United States Patent [191
`Suzuki et a1.
`
`l||l|lllllllllllllllllllllll] llllllllllllllllllllllllllllllllllllllllllll
`
`SOO5600462A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,600,462
`Feb. 4, 1997
`
`[54] OPTICAL FILM AND LIQUID CRYSTAL
`DISPLAY DEVICE USING THE FILM
`
`[75] Inventors: Masaru Suzuki, Yamato; Fumihisa
`Hanzawa, Sagamihara; Manabu Mogi,
`Yamato, all of Japan
`
`[73] Assignee: International Business Machines
`Corporation, Armonk, NY.
`
`[21] Appl. N0.: 121,657
`[22] Filed:
`Sep. 14, 1993
`[30]
`Foreign Application Priority Data
`
`Sep. 16, 1992
`
`[JP]
`
`Japan .................................. .. 4-246225
`
`[51] Int. Cl.6 ................................................. .. G02F 1/1343
`[52] US. Cl. ......................................... .. 349/112; 349/62
`[58] Field of Search ................................ .. 359/69, 48, 49,
`359/50, 42, 599; 362/31, 330, 333; 385/146,
`901
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,832,541
`
`8/1974 Basset et al. ......................... .. 362/333
`
`4,648,690
`
`3/1987 Ohe . . . . . . . . . . . . . .
`
`. . . .. 362/31
`
`4,725,134
`
`2/1988 Ogino . . . . . . . . . . . . . . _ _ . . .
`
`. . _ .. 353/74
`
`4,791,540 12/1988 Dreyes, Jr. et al. . . . . .
`
`. . . .. 362/31
`
`5,005,168
`4/1991 Pritash et a1. . . . . . . . .
`. . . .. 362/31
`5,159,478 10/1992 Akiyarna et al. ....................... .. 359/68
`
`5,206,746 4/1993 Ooi et a1. . . . .
`
`. . . . .. 359/40
`
`5,262,880 11/1993 Abileah . . . . . . . . . . .
`
`. . . . .. 359/68
`
`5,289,351
`5,390,276
`
`.... .. 362/31
`2/1994 Kashima et a1.
`2/1995 Tai et a1. ............................... .. 385/146
`
`FOREIGN PATENT DOCUMENTS
`
`0534140 3/1993 European Pat. Off. ............... .. 359/50
`
`OTHER PUBLICATIONS
`“Polarized Backlight for Liquid Crystal Display”, IBM
`Technical Disclosue Bulletin, vol. 33 No. 1B, Jun. 1990, pp.
`143—144.
`
`Primary Examiner—William L. Sikes
`Assistant Examiner—James A. Dudek
`Attorney, Agent, or Firm—David Aker; Jay P. Sbrollini;
`James E. Murray
`
`[57]
`
`ABSTRACT
`
`The liminance within the viewing angle of an LCD device
`is increased by using an optical ?lm of transparent material.
`The ?lm has a ?rst surface having a wave structure including
`a plurality of isosceles triangle prisms arranged side-by-side,
`and a second surface having an optically rough structure for
`performing di?’use transmission. The ?lm may also have a
`?rst surface having a structure including a plurality of
`quadrangular prisms arranged side-by-side, and a second
`surface having an optically rough structure for performing
`diffuse transmission.
`
`7 Claims, 8 Drawing Sheets
`
`1
`
`P4
`
`LGD_001147
`
`LG Display Ex. 1034
`
`

`
`US. Patent
`
`Feb. 4, 1997
`
`Sheet 1 of 8
`
`5,600,462
`
`FIG. 1
`
`1{ “\1
`.
`
`7
`
`6 0
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`'f4 2
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`FIG 2
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`Bi
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`QQOOOOOOQO 00000000000
`QOOOOOOOQOQGOQOOOOOOOQ
`QOQOOOOOQQQOQOOOOOOQQQ
`OOOOOOOOOGGOGOOOOOOOOO
`0000000000 00000000000
`oooOOOOOOQ 00600000000
`QOOOOOOOQQOQQQOOOOOOQQ
`MIN
`MAX
`MIN
`1.2mmgb
`2.0mm¢
`1.2mm¢
`
`LGD_001148
`
`

`
`US. Patent
`
`Feb. 4, 1997
`
`Sheet 2 of 8
`
`5,600,462
`
`FIG. 3
`
`L
`
`2
`
`2
`
`32
`
`4V4
`
`1W
`
`r5
`
`9
`
`31
`
`DOT
`
`
`
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`
`1
`
`1
`
`I’ 1
`
`1
`
`l
`
`l
`
`J
`
`I
`
`l
`
`LGD_001149
`
`

`
`US. Patent
`
`Feb. 4, 1997
`
`Sheet 3 of 8
`
`5,600,462
`
`LGD_001150
`
`

`
`U.S. Patent
`
`Feb. 4, 1997
`
`Sheet 4 of 8
`
`5,600,462
`
`FIG . 6
`
`
`
`—=+— B=90"
`—o— B=95"
`—-— a=1oo°
`—x—— B=110:
`3:190
`
`30
`70
`
`INTENSITY
`
`so
`
`40
`
`30
`
`20
`
`o
`
`
`
`FIG. 7
`
`—u—— B=g0°
`—~o— a=95°
`—-— B=100°
`+ B=110"
`—— B=120“
`
`
`
`LGD_001151
`
`LGD_001151
`
`

`
`US. Patent
`
`Feb. 4, 1997
`
`Sheet 5 of 8
`
`5,600,462
`
`FIG. 8
`
`FIG. 9
`
`LGD_001152
`
`

`
`U.S. Patent
`
`Feb. 4, 1997
`
`Sheet 6 0f 8
`
`5,600,462
`
`FIG. 10
`
`FIG. 11
`
`—°— ARRANGEMENTS IN FIGS. 9 AND 10
`+ ARRANGEMENT IN FIG. 13
`—-+— ARRANGEMENT IN FIG. 8
`+ ARRANGEMENT IN FIG. 12
`
`150 —
`
`140 —
`
`120 4
`
`100 —
`
`LGD_001153
`
`

`
`U.S. Patent
`
`Feb. 4, 1997
`
`Sheet 7 of 8
`
`5,600,462
`
`FIG. 12
`
`l/'\
`.
`
`A
`
`A
`
`81
`
`
`
`1
`
`
`
`5
`
`10
`
`FIG. 13
`
`81
`
`7 1 1
`L§’% %
`§\
`
`LGD_001154
`
`LGD_001154
`
`

`
`U.S. Patent
`
`Féb. 4, 1997
`
`Sheet 8 of 8
`
`5,600,462
`
`FIG . 15
`
`LGD_001155
`
`LGD_001155
`
`

`
`1
`OPTICAL FILM AND LIQUID CRYSTAL
`DISPLAY DEVICE USING THE FILM
`
`5,600,462
`
`2
`In accordance with the invention, an optical ?lm of
`transparent material comprises a ?rst surface having a wave
`structure including a plurality of isosceles triangle prisms
`arranged side-by-side, and a second surface having an
`optically rough structure for performing diffuse transmis
`sion. A top angle of said isosceles triangle prisms is in a
`range of 95 degrees to 120 degrees.
`An optical ?lm of transparent material of the present
`invention may also comprise a ?rst surface having a struc
`ture including a plurality of quadrangular prisms arranged
`side-by-side, and a second surface having an optically rough
`structure for performing diffuse transmission. A top angle of
`said quadrangular prisms is in the range of 95 degrees to 120
`degrees.
`The liquid crystal display device of the present invention
`includes a liquid crystal display panel and a back light
`device. The back light device comprises a light source for
`emitting light; a light guide means having a top surface
`facing a back surface of the liquid crystal display panel and
`a side surface receiving the light from said light source; a
`re?ector means provided on a back surface of the light guide
`means; and an optical ?lm of transparent material positioned
`between the back surface of the liquid crystal display panel
`and the top surface of the light guide means, including a ?rst
`surface having a wave structure including a plurality of
`isosceles triangle prisms arranged side-by-side and a second
`surface having an optically rough structure for performing
`diffuse transmission. A top angle of the isosceles triangle
`prisms is in the range of 95 degrees to 120 degrees. A
`polarizer is positioned between the liquid crystal display
`panel and the optical ?lm. The direction along which peaks
`and valleys of the isosceles triangle prisms are oriented is
`aligned in parallel to a polarizing axis of the polarizer.
`The liquid crystal display device of the present invention
`also includes a liquid crystal display panel and a back light
`device wherein the back light device comprises a light
`source for emitting light; a light guide means having a top
`surface facing a back surface of the liquid crystal display
`panel and a side surface receiving the light from said light
`source; a re?ector means provided on a back surface of the
`light guide means; and two optical ?lms of transparent
`material positioned between the back surface of the liquid
`crystal display panel and the top surface of the light guide
`means, each of the optical ?lms including a ?rst surface
`having a wave structure including a plurality isosceles
`triangle prisms arranged side-by-side and a second surface
`having an optically rough structure for performing diffuse
`transmission. A top angle of the isosceles triangle prisms is
`in the range of 95 degree to 120 degrees.
`The direction along which peaks and valleys of the
`isosceles triangle prisms of one of the two optical ?lms are
`oriented is at an angle with respect to a direction along
`which peaks and valleys of the isosceles triangle prisms of
`the other of the two optical ?lms are oriented.
`A polarizer is positioned between the liquid crystal dis
`play panel and the two ?lms, and the direction along which
`the peaks and valleys of the isosceles triangles prisms of the
`optical ?lm nearer to said polarizer is oriented is parallel to
`a polarizing axis of the polarizer.
`The liquid crystal display device of the present invention
`also includes a liquid crystal display panel and a back light
`device wherein the back light device comprises a light
`source for emitting light; a light guide means having atop
`surface facing a back surface of the liquid crystal display
`panel and a side surface receiving the light from the light
`source; a re?ector means provided on a back surface of the
`
`FIELD OF THE INVENTION
`
`This invention relates to liquid crystal displays. More
`particularly it relates to an optical ?lm used in a liquid
`crystal display (LCD) device and an LCD device including
`an LCD panel and back light device having the optical ?lm,
`so as to increase luminance of light within the viewing
`angle.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`An LCD device includes an LCD panel, polarizers, a
`backlight device, and an LCD driver circuit. The backlight
`device includes one or two ?uorescent lamps, and a light
`guide. The light guide receives light from the ?uorescent
`lamp to transmit light of uniform luminance to the LCD
`panel.
`The light guide has a lower surface for performing diffuse
`re?ection and an upper surface for performing diffuse trans
`mission, whereby light of uniform luminance is directed to
`the LCD panel from the upper surface.
`To enhance the uniformity of the light, various technolo
`gies have been developed. One of these technologies is
`shown in Japanese utility model application 61-197685
`(Published
`unexamined utility
`model application
`63-101921), wherein a backlight device includes a diffuse
`transmission plate facing an LCD panel, a backside re?ector
`and a lamp positioned between the diffuse transmission plate
`and the backside re?ector. A center portion of the diffuse
`transmission plate has a Fresnel lens arranged parallel to the
`lamp.
`Recently, color LCD devices used in portable personal
`computers have required a backlight device of high lumi
`nance and low power consumption. The high luminance and
`the low power consumption are inconsistent with each other,
`because as small a battery as possible is required in a
`portable personal computer.
`Further, recently portable personal computers have been
`reduced to A4 size. The size of the LCD device, and
`particularly the size of the backlight device, must also be
`reduced.
`U.S. Pat. No. 4,906,070 discloses a thin ?exible ?lm made
`of a transparent polymeric material including a structured
`surface and an opposite smooth surface, wherein light strik
`ing either surface within certain angular ranges, is totally
`internally re?ected. The structured surface includes a linear
`array of miniature substantially right angled isosceles prisms
`arranged side-by-side to form a plurality of peaks of
`grooves. Viewed in a broad conceptual sense, the structured
`surface of the ?lm described in this patent may be perceived
`as resembling the structure of one surface of the optical ?lm
`of the present invention. However, the optical ?lm of the
`present invention differs remarkably from the ?lm disclosed
`in U.S. Pat. No. 4,906,070. Further, the use of the optical
`?lm in the particular position in the LCD device according
`to the present invention is not disclosed or suggested by this
`patent.
`
`20
`
`25
`
`35
`
`40
`
`45
`
`55
`
`SUMMARY OF THE INVENTION
`
`It is an object of the invention to provide a high luminance
`and small size backlight device for the color LCD used in
`portable personal computers.
`
`65
`
`LGD_001156
`
`

`
`5,600,462
`
`3
`light guide means; and an optical ?lm of transparent material
`positioned between the liquid crystal display panel and the
`light guide means, including a ?rst surface having a structure
`including a plurality of quadrangular prisms arranged side
`by-side and a second surface having an optically rough
`structure for performing diffuse transmission. A top angle of
`the quadrangular prisms is in the range of 95 degrees to 120
`degrees.
`A polarizer is positioned between the liquid crystal dis
`play panel and the optical ?lm. The direction along which
`peaks and valleys of the quadrangular prisms of the optical
`?lm is parallel is oriented to a polarizing axis of said
`polarizer.
`The liquid crystal display device of the present invention
`also includes a liquid crystal display panel and a back light
`device wherein the light device comprises a light source for
`emitting light; a light guide means of transparent material
`having a top surface facing a back surface of the liquid
`crystal display panel, a bottom surface and a side surface
`receiving the light from the light source; a re?ector means
`provided on a back surface of the light guide means; and a
`?lm of material performing diffuse transmission formed on
`an edge area of the bottom surface of said light guide means
`adjacent to said light source.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic cross sectional view of an LCD
`device in accordance with the present invention.
`FIG. 2 is a plan view of the light guide of the present
`invention.
`FIG. 3 is an enlarged cross sectional view of a portion of
`the LCD device of the present invention.
`FIG. 4 is an perspective view of the structure of the optical
`?lm of the present invention.
`FIG. 5 illustrates various light paths in the optical ?lm of
`the present invention.
`FIG. 6 illustrates the characteristic curves of the optical
`?lm of the present invention.
`FIG. 7 illustrates additional characteristic curves of the
`optical ?lm of the present invention.
`FIG. 8 is a schematic representation of a ?rst arrangement
`according to present invention.
`FIG. 9 is a schematic representation of a second arrange
`ment according to the present invention.
`FIG. 10 is a schematic representation of a third arrange
`ment according to the present invention.
`FIG. 11 illustrates further characteristic curves of the
`optical ?lms of the present invention.
`FIG. 12 is a schematic representation of a fourth arrange
`ment according to present invention.
`FIG. 13 is a schematic representation of a ?fth arrange
`ment according to the present invention.
`FIG. 14 illustrates an alternative structure of the optical
`?lm of the present invention.
`FIG. 15 is an enlarged cross sectional view of a prior art
`structure.
`FIG. 16 is an enlarged cross sectional view of a structure
`in accordance with the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`FIG. 1 shows an LCD device which comprises an LCD
`glass panel 1, polarizers 4 and 5 and a backlight device 6.
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`The LCD glass panel 1 includes two glass substrates 2 and
`3, peripheral edges of which are sealed by an edge seal 11,
`such as epoxy resin, and liquid crystal material retained
`between the glass substrates 2 and 3. The polarizing axis of
`polarizer 4 is perpendicular to the polarizing axis of polar
`izer 5.
`The backlight device 6 includes ?uorescent lamps 7, a
`light guide 8 and a thin optical ?lm 9. The light guide 8 is
`made of transparent material, such as polymethylmethacry
`late. On a ?rst or bottom surface of the light guide 8, a
`re?ective dot pattern of a re?ective material e.g. alumina is
`printed, as shown in FIG. 2. The size of the printed dots are
`varied from about 2 mm at the center portion to about 1.2
`mm at the peripheral portion. The second or upper surface of
`the light guide 8 is not provided with the printed dots. The
`second surface of the light guide faces the optical ?lm 9, as
`shown in FIG. 3. Each dot performs a diffuse re?ection
`which is shown by a reference number 31.
`The printed dots cause the light from the ?uorescent
`lamps 7 to be re?ected toward the LCD panel 1 with
`substantially uniform luminance.
`The structure of the optical ?lm 9 is shown in FIGS. 4 and
`5. The optical ?lm 9 is made of a transparent material which
`has a total transmission equal to or more than 98% and a
`refractive index of 1.5 to realize a large critical angle from
`the material to air. Examples of materials for the optical ?lm
`9 are polymethylmethacrylate or polycarbonate.
`A lower or ?rst surface of the optical ?lm 9 has an
`optically rugged or rough structure which is similar to the
`surface of frosted glass. The optically rough structure is
`required to provide a diffuse transmission wherein light of
`the same intensity or luminance is di?used in substantially
`all directions within a range of 180 degrees, as shown by a
`reference number 32 in FIG. 3. The distance between peaks
`of the rough structure is about 10 micrometers. If the ?rst or
`bottom surface is optically smooth, diffuse transmission
`does not occur. Diffuse transmission at the ?rst surface is
`required to prevent the printed dot pattern of the light guide
`8 from being observed by an operator. More particularly, the
`light form the ?uorescent lamps 7 is diffuse-re?ected at each
`dot. The, the light is diffused by the diffuse transmission of
`the optically rough structure of the optical ?lm 9. If the
`optically rough structure is not formed on the ?rst surfaces
`of the optical ?lm 9, the printed dot pattern of the light guide
`8 is observed by human eyes. This degrades the quality of a
`displayed image.
`An upper or second surface of the optical ?lm 9 has a
`wave structure, as shown in FIGS. 3, 4 and 5, which includes
`a plurality of isosceles triangle prisms arranged side-by-side
`with peaks and valleys being arranged in parallel to each
`other.
`In order to decrease loss of light transmitted to the LCD
`panel 1, the direction of the peaks and valleys is aligned with
`a polarizing axis 5A of the polarizer 5, as described below
`with refer to FIGS. 8, 9, 10 and 12.
`FIG. 5 shows various light paths inside and outside of the
`optical ?lm 9. As described below, the incidental light into
`the rough structure of the bottom of the optical ?lm 9 is
`diffuse-transmitted. The diffused light 55, 56, 57, 58 reaches
`surfaces 51, 52 at various angles and is refracted, as shown
`in FIG. 5. The refracted light 55A, 56A, 57A, 58A is
`gathered within a range de?ned by an angle 2A.
`Angle A depends upon the top angle B of the isosceles
`triangle prism, as shown in FIG. 6 and 7, which illustrate
`experimentally measured results.
`Before describing the curves in FIG. 6, the viewing angle
`of the LCD device is described. As well known in the art, the
`
`LGD_001157
`
`

`
`5,600,462
`
`5
`viewing angle is de?ned as an angle with respect to a line
`normal to the surface of the LCD device at which an image
`displayed on the LCD device can be satisfactorily observed
`by the operator. The viewing angle in the vertical direction
`is at least i35 degrees, and the viewing angle in the
`horizontal direction is approximately i55 degrees. The light
`from the ?uorescent lamps is gathered within the viewing
`angle of the LCD device by the optical ?lm 9 of the present
`invention. In other words, the luminance of light within the
`viewing angle of the LCD device is increased and the
`luminance of light outside the viewing angle of the LCD
`device is decreased by the optical ?lm 9.
`Referring again to FIG. 6, the vertical axis represents the
`relative luminance (represented as a percentage), and the
`horizontal axis represents the angle A. Angle A was mea
`sured by arranging the peaks and valleys of the optical ?lm
`9 in the horizontal direction of the LCD panel 1 and by
`rotating the LCD device around an axis 81, as shown in FIG.
`8. Since the purpose of the FIGS. 8, 9, 10, 12 and 13 is to
`show the direction of peaks and valleys of the optical ?lm 9
`or 10 with respect to the polarizing axis 5A of the polarizer
`5 and the LCD panel 1, the polarizer 4 and the backlight
`device 6 are not shown in these ?gures.
`The value 100% on the vertical axis in FIG. 6 represents
`the luminance when the optical ?lm 9 is not inserted
`between the polarizer 5 and the light guide 8. The ?ve curves
`represent the cases of various top angles B of the isosceles
`triangle prism of the optical ?lm 9. It is apparent in FIG. 6
`that the luminance within the particular angle is increased by
`inserting the optical ?lm 9 between the polarizer 5 and the
`light guide 8. The increase of the luminance is caused by the
`light gathering effect of the optical ?lm 9.
`In accordance with the present invention, a preferred top
`angle B is 95 degrees to 120 degrees, for the following
`reasons.
`Comparing the ?ve curves, in the case of the angle B=90
`degrees, the luminance at the required vertical viewing angle
`A235 degrees is 30%. The luminance required for a practical
`use of the LCD device for providing a minimum image
`quality is 65—70%. Therefore, the luminance of 30% at a top
`angle B=90 degrees at A=35 degrees causes a degradation of
`image quality. The luminance of the angle B?S degrees at
`the view angle A=35 degrees is 70%, so that the required
`minimum image quality is realized.
`The curve of B=l20 degrees shows a highest luminance
`value of 117%. But, if the angle B is more than 120 degrees,
`the luminance is decreased to a level of approximately
`100%, and there is less merit in using the optical ?lm 9.
`Based upon this discovery, the upper limit of the angle B is
`selected to be 120 degrees.
`FIG. 7 shows the relationship between the relative 1umi—
`nance and the angle A measured by arranging the peaks and
`valleys of the optical ?lm 9 in the horizontal direction of the
`LCD panel 1 and by rotating the LCD device around an axis
`82, as shown in FIG. 8. The value of 100% on the vertical
`axis in FIG. 7 represents a luminance when the optical ?lm
`9 is not inserted between the polarizer 5 and the light guide
`8.
`Comparing the ?ve curves in FIG. 7; in the case of the
`angle B=90 degrees, the luminance at the required horizontal
`viewing angle A=55 degrees is 30%. In the case of the angle
`B=95 degrees, the luminance at the viewing angle A=55
`degrees is 65%. Since the luminance required for the prac
`tical use of the LCD device for providing a minimum image
`quality is 65-70%, as stated above, the angle B=95 degrees
`is selected.
`
`45
`
`55
`
`65
`
`20
`
`25
`
`30
`
`35
`
`6
`The curve of B=l20 degrees shows a highest luminance
`value of 115%. If an angle B of more than 120 degrees is
`used, the luminance is decreased to approximately 100%.
`Based upon this discovery, the upper limit of the angle B is
`selected to be 120 degrees.
`Based upon the experimental data shown in the FIGS. 6
`and 7, the preferred top angle B is from 95 degrees to 120
`degrees.
`FIG. 11 shows four characteristic curves 84, 85, 85 and
`87. Curve 84 was obtained by rotating the arrangement
`shown in FIGS. 9 and 10 around axis 81. Curve 85 was
`obtained by rotating the arrangement shown in FIG. 8
`around axis 82. Curve 86 was obtained by rotating the
`arrangement shown in FIG. 12 around axis 81. Curve 87 was
`obtained by rotating the arrangement shown in FIG. 13
`around axis 81 or 82.
`In FIG. 8, the direction of the peaks and valleys of the
`optical ?lm 9 is oriented along the polarizing axis 5A of the
`polarizer 5. In FIG. 12, the direction of the peaks and valleys
`of the optical ?lm 9 is oriented along the polarizing axis 5A
`of the polarizer 5. In FIG. 9, two optical ?lms 9 overlap each
`other, and, the direction of the peaks and valleys of one of
`the two optical ?lms 9 is perpendicular to the direction of the
`peaks and valleys of the other of the two optical ?lms 2.
`The optical ?lm 10 used in FIG. 10 is shown in FIG. 14.
`Optical ?lm 10 has a plurality of quadrangular prisms
`arranged side-by-side in a regular array of rows and col
`umns. A cross-section along line 14A-14B and line
`14C-14D is the same as that shown in FIG. 5. The material
`of the optical ?lm 10 is the same as that of the optical ?lm
`9. The single optical ?lm 10 performs the same function as
`that of the two optical ?lms 9 arranged orthogonal to each
`other.
`The optical ?lms 9 and 10 used in the arrangements
`shown in FIGS. 8, 9, 10, 12 and 13 have the angle B=ll0
`degrees.
`The arrangement shown in FIG. 8 has horizontal viewing
`angle A=0—60 degrees in which luminance of about
`l30%—70% is maintained, (as shown in FIG. 7) and has
`vertical viewing angles A:0-43 degrees in which luminance
`of about l27%—70% is maintained (as shown in FIG. 6).
`The ?lm arrangement shown in FIG. 12 has horizontal
`viewing angles A=0—43 degrees in which luminance of
`about 130%—70% is maintained (as shown in FIG. 6) and
`has a vertical viewing angles A=0—60 degrees in which
`luminance of about 128%—70% is maintained (as shown in
`FIG. 7).
`The arrangement shown in FIGS. 9 and 10 has a higher
`luminance (approximately 148%—125%) in the viewing
`angles A=0—30 degrees, although the luminance in the
`angles A=3l—60 degrees is lower than that represented by
`curve 85, as shown in FIG. 11.
`The arrangement shown in FIG. 13 has a luminance
`comparable to that of the arrangement shown in FIGS. 9 and
`10 at angles of 0-36 degrees, and has a higher luminance
`than that of the arrangement of FIGS. 9 and 10 at angles of
`37-45 degrees.
`It is apparent that the optical ?lms 9 and 10 of the present
`invention increases the luminance of the LCD device at the
`ordinary viewing angles Ari-35 degrees and A=i55 degrees.
`The distance between the peaks of the optical ?lm 9 and
`10 is de?ned as the pitch, as shown in FIG. 5. The pitch is
`selected to prevent interference fringes from being observed
`by an operator when the optical ?lm 9 or 10 is used in the
`LCD panel 1. The panel may have gate lines spaced from
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`7
`each other by 330 micrometers and data lines spaced from
`each other by 110 micrometers. It has been experimentally
`found by the inventors of the present invention that no
`interference fringes have been observed if the pitch is equal
`to or less than 150 micrometers.
`FIG. 15 shows a problem encountered in a prior structure
`for mounting the ?uorescent lamp 7 to the light guide 8. The
`optical plate 9 or 10 and the re?ector plate 12 are mechani
`cally pressed to the light guide 8 by a cylindrical re?ector
`plate 13. The ?uorescent lamp 7 is mounted on the re?ector
`plate 13 by supporting members (not shown). When these
`parts are assembled, end portions 9A or 10A of the optical
`?lm 9 or 10 are forced into close contact with the upper
`surface of the light guide 8, and end portion 12A of the
`re?ector plate 12, which is a white ?lm, is forced into close
`contact with the lower surface of the light guide 8.
`It has been found by the inventors of the present invention
`that the luminance 71 at the edge portion of the LCD panel
`is undesirably increased, as shown in FIG. 15, so that image
`quality is remarkably degraded, and that such increase in the
`luminance is caused by a separation of end portions 9A, 10A
`and 12A from the surface of the light guide 8 over a period
`of time due to the high temperature of the ?uorescent lamp
`7. More particularly, it has been found that an undesired
`peak in the luminance is caused mainly by light along paths
`72 and 73 which directly passes at a sharp angle X from
`bottom surface of the light guide 8 without diffuse trans
`mission to the LCD panel, and light along paths 74 and 75
`which passes at angle X from the upper surface of the light
`guide 8.
`In accordance with the present invention, to solve the
`problem, thin coatings 88 are deposited or coated on the
`edge of the bottom and upper surface of the light guide 8, as
`shown in FIGS. 2 and 16. Coating 88 is formed of a material
`which has low transmittance and performs perfect di?'use
`transmission. Coatings 88 prevent the light from paths 72,
`73 and 75 from being directly transmitted from the bottom
`and upper surfaces of the light guide 8, whereby a uniform
`or even luminance 82A can be realized, as shown in FIG. 16.
`The coatings 88 are made of alumina (aluminum oxide) or
`white paint.
`While the invention has been described in connection
`with speci?c embodiments, it will be understood that those
`with skill in the art may be able to develop variations of the
`disclosed embodiments, without departing from the spirit of
`the invention or the scope of the following claims.
`What is claimed is:
`1. A liquid crystal display device including a liquid crystal
`display panel and a back light device, said back light device
`comprising:
`a light source for emitting light;
`a light guide means having a top surface facing a back
`surface of said liquid crystal display panel and a side
`surface receiving said light from said light source;
`a re?ector means provided on a back surface of said light
`guide means; and
`an optical ?lm of transparent material positioned between
`said back surface of said liquid crystal display panel
`and said top surface of said light guide means, includ
`ing a ?rst surface having a wave structure including a
`plurality of regularly spaced isosceles triangles prisms
`arranged side-by-side, the prisms having smooth sur
`faces, and a second surface having an optically rough
`structure for performing dilfuse transmission, wherein
`a top angle of said isosceles triangle prisms is in a range
`of 95 degrees to 120 degrees for ?at, angles prism
`
`25
`
`30
`
`35
`
`40
`
`45
`
`55
`
`65
`
`8
`surfaces to gather light from the diffuse transmission
`into a desired viewing angle for the liquid crystal
`display panel.
`2. A liquid crystal display device according to claim 1,
`wherein a polarizer is positioned between said liquid crystal
`display panel and said optical ?lm, and a direction along
`which said peaks and valleys of said isosceles triangle
`prisms are oriented is aligned in parallel to a polarizing axis
`of said polarizer and the tops of the isosceles triangle prisms
`are not farther than 160 um apart.
`3. A liquid crystal display device including a liquid crystal
`display panel and a back light device, said back light device
`comprising:
`a light source for emitting light;
`a light guide means having a top surface facing a back
`surface of said liquid crystal display panel and a side
`surface receiving said light from said light source;
`a re?ector means provided on a back surface of said light
`guide means; and
`two optical ?lms of transparent material positioned
`between said back surface of said liquid crystal display
`and said top surface of said light guide means, each of
`said optical ?lms including a ?rst surface having a
`wave structure including a plurality of isosceles tri
`angle prisms arranged side-by-side, the prism having
`smooth surfaces, and a second surface having an opti
`cally rough structure for performing di?use transmis
`sion wherein a top angle of said isosceles triangle
`prisms is in a range of 95 degrees to 120 degrees for
`?at, angled prism surfaces to gather light from the
`di?fuse transmission by the second surface into a
`desired viewing angle for the liquid crystal display
`panel.
`4. A liquid crystal display device according to claim 3,
`wherein a direction along which peaks and valleys of said
`isosceles triangle prisms of one of said two optical ?lms is
`oriented is at an angle with respect to a direction along
`which peaks and valleys of said isosceles triangles prisms of
`another of said two optical ?lms are oriented.
`5. A liquid crystal display device according to claim 3,
`wherein a polarizer is positioned between said liquid crystal
`display panel and said two optical ?lms, and a direction
`along which peaks and valleys of said isosceles triangle
`prisms of said optical ?lm closer to said polarizer is oriented
`in parallel to a polarizing axis of said polarizer and the tops
`of the isosceles triangle prisms are no more than 160 pm
`apart.
`6. A liquid crystal display device including a liquid crystal
`display panel and a back light device, said back light device
`comprising:
`a light source for emitting light;
`a light guide means having a top surface facing a back
`surface of said light crystal display panel and a side
`surface receiving said light from said light source;
`a re?ector means provided on a back surface of said light
`guide means; and
`an optical ?lm of transparent material positioned between
`said liquid crystal display panel and said light guide
`means, including a ?rst surface having a structure
`including a plurality of quadrangular prisms, which are
`substantially the same size and shape, in an orderly
`matrix of equally spaced prisms, the prisms having
`smooth surfaces, and a second surface having an opti
`cally rough structure for performing diffuse transmis
`sion wherein a top angle of said quadrangular prisms is
`in a range of 95 degrees to 120 degrees for ?at, angles
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`sides of the prisms to gather the light from the diffuse
`transmission of the second surface into the desired
`viewing angle for the liquid crystal display device.
`7. A liquid crystal display device according to claim 6,
`wherein a polan'zer is positioned between said liquid crystal 5
`di