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`US005384658A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,384,658
`Jan. 24, 1995
`
`FOREIGN PATENT DOCUMENTS
`2054433 5/1972 Germany .
`349167 11/1959 Japan .
`498711 2/1974 Japan .
`57-128383 8/1982 Japan .
`58-155947 9/1983 Japan .
`59-8809 2/1984 Japan .
`61-276831 12/1986 Japan .
`2126501 5/1990 Japan .
`
`OTHER PUBLICATIONS
`English language translation of claim of JP-A-34-9167.
`English language translation of claim of JP-A--49-8711.
`English language translation of claim of JP-B-59-8809.
`English language translation of claims of JP-A-5-
`7-128383.
`language Abstract of JP reference No.
`Engfish
`2-126501.
`English language translation of Claims of JP-A-S-
`8-155947.
`English language Abstract of JP Reference No.
`61-276831.
`Derwent Publications Ltd. London, GB; AN
`89-183663 & JP 1 123 201 May 16, 1989.
`Derwent Publications Ltd. London, GB AN 86-058934
`& JP 61 010499 Jari. 17, 1986.
`Primary Examiner-Scott J. Sugarman
`Assistant Examiner-Thong Nguyen
`Attorney, Agent, or Firm-Ladas & Parry
`[57]
`ABSTRACT
`There is provided a plastics optical member having on
`its surface a light-diffusing layer composed of a layer of
`micro-joints formed by immersing a plastics material
`successively in a good solvent and a poor solvent, and a
`plastics light-quantity-controlling member having on its
`surface a white pattern composed of the micro-joints.
`The superiority in light diffuse reflection, transmission
`and light-quantity-controlling property permit their
`incorporation into various optical systems such as
`screens for projection, various displays, light-source(cid:173)
`directly-under-an-illumination-surface type light boxes,
`edge light type light boxes, etc.
`
`8 Claims, 6 Drawing Sheets
`
`United States Patent £19]
`Ohtake et al.
`
`[54] PLASTIC OPTICAL MEMBER AND
`LIGHT-QUANTITY-CONTROLLING
`MEMBER EACH HAVING A
`LIGHT-DIFFUSING LAYER ON ITS
`SURFACE
`
`[75]
`
`Inventors: Genji Ohtake, Yokohama; Hisao
`Morohashi, Kawasaki; Souichi
`Hasegawa, Yokohama, all of Japan
`
`[73] Assignee: Ohno Research & Development
`Laboratories Co. Ltd., Yokohama,
`Japan
`
`[21] Appl. No.:
`
`849,398
`
`[22] PCT Filed:
`
`Sep.3, 1991
`
`[86] PCT No.:
`§ 371 Date:
`§ 102(e) Date:
`
`PCf/JP91/01177
`Apr. 27, 1992
`Apr. 27, 1992
`
`[87] PCT Pub. No.:
`W092/04400
`PCT Pub. Date: Mar.19, 1992
`
`Foreign Application Priority Data
`[30]
`Sep. 4, 1990 [JP]
`Japan .................................. 2-233978
`Oct. 30, 1990 [JP]
`Japan .................................. 2-292991
`
`Int. CI.6 ...........•...••...•... G02B 13/20; G02B 5/02;
`[51]
`A61G 13/00
`[52] u.s. Cl •.................................... 359/707; 359/599;
`362/33
`[58] Field of Search ............... 359/599, 707, 893, 894, -
`359/619; 362/26, 31, 33
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,671,101 6/1972 Finch .................................. 359/599
`3,850,667 11/1974 Tani et al ........................... 117/36.7
`4,114,983 9/1978 Maffitt et al ........................ 359/599
`4,310,219 1/1982 Jaccard ............................... 359/599
`4,427,265 1/1984 Suzuki et al ........................ 359/599
`4,529,563 7/1985 McGinniss ............................ 264/84
`4,573,766 3/1986 Boumay, Jr. et al ................. 362/26
`4,989,933 2/1991 Duguay ............................... 359/599
`
`2
`
`2
`
`LGD_000885
`
`LG Display Ex. 1022
`
`

`
`U.S. Patent
`
`Jan.24, 1995
`
`Sheet 1 of 6
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`5,384,658
`
`F I G.
`
`I
`
`FIG. 2
`
`T
`
`LGD_000886
`
`

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`U.S. Patent
`
`Jan. 24, 1995
`
`Sheet 2 of 6
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`5,384,658
`
`FIG. 3
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`LGD_000887
`
`

`
`U.S. Patent
`
`Jan. 24, 1995
`
`Sheet 3 of 6
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`5,384,658
`
`FIG. 4
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`2
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`2
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`LGD_000888
`
`

`
`U.S. Patent
`
`Jan.24, 1995
`
`Sheet 4 of 6
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`5,384,658
`
`FIG. 5
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`6
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`3
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`4
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`5
`
`FIG. 6
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`6
`
`10
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`II
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`5
`
`FIG. 7
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`LGD_000889
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`

`
`U.S. Patent
`
`Jan. 24, 1995
`
`Sheet 5 of 6
`
`5,384,658
`
`FIG. 8
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`2
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`12
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`14
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`2
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`FIG. 9
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`2
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`12
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`15
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`2
`
`FIG. 10
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`151
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`16
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`13
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`15
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`2
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`LGD_000890
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`

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`U.S. Patent ·
`
`Jan. 24, 1995
`
`Sheet 6 of 6
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`5,384,658
`
`F I G. I I
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`4
`
`LGD_000891
`
`

`
`1
`
`5,384,658
`
`TECHNICAL FIELD
`
`PLASTIC OPTICAL MEMBER AND
`LIGHT-QUANTITY-CONTROLLING MEMBER
`EACH HAVING A LIGHT-DIFFUSING LAYER ON
`ITS SURFACE
`
`2
`When a light-diffusing white coating material is ap(cid:173)
`plied on a plastic material, the spectral absorption char(cid:173)
`acteristics of the coated material tend to be imbalanced
`because the light-diffusing white coating material is
`5 composed of a white pigment, a binder, various addi(cid:173)
`tives, etc. The reason is that in the case of such a coating
`material, coating properties should be given priority. It
`is difficult to make the content of the white pigment in
`the coating material much higher than that of the binder
`The present invention is a technique utilizing light
`diffusion, and relates to a plastic optical member having 10 from the viewpoint of the film-forming properties of the
`a specific micro-joints layer on its surface, and a light-
`coating material. In addition, in this case, it is the
`quantity-controlling member having a white pattern of
`boundary surface between the binder and the white
`the aforementioned micro-joints on its surface.
`pigment that scatters light. A small difference between
`Here, the term "optical member" refers to an optical
`their refractive indexes results in a small light-scattering
`member used in any part of various optical systems in 15 effect. Therefore, if a large light diffuse reflection effect
`which light is desired to be subjected to diffuse reflec-
`is desired, it is necessary to thicken the coating of the
`tion or diffuse transmission. Specific examples thereof
`light-diffusing white coating material, so that a thin
`light diffuse reflective layer cannot be realized. More-
`are optical members used in planar light sources, pro-
`over, the light-diffusing white coating material is diffi-
`jection screens, window materials for building, various
`displays, office automation appliances, and precision 20 cult to be applied on a curved surface.
`On the other hand, conventional thin light boxes
`optical instruments. The term "light-quantity-controll-
`ing member" refers to a specific optical member used
`requiring a uniform illumination surface includes the
`for controlling the quantity of light. Specific examples
`following two types of light boxes: the so-called edge
`thereof are optical members for homogenizing the light-
`light type (see, for example, JP-A-57-128383 and JP-A-
`ness of the illumination surface in the so-called "thin 25 2-126501) which comprises a light-source provided at a
`side end of an illumination surface, and a light-quantity-
`light box" such as a backlight used in liquid crystal
`displays of personal co~pute~, a ~cha~ten for medi-
`controlling member with a light guide panel having a
`cal treatment~ an electnc.ally illummat~ display, etc., to
`reflective pattern formed thereon, which member dif-
`rend~r the bng~tness unifo.rm. The optical member an?
`fuses light from the light source uniformly on the illumi-
`t~e ligh~-quan~ty~ontrolling. m~mber commonly u~- 30 nation surface; and the so-called lighting curtain type
`~e the hght-diffusmg properties ~p~ed b~ ~he proVI-
`(see, for example, JP-B-59-8809) which comprises a flat
`SIOn of the aforementioned specific micro-Jomts layer
`housing with an illumination surface on its open side, a
`on the whole or a part of therr surfaces.
`light source provided in the flat housing and a light-
`BACKGROUND ART
`quantity-controlling member provided right above the
`35 light source and containing a translucent reflector
`called "lighting curtain" which has a reflecting pattern,
`Using conventional such as surface chemical treat-
`so as to allow light from the light source to reflect
`ment, sandblasting method (see JP-B-49-8711), coating
`repeatedly between the inner surface of the housing and
`with a light-diffusing white coating material (see, for
`the light-quantity controlling member to achieve uni-
`example, JP-B-34-9168), etc., it is difficult to obtain on
`the surface of a plastic material a thin light-diffusing 40 form illumination.
`The edge light type light box is disadvantageously
`layer which can be formed partly or in the shape of a
`pattern, permits adjustment of light diffuse reflection
`heavy because it is used together with a light guide
`panel made of transparent plastic, glass or the like. The
`properties and light diffuse transmission properties, and
`can be formed not only on planar surfaces but also on
`edge light type light box is disadvantageous in that the
`any curved surfaces. The reason for the difficulty is 45 area of light guide portion cannot be increased in pro-
`portion to the enlargement of the illumination surface
`explained below for each method.
`In the surface chemical treatment, a violent reaction
`since light is introduced through the end face of the
`is carried out using a strong acid, a strong base, etc., so
`light guide panel, so that the light box is dark.
`that the spectral absorption characteristics of the sur-
`The lighting curtain type light box is advantageous in
`face of the resulting light-diffusing layer are changed by 50 that the light box is lightweight and very light because
`no light guide panel is used therein. However, its assem-
`the chemical reaction. In other words, coloring and the
`bling is complicated because the positional relationship
`like are caused. Moreover, the chemical treatment
`between the illumination surface and the reflective sur-
`merely forms depressions and protuberances in the sur-
`face at the inner surface of the housing must be strictly
`face and hence does not impart significant light diffuse
`reflection properties or light diffuse transmission prop- 55 set to obtain a uniform illumination.
`In both the edge light type light box and the lighting
`erties to the surface. Furthermore, when the treatment
`is carried out so as to form a pattern, it is difficult to
`curtain type light box, a regularly reflecting pattern
`prepare a resist which is resistant to the above-men-
`such as a conventional aluminum-deposited pattern as
`well as a printed pattern with a white pigment is con-
`tioned violent chemiCal reaction.
`In the sandblasting method, sand grains are caused to 60 ventionally used as the reflecting pattern on the light-
`collide with a plastic material at high speeds. For the
`quantity-controlling member. However, the regularly
`collision of the grains, high energy is required. In addi-
`reflecting pattern has the shortcoming that parallax is
`tion, the finer the sand grains, the greater the air resis-
`caused on the illumination surface, depending on the
`tance, so that the fmeness of the plastic material surface
`angle from which the illumination surface is looked.at,
`achieved by the treatment has its own limit and it can- 65 due to its directivity, although the regularly reflectmg
`not be expected to impart very high light diffuse reflec-
`pattern has a high reflectance. The printed pattern with
`tion properties or light diffuse transmission properties
`a white pigment is disadvantageous in that the reflection
`to the surface of the plastic material.
`wavelength characteristics of the pigment are shifted by
`
`LGD_000892
`
`

`
`5,384,658
`
`3
`various additives added to an ink for printing in order to
`maintain the printability, resulting in the so-called color
`shift. Furthermore, in the case of a white pattern ob(cid:173)
`tained by printing, the size of dots constituting the pat(cid:173)
`tern is restricted by printing techniques and cannot be
`made very small. Therefore, the illumination surface
`requires an additional light diffuser panel for hiding a
`dot image since printing of dots with an invisible size is
`difficult.
`SUMMARY OF THE INVENTION
`The present invention seeks to solve the various prob(cid:173)
`lems in the formation of a thin light-diffusing layer on
`the surface of a plastic as present material in the prior
`art, and provides a plastic optical member comprising a
`plastic material as present having on the surface a light(cid:173)
`diffusing layer which bas adjustable diffuse reflection
`properties or light diffuse transmission properties, can
`be formed partly or in the shape of a pattern, on any
`curved surfaces, and bas a flat spectral absorption distri- 20
`bution. In addition, the present invention is seek to solve
`the above-mentioned various problems in conventional
`light-quantity-controlling means in light-quantity-con(cid:173)
`trolling members for adjusting the quantity of transmit(cid:173)
`ted light and the quantity of reflected light, and pro- 25
`vides a light-quantity-controlling member which makes
`it possible to obtain a bright box that is lightweight,
`very light, and easy to construct and hardly causes
`parallax, and which member is free from color irregu-
`larities and bas a simple structure.
`Further in the present invention, these objects are
`achieved by forming a light-diffusing layer on the sur(cid:173)
`face of a plastic material. The light-diffusing layer com(cid:173)
`prises micro-joints formed on the surface of the plastic
`material.
`For forming such micro-joints, the plastic material is
`immersed in a good solvent (hereinafter referred to also
`as first solvent) for the constituent plastic of the plastic
`material, and then in a poor solvent (hereinafter re(cid:173)
`ferred to also as second solvent) for the constituent 40
`plastic which is compatible with the fust solvent. By
`this procedure, the light-diffusing layer is formed on the
`surface of the plastic material. Each solvent may be
`either a single solvent or a mixed solvent. In general,
`when an object is increased in volume by swelling, 45
`dissolution, melting, etc., and returns to its original
`state, it shrinks toward the centers of shrinkage scat(cid:173)
`tered in the object. Owing to the cracks or spaces
`formed by the shrinkage between the centers of shrink(cid:173)
`age, numerous laminated structures in a pillar shape, a 50
`string shape, a plate shape or a spherical shape are
`formed near the surface of the object, and they are
`called joints.
`In some cases, the joint structures become spongy or
`felt-like depending on the shrinkage rate and the flexi- 55
`bility and brittleness of the object. Their sizes reach
`several micrometers to several tens of centimeters in
`some cases.
`The present inventors found that by the above-men(cid:173)
`tioned immersion of the plastic material in the good 60
`solvent and then in the poor solvent, extremely minute
`joints are formed on the surface of the plastic material.
`These micro-joints are assemblies of very small
`cracks formed by expansion of a surface portion of the
`plastic material, followed by the rapid shrinkage 65
`thereof. The cracks are formed in a very small size with
`very high density in the surface portion of the plastic
`material. Furthermore, the cracks are formed in multi-
`
`4
`layers in the direction of the depth of the plastic mate(cid:173)
`rial. The cracks provide a very large boundary surface
`area between air and the constituent plastic of the plas(cid:173)
`tic material. Since light is scattered at the boundary
`5 surfaces, there are an extremely large number of por(cid:173)
`tions from which light is scattered. Therefore, a large
`light-diffusing effect can be obtained even by using a
`thin light-diffusing layer.
`When such a light-diffusing layer comprising micro-
`tO joints as described above is formed on the surface of a
`plastic material by treatment with the solvents, the
`surface portion of the plastic material is at fust swollen
`by the immersion of the plastic material in a ftrst solvent
`which is a good solvent for the constituent plastic of the
`15 plastic material. The swelling is caused by the expansion
`of the surface portion of the plastic material. This ex(cid:173)
`pansion is caused by the intrusion of the molecules of
`the solvent between the molecules of the constituent
`plastic. When the plastic material in this state is im(cid:173)
`mersed in a second solvent which is a poor solvent for
`the plastic, the molecules of the first solvent present
`between the molecules of the plastic are replaced by the
`molecules of the second solvent, so that the molecules
`of the plastic are rapidly shrunk and solidified. There(cid:173)
`fore, a large number of cracks are formed and become
`micro-joints.
`In this case, unless the first solvent and the second
`solvent are compatible with each other, the molecules
`of the fust solvent are not smoothly replaced by the
`molecules of the second solvent, and hence no uniform
`micro-joints are formed. Therefore, the fust solvent and
`the second solvent must be compatible with each other.
`Owing to this compatibility, the molecules of the fust
`solvent are gradually replaced by the molecules of the
`second solvent as the ratio of the former to the latter is
`changed, so that uniform micro-joints are formed.
`Thus, the micro-joints according to the present in-
`vention have a structure composed of very small cracks
`and spaces integrated closely with high density. The
`appearance of the micro-joints slightly varies depending
`on the kind of the constituent plastic to be treated, the
`kinds of the fust and second solvents, immersion
`method, immersion temperature, immersion time, etc.
`For example, the micro-joints appear as assemblies of
`very small cracks or appear spongy or felt-like. How(cid:173)
`ever, the micro-joints serve as the light-diffusing layer
`of an optical member, irrespective of their appearance.
`The average thickness (depth) of the micro-joints
`layer obtained in the present invention is 1 to 30 J.Lm. A
`micro-joints layer having an average thickness of 10 to
`20 J.Lm is particularly preferable for a light-diffusing
`layer. The average distance between the cracks or
`spaces formed is 0.01 to 50 J.Lm, and in particular, in an
`optical member is preferably 0.02 to 20 J.Lm.
`The outstanding advantages of the micro-joints are as
`follows: the micro-joints provide a very high light dif(cid:173)
`fuse reflectance (a high whiteness degree of the sur(cid:173)
`face); they do not peel off because they are integral with
`the substrate (they have the same quality as that of the
`substrate) and provide a higher surface strength as com(cid:173)
`pared with the coating with a white pigment, etc. The
`micro-joints also have advantages in their production.
`For example, their characteristics can easily be chosen
`by changing the kinds of the solvents employed and the
`conditions of the treatment.
`As regards the plastic material used as the optical
`member or the light-quantity-controlling member of the
`present invention, the micro-joints layer can be formed
`
`30
`
`35
`
`LGD_000893
`
`

`
`5,384,658
`
`5
`6
`on the surface of any kind of plastic so long as there is
`The removal of the solvent on the surface and of the
`a good solvent for the plastic. Examples of plastic in-
`resist forms a sharp pattern composed of a micro-joints
`elude homopolymers such as acrylic resin (e.g.
`layer on the surface of the plastic material. The micro-
`PMMA), polyester resin (e.g. PET), polycarbonate,
`joints in the white pattern constitute a multiple voids
`polyolefin (e.g. polyethylene), polystyrene, polyamide, 5 layer having an average distance between the voids of
`polyoxymethylene, polyvinyl chloride and the like, and
`0.01 to 50 JLm and an average layer thickness of 1 to 30
`copolymers such as ABS, AS and the like. These plastic
`JLm.
`can be used even when they are not particularly trans-
`When the pattern comprising the micro-joints layer
`parent.
`formed on the surface of the plastic material in the
`As a method for allowing the first solvent and the 10 manner as described above is irradiated with light, the
`second solvent to act on the plastic, any methods can be
`cast light carries out repeated reflection and refraction
`employed, such as immersion in each of the solvents,
`due to the existence of a large number of the very small
`exposure to the vapor of each of the solvents, spraying
`cracks which are densely and irregularly overlaid in the
`each of the solvents onto the surface, coating of the
`direction of the depth of the plastic material. Therefore,
`surface with each of the solvents, etc. Since the second 15 in spite of the thinness of the layer of the micro-joints,
`solvent replaces the first solvent which has already
`the pattern has a high whiteness and a very high non-
`swollen the surface portion of the plastic, it is necessary
`directional reflectance.
`to supply a large amount of the second solvent to the
`As described above, the white pattern comprising a
`surface.
`micro-joints layer repeats reflection and refraction very
`As described above, the plastic material having a 20 many times in three dimensions in the thin layer. There-
`light-diffusing layer comprising the micro-joints as a
`fore, the white pattern exhibits extremely excellent
`white surface for carrying out light diffuse reflection
`effective diffusion, per unit transmittance, than does an
`provides highly efficient and dense diffuse-reflected
`etched surface or a sandblasted surface, whose diffusion
`light. The plastic material can be employed for the
`is mainly attributable to planar reflection and refraction.
`production of a diffuse reflector used in various illumi- 25 Accordingly, the white pattern layer can be made thin-
`nators, a material constituting the inner surface of a
`ner, whereby light loss is reduced. Therefore, the light-
`light box, a projection screen, etc. Particularly when
`quantity-controlling member of the present invention
`the material is utilized as a projection screen, a light and
`having the white pattern comprising micro-joints effi-
`highly clear projected image can be obtained owing to
`ciently reflects or refracts the light thereby to enable
`the thin light-diffusing layer and the dense surface 30 accurate and non-directional control of the light.
`thereof.
`The light-quantity-controlling member of the present
`The light-quantity-controlling member of the present
`invention does not contain any factor which changes
`invention for adjusting the quantity of transmitted light
`the wavelength characteristics of the light at the time
`and for adjusting the quantity of reflected light has a
`when the member reflects or refracts the light. There-
`light-quantity-controlling means comprising a white 35 fore, the member does not cause color shift.
`pattern comprising micro-joints formed on the surface
`Embodiments of the present invention are explained
`of a plastic material. Such a light-quantity-controlling
`below.
`means can be formed as follows. Before forming the
`BRIEF DESCRIPTION OF THE DRAWINGS
`white pattern comprising the micro-joints, the surface
`of the plastic material is masked with a resist such as 40
`FIG. 1 is a schematic view of one example of the
`photoresist which is insoluble in both the fust and sec-
`surface of a light-diffusing layer comprising the micro-
`and solvents, wherein the first solvent is a good solvent
`joints of the present invention observed under a micro-
`for the constituent plastic, and the second solvent is a
`scope;
`poor solvent for the constituent plastic and is compati-
`FIG. 2 is a schematic view of one example of a sec-
`ble with the first solvent. The plastic material thus 45 tion of the light-diffusing layer comprising the micro-
`treated is immersed successively in the first solvent and
`joints;
`then in the second solvent.
`FIG. 3 is a schematic illustration of the micro-joints
`In the present invention, the surface of the plastic
`of the present invention at a magnification of 5,000;
`material in which the white pattern is to be formed is
`FIG. 4 is a partial perspective view of the light-quan-
`masked with a resist insoluble in both the first solvent (a 50 tity-controlling member of the present invention;
`good solvent for the plastic material) and the second
`FIG. 5 shows another example of the light-quantity-
`solvent (a poor solvent for the plastic material), while
`controlling member of the present invention. FIG. 6,
`leaving a portion to be patterned. Then, the plastic
`FIG. 7, FIG. 8, FIG. 9, FIG. 10 and FIG. 11 are sec-
`material thus treated is immersed in the first solvent. At
`tiona! side views of other examples of the present inven-
`this time, a portion not masked with the resist is allowed 55 tion.
`to swell and increase in volume.
`The symbols employed in these figures denote as
`The immersion of the plastic material in the second
`follows.
`solvent causes rapid replacement of the first solvent by
`1 - - - housing,
`the second solvent due to the compatibility between
`2 - - - light source,
`these solvents, and results in the rapid shrinkage and 60
`3 - - - light-quantity-controlling member,
`solidification of the swollen portion, which lead to the
`4 - - - light diffuser panel,
`formation of numerous extremely minute cracks. Since
`5 - - - reflective surface,
`the plastic material has low crystallinity, the cracks
`6- --white pattern,
`constitute a layer of the so-called micro-joints which
`7 - - - light-quantity-controlling member,
`are irregularly placed one upon another in a large num- 65
`8 - - - white layer,
`ber in the direction of the depth of the plastic material,
`9 - - - transparent member,
`as can be seen from FIG. 2 and FIG. 3 given hereinaf-
`10 - - - light-quantity-controlling member,
`ter.
`11 - - - light diffuse transmission member,
`
`LGD_000894
`
`

`
`8
`amorphous structure of the polymethyl methacrylate
`plate.
`For comparison, the same polymethyl methacrylate
`plate is sandblasted on one side and the above-men-
`s tioned reflection optical density was measured. The
`reflection optical density was 0.62. When visually ob(cid:173)
`served, the sandblasted surface was not so white and
`was rough.
`Next, a white coating material comprising titanium
`10 oxide as a white pigment was applied on one side of a
`polymethyl methacrylate plate to a thickness of 20 p.m.
`The thickness was the same as that of the aforesaid
`light-diffusing layer composed of the micro-joints
`formed by the method of the present example. In this
`1S case, the reflection optical density was 0.08. For further
`reducing the reflection optical density, the coating
`thickness or the titanium oxide content in the white
`coating material may be increased. However, even
`when either of these factors is increased, the application
`of the white coating material becomes difficult and the
`formation of a dense layer on the surface of the poly-
`methyl methacrylate plate becomes impossible.
`In the present example, the transparent polymethyl
`methacrylate plate was used merely for showing the
`result of evaluation of the formed light-diffusing layer
`composed of the micro-joints in terms of the reflection
`optical density. As mentioned above, the plastic mate(cid:173)
`rial used in the present invention is not limited to trans-
`parent plastic plates.
`In the method of the present example, it is sufficient
`to immerse the plastic material successively in the first
`solvent and then in the second solvent. Therefore,
`whatever shape the plastic material may have, the light-
`diffusing layer can be formed on the surface of the
`plastic material in the same manner as described above.
`EXAMPLE2
`A light diffuse reflective layer of micro-joints was
`formed on the surface of each of 3 kinds of plastic plates
`in the same manner as in Example 1, except for replac(cid:173)
`ing the first solvent and the second solvent. The reflec(cid:173)
`tion optical density was measured in the same manner as
`in Example 1.
`For choosing solvents as the first solvent and the
`second solvent, respectively, the value of solubility
`parameter can be used.
`The definition of solubility parameter is shown be(cid:173)
`low:
`
`2s
`
`40
`
`5,384,658
`
`7
`
`12 - - - reflective sheet,
`13 - - - light guide member,
`14 - - - reflecting pattern,
`15 and 15' - - - white pattern comprising a micro-
`joints layer,
`16 - - - light-quantity-controlling member,
`17 - - - light-quantity-controlling member,
`18 - - - housing.
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`The present invention is further illustrated below
`with examples.
`
`EXAMPLE 1
`A 30 mm X 80 mm transparent polymethy1 methacry(cid:173)
`late plate was immersed in dichloromethane (a first
`solvent) for 30 seconds and then in n-hexane (a second
`solvent) for 60 seconds, and dried to form a white light(cid:173)
`diffusing layer on the surface of the polymethyl methac- 20
`rylate plate. In the formation of the light-diffusing layer,
`one side of the polymethyl methacrylate plate was cov(cid:173)
`ered with an adhesive tape made of polyester so that the
`light-diffusing layer was formed only on the other side
`of the polymethyl methacrylate plate which was ex(cid:173)
`posed to the solvents. After the formation of the light(cid:173)
`diffusing layer, the adhesive tape was peeled off.
`The reflection optical density of the light-diffusing
`layer thus formed on the surface of the polymethyl 30
`methacrylate plate was measured with a photographic
`densitometer (Model P-2, mfd. by Fuji Photo Film Co.,
`Ltd.) [the reflection optical density is expressed by the
`equation D=log (loll) (log: common logarithm)
`wherein D is the value of the reflection optical density, 3s
`Io is the intensity of an incident light, and I is the inten(cid:173)
`sity of a reflected light]. A sheet of black felt was
`brought into close contact with the reverse side of a
`sample to be measured, whereby the light transmitted
`through the light-diffusing layer was absorbed.
`The reflection optical density of the light-diffusing
`layer formed on the surface of the polymethyl methac(cid:173)
`rylate plate was 0.03, and the value corresponds to a
`reflectance of 93%. Thus, the light-diffusing layer has
`sufficient light diffuse reflection properties and hiding 4S
`power as a white layer.
`Observation of the light-diffusing layer formed on the
`surface of the polymethyl methacrylate plate under a
`microscope demonstrated that there were micro-joints
`composed of numerous extremely minute and irregular so
`cracks on the surface of the polymethyl methacrylate
`plate. As mentioned above, the micro-joints observed
`under the microscope are schematically shown in FIG.
`1.
`
`Microscopic observation of a section of the poly- ss
`methyl methacrylate plate having the light-diffusing
`layer on the surface revealed that the thickness of the
`light-diffusing layer was about 20 p.m. The section ob(cid:173)
`served under a microscope is schematically shown in
`FIG. 2. Each of FIG. 1 and FIG. 2 merely shows an 60
`example, and the size and shape of the micro-joints vary
`depending on the conditions at the formation of the
`light-diffusing layer.
`When visually observed, the light-diffusing layer
`formed on the surface of the polymethyl methacrylate 6S
`plate was pure-white, had a very dense surface shape,
`and showed no directional property of surface proflle at
`all. This is considered to be due to the non-crystalline,
`
`SP = ~ z (/lil- R1)
`
`SP: solubility parameter,
`AH: latent heat of vaporization,
`R: gas constant (cal/mole),
`C: density (glee),
`M: gram-molecular weight (g!mol),
`T: absolute temperature.
`When a polymethyl methacrylate plate is used as a
`transparent plastic plate, the value of solubility parame(cid:173)
`ter (hereinafter referred to as SP value) of the first sol(cid:173)
`vent,. i.e. the good solvent, is preferably between 9.0 and
`9.8, more preferably between 9.3 and 9.7.1t is sufficient
`that the SP value of the second solvent, i.e. the poor
`solvent, is 9.9 or more, or 8.8 or less. As the second
`solvent, there can be exemplified alcohols, glycols,
`chain hydrocarbons and cyclic hydrocarbons.
`
`LGD_000895
`
`

`
`9
`Table 1 shows examples of the formation of a light
`diffuse reflective layer of micro-joints in the surface of
`a polymethyl methacrylate plate by immersion of the
`polymethyl methacrylate plate in each of the first sol(cid:173)
`vents having different solubility parameters for 30 sec- 5
`onds and then in ethanol (SP value= 12.7) as the second
`solvent for 60 seconds. For the layers of micro-joints
`which remained transparent, the symbol T is written in
`the table instead of measuring the reflection optical
`density.
`
`10
`
`5,384,658
`
`10
`first solvents for 30 seconds and then in n-hexane as the
`second solvent for 60 seconds.
`TABLE4
`
`First solvent
`
`SP value
`
`Carbon tetrachloride
`Xylene
`Benzene
`Chloroform
`Dichloromethane
`Dioxane
`Acetone
`
`8.5
`8.8
`9.2
`9.3
`9.7
`9.9
`10.0
`
`Reflection
`optical density
`T
`0.02
`Nonuniform
`Nonuniform
`Nonuniform
`0.24
`T
`
`First solvent
`
`Xylene
`Benzene
`Chloroform
`Chlorob