United States Patent [19]
`Ishikawa
`
`US005921651A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,921,651
`Jul. 13, 1999
`
`[54] SURFACE LIGHT SOURCE DEVICE OF SIDE
`LIGHT TYPE HAVING DIFFUSING
`ELEMENT WITH IMPROVED
`DISTRIBUTION PATTERN OF LIGHT
`
`[75] Inventor: Tsuyoshi Ishikawa, Tokyo, Japan
`
`[73] Assignee: Enplas Corporation, KaWaguchi, Japan
`
`[21] Appl. No.: 08/624,313
`[22] Filed:
`Mar. 27, 1996
`[30]
`Foreign Application Priority Data
`
`Mar. 31, 1995
`
`[JP]
`
`Japan .................................. .. 7-097450
`
`Int. Cl.6 ...................................................... .. F21V 8/00
`[51]
`[52] US. Cl. ............................................... .. 362/31; 362/26
`[58] Field of Search ................................. .. 362/31, 26, 27
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,937,709
`4,985,809
`5,093,765
`5,134,549
`5,178,447
`5,283,673
`
`6/1990 Yanagi et a1. ........................... .. 362/31
`1/1991 Matsui et a1.
`.. 362/31
`3/1992 Kashima et a1. .
`362/31
`7/1992 Yokoyama ..... ..
`362/31
`1/1993 Murase et a1.
`362/31
`2/1994 Murase et a1.
`.. 362/31
`
`5,357,405 10/1994 Park . . . . . . . . . . . . . . . . .
`
`. . . . .. 362/31
`
`5,386,347
`
`1/1995 Matsumoto . . . . . .
`
`. . . . .. 362/31
`
`5,408,387
`
`4/1995 Murase et a1. .......................... .. 362/31
`
`5,408,388
`5,641,219
`5,649,754
`5,667,289
`
`4/1995 Kobayashi et a1. ..................... .. 362/31
`6/1997 MiZobe ........ ..
`362/31
`7/1997 Matsumoto
`362/31
`9/1997 Akahane et a1. ........................ .. 362/31
`
`FOREIGN PATENT DOCUMENTS
`
`2/1992 Japan .
`4-62519
`9/1992 Japan .
`4-268506
`40616062 6/1994 Japan ..................................... .. 362/31
`
`Primary Examiner—Laura Tso
`Attorney, Agent, or Firm—Staas & Halsey
`
`[57]
`
`ABSTRACT
`
`Adot pattern of a light guide plate is optimally designed for
`a surface light source device of side light type. A lamp 1 is
`disposed at the side of the light guide plate 2 and light
`introduced from a light incidence surface into the light guide
`plate 2 is emitted from a luminous surface. The dot pattern
`is arranged on the rear surface of the light guide plate 2
`according to the folloWing principles: (1) there is a
`(gradient) portion Where the rate of area occupation of the
`dot pattern increases from a region (usually a far region)
`loWly eXposable to light from the lamp 1; and (2) there is no
`sharp variation in the rate of area occupation of the dot
`pattern. At a corner of the light guide plate 2, a substantially
`triangular dot pattern is disposed for compensating any
`luminance drop due to the location in the vicinity of an
`electrode portion 1a of the lamp 1.
`
`3 Claims, 9 Drawing Sheets
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 1 of9
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`5,921,651
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`FIG. 1
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`PRIOR ART
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 2 of9
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`5,921,651
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 3 of9
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`5,921,651
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 4 of9
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`U.S. Patent
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`Jul. 13,1999
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`SheetS 0f9
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 6 of9
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`5,921,651
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 7 of9
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`5,921,651
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 8 of9
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`5,921,651
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`U.S. Patent
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`Jul. 13,1999
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`Sheet 9 of9
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`5,921,651
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`
`1
`SURFACE LIGHT SOURCE DEVICE OF SIDE
`LIGHT TYPE HAVING DIFFUSING
`ELEMENT WITH IMPROVED
`DISTRIBUTION PATTERN OF LIGHT
`
`BACKGROUND
`
`1. Field of the Invention
`The present invention relates to a surface light source
`device of a type in Which a primary light source is disposed
`at the side of a light guide plate and one surface of the light
`guide plate serves as a luminous surface, i.e., a “surface light
`source device of side light type”, and more particularly to a
`side-light-type surface light source device for improving a
`distribution pattern of a light diffusing element on a rear
`surface of the light guide plate. The side-light-type surface
`light source device according to the present invention is
`particularly useful When applied to back lighting of a liquid
`crystal display.
`2. Related Art
`A general structure of a surface light source device of a
`side light type and its operation Will be described. FIG. 1 is
`a fragmentary cross-sectional vieW shoWing the structure of
`a knoWn surface light source device of side light type. In
`FIG. 1, a reference numeral 2 designates a light guide plate
`usually made of transparent plastic material. At the side of
`the light guide plate 2, a rod-shaped light source element 1
`is disposed along a side edge surface (an incidence surface)
`2a of the light guide plate 2, as a primary light source.
`Generally, a cold-cathode tube is used as the light source
`element 1. The light source element 1 is hereinafter simply
`referred to as a lamp.
`One surface of the light guide plate 2 serves as a luminous
`surface on Which at least one sheet-shaped element 3
`(hereinafter referred to as “an additional element”) is addi
`tionally disposed. The additional element 3 is provided in
`the form of a light diffusing sheet, a prism sheet, a prism
`sheet protection sheet or an interference fringe prevention
`sheet, etc.
`On a surface opposite to the luminous surface (hereafter
`referred to as “a rear surface” and also referred to as a
`re?ecting surface), a re?ecting sheet 4 is disposed. The
`re?ecting sheet 4 is provided in the form of a silver foil, an
`aluminum foil or a White sheet. At the back surface of the
`lamp 1, a non-illustrated re?ector is disposed for improving
`the efficiency of incidence of light on the light guide plate 2.
`A major part of the light emitted from the lamp 1 is
`introduced into the light guide plate 2 directly or indirectly
`from the incidence surface 2a. The light introduced into the
`light guide plate 2 is guided toWard the back surface
`opposite to the incidence surface 2a as repeatedly re?ected
`on the front and rear surfaces of the light guide plate 2. The
`light is gradually taken out from the luminance surface at
`this stage and emitted as illuminating light as being sub
`jected to an action (a light diffusing action) of the additional
`element 3. This illuminating light is utiliZed as, for example,
`a back lighting of a liquid crystal display.
`The foregoing are the most basic structure and operation
`of the surface light source device of a side light type. With
`the simple structure and operation as described above, it is
`dif?cult to secure evenness of luminance of the upper
`surface of the additional element 3, Which is served as a
`luminous surface. Speci?cally, a gradient of the luminance
`is caused on the luminous surface according the distance
`from the incidence surface 2a. Further, the function of
`directing the light introduced into the light guide plate 2 to
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`the luminous surface is inadequate, so that the luminance
`level tends to be de?cient. In order to avoid this problem, it
`has been adopted to arrange a number of light-diffusive
`elements in a predetermined distribution pattern on the rear
`surface of the light guide plate 2. As a number of dot-shaped
`elements are arranged according to a predetermined concept,
`the distribution pattern is called a dot pattern.
`FIG. 2 is a plan vieW, as seen from the rear side, of the
`light guide plate 2 on Which the light diffusing elements are
`arranged in a most basic distribution pattern. In FIG. 2, large
`and small circles shoW light diffusing elements in the form
`of a print layer of light-diffusive ink, an aventurine surface
`(a ?nely roughed surface), etc. A number of light diffusing
`elements are arranged in roWs parallel to the lamp 1. A
`density of existence of light diffusing elements (a rate of area
`occupation of light-diffusive region) decreases gradually
`according to the distance from the lamp 1.
`Adopting the light guide plate 2 as shoWn in FIG. 2, it is
`possible to suppress the luminance difference depending on
`the distance from the lamp 1. HoWever, the luminance of a
`portion (indicated by S) near the electrode portion 1a of both
`ends of the lamp 1 Would be insuf?cient. For example, in the
`case of a lamp of 217 mm in full length, Which is available
`on the market, the length of an emitting portion is 203 mm
`and there is an electrode portion (a non-emitting portion) of
`7 mm Wide at each end of the lamp. In the vicinity of such
`a non-emitting portion, the luminance Would be insuf?cient
`as a matter of course.
`If the length of the emitting portion of the lamp 1 is set
`substantially equal to the Width d of the light guide plate 2,
`it is possible to somehoW minimiZe the foregoing problem
`and, on the other hand, adopting a lamp having the emitting
`portion of such length is very unpreferable in vieW of
`making both shape and siZe of the surface light source
`device compact.
`Various proposals have been made to overcome the above
`problems. FIG. 3 illustrates one of the proposals. In this
`conventional art, tWo corner portions F of the rear surface of
`the light guide plate 2 shoWn in FIG. 2, Which are nearer to
`the electrode portion 1a of the lamp 1 are treated differently
`from the other corner portions With respect to Width t1 and
`depth t2. Namely, in these corner portions F, the light
`diffusing elements are arranged in an increased density as
`compared With the circumference thereof. Within the corner
`portions F, as shoWn, density of the light diffusing elements
`may also be graded in the depthWise direction, if necessary.
`HoWever, this method of de?ning the distribution pattern
`also involves a problem. Namely, since the corner portions
`F are treated specially, it is inevitable that a sharp difference
`occurs in density of occupation of light diffusing elements
`betWeen portions contiguous to these corners F. This differ
`ence may cause unevenness of luminance in the vicinity of
`the corner portions.
`FIG. 4 illustrates another conventional art according to a
`different concept from the example shoWn in FIG. 3. In this
`conventional art, the rear surface of the light guide plate 2
`shoWn. in FIG. 2 is divided into a region I near the lamp 1
`and a region II far from the lamp 1. In the region I, the light
`diffusing elements are arranged in such a manner that they
`radially rodite about a point G Which confronts a central
`point of the lamp 1. Namely, Within the region I, the
`distribution pattern is de?ned so that the density of occu
`pation of light diffusing elements is minimal in the vicinity
`of the point G, and gradually increases in accordance With
`the distance from the point G.
`MeanWhile, Within the region II, a concept similar to that
`in FIG. 2 is adopted. Namely, the light diffusing elements are
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`LGD_001000
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`arranged in roWs parallel to the lamp 1, and its density of
`occupation decreases gradually according to the distance
`from the lamp 1.
`This method of de?ning the distribution pattern has
`another problem. Namely, since the density of occupation of
`light diffusing elements is graded radially in the region I
`While it is graded linearly in the depthWise direction of the
`light guide plate 2 in the region II, the tWo patterns can not
`be matched With each other in the vicinity of the junction of
`the regions I and II, so that a smooth variation of density of
`occupation of the light diffusing elements is not achieved.
`This mismatching might cause the luminance unevenness in
`the vicinity of the junction of the regions I and II.
`Further, if the gradient in the region I is set so as not to
`cause inadequate luminance in the vicinity of the electrode
`portion 1a and the magnitude and gradient of the density of
`occupation of the light diffusing elements are set indepen
`dently With its appropriateness taken precedence of the other
`things in the region II, portions indicated by Z might be
`reversely graded in density of occupation of the light dif
`fusing elements.
`Thus, in the conventional art, if the length of the emitting
`portion of the rod-shaped lamp disposed at the side of the
`light guide plate is smaller than the Width of the light guide
`plate, it is difficult to de?ne an appropriate distribution
`pattern of light diffusing elements on the rear surface of the
`light guide plate, Which secures the uniform luminance in
`both depthWise and WidthWise directions of the light guide
`plate and prevents local occurrence of sharp gradients and
`reverse gradients of the density of occupation of the light
`diffusing elements.
`SUMMARY OF THE INVENTION
`It is an object of the present invention to provide a
`technological measure of solving the foregoing problems in
`the conventional surface light source device of side light
`type, and more particularly to provide a surface light source
`device of side light type in Which an appropriate distribution
`pattern of light diffusing elements is de?ned on the rear
`surface of a light guide plate so as not to cause the luminance
`unevenness over the entire luminous surface under the
`condition that the length of an emitting portion of a rod
`shaped lamp as a primary light source is smaller than the
`Width of the light guide plate.
`The present invention provides a surface light source
`device of side light type having a light guide plate and a
`primary light source disposed at the side of the light guide
`plate to solve the above-mentioned technical problems on
`condition that the length of the emitting portion of the
`primary light source is smaller than the Width of the light
`guide plate. According to the present invention, the folloW
`ing conditions are set to a pattern of the light diffusing
`elements arranged on the rear surface of the light guide
`plate.
`(1) the pattern of the light diffusing elements eXcludes a
`portion in Which the rate of area occupation of the light
`diffusing elements decreases from a region highly
`eXposable to light from the primary light source to a
`region loWly eXposable to light from the primary light
`source; and
`(2) a pattern of the light diffusing elements eXcludes a
`portion in Which the rate of area occupation of the light
`diffusing elements varies sharply.
`The pattern of the light diffusing elements can be de?ned
`according to the folloWing stages:
`[1] determining a distribution pattern of light diffusing
`element With a gradient of area occupation rate of the
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`light diffusing element so that the distribution of lumi
`nance is uniformed in a depthWise direction of the light
`guide plate;
`[2] measuring the luminance distribution in a WidthWise
`direction of the light guide plate in the vicinity of the
`primary light source With the light diffusing element
`pattern de?ned in stage [1];
`[3] determining a siZe of the light diffusing element A at
`the corner of the light guide plate in the vicinity of the
`non-emitting portion in consideration of the magnitude
`of the luminance drop at the corner measured in stage
`[2], and determining a border for dividing a portion in
`the vicinity of the primary light source on the rear
`surface of the light guide plate into a near block having
`no substantial drop of luminance of the light guide plate
`and a near block having a substantial drop of luminance
`of the light guide plate, based on the determined siZe of
`the light diffusing element A;
`[4] selecting a light diffusing element A‘ having a siZe
`equal to or slightly larger than the siZe of the light
`diffusing element A in the distribution pattern de?ned
`in the step [1], and determining a line passing near the
`light diffusing element A‘ and extending WidthWise of
`the light guide plate as a border betWeen the far block
`remote from the primary light source and each of the
`near blocks;
`[5] adopting the distribution pattern de?ned in the step [1]
`for the light diffusing elements in the block and in the
`near block having no substantial luminance drop; and
`[6] determining the distribution pattern of the light dif
`fusing element in a portion B ranging from the border
`of the near block having no substantial luminance drop
`to the portion of the light diffusing element A, in such
`manner that the rate of area occupation of the light
`diffusing element smoothly varies based on the result of
`the measuring in stage [2], and determining the distri
`bution pattern of the light diffusing elements for a
`portion C ranging from the portion B to the border of
`the far block, and for a portion D ranging from the
`portion of the light diffusing element Ato the border of
`the far block, in such a manner that the rate of area
`occupation of the light diffusing element smoothly
`varies.
`It is preferable that the siZe of the light diffusing element
`A is determined so that the distribution pattern of the light
`diffusing element for the portion B causes no unevenness of
`luminance in the portion B. At a corner portion of the light
`guide plate in the vicinity of the non-emitting portion of the
`primary light source there is provided an additional pattern
`of the light diffusing element for compensating a luminance
`drop to be caused due to the vicinity of said non-emitting
`portion of said primary light source. It is preferable that the
`siZe and/or shape of said pattern of said light diffusing
`element is determined after stage
`According to the present invention, the technological
`measures for resolving the above-mentioned problems of the
`conventional side-light-type surface light source device.
`Namely, an appropriate distribution pattern of the light
`diffusing elements is de?ned on the rear surface of the light
`guide plate so that any luminance unevenness is not caused
`over the entire luminous surface under the condition that the
`length of the emitting portion of the rod-shaped lamp as a
`primary light source is smaller than the Width of the light
`guide plate.
`For the above purpose, the pattern of the light diffusing
`elements is designed to eXclude a portion in Which the rate
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`of area occupation of the light diffusing elements decreases
`from a region highly eXposable to light from the primary
`light source to a region loWly eXposable to light from the
`primary light source. At the same time, the condition is
`satis?ed that the pattern of the light diffusing elements
`excludes a portion in Which the rate of area occupation of the
`light diffusing elements varies sharply. The procedure of
`designing the pattern of light diffusing elements for satis
`fying these conditions Will be disclosed in the speci?cation.
`The present invention Will be described in greater detail
`With reference to the accompanying draWings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a fragmentary cross-sectional vieW shoWing a
`conventional surface light source device of side light type;
`FIG. 2 is a plan vieW of a light guide plate, as seen from
`the rear side thereof, on Which light diffusing elements are
`arranged in the most basic distribution pattern according to
`the conventional art;
`FIG. 3 is a plan vieW of a light guide plate shoWing one
`eXample of the distribution pattern for the uniform lumi
`nance according to the conventional art;
`FIG. 4 is a plan vieW of a light guide plate shoWing
`another eXample of the distribution pattern for the uniform
`luminance according to the conventional art;
`FIG. 5 is a schematic vieW shoWing a ?rst stage of the
`procedure of de?ning a dot pattern according to the concept
`of the present invention;
`FIG. 6 is a schematic vieW shoWing a second stage of the
`procedure of de?ning a dot pattern according to the concept
`of the invention;
`FIG. 7 is a schematic vieW shoWing a third stage of the
`procedure of de?ning a dot pattern according to the concept
`of the invention;
`FIG. 8 is a schematic vieW shoWing a fourth stage of the
`procedure of de?ning a dot pattern according to the concept
`of the invention;
`FIG. 9 is a schematic vieW shoWing a ?fth stage of the
`procedure of de?ning a dot pattern according to the concept
`of the invention;
`FIG. 10 is a schematic vieW shoWing a ?rst step of a siXth
`stage of the procedure of de?ning a dot pattern according to
`the concept of the invention;
`FIG. 11 is a schematic vieW shoWing a second step of the
`siXth stage of the procedure of de?ning a dot pattern
`according to the concept of the invention;
`FIG. 12 is a schematic vieW shoWing a third step of the
`siXth stage of the procedure of de?ning a dot pattern
`according to the concept of the invention;
`FIG. 13 is a schematic vieW shoWing a seventh stage of
`the procedure of de?ning a dot pattern according to the
`concept of the invention; and
`FIG. 14 is a schematic vieW shoWing an eXample of the
`dot pattern completed through the stages 1—7.
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`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`The frameWork of a side-light-type surface light source
`device of the present invention is basically the same as that
`of FIG. 1. HoWever, a lamp 1 having an emitting portion of
`a length smaller than a Width of a light guide plate 2 is used
`so as to prevent the electrode portions from projecting from
`a depthWise side of the light guide plate and thereby not
`deteriorating the compactness of the Whole device. Further,
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`in general, the shorter the lamp 1 is, the smaller the elec
`tricity consumption Would become and therefore the above
`arrangement results in vieW of saving poWer.
`The present invention is directed to hoW to de?ne a
`distribution pattern (hereinafter simply referred to as “a dot
`pattern”) of the light diffusing elements arranged on the rear
`surface of the light guide plate. First, the procedure of
`de?ning the dot pattern according to the concept of the
`present invention Will be described. For convenience of the
`description, the procedure of de?ning a dot pattern is divided
`into seven stages and Will be described referring to FIGS.
`5—13, Which corresponds to the individual stages eXcept the
`siXth stage Which includes steps. Illustration in each ?gure
`is limited to minimal essentials.
`Further, a unit element of the dot pattern is represented by
`a rectangular mark rather than a circular mark. In general, as
`long is a gradient of light-diffusing ability per unit area is
`realiZed as designed, there is no limitation in shape of the
`unit element of the dot pattern. Here, the gradient of light
`diffusing ability per unit area is given in terms of the rate of
`area occupation of the dot pattern. Namely, in the absence of
`any proviso, it is assumed that the light-diffusing ability per
`unit area of each dot pattern element is uniform.
`[First Stage]; With reference to FIG. 5
`First, a dot pattern is de?ned for the entire rear surface of
`the light guide plate 2 by determining a gradient in the
`direction of depth of the light guide plate 2. A gradient of the
`rate of area occupation is determined depthWise in vieW of
`the luminance at the center (indicated by Q) of the light
`guide plate 2. Namely, the dot-pattern de?ning concept of
`FIG. 2 is adopted. The de?ned dot pattern Will become larger
`according to the distance from the lamp 1. As a matter of
`course, at this stage, a luminance drop in the vicinity of the
`both ends 1a of the lamp 1 is not considered.
`[Second Stage]; With reference to FIG. 6
`In the dot pattern de?ned in the ?rst stage, the luminance
`distribution in the direction of Width of the light guide plate
`2 (indicated by an arroW K) is measured in the vicinity of the
`lamp 1.
`[Third Stage]; With reference to FIG. 7
`Considering the magnitude of the luminance drop in the
`vicinity of the lamp ends 1a as measured at Stage 2, the siZe
`of the dots A at those portions is determined.
`Then, the entire rear surface of the light guide plate 2 is
`divided into near blocks and a far blocks (blocks near the
`lamp 1 and block far from the lamp 1) according to the
`distance from the lamp 1. Here, since the lamp 1 has
`non-emitting portions at both ends, the near blocks are
`de?ned as blocks 1—3 While the far block is de?ned as a
`block 4, as shoWn in FIG. 7. At this stage, a border of the
`blocks 1 and 2 and a border of the blocks 2 and 3 are
`determined in the folloWing manner.
`Lines of opposite ends (eXtending in the depthWise
`direction) of a WidthWise range in Which a luminance drop
`of the light guide plate 2 is discriminated to be substantially
`Zero according to the measurement at the second stage are
`regarded as a border of the blocks 1 and 2 and a border of
`the blocks 2 and 3.
`If the luminance in the vicinity of the lamp ends 1a is
`extremely loW as compared to the luminance level of block
`2, the siZe of the dot A is determined by considering a
`measured value of luminance of the portion near the border
`of the block 2, neglecting a measured value of luminance in
`the vicinity of the lamp ends 1a. It is preferable to decide
`Whether or not the above procedure should be taken in
`consideration of the distance betWeen the borders of the
`block 2 and the WidthWise ends.
`
`LGD_001002
`
`

`
`5,921,651
`
`15
`
`25
`
`7
`Namely, if the luminance drop in the vicinity of the lamp
`ends 1a is large and the above distance is short, the fore
`going procedure is taken. On the contrary, if the luminance
`drop in the vicinity the lamp ends 1a is small and the above
`distance is long, the foregoing procedure is not needed. In an
`intermediate case, the discrimination is made in consider
`ation of the luminance drop and the distance.
`If the luminance drop is even, necessity of the foregoing
`procedure increases as the distance is shorter. The purpose of
`the procedure is to prevent the rate of variation of the dot
`siZe in the portion B Which is referred to at the later stage 6
`from being made excessive to occur the luminance uneven
`ness in that portion. In other Words, it is preferable to adopt
`the substantially maximal dot siZe for the dot A in a range
`Where no luminance unevenness occurs in the portion B.
`[Fourth Stage]; With reference to FIGS. 7 and 8
`Aborder of the block 4 and the blocks 1—3 shoWn in FIG.
`7 is determined as folloWs: Considering the depthWise
`gradient of the area occupation rate determined at stage 1
`and the result of measurement at stage 2, a dot A‘ equal to
`or slightly larger than the dotAis selected, and a line passing
`(WidthWise of the light guide plate 2) near the dot A‘ is
`de?ned to be the border of the block 4 and the blocks 1—3.
`At that time, attention should be paid not to select a dot
`smaller than the dot A. If a dot smaller than the dot A is
`selected as the dot A‘, a reverse gradient (a gradient that the
`siZe of dots far from the lamp 1 is smaller than that of the
`dots near the lamp 1) Would have occurred in the vicinity of
`the border of the block 4 and the blocks 1—3.
`[Fifth Stage]; With reference to FIG. 9
`Dots Within the blocks 2 and 4 (indicated by diagonal
`lines) are determined in conformity With those in the dot
`pattern de?ned at the stage 1.
`[Sixth Stage]; With reference to FIGS. 10—12
`In step 1 (FIG. 10), based on the result of measurement at
`stage 2, a dot pattern for the portion B (also indicated in FIG.
`35
`7), ranging from the border of the block 2 toWard the dot A
`is de?ned. Since the siZe of the dots at the border of the block
`2 and the siZe of the dot A are already determined, the
`gradient of the rate of area occupation is determined so as to
`link these portions smoothly. This gradient is not necessarily
`linear but may be in a desired form in consideration of the
`measurement result at stage 2.
`So as to con?rm that no luminance unevenness Would
`occur in the portion B, a luminance distribution of this
`portion is measured again With the de?ned dot pattern. If the
`luminance unevenness (usually, a stria light-and-shade
`pattern) is observed, it means that the dot A is too large, as
`mentioned at stage 3. Thus, in such case, after appropriately
`reducing the siZe of the dot A, the dot pattern for the portion
`B is de?ned again and con?rm that the luminance uneven
`ness is eliminated.
`In step 2 (FIG. 11), a dot pattern for a portion C ranging
`from the portion B toWard the border of the block 4 is
`de?ned. Since the siZe of dots in the portion B and the siZe
`of dots at the border of the block 4 are already determined,
`the gradient of the rate of area occupation is determined so
`as to link these tWo portions smoothly.
`In step 3 (FIG. 12), a dot pattern for a portion D ranging
`from the portion A toWard the border of the block 4 is
`de?ned. Since the siZe of dots in the portion B and the siZe
`of dots at the border of the block 4 are already determined,
`the gradient of the rate of area occupation is determined so
`as to link these tWo portions smoothly. According to the Way
`of determining the border of the block 4 at stage 4, this
`gradient Would generally be small. If the siZe of the dot A‘
`selected at stage 4 is equal to the siZe of the dot A, the
`gradient is made 0.
`
`8
`[Seventh Stage]; With reference to FIG. 13
`This stage is an additional stage to be added When the siZe
`of the dot Ais determined not based on the luminance value
`(executed in Stage 2) at the position of the dot A (in the
`vicinity of the lamp ends 1a) but in consideration of a
`measured value of luminance in the vicinity of the border of
`the block 2. In other Words, When the siZe of the dot A is
`determined based on the luminance value at the position of
`the dot A (in the vicinity of the lamp ends 1a), this stage is
`unnecessary in general.
`At this stage, the portion of the dot A (in the vicinity of
`the lamp ends 1a) is regarded as a special region E. In the
`region E, a light diffusing element (a large-area dot or a dot
`group) extending from an apex of the corner toWard a central
`portion of the light guide plate 2 is provided in the form of
`triangle, fan-shape or a V-shape. The siZe and shape of the
`region E are adjusted to compensate for a remarkable
`luminance drop in the vicinity of the lamp ends 1a.
`Strictly speaking, the dots provided on the rear surface of
`the light guide plate 2 do not independently affect the
`luminance of that portion but are in?uenced by the dots in
`other portions. Especially, When the region E is set at the
`corner, a considerable in?uence is given on its circumfer
`ential portion. Accordingly, in designing the region E, it is
`preferable to adjust the siZe and shape (also, diffusing ability
`per unit area, if necessary), taking the above in?uence into
`consideration.
`FIG. 14 shoWs an example of dot pattern completed
`through the above-described stages 1—7. As is apparent from
`FIG. 14, the folloWing principles are satis?ed on the entire
`rear surface of the light guide plate 2.
`(1) There is not any (reverse gradient) portion Where the
`rate of area occupation of the dot pattern decreases from a
`region highly exposable to light from the lamp 1 (usually
`nearer to the lamp 1) to a region loWly exposable to light
`from the lamp 1 (usually far from the lamp 1).
`(2) There is not any portion Where the rate of area
`occupation of the dot pattern varies sharply.
`The above features have not been realiZed in the conven
`tional art, as mentioned previously. In a special case (e.g.,
`When the depth