United States Patent [191
`Kobayashi et a1.
`‘
`
`USO05408388A
`[11] Patent Number: ,
`[45] Date of Patent:
`
`5,408,388
`Apr. 18, 1995
`
`[54] PLANAR ILLUMINATING DEVICE
`
`[75] Inventors: Uruo Kobayashi, Tokyo; Yoji Oki,
`Yokohama; Nobuo Matsui,
`Yokohama; Yoshinori Go,
`Yokohama, all of Japan
`
`[73] Assignee: Stanley Electric C0., Ltd., Tokyo,
`Japan
`
`[21] Appl. No.: 9,208
`
`[22] Filed:
`
`Jan. 22, 1993
`
`Foreign Application Priority Data
`[30]
`Jan. 24, 1992 [JP]
`Japan ................................ .. 4032951
`
`[51] Int. Cl.6 .............................................. ..1=21v 8/00
`[52] us. 01. ...................................... .. 362/31; 362/26;
`362/27; 362/331
`[58] Field Of Search ..................... .. 362/26, 27, 29, 30,
`362/31, 326, 327, 328, 330, 331, 339, 223
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,233,651 11/1980 Fabbri ............................... .. 362/ 330
`
`
`
`4,242,723 12/1980 Fabbri et al. . 4,257,084 3/ 1981 Reynolds
`
`4,277,817 7/1981 Hehr . . . . . . . . . . . .
`
`. . . .. 362/31
`
`362/331
`4,791,540 12/1988 Dreyer et al.
`. . . ,. 362/31'
`4,792,884 12/1988 Suman ct al. . . . .
`4,975,807 12/ 1990 Ohashi .................... .. 362/26
`5,050,946 9/1991 Hathaway et a1. ..
`362/31
`5,178,447 I/ 1993 Murase et a1. ...................... .. 362/31
`
`FOREIGN PATENT DOCUMENTS
`
`30346 5/ 1926 France ................................ .. 362/31
`475508 8/1969 Switzerland ...................... .. 362/327
`Primary Examiner—Ira S. Lazarus
`Assistant Examiner--Y. Quach
`Attorney, Agent, or Firm—Louis Weinstein
`[57]
`ABSTRACT
`A planar illuminating device, comprising a light-trans
`mitting plate made of a transparent material and having
`opposite side edges and opposing front and rear faces,
`two sources of light each arranged adjacent to one of
`the side edges of the plate, a re?ector arranged behind
`the plate rear face and re?ecting rays of light from the
`plate to the front face of the plate, a light diffuser ar
`ranged in front of the plate and diffusing rays of light
`from the plate. The device comprises a re?ecting ?nish
`applied directly to the rear face of the plate. A re?ect
`ing surface of the re?ecting ?nish faces substantially in
`the direction of the front face of the plate. The re?ec
`tance of part‘ of the re?ecting ?nish depends on the
`distance of that part from either of the two sources of
`light. The device further comprises parallel prismatic
`cuts along the front face of the plate. The longitudinal
`axes of the prismatic cuts are substantially parallel to
`rays of light passing from the sources of light through
`the plate. A combination of the re?ecting ?nish and the
`re?ector increases the surface illuminance and the uni
`formity in the surface illuminance of the planar illumi
`nating device.
`
`11 Claims, 5 Drawing Sheets
`
`LG Display Ex. 1008
`
`LGD_001176
`
`

`

`.,/U.S. Patent
`
`Apr. 18, 1995
`
`Sheet 1 of 5
`
`5,408,388
`
`FIG. I
`
`FIG. 2
`
`3
`
`
`
`00000 099006
`
`@QQQQQQQQQmU.
`
`@mWmVAwmwQQQAwQMW»
`
`v@®®®@@®@mw@@m
`
`22
`
`LGD_001177
`
`

`

`US. Patent
`
`Apr. 18, 1995
`
`Sheet 2 of 5
`
`5,408,388
`
`FIG. 3
`/23
`
`FIG. 4
`
`LGD_001178
`
`

`

`US. Patent
`
`Apr. 18, 1995
`
`Sheet 3 of s
`
`5,408,388
`
`FIG.5
`
`LGD_001179
`
`

`

`US. Patent
`
`Apr. 18, 1995
`
`Sheet 4 of 5
`
`5,408,388
`
`_
`
`2
`
`I
`
`LESS THAN
`CRITICAL
`
`ANGLE
`
`LGD_001180
`
`

`

`‘ US. Patent
`
`Apr. 18, 1995
`
`Sheet 5 of 5
`
`5,408,388
`
`F|G.7
`(PRIORART)
`
`94 91c
`
`9Q
`
`_
`
`8
`C)
`
`I
`s
`93
`
`/
`/
`A v,
`(
`Y
`em 9|
`
`8
`92.
`(T
`\
`9|o
`
`FIG.8'
`(PRIOR ART)
`
`LGD_001181
`
`

`

`1
`
`PLANAR ILLUMINATING DEVICE
`
`5,408,388
`
`2
`tional to the distance of a spot from the nearest edge
`surface 910 , i.e. the size of spots close to the nearest end
`is smaller in size than a spot further away from its near
`est edge surface, as shown in FIG. 8. This arrangement
`involves a new drawback that the surface illuminance of
`the entire light diffusing sheet 94 is equalized to that of
`the darkest part of the light diffusing sheet 94 to reduce
`ef?ciency of the entire planar illuminating device.
`Therefore, measures for eliminating both the drawbacks
`have been desired but not achieved prior to the present
`invention.
`
`10
`
`25
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to a planar illuminating
`device used as a back light for liquid crystal displays of
`portable of?ce automation apparatuses such as a word
`processor or a personal computer.
`2. Background Art
`FIG. 7 illustrates an arrangement of such a conven
`tional planar illuminating device 90. The planar illumi
`nating device 90 comprises a rectangular ?at light trans
`mitting plate 91, two sources of light 92, 92 a ?at light
`re?ecting sheet 93 and a light diffusing sheet 94. The
`light transmitting plate 91 is made of a high-transparent
`material, such as acrylic resin, in the form of a thick
`plate. The sources of light 92 comprise, e.g., tubular
`?uorescent lamps and are arranged adjacent to the sur
`faces 91a, 91a of opposite side edges of the light trans
`mitting plate 91. The light re?ecting sheet 93 has the
`same shape as the light transmitting plate 91, is attached
`to and covers the entire rear surface of the light trans
`mitting plate 91. The light re?ecting sheet 93 has, e. g.,
`a {white re?ecting front surface having a suitable diffus
`ing force or diffusivity. The light diffusing sheet 94 has
`the same shape as the light transmitting plate 91 and a
`milky color and covers the entire front surface of the
`light transmitting plate 91 with a suitable clearance
`therebetween.
`In the operation of the light transmitting plate 91,
`portions of rays of light from the sources of light 92
`incident upon the edge surface 91c of the light transmit
`ting plate 91 which has an angle of incidence less than a
`critical angle of incidence (approximately 45°) is re
`?ected by the front surface 91c due to a difference of the
`indices of refraction of the light transmitting plate 91
`and the air to travel to the light re?ecting sheet 93 and
`be re?ected again by the light re?ecting sheet 93 to the
`front surface 910.
`Since the light re?ecting sheet 93 has a the suitable
`diffusivity, rays of light re?ected from the light re?ect
`ing sheet 93 to the front surface 91c involve rays of light
`of the critical angle or more which pass through the
`front surface 91c and the light diffusing sheet 94 to
`illuminate the light diffusing sheet 94. Thus, the light
`transmitting plate 91 illuminates the entire light diffus
`ing sheet 94 to illuminate the entire liquid crystal dis
`play (not shown) with the planar illuminating device 90
`from behind the display even if part of the light diffus
`ing sheet 94 is remote from the sources of light 92.
`However, the planar illuminating device 90 involves
`a drawback that direct rays of light from the sources of
`light 92 more intensively illuminate part of the light
`transmitting plate 91 near the edge surfaces 91a of the
`55
`light transmitting plate 91.0pposite to the sources of
`light 92 than the other part of the light transmitting
`plate 91 to produce such nonuniformity in the illumina
`tion of that the illuminance of part of the light diffusing
`sheet 94 is as small as that part is remote from the
`sources of light 92.
`In order to eliminate the above-described drawback,
`there was proposed a planar illuminating device in
`which the rear surface 91b of the light transmitting plate
`91 has, e.g., a large number of spot-shaped re?ecting
`layers 93 attached thereto and the area of part of the
`rear surface 91b occupied by the spot-shaped re?ecting
`layers 93 is such that the size of a spot is directly propor
`
`45
`
`60
`
`SUMMARY OF THE INVENTION
`Therefore, a primary object of the present invention
`is to provide a planar illuminating device having an
`increased illuminance.
`In order to achieve this object, a planar illuminating
`device of a ?rst aspect of the present invention com
`prises a light-transmitting plate made of a transparent
`material and having opposite side edges, two sources of
`light arranged adjacent to the side edges of the light
`transmitting plate, a re?ector arranged behind the light
`transmitting plate for re?ecting rays of light from the
`light transmitting plate to the front of the light transmit
`ting plate, a light diffuser arranged in front of the light
`transmitting plate and diffusing rays of light from the
`light transmitting plate,a re?ecting ?nish applied di
`rectly to the rear surface of the light transmitting plate,
`a re?ecting surface of the re?ecting ?nish for re?ecting
`light substantially in a direction toward the front of the
`light transmitting plate, the re?ectance of part of the
`re?ecting ?nish varying in direct proportion to the
`distance of the re?ective surface from the closer of the
`two sources of light, and parallel prismatic cuts on the
`front surface of the light transmitting plate, the longitu
`dinal axes of the prismatic cuts being substantially paral
`lel to rays of light passing from the sources of light
`through the light transmitting plate.
`The prismatic cuts may be a knurled or hairline ?nish.
`The re?ecting ?nish of the planar illuminating device
`of the ?rst aspect of the present invention may comprise
`an assembly of spot-shaped layers applied to the rear
`surface of the light transmitting plate. The spot-shaped
`layers have a higher re?ectance than the re?ector. The
`size of a spot-shaped layer is in direct proportion to the
`distance of the spot-shaped layer from the nearer
`sources of light.
`The re?ecting ?nish of the planar illuminating device
`of the ?rst aspect of the present invention may alterna
`tively comprise a satin'fmish on the rear surface of the
`light transmitting plate, and an assembly of spot-shaped
`layers made of a transparent paint applied to the satin
`?nish. The area of part of the satin ?nish occupied by a
`spot-shaped layer is as small as that part is remote from
`the sources of light i.e., the size of each satin ?nish is
`inversely proportion to the distance of each satin ?nish
`from the nearest light source.
`A further object of the present invention is to provide
`a planar illuminating device having an increased unifor
`mity in surface illumination.
`In order to achieve this object, a planar illuminating
`device of a second aspect of the present invention com
`prises in addition to the planar illuminating device of
`the ?rst aspect of the present invention the re?ecting
`?nish comprising an assembly of spot-shaped layers
`applied to the rear surface of the light transmitting
`plate, the spot-shaped layers having a higher re?ectance
`
`LGD_001182
`
`

`

`4
`FIG. 8 is a cross section of a prior art planar illumi
`nating device.
`
`5,408,388
`3
`than the re?ector, the area of part of the rear surface of
`the light transmitting plate occupied by a spot-shaped
`layer being as large as that part is remote from the
`sources of light, the rear portion of the light transmit
`ting plate having a tapering portion tapering in the
`thickness from the sources of light.
`The re?ecting ?nish of the planar illuminating device
`of the second aspect of the present invention may alter
`natively comprise a satin ?nish on the rear surface of the
`light transmitting plate, and an assembly of spot-shaped
`layers made of a transparent paint applied to the satin
`?nish. The area of part of this satin ?nish occupied by a
`spot-shaped layer is as small as that part is remote from
`the sources of light.
`In order to achieve the further object, the planar
`illuminating device of the second aspect of the present
`invention may further comprise a prismatic sheet made
`of the same material as the light transmitting plate and
`arranged between the light transmitting plate and the
`light diffuser. The front surface of the prismatic sheet
`has substantially the same prismatic cuts as the front
`surface of the light transmitting plate. The longitudinal
`axes of the prismatic cuts of the prismatic sheet are
`transverse to the longitudinal axes of the prismatic cuts
`of the light transmitting plate.
`In the present invention, the combination of the re
`?ecting ?nish having the higher re?ectance on the rear
`surface of the light transmitting plate and the re?ector
`having the lower reflectance, and the provision of the
`30
`prismatic cuts on the front surface of the light transmit
`ting plate increase the luminous energy of the rays of
`light from the sources of light to the light diffuser and
`the uniformity in the surface illuminance of the planar
`illuminating device.
`In addition, the provisions of the prismatic sheet and
`the tapering portion further increase the uniformity in
`the surface illuminance of the planar illuminating de
`vice.
`Other objects, features and advantages of the present
`invention will be apparent from a consideration of the
`following description, taken in connection with the
`accompanying drawing.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an exploded perspective view of a planar
`illuminating device according to a ?rst embodiment of
`the present invention;
`FIG. 2 is a rear elevation of a light transmitting plate
`of the planar illuminating device of FIG. 1;
`FIG. 3 is a fragmentary longitudinal section of a light
`transmitting plate of a planar-illuminating device ac
`cording to a second embodiment of the present inven
`tion;
`FIG. 4 is an exploded perspective view of a planar
`illuminating device according to a third embodiment of
`the present invention;
`FIG. 5 is a fragmentary longitudinal section of a light
`transmitting plate of a planar illuminating device ac
`cording to a fourth embodiment of the present inven
`tion;
`FIGS. 51: and Sb show the manner in which light rays
`are re?ected and refracted by the apparatus of FIG. 5.
`FIG. 6 is a fragmentary cross section of a planar
`illuminating device according to a fifth embodiment of
`65
`the present invention.
`FIG. 7 is a cross section of a prior art planar illumi
`nating device; and
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`Preferred embodiments of the present invention wil
`be described with reference to the drawings.
`'
`FIG. 1 shows a planar illuminating device according
`to a ?rst embodiment of the present invention. A planar
`illuminating device 1 comprises a rectangular light
`transmitting plate 2 of a transparent material such as
`acrylic resin or polycarbonate, a pair of tubular ?uores
`cent lamps 3, 3 arranged along opposite side edge sur
`faces 2a, 2a of the light transmitting plate 2, a light
`diffusing sheet or plate 4 having the same rectangular
`front surface size as the light transmitting plate 2 and
`arranged to the front surface of the light transmitting
`plate 2 with a clearance, and a re?ecting sheet 5 having
`the same rectangular front surface size as the light trans
`mitting plate 2 and arranged relative to the rear surface
`of the light transmitting plate 2 with a clearance. Air
`?lls the clearance regions. The re?ecting sheet 5 has a
`light re?ecting layer made, e.g., of a white paint such as
`titanium oxide or dioxide or aluminum oxide, or alumi
`num vapor deposition. The front portion of the light
`transmitting plate 2 has prismatic cuts 21. The rear
`surface of the light transmitting plate 2 has a re?ecting
`?nish, e.g., an array of spot-shaped light re?ecting lay~
`ers 22 applied directly thereto, as best shown in FIG.2.
`The arrangement of the light transmitting plate 2 in
`creases both the surface illuminance and the uniformity
`in the surface illuminance of the diffusing sheet 4.
`In detail, each of the prismatic cuts 21 on the front
`portion of the light transmitting plate 2 has a triangular
`cross section and extends between the side edge sur
`faces 2a of the light transmitting plate 2 in the direction
`of longitudinal axis Z of a prismatic cut transversely to
`the longitudinal axis of the light transmitting plate 2.
`The prismatic cuts 21 are arranged close to one another
`at an equal pitch lengthwise of the light transmitting
`plate 2 and cover the entire front portion of the light
`transmitting plate 2 so that the prismatic cuts 21 collec
`tively de?ne a knurled ?nish or the cross sections of the
`prismatic cuts 21 are arranged in the form of roulette.
`The array of spot-shaped re?ecting layers 22 is made,
`e.g., of a white paint such as titanium oxide or dioxide
`or aluminum oxide, or aluminum vapor deposition. The
`array of spot-shaped re?ecting layers 22 may alterna
`tively be made of a material of a higher re?ectance than
`re?ecting sheet 5. As shown in FIG. 2, the rear surface
`of the light transmitting plate 2 is provided with re?ect
`ing spots 22 of difference sizes. The arrangement of the
`different size spots is such that the spots further re
`moved from both of the sources of light 3 are of an
`increased diameter. Alternatively, the density of re?ect
`ing spots 22 increases in moving toward the center of
`the rear surface of the light transmitting plate 2 which is
`remote from both the sources of light 3. Thus, the array
`of re?ecting spots 22 increases the re?ectance of the
`central of the rear surface of the light transmitting plate
`2 at and near the longitudinal axis of the light transmit
`ting plate 2 more so than the re?ectance of part near
`each of the sources of light 3. The re?ecting sheet 5 is
`arranged to the rear surface of the light transmitting
`plate 2 so as to be in contact with the array of re?ecting
`spots 22.
`The operation and advantages of the planar illuminat
`ing device 1 according to the ?rst embodiment will be
`
`25
`
`35
`
`60
`
`LGD_001183
`
`

`

`0
`
`15
`
`20
`
`5,408,388
`5
`described hereinafter. Since the array of re?ecting spots
`22 is applied directly to the rear surface of the light
`transmitting plate 2, the array of re?ecting spots 22
`re?ects all of rays of light arriving thereat at the array.
`On the other hand, since the re?ecting sheet 5 has the
`small clearance or spacing corresponding to the thick
`ness of the array of re?ecting spots 22 with the rear
`surface of the light transmitting plate 2, only rays of
`light with a critical angle of incidence or more arriving
`at the rear surface of the light transmitting plate 2 in
`contact with the air travel through the light transmit
`ting plate 2 and are re?ected by the re?ecting sheet 5.
`Even if the array of re?ecting spots 22 is made, e.g., of
`aluminum vapor deposition and the re?ecting sheet 5
`has a re?ecting layer of the same material as the array of
`re?ecting spots 22, the effective re?ectance of the array
`of re?ecting spots 22 is greater than that of the re?ect
`ing sheet 5.
`The prismatic cuts 21 provide faces constituting an
`elevation angle to the travel direction of rays of light
`from the sources of light 3 propagating through the
`light transmitting plate 2. Since the faces constituting
`the elevation angle re?ect the total rays of light incident
`upon the faces of the prismatic cuts 21, rays of light
`totally re?ected by the front portion of the light trans
`mitting plate 2 include more component rays of light
`traveling across the thickness of the light transmitting
`plate 2 than rays of light totally re?ected by the ?at
`front surface of a prior art light transmitting plate do.
`Rays of light re?ected by the array of re?ecting spots
`22 and the re?ecting sheet 5 thus include more compo
`nent rays of light traveling across the thickness of the
`light transmitting plate 2, i.e., in the direction of a view
`ing of an operator of an of?ce automation apparatus
`comprising the planar illuminating device 1 than rays of
`35
`light re?ected by a re?ecting device attached to the
`rear surface of the prior art light transmitting plate.
`Consequently, the effective ?ux-of-light availability of
`the planar illuminating device 1 is higher than that of
`the prior art light transmitting plate.
`40
`In a trial manufacture and a study of the planar illumi
`nating device 1 by the present inventor, the prismatic
`cuts 21 still advantageously operate even if the pitch of
`the prismatic cuts 21 is very small. The inventor con
`?rmed that the so-called hairline ?nish 23 according to
`45
`a second embodiment of the present invention in which
`the prismatic cuts 21 constitute an assembly of hairline
`grooves extending in the direction of the arrow Z and
`which is illustrated at a very enlarged scale in FIG. 3 is
`still suf?ciently operated. In the ?rst and second em
`bodiments, the rays of light re?ected by the array of
`re?ecting spots 22 and the re?ecting sheet 5 which
`travel through the prismatic cuts 21 to the light diffus
`ing sheet 4 are refracted only in a direction transverse to
`the direction of the arrow Z when traveling through the
`prismatic cuts 21 and then diffused by the light diffusing
`sheet 4.
`This causes the planar illuminating device 1 to have
`different angles of illumination in two directions trans
`verse to each other, e.g., the vertical and horizontal
`directions when the planar illuminating device 1 is in a
`vertical position in such a manner that the sources of
`light 3 are right-hand and left-hand. Thus, a liquid crys
`tal display with the planar illuminating device 1 has
`visual ?elds of a different extent in the vertical and
`horizontal directions.
`‘
`FIG. 4 shows a planar illuminating device 1 accord
`ing to a third embodiment of the present invention
`
`6
`which is provided in order to improve the anisotropy in
`the angle of illumination of the illuminating device 1
`according to the ?rst and second embodiments. The
`planar'illuminating device 1 according to the third em
`bodiment further provides between the light transmit
`ting plate 2 and the light diffusing sheet 4 a prismatic
`sheet 6 the front portion of which has substantially the
`same prismatic cuts as the prismatic cuts 21 but has the
`axes Y of the prismatic cuts transverse to the axes Z of
`the prismatic cuts 21. This causes the planar illuminat
`ing device 1 according to the third embodiment to have
`the visual ?elds of the same extent in the vertical and
`horizontal directions when the planar illuminating de
`vice 1 is in a vertical position in such a manner that the
`sources of light 3, 3 are right-hand and left-hand. The
`planar illuminating device 1 according to the third em
`bodiment has the same operations and advantages as the
`planar illuminating device 1 according to the ?rst and
`second embodiments except that the former has the
`visual ?elds of the same extent in the vertical and hori
`zontal directions. When the prismatic cuts 21 comprise
`the hairline grooves or cuts 23 as illustrated in the sec
`ond embodiment, the prismatic cuts of the prismatic
`sheet 6 likewise comprise the same hairline cuts or
`grooves.
`FIG. 5 illustrates a main part of a planar illuminating
`device according to a fourth embodiment of the present
`invention and FIGS. 5a and 5b illustrate the manner in
`which rays are refracted and re?ected by the apparatus
`of FIG. 5. The entire rear portion of the light transmit
`ting plate 2 has a satin ?nish 24 which comprises minute
`depressions having suitable shapes such as a concavity
`or a hemisphere. The satin ?nish 24 of the light trans
`mitting plate 2 has, e.g., a large number of spot-shaped
`layers 25 which are made of a transparent paint of sub
`stantially the same index of refraction as the light trans
`mitting plate 2 and applied to the satin ?nish 24. Each
`spot-shaped layer 25'fully ?lls depressions of the satin
`?nish 24. The rear surfaces 25a of the spot-shaped layers
`25 are ?at. The area of a spot-shaped layer 25 is as small
`as the spot-shaped layer 25 is remote from both the
`sources of light 3. The re?ecting sheet 5 is arranged
`relative to the rear surface of the light transmitting plate
`2 to provide with a narrow clearance between the lay
`ers 25 and the front surface of the re?ecting sheet 5. Air
`?lls this region.
`In the operation of the planar illuminating device
`according to the fourth embodiment, all of the rays of
`light emitted from the sources of light 3 to the satin
`?nish 24 through the light transmitting plate 2 are re
`?ected by the surfaces of the minute depressions of the
`satin ?nish 24, so that the satin ?nish 24 has a higher
`re?ectance than the rear surface 25a of each of the
`spot-shaped layers 25 (see FIG. 50).
`On the other hand, since the rear surfaces 250 of the
`spot-shaped layers 25 applied to the satin ?nish 24 are
`?at, only rays of light incident at the critical angle or
`less upon the rear surfaces 25a of the spot-shaped layers
`25 pass through the spot-shaped layers 25 to the re?ect~
`ing sheet 5 and are re?ected by the re?ecting sheet 5, so
`‘that the spot-shaped layers 25 have a lower re?ectance
`than the satin ?nish 24 (see FIG. 5b . Therefore, the
`occupation ratio between part of the'satin ?nish 24
`having no spot-shaped layer 25 and part of the satin
`?nish 24 having a spot-shaped layer 25 is as appropri
`ately varied as these parts are remote from both the
`sources of light 3 so that the light diffusing sheet 4 has
`a uniform surface illuminance.
`
`25
`
`60
`
`65
`
`LGD_001184
`
`

`

`10
`
`5
`
`20
`
`5,408,388
`7
`8
`FIG. 6 illustrates a main part of a planar illuminating
`front surface of said light transmitting plate, said
`device according to a ?fth embodiment of the present
`front surface being said other major face, the re
`invention. This planar illuminating device is different
`?ectance of part of said re?ecting ?nish increasing
`from the planar illuminating devices according to the
`with increasing distance of said re?ecting ?nish
`?rst to fourth embodiments in that the latter comprise 5
`part from at least one of said two sources of light;
`the light transmitting plate 2 having the parallel front
`parallel prismatic cuts being provided in the front
`surface of said light transmitting plate, the longitu
`and rear surfaces but the former comprises a light trans
`mitting plate 7 having a rear portion comprising two
`dinal axes of said prismatic cuts being substantially
`tapering portions 7a, 7a tapering in thickness moving
`parallel to a direction of an illumination of said
`away from both sources of light 3, 3 as part of the rear
`source of light into said light transmitting plate;
`portion of the light transmitting plate 7 is remote from
`and
`both sources of light 3. The front portion of the light
`said re?ecting ?nish comprising a satin ?nish on the
`transmitting plate 7 has the prismatic cuts 21 or hairline
`rear surface of said light transmitting plate, and an
`?nish 23. The rear portion of the light transmitting plate
`assembly of spot-shaped layers made of a transpar
`7 has a re?ecting ?nish of an array of re?ecting spots 22
`ent paint applied to said satin ?nish an area of part
`or of a combination of the satin ?nish 24 and the assem
`of said satin ?nish occupied by a spot-shaped layer
`bly of spot-shaped layers 25.
`comprising spots whose size increase with increas
`The planar illuminating device according to the ?fth
`ing distance of each spot from at least one of said
`embodiment comprises the ?at re?ecting sheet 5 adja
`sources of light.
`cent to the rear side edges of the light transmitting plate
`2. The planar illuminating device as recited in claim 1,
`7 with a narrow clearance. Air ?lls the clearance space
`wherein said rear surface of said light transmitting plate
`between the tapering portions 7a and the re?ecting
`having a plurality of tapering portions tapering in thick
`sheet 5.
`ness from the side edges adjacent to said sources of light
`In the operation of the light transmitting plate 7, the
`toward a central region of the light transmitting plate.
`tapering portions 70 of the light transmitting plate 7
`3. The planar illuminating device as recited in claim 2,
`25
`further comprising:
`facilitates the array of re?ecting spots 22 or the re?ect
`ing assembly of the satin ?nish 24 and the spot-shaped
`a prismatic sheet made of the same material as said
`layers 25 to remarkably increase a re?ectance of the
`' light transmitting plate and arranged between said
`planar illuminating device towards the light diffusing
`light transmitting plate and said light diffusing
`sheet 4 to remarkably increase the surface illuminance
`sheet, a front surface of said prismatic sheet having
`of the planar illuminating device. In addition, the provi
`substantially the same prismatic pattern as said
`sionlof the tapering portions 70 increases an adjustable
`front surface of said light transmitting plate, longi
`range in the surface illuminance of the planar illuminat
`tudinal axes of prismatic cuts of said prismatic sheet
`ing device by the array of re?ecting spots 22 or the satin
`being transverse to longitudinal axes of prismatic
`?nish 24 to increase the uniformity of the illuminance of
`cuts of said light transmitting plate.
`35
`the planar illuminating device.
`4. The planar illuminating device as recited in claim 1,
`Each of the planar illuminating devices according to
`further comprising:
`the fourth and ?fth embodiments may further comprises
`a prismatic sheet made of the same material as said
`the prismatic sheet 6 of the planar illuminating device
`light transmitting plate and arranged between said
`according to the third embodiment.
`light transmitting plate and said light diffusing
`The present invention is not rigidly restricted to the
`sheet, a front surface of said prismatic sheet having
`embodiments described above. It is to be understood
`substantially the same prismatic pattern as said
`that a person skilled in the art can easily change and
`front surface of said light transmitting plate, longi
`modify the present invention without departing from
`tudinal axes of prismatic cuts of said prismatic sheet
`the scope of the invention de?ned in the appended
`being transverse to longitudinal axes of prismatic
`45
`claims.
`cuts of said light transmitting plate.
`What is claimed is:
`5. A planar illuminating device, comprising:
`1. A planar illuminating device, comprising:
`a light transmitting plate made of a transparent mate
`a light transmitting plate made of a transparent mate
`rial and having opposing side edges and a pair of
`opposing major faces;
`rial and having opposing side edges and a pair of 50
`opposing major faces;
`'
`two sources of light each arranged adjacent to one of
`two sources of light each arranged adjacent to one of
`the side edges of said light transmitting plate;
`the side edges of said light transmitting plate;
`a light re?ecting sheet arranged adjacent to one
`a light re?ecting sheet arranged adjacent to one
`major face of said light transmitting plate, and
`major face of said light transmitting plate, and
`having a re?ecting surface, said re?ecting surface
`having a re?ecting surface, said re?ecting surface
`of said light re?ecting sheet facing in a direction of
`of said light re?ecting sheet facing in a direction of
`said light transmitting plate;
`said light transmitting plate;
`a light diffusing sheet adjacent to another one major
`a light diffusing sheet adjacent to another one major
`face of said light transmitting plate for diffusing
`face of said light transmitting plate for diffusing
`rays of light emitted from said light transmitting
`plate;
`rays of light emitted from said light transmitting
`plate;
`said one major face being a rear surface of the light
`transmitting plate;
`said one major face being a rear surface of the light
`transmitting plate;
`a re?ecting ?nish applied directly to the rear surface
`a re?ecting ?nish applied directly to the rear surface
`of said light transmitting plate, a re?ecting surface
`of said light transmitting plate, a re?ecting surface
`of said re?ecting ?nish substantially directing re
`of said reflecting ?nish substantially directing re
`?ected light substantially in a direction toward a
`?ected light substantially in a direction toward a
`front surface of said light transmitting plate, said
`
`55
`
`65
`
`LGD_001185
`
`

`

`20
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`35
`
`5,408,388
`9
`front surface being said other major face, the re
`?ectance of part of said re?ecting ?nish increasing
`with increasing distance of said re?ecting ?nish
`part from at least one of said two sources of light;
`parallel prismatic cuts being provided in the front
`surface of said light transmitting plate, the longitu
`dinal axes of said prismatic cuts being substantially
`parallel to a direction of an illumination of said
`source of light into said light transmitting plate;
`said prismatic cuts collectively forming a knurled
`?nish; and
`said re?ecting ?nish comprising a satin ?nish on the
`rear surface of said light transmitting plate, and an
`assembly of spot-shaped layers made of a transpar
`ent paint applied to said satin ?nish, an area of part
`of said satin ?nish