” is used to mean
`“acryloyl” or “methacryloyl” throughout the specification.
`Such prepolymer and oligomer include an acrylate such as
`urethane (meth)acrylate, polyester (meth)acrylate epoxy
`(meth)acrylate, and an unsaturated polyester. Further, a
`dipentaerythritol penta (meth)acrylate can be exemplified as
`the monomer used in this case.
`
`It is important that these materials contain absolutely no
`light-diffusing particles, unlike in the ordinary light-
`diffusing plate (such as that cited in U.S. Pat. No. 4,729,067)
`For a light-diffusing sheet 8 for an edge-lit fiat
`light
`source, a sheet of the above described transparent material
`that has a thickness on the order of 5 to 200 pm is used.
`Alternatively,
`for a back-lit
`light source,
`the light-
`diffusing sheet 8 must support its own weight and bear
`external forces, so it should have a thickness on the order of
`1 to 10 mm to prevent distortion.
`The light-diffusing sheet 8 could have a single-layer
`structure as shown in FIG. 1, or a double-layer structure as
`shown in FIG. 13, or even a multi-layer structure with three
`or more layers.
`The projections 41 on the surface of the light-diffusing
`sheet 8 are formed as fine irregularities of a height that is at
`least the wavelength of the light from the light source, but
`no more than 100 gm. These irregularities could be formed
`directly by subjecting the surface of the optically transmis-
`sive material to embossing by a thermal press or to sand-
`blasting, or they could be formed by a method such as
`casting. Alternatively, a layer of an optically transmissive
`material having these projections 41 could be formed on the
`flat surface of the optically transmissive plate. More
`specifically, a method such as that described in Japanese
`Patent Laid Open No. 5-169,015 and U.S. Pat. No. 4,576,
`850 could be used, in which projections 41 are formed on the
`surface of an optically transmissive base film 12 by using
`rolls and the above ultraviolet- or electron-beam-curable
`
`(hardenable) plastic.
`transmitted thereto
`In addition to diffusing the light
`isotropically,
`the projections 41 formed on the light-
`diffusing sheet 8 are designed to create the gap 9 of at least
`the wavelength 7» of the light
`from the light source
`(dime