Ulllted States Patent [19]
`Saito
`
`US005890791A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,890,791
`*Apr. 6, 1999
`
`[54] LIGHT CONTROL SHEET, SURFACE LIGHT
`SOURCE UNIT AND LIQUID CRYSTAL
`DISPLAY DEVICE
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`[75] Inventor: Shinichiro Saito, Hachioji, J apan
`
`[73] Assignee: Konica Corporation, Tokyo, Japan
`
`3,716,710
`5,032,963
`5,303,322
`5,506,929
`
`2/1973 Clostermann et a1. ............... .. 362/330
`7/1991 Granstrom ................ .. 362/337
`4/1994 Winston et a1.
`385/146
`4/1996 Tai et a1. ............................... .. 385/146
`
`[*] 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).
`
`FOREIGN PATENT DOCUMENTS
`W094/06051 3/1994 WIPO .................................... .. 349/65
`
`Primary Examiner—Alan Cariaso
`Attorney, Agent, or Firm—Finnegan, Henderson, FaraboW,
`Garrett & Dunner, LLP
`
`[22]
`[30]
`
`Oct‘ 9’ 1996
`Flled:
`Foreign Application Priority Data
`
`[JP]
`
`Oct. 13, 1995
`6
`IIlt. Cl- ...................................................... ..
`
`Japan .................................. .. 7-265530
`
`[52] US. Cl. .......................... .. 362/31; 362/330; 362/339;
`385/146
`
`A light control sheet for receiving a light incident on one
`surface thereof and for emitting the received light from the
`other surface thereof, includes plural prisms Which is pro
`Vided at least one of Surfaces of the Sheet' Each of the prisms
`has a generatriX in parallel to each other, and has tWo side
`Surfaces formed on
`Sides
`respect to a plane Surface
`Which includes the generatriX and is perpendicular to the
`light control sheet. The tWo side surfaces have asymmetrical
`shapes each other With respect to the plane surface, and at
`least one of the tWo side surfaces is nonplanar.
`
`[58] Field of Search ........................ .. 349/62—66; 362/26,
`362/31, 330, 337, 339; 385/146
`
`19 Claims, 24 Drawing Sheets
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`14
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`I
`12a 12
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`12b
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`13a
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`LGD_001162
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`LG Display Ex. 1035
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 1 0f 24
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`5,890,791
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`LGD_001163
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 2 0f 24
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`5,890,791
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`A.
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`#955?
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`6838+
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`LGD_001164
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 3 0f 24
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`5,890,791
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`13c
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`POINT B
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`POINT A 20
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`FIG. 3
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`LGD_001165
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 4 0f 24
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`5,890,791
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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 5 0f 24
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`5,890,791
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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 6 0f 24
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`5,890,791
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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 7 0f 24
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`5,890,791
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`U.S. Patent
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`Apr. 6, 1999
`
`Sheet 8 0f 24
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`5,890,791
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 9 0f 24
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`5,890,791
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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 10 0f 24
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`5,890,791
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 11 0f 24
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`U.S. Patent
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`5,890,791
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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 13 of 24
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`U.S. Patent
`
`Apr. 6, 1999
`
`Sheet 14 0f 24
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`5,890,791
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`U.S. Patent
`
`Apr. 6, 1999
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`Sheet 15 0f 24
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`5,890,791
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`

`
`U.S. Patent
`
`Apr. 6, 1999
`
`Sheet 16 0f 24
`
`5,890,791
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`FIG. 16
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`LGD_001178
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 17 0f 24
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`5,890,791
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`LGD_001179
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`

`
`U.S. Patent
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`5,890,791
`
`FIG. 18(
`
`FIG.18(b)
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`LGD_001180
`
`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 19 0f 24
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`5,890,791
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`2b
`
`FIG. 19
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`LGD_001181
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`

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`U.S. Patent
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`Apr. 6, 1999
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`Sheet 20 of 24
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`5,890,791
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`LGD_001182
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`

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`U.S. Patent
`
`Apr. 6, 1999
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`Sheet 21 of 24
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`5,890,791
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`U.S. Patent
`
`Apr. 6, 1999
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`Sheet 22 of 24
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`5,890,791
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`LGD_001184
`
`

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`U.S. Patent
`
`Apr. 6, 1999
`
`Sheet 23 of 24
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`5,890,791
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`FIG.23(b)
`
`3e
`
`3b
`
`FIG.23(a)
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`LGD_001185
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`LGD_001185
`
`

`
`U.S. Patent
`
`Apr. 6, 1999
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`Sheet 24 of 24
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`5,890,791
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`
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`LGD_001186
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`

`
`5,890,791
`
`1
`
`LIGHT CONTROL SHEET, SURFACE LIGHT
`SOURCE UNIT AND LIQUID CRYSTAL
`DISPLAY DEVICE
`
`BACKGROUND OF THE INVENTION
`
`invention relates to a light control sheet
`The present
`wherein a plurality of prisms are formed on at least one side
`of the sheet in a manner that generatrices forming the prisms
`are in parallel each other so that an incident light entering the
`light control sheet through the prism-formation side of the
`light control sheet emerges out of the other side thereof, and
`it relates also to a surface light source unit and a liquid
`crystal display device both employing the light control
`sheet.
`
`Next, an example of a conventional light control sheet
`will be explained as follows, referring to drawings. FIG. 19
`is a diagram showing how a liquid crystal display device
`employing a conventional light control sheet is structured,
`and FIG. 20 is an enlarged section of the light control sheet
`shown in FIG. 19.
`
`In the drawing, the numeral 1 represents a cold-cathode
`tube, and 2 represents a light-guiding plate wherein prism
`projection 2b is formed on transmission surface 2a so that
`light emitted from the cold-cathode tube may emerge
`obliquely to light control sheet 3.
`The light control sheet 3 makes light emerged from the
`transmission surface 2a to be emerged almost perpendicular
`to the surface of the light control sheet, to illuminate liquid
`crystal display element 4.
`With regard to the light control sheet 3, in this case, a
`plurality of prisms of the same shape 3a are formed on
`surface (incident surface) 3a’ which faces the light-guiding
`plate 2 in a manner that generatrices are in parallel each
`other as shown in FIG. 20. Further, a shape on the section of
`each prism 3a viewed in the direction that is in parallel with
`its generatrix is an equilateral triangle.
`Namely, in prism 3a, a length of boundary surface at light
`source side 3b through which light coming from light-
`guiding plate 2 enters is the same as a length of boundary
`surface on the side opposite to a light source 3c totally
`reflecting light toward liquid crystal display element 4.
`In a liquid crystal display device structured in the afore-
`said manner, light emitted from light source 1 enters light-
`guiding plate 2, then, is guided through the light-guiding
`plate 2, and emerges out of the light-guiding plate through
`its transmission surface 2a. Then, the light enters through
`boundary surface at the light source side 3b of prism 3a on
`light-converging sheet 3, and a part of the light directly
`emerges out of emergent surface 3e and the other part
`thereof is reflected on boundary surface on the side opposite
`to a light source 3c and both of them emerge out of emergent
`surface 3e, to illuminate liquid crystal display element 4.
`FIG. 22 shows an example of light distribution on emer-
`gent surface 3e indicated when light having characteristics
`shown in FIG. 21 enters an incident surface of the light
`control sheet 3.
`Incidentally, specifications of the light
`control sheet 3 used for the present conventional example
`are as follows.
`
`. Polycarbonate (refractive index n=1.59)
`.
`Material .
`Vertical angle (61) of boundary surface at light source
`side 3b of prism 3a=34°
`Vertical angle (62) of boundary surface on the side
`opposite to a light source 3c of prism 3a=34°
`Both boundary surface at light source side 3b and bound-
`ary surface on the side opposite to a light source 3c are
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`2
`shown on the expression wherein coordinate axes are estab-
`lished so that an origin of the coordinate passes through a
`vertex of the prism 3a and generatrices of the prism 3a are
`perpendicular to X-Y plane, as shown in FIG. 20.
`Boundary surface at light source side 3b .
`.
`. Y=—1.540X
`(-0.649 mm§X§0 mm)
`Boundary surface on the side opposite to a light source 3c
`. .Y=1.540X (0§X§0.649 mm)
`On the light control sheet 3 of the present conventional
`example, the maximum luminance was 2278 (cd/m2) and a
`half-value degree (angular range having the luminance that
`is more than a half of the maximum luminance) was 25 .63°.
`An optical system of the light control sheet 3 mentioned
`above will be explained as follows, referring to FIGS. 23(a)
`and 23(b). FIG. 23(a) is a diagram explaining an optical
`system wherein light shown in FIG. 21 enters light control
`sheet 3 shown in FIG. 20, while FIG. 23(b) is a diagram on
`which the optical system shown in FIG. 23(a) is developed.
`FIG. 23(b) shows an optical system wherein light enters
`a slanted transparent plate and then emerges from it. In the
`case of this type of optical system, when an inclination angle
`is changed, an angle of emergence from emergent surface 3e
`only changes. Namely, a graphical shape of light distribution
`characteristics as shown in FIG. 22 is only subjected to
`parallel displacement in the direction of an axis of the angle
`of emergence, and the shape itself can not be controlled.
`On the other hand, FIG. 23(a) is equivalent to FIG. 23(b)
`wherein light enters a slanted transparent plate and emerges
`from it. Therefore, even when vertical angles (61, 62) of
`prism 3a corresponding to a slanted angle are changed, a
`graphical shape itself can not be controlled though an angle
`of emergence only changes. Namely, neither the maximum
`luminance nor the half-value degree can be controlled, and
`further, side lobe light (undesired light that usually does not
`enter an eye of a liquid crystal observer) can not be con-
`trolled either, which is a problem.
`When light emerging from light control sheet 3 is highly
`directive under the condition that an amount of light emerg-
`ing from light control sheet 3 is constant independently of a
`distance from a light source, especially when the emergence
`luminance in the +10° direction falls rapidly under the
`condition that light which emerges in the direction to the
`front (0° direction) is intense, there is a possibility that a
`liquid crystal observer observes an occurrence of luminance
`unevenness.
`
`For example, when a liquid crystal image plane in A4 size
`is observed from the point which is away from the image
`plane by about 500 mm and is perpendicular to the image
`plane at its center, an edge of the image plane is inclined by
`about 10°.
`
`FIG. 24 is a diagram showing light distribution charac-
`teristics of a light control sheet and luminance unevenness
`that is caused by the change of an apparent angle to a liquid
`crystal observer.
`Therefore, it is efficient when a peak of emergence is
`directed to a liquid crystal observer at various points on an
`image plane instead of directing the peak of emergence to
`the 0° direction all together on the entire image plane.
`SUMMARY OF THE INVENTION
`
`The present invention has been achieved in view of the
`problems mentioned above, and its first object is to provide
`a light control sheet capable of controlling the maximum
`luminance, a half-value degree and a side lobe light, a
`surface light source unit and a liquid crystal display device.
`The second object is to provide a light control sheet that
`is free from luminance unevenness.
`
`LGD_001187
`
`LGD_001187
`
`

`
`5,890,791
`
`3
`A light control sheet of the invention for solving the
`problems mentioned above is represented by a light control
`sheet having on at least one side of the sheet a plurality of
`prisms formed in a manner that generatrices forming the
`prisms are in parallel each other so that an incident light
`entering the light control sheet through the prism-formation
`side of the sheet emerges out of the other side thereof,
`wherein each of the aforesaid prisms has, when viewed in
`the direction that is in parallel with the generatrices, two
`boundary surfaces which meet at a vertex of the prism and
`are asymmetric about a straight line which passes through
`the vertex of the prism and is perpendicular to the sheet
`surface, and at least one of the two boundary surfaces is
`nonplanar.
`Due to an arrangement that at least one of two boundary
`surfaces is made to be nonplanar, when a shape of the
`nonplanar surface is changed, a graphical shape of lumi-
`nance of light emerging from a light control sheet can be
`changed, and thereby it is possible to control a maximum
`luminance, a half-value degree and a side lobe light so that
`they may meet the specifications of a liquid crystal display
`device.
`
`As a preferable example, there is one wherein a sectional
`shape of the prism is made to be almost a triangle having the
`first and second boundary surfaces and at least one of these
`two boundary surfaces is made to be a curved surface, and
`there further is one wherein the prism is formed on each of
`both sides of an incident light.
`In an example which is further preferable, an incident
`light to one side of a sheet is a directive light having a peak
`of luminance in the direction inclined against the sheet
`surface, and aforesaid first boundary surface is a plane for
`guiding aforesaid light to the inside of the prism, while,
`aforesaid second boundary surface is a plane for reflecting
`the light guided through the first boundary surface toward a
`surface on the other side of the sheet.
`
`Constitution of the prism such as one mentioned above
`includes the following.
`(1) Aforesaid first boundary surface of the prism is a plane
`surface and aforesaid second boundary surface thereof is
`a curved surface that is convex.
`
`(2) Aforesaid first boundary surface of the prism is a plane
`surface and aforesaid second boundary surface thereof is
`a curved surface that is concave.
`
`(3) Aforesaid first boundary surface of the prism is a curved
`surface that is convex and aforesaid second boundary
`surface is a plane surface.
`(4) Aforesaid first boundary surface of the prism is a curved
`surface that is concave and aforesaid second boundary
`surface thereof is a plane surface.
`(5) One of the first and second boundary surfaces of the
`prism is a curved surface that is convex while the other is
`a curved surface that is concave.
`
`(6) Both of the first and second boundary surfaces of the
`prism represent a curved surface that is convex.
`(7) Both of the first and second boundary surfaces of the
`prism represent a curved surface that is concave.
`Further,
`for eliminating luminance unevenness when
`viewed from an observer, it is preferable to change a shape
`of the section of the prism so that an intensity peak of light
`entering the sheet may be inclined against the sheet plane,
`and light emerging from the sheet may pass through the
`center portion of the sheet and may cross a plane that is in
`parallel with generatrices of the prism and is perpendicular
`to the sheet surface.
`
`Asurface light source unit of the invention is represented
`by a surface light source unit composed of a light source, a
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`4
`light-guiding plate that guides light emitted from the light
`source and makes it to emerge in the prescribed direction,
`and a light control sheet in which a plurality of prisms are
`formed on at least one side of the sheet in a manner that
`
`generatrices forming the prisms are in parallel each other
`and light coming from the light-guiding plate is made to
`enter one side of the sheet and to emerge from the other side
`of the sheet, wherein each of the aforesaid prisms of the light
`control sheet has, when viewed in the direction that is in
`parallel with the generatrices, two boundary surfaces which
`meet at a vertex of the prism and are asymmetric about a
`straight line which passes through the vertex of the prism
`and is perpendicular to the sheet surface, and at least one of
`the two boundary surfaces is nonplanar.
`Due to an arrangement that at least one of two boundary
`surfaces is made to be nonplanar, when a shape of the
`nonplanar surface is changed, a graphical shape of lumi-
`nance of light emerging from a light control sheet can be
`changed, and thereby it is possible to control a maximum
`luminance, a half-value degree and a side lobe light so that
`they may meet the specifications of a liquid crystal display
`device.
`
`A liquid crystal display device of the invention is repre-
`sented by a liquid crystal display device composed of a light
`source, a light-guiding plate that guides light emitted from
`the light source and makes it to emerge in the prescribed
`direction, a light control sheet in which a plurality of prisms
`are formed on at least one side of the sheet in a manner that
`
`generatrices forming the prisms are in parallel each other
`and light coming from the light-guiding plate is made to
`enter one side of the sheet and to emerge from the other side
`of the sheet, and liquid crystal display elements provided at
`both sides on the other side of the light control sheet,
`wherein each of the aforesaid prisms of the light control
`sheet has, when viewed in the direction that is in parallel
`with the generatrices, two boundary surfaces which meet at
`a vertex of the prism and are asymmetric about a straight line
`which passes through the vertex of the prism and is perpen-
`dicular to the sheet surface, and at least one of the two
`boundary surfaces is nonplanar.
`Due to an arrangement that at least one of two boundary
`surfaces is made to be nonplanar, when a shape of the
`nonplanar surface is changed, a graphical shape of lumi-
`nance of light emerging from a light control sheet can be
`changed, and thereby it is possible to control a maximum
`luminance, a half-value degree and a side lobe light.
`It is preferable to provide a diffusion sheet between a
`light-guiding plate and a light control sheet so that an image
`plane of a liquid crystal element may be illuminated uni-
`formly in terms of luminance.
`Further, for the purpose of preventing a moire caused by
`a light-guiding plate and a light control sheet, it is possible
`to provide the light control sheet on the surface of the liquid
`crystal display element that is opposite to its surface facing
`the light-guiding plate.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a diagram showing the structure of a liquid
`crystal display device that is an example of the invention.
`FIG. 2 is an enlarged section of a light control sheet
`shown in FIG. 1.
`
`FIG. 3 is a diagram explaining a curved surface of a prism
`in the example.
`FIG. 4 is a diagram showing light distribution character-
`istics of an emergent light in the case where a boundary
`surface positioned at the light source side is a plane surface
`
`LGD_001188
`
`LGD_001188
`
`

`
`5,890,791
`
`5
`and a boundary surface on the opposite side is a curved
`surface (convex).
`FIG. 5 is a diagram showing light distribution character-
`istics of an emergent light in the case where a boundary
`surface positioned at the light source side is a plane surface
`and a boundary surface on the opposite side is a curved
`surface (concave).
`FIG. 6 is a diagram showing light distribution character-
`istics of an emergent light in the case where a boundary
`surface positioned at the light source side is a curved surface
`(convex) and a boundary surface on the opposite side is a
`plane surface.
`FIG. 7 is a diagram showing light distribution character-
`istics of an emergent light in the case where a boundary
`surface positioned at the light source side is a curved surface
`(concave) and a boundary surface on the opposite side is a
`plane surface.
`FIG. 8 is a diagram showing light distribution character-
`istics of an emergent light in the case where a boundary
`surface positioned at the light source side is a curved surface
`(convex) and a boundary surface on the opposite side is a
`curved surface (convex).
`FIG. 9 is a diagram showing light distribution character-
`istics of an emergent light in the case where a boundary
`surface positioned at the light source side is a curved surface
`(concave) and a boundary surface on the opposite side is a
`curved surface (concave).
`FIG. 10 is a diagram showing light distribution charac-
`teristics of an emergent light in the case where a boundary
`surface positioned at the light source side is a curved surface
`(convex) and a boundary surface on the opposite side is a
`curved surface (concave).
`FIG. 11 is a diagram showing light distribution charac-
`teristics of an emergent light in the case where a boundary
`surface positioned at the light source side is a curved surface
`(concave) and a boundary surface on the opposite side is a
`curved surface (convex).
`FIG. 12 is a diagram in which FIGS. 4, 5 and 22 are
`summarized.
`
`FIG. 13 is a diagram in which FIGS. 6, 7 and 22 are
`summarized.
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`FIG. 14 is a diagram in which FIGS. 8, 9 and 22 are
`summarized.
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`FIG. 15 is a diagram in which FIGS. 10, 11 and 22 are
`summarized.
`
`FIG. 16 is a diagram showing the structure of a liquid
`crystal display device in which a diffusion sheet is used.
`FIGS. 17(a) and 17(b) are diagrams showing an arrange-
`ment wherein a prism portion of a light control sheet is
`arranged so that it faces a liquid crystal display element.
`FIGS. 18(a) and 18(b) are diagrams showing other
`examples wherein a boundary surface on a prism portion of
`a light control sheet is made to be aspheric.
`FIG. 19 is a diagram showing a liquid crystal display
`device employing a conventional light control sheet.
`FIG. 20 is an enlarged section of the light control sheet
`shown in FIG. 19.
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`FIG. 21 is a diagram of light distribution characteristics of
`light entering an incident surface of the light control sheet
`shown in FIG. 19.
`
`FIG. 22 is a diagram showing an example of light
`distribution characteristics observed on the surface of emer-
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`gence when the light having characteristics shown in FIG.
`21 enters a light control sheet shown in FIG. 19.
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`FIGS. 23(a) and 23(b) represent a diagram explaining an
`optical system of the light control sheet shown in FIG. 19.
`FIG. 24 is a diagram explaining the relation between a
`luminance of light emerging from the light control sheet
`shown in FIG. 19 and a distance from a light source.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Structure of an example of the invention will be explained
`as follows, referring to the drawings. FIG. 1 is a diagram
`showing the structure of a liquid crystal display device that
`is an example of the invention, and FIG. 2 is an enlarged
`section of a light control sheet shown in FIG. 1.
`In the drawing, the numeral 11 represents a cold-cathode
`tube which is a light source, and 12 is a light-guiding plate
`wherein prism projection 12b is formed on transmission
`surface 12a so that light emitted from the cold-cathode tube
`11 may emerge obliquely toward light control sheet 13.
`On the transmission surface 12a of the light-guiding plate
`12, there is provided light control sheet 13 which makes
`light emerging from the transmission surface 12a of the
`light-guiding plate 12 to emerge in the prescribed direction.
`These items including the cold-cathode tube 11,
`the
`light-guiding plate 12 and the light control sheet 13 consti-
`tute a surface light source unit that illuminates liquid crystal
`display element 14.
`in this case, a
`With regard to the light control sheet,
`plurality of prisms 13a are formed on the surface (incident
`surface) 13d that faces the light-guiding plate 2, in a manner
`that generatrices are in parallel each other as shown in FIG.
`2.
`
`Each of prisms 13a in the example of the invention is
`almost a triangle in shape which is asymmetric about a plane
`perpendicular to the light control sheet 13 when viewed in
`the direction parallel with generatrices forming the prism
`13a, and boundary surface 13b positioned at the light source
`side is a plane surface and boundary surface 13c on the side
`opposite to a light source is a curved surface which is
`concave.
`
`Light control sheet 13 in the example of the invention is
`manufactured through a method of molding by means of a
`metallic mold in which polycarbonates or acrylic resins are
`used, or
`through a method for forming a prism on a
`transparent base board using UV-hardening resin composi-
`tion disclosed in Japanese Patent Publication Open to Public
`Inspection No. 174911/1995 (hereinafter referred to as Japa-
`nese Patent O.P.I. Publication).
`Further, a form of the section of each prism 13a is
`established to take a shape which makes the light emerging
`from light control sheet 13 to pass through the center portion
`of the light control sheet 13 and to intersect a plane which
`is in parallel with generatrices of the prism 13 and is
`perpendicular to the light control sheet plane.
`In a liquid crystal display device structured in the afore-
`said manner,
`light emerging from light source 11 enters
`light-guiding plate 12, then, is guided through the light-
`guiding plate 12, and emerges out through its transmission
`surface 12a. Then, the light enters through boundary surface
`at light source side 13b of prism 13a on light-converging
`sheet 13, and a part of the light directly emerges out of
`emergent surface 13e and the other part thereof is reflected
`on boundary surface on the side opposite to a light source
`13c and emerges out of emergent surface 13e, both to
`illuminate liquid crystal display element 14.
`Incidentally, a difference between prism 13a of light
`control sheet 13 in the example of the invention and a prism
`
`LGD_001189
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`LGD_001189
`
`

`
`5,890,791
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`7
`of a Fresnel lens lies in that a boundary surface at only one
`side of a Fresnel lens prism is used, while, light enters
`boundary surface 13b at the light source side and boundary
`surface 13c on the side opposite to a light source reflects
`light in the prism 13a in the example of the invention,
`resulting in a difference between them in terms of optical
`actions.
`
`The light control sheet 13 structured in aforesaid manner
`is equivalent to the occasion where an incident surface of a
`slanted transparent plate shown in FIG. 23(b) is a curved
`surface which is concave, and when this curved surface is
`changed, light distribution characteristics of emergent light,
`namely, a maximum luminance, a half-value degree and a
`side lobe are changed. Even in aforesaid constitution,
`therefore, it is possible to change light distribution charac-
`teristics of an emergent
`light by making the boundary
`surface 13c on the side opposite to a light source of prism
`13a on the light control sheet 13 to be a concave curved
`surface.
`
`Further, due to an arrangement wherein a sectional form
`of each prism 13a is established to take a shape which makes
`the light emerging from light control sheet 13 to pass
`through the center portion of the light control sheet 13 and
`to intersect a plane which is in parallel with generatrices of
`the prism 13 and is perpendicular to the light control sheet
`plane, a luminance peak of light emerging from emergent
`surface 13e of the light control sheet 13 moves from the
`minus direction to the plus direction as a distance from light
`source 11 increases, and the peak of light is concentrated to
`an eye of an observer looking at almost the center of liquid
`crystal display element 14, resulting in elimination of lumi-
`nance unevenness.
`
`To be concrete, it is preferable to make an arrangement so
`that an angle of emergent peak of light emerging from the
`light control sheet 13 takes —10:5° at the position near the
`light source, 15° at the center of an image plane, and 10:5°
`at the position far from the light source, under consideration
`about a margin that is for the occasion that an eye of an
`observer is deviated from the center of an image plane.
`Incidentally, the invention is not limited to the example
`mentioned above. In aforesaid example, boundary surface
`13b at the light source side on the prism 13a is a plane
`surface and boundary surface 13c on the side opposite to a
`light source thereof is a concave curved surface. However,
`the invention is not
`limited to this, and the following
`combinations are also satisfactory.
`(1) Aboundary surface at the light source side on the prism
`is a plane surface and a boundary surface on the side
`opposite to a light source on the prism is a convex curved
`surface.
`
`(2) A boundary surface at the light source side of the prism
`is a convex curved surface and a boundary surface on the
`side opposite to a light source thereof is a plane surface.
`(3) A boundary surface at the light source side of the prism
`is a concave curved surface and a boundary surface on the
`side opposite to a light source thereof is a plane surface.
`(4) One of a boundary surface at the light source side of the
`prism and a boundary surface on the side opposite to a
`light source thereof is a convex curved surface and the
`other is a concave curved surface.
`
`(5) Both of a boundary surface at the light source side and
`a boundary surface on the side opposite to a light source
`represent a convex curved surface.
`(6) Both of a boundary surface at the light source side and
`a boundary surface on the side opposite to a light source
`represent a concave curved surface.
`Though a form of the section of prism 13a is mostly a
`triangle in the example stated above, the invention is not
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`limited to this, and a form of the section of prism 13a which
`is mostly a trapezoid having a flat portion at a vertex of the
`prism as shown in FIG. 3 is also satisfactory provided that
`a length of the flat portion is shorter than a pitch of the prism
`13a. In this case, when the vertex of the prism is positioned
`to be farthermost from the sheet surface, prism vertex 20 is
`represented by two points (point A and point B), and a
`boundary surface at the light source side is positioned on the
`left side of the point A, while a boundary surface on the side
`opposite to a light source is positioned on the right side of
`the point B.
`In the liquid crystal display device mentioned above, a
`diffusion sheet may be provided between light-guiding plate
`12 and light control sheet 13 so that an image plane of liquid
`crystal display element 14 can be illuminated uniformly in
`terms of luminance.
`
`Further, it is also possible to provide light control sheet 13
`on the surface of liquid crystal display element 14 opposite
`to its surface facing light-guiding plate 12 for the purpose of
`preventing a moire caused by both light-guiding plate 12 and
`light control sheet 13.
`
`EXAMPLE
`
`Inventors of the invention examined light distribution
`characteristics of an emergent light by changing the forms of
`boundary surface 13b at the light source side and boundary
`surface 13c on the side opposite to a light source both of
`light control sheet 13. The results of this examination are
`shown in FIGS. 4-11.
`
`In these drawings, light entering the light control sheet 13
`is the same as those in FIG. 21. Specifications of light
`control sheet 13 are as follows.
`
`. Acrylic resin (refractive index n=1.492) or,
`.
`Material .
`polycarbonate resin (refractive index n=1.59)
`Height of prism 13a .
`.
`. 1 mm
`Further, a plane surface and a curved surface are repre-
`sented by expressions wherein there are established X-Y
`axes of coordinates in which an origin passes through the
`vertex of prism 13a and generatrices of prism 13a are
`perpendicular to X-Y plane.
`(1) FIG. 4
`. Acrylic resin
`.
`Material .
`the light source side (plane
`Boundary surface 13b at
`surface) Y=—1.540X (-0.649 mm§X§0 mm)
`Boundary surface 13c on the side opposite to a light
`source (convex)
`(X+5)2+(Y—3.247)2=35.543 (0
`mm§X§0.522 mm)
`(2) FIG. 5
`. Acrylic resin
`.
`Material .
`the light source side (plane
`Boundary surface 13b at
`surface) Y=—1.540X (-0.649 mm§X§0 mm)
`Boundary surface 13c on the side opposite to a light
`so