(12) U n i t e d States P a t e n t
`Pelka et al.
`
`US006473554B1
`US 6,473,554 Bl
`(io) Patent No.:
`*Oct. 29,2002
`(45) Date of Patent:
`
`(54) LIGHTING APPARATUS HAVING LOW
`PROFILE
`
`(75)
`
`Inventors: David G. Pelka, Los Angeles, CA
`(US); John Popovich, Del Mar, CA
`(US)
`
`(73) Assignee: Teledyne Lighting and Display
`Products, Inc., Hawthorne, CA (US)
`
`( * ) Notice:
`
`This patent issued on a continued pros(cid:173)
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent
`term provisions ol 35 U.S.C.
`154(a)(2).
`
`Subject to any disclaimer, the term ol this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 08/936,717
`
`(22) Filed:
`
`Sep. 24, 1997
`
`Related U.S. Application Data
`
`(63) Continuation-in-part of application No. 08/764,298, filed on
`Dec. 12, 1996.
`Int. Cl.7
`(51)
`(52) U.S. Cl
`(58) Field of Search
`
`G02B 6/00
`385/146; 385/901; 365/555
`362/555, 559,
`362/560, 561; 385/146, 901, 14
`
`(56)
`
`References Cited
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`
`U.S. Patent Application No. 09/620,015 entitled Lighting
`Apparatus filed Jul. 20, 2000.
`U.S. Patent Application No. 09/334,848 entitled Lighting
`Apparatus Having Low Profile filed Jun. 16, 1999.
`3M Optical Systems Brightness Enhancement Film (BEF), 2
`pgs., 1993.
`
`Primary Examiner—Hung N. Ngo
`(74) Attorney, Agent, or Firm—Knobbe, Martens, Olson &
`Bear LLP
`(57)
`
`ABSTRACT
`
`Disclosed is a low profile lighting apparatus that is particu(cid:173)
`larly advantageous for use as a backlight for illuminating a
`display. The
`lighting apparatus
`includes a waveguide
`coupled
`to a light source for
`injecting
`light into
`the
`waveguide. The waveguide includes a plurality of elongate
`structures for ejecting
`light propagating within
`the
`waveguide
`through a predetermined surface of
`the
`waveguide. Another embodiment of the waveguide includes
`a central region of reduced thickness that redirects light
`propagating within the waveguide. The lighting apparatus
`has a low profile so it is particularly useful in areas of limited
`space.
`
`(List continued on next page.)
`
`48 Claims, 13 Drawing Sheets
`
`LGD_000494
`
`LG Display Ex. 1009
`
`

`
`US 6,473,554 Bl
`Page 2
`
`U.S. PATENT DOCUMENTS
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`4,972,394
`
`A
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`A
`A
`A
`A
`A
`A
`A
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`A
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`A
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`
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`Eberhardt et al
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`Trcka et al
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`
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`
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`
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`
`A
`A
`A
`A
`A
`A
`A
`E
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`A
`
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`4/1991
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`9/1991
`12/1991
`* 1/1992
`7/1992
`7/1992
`9/1992
`11/1992
`11/1992
`12/1992
`4/1993
`8/1993
`12/1993
`4/1994
`8/1994
`1/1995
`4/1995
`5/1995
`8/1995
`8/1995
`1/1996
`1/1996
`11/1996
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`* 10/1997
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`
`Horiuchi
`Russay et al
`Sanai et al
`De Vaan
`Hathaway et al
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`Pasco
`Suzawa
`Abileah et al
`Farrell
`Abileah et al
`Elderfield
`Palmer
`Arego et al
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`McKillip
`Winston et al
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`
`cited by examiner
`
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`
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`
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`
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`
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`362/31
`
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`
`313/110
`385/37
`362/31
`349/65
`
`LGD_000495
`
`

`
`U.S. Patent
`
`Oct. 29,2002
`
`Sheet 1 of 13
`
`US 6,473,554 Bl
`
`J%r.i
`
`20
`
`l&r.M
`
`LGD_000496
`
`

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`U.S. Patent
`
`Oct. 29,2002
`
`Sheet 2 of 13
`
`US 6,473,554 B l
`
`&&.2
`
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`
`LGD_000497
`
`

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`U.S. Patent
`
`Oct. 29,2002
`
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`US 6,473,554 B l
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`
`Oct. 29,2002
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`U . S . P a t e n t
`
`Oct. 29,2002
`
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`U.S. Patent
`
`Oct. 29,2002
`
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`U.S. Patent
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`Oct. 29,2002
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`U.S. Patent
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`Oct. 29,2002
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`Oct. 29,2002
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`Oct. 29,2002
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`US 6,473,554 Bl
`
`LIGHTING APPARATUS HAVING LOW
`PROFILE
`
`This application is a continuation-in-part of U.S. patent
`application Ser. No. 08/764,298, filed Dec. 12,1996, entitled 5
`"Waveguide with Light Emitting Diodes for Illumination
`and Display."
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to a lighting apparatus.
`More particularly, the present invention relates to a low
`profile lighting apparatus utilizing a waveguide for illumi(cid:173)
`nation. The invention is particularly advantageous for use as
`a backlight for illuminating a display.
`Backlights may be used to illuminate both mechanical
`displays, such as on analog watches or automobile gauges,
`as well as electronic displays, such as liquid crystal displays
`used with cellular phones, and pagers, and personal digital
`assistants. Because many backlight applications involve 20
`smaller displays where space is at a premium, it is desirable
`to reduce the thickness of such backlights while still main(cid:173)
`taining the area of illumination. Backlights thus require
`reduced aspect ratios, defined as the ratio of the thickness of
`the backlight to the length of the illumination area.
`One type of a backlight utilizes of a light source, such as
`a light-emitting diode (LED), that is coupled to a waveguide
`into which light is injected. The light source is typically
`mounted at an outer peripheral edge of the waveguide and is
`energized to emit light into the waveguide. The light under- 30
`goes several reflections between
`the surfaces of
`the
`waveguide until being transmitted through a top surface to
`illuminate the display.
`One difEculty associated with such backlights is they do
`not produce a uniform intensity across the surface of the 3 5
`waveguide. The light loses intensity as it propagates outward
`from the light source. Consequently, as the distance from the
`light source increases, the intensity of the light transmitted
`out of the waveguide decreases. This results in the portions
`of the waveguide distal of the light source having reduced 4 0
`intensity.
`There is therefore a need for an efEcient backlight having
`a low aspect ratio that provides a substantially uniform
`illumination profile across the entire area of illumination
`
`25
`
`45
`
`SUMMARY OF THE INVENTION
`
`One aspect of the present invention relates to a lighting
`apparatus for illuminating an illumination area of a display.
`The lighting apparatus comprises a waveguide adapted for 50
`mounting adjacent the display so as to illuminate the illu(cid:173)
`mination area of the display. The waveguide comprises a top
`surface having an optical output area corresponding in size
`to the illumination area, a bottom surface spaced apart from
`the top surface, and a side surface extending between the top 55
`and bottom surfaces. Reflective material is positioned adja(cid:173)
`cent the bottom and side surfaces of the waveguide. At least
`one light source is mounted to input light proximate to a
`periphery of the waveguide between the top and bottom
`surfaces. The waveguide further comprises a light ejector on go
`one of the top and bottom surfaces configured to redirect
`light propagating between the surfaces towards the top
`surface for transmission therethrough. The light ejector is
`arranged to provide a preselected illumination profile across
`the optical output area of the top surface.
`
`55
`
`Another aspect of the invention relates to a lighting
`apparatus comprising a waveguide having pair of opposed
`
`surfaces. Each of the surfaces is at least partially reflective
`and at least one of the surfaces is partially transmissive.
`Each of the surfaces have a reflectivity greater than the
`transmissivity of the at least one surface.
`Another aspect of the invention relates to a lighting
`apparatus comprising a planar waveguide having a periph(cid:173)
`eral edge and a light source mounted proximate to the
`peripheral edge so as to direct light into the waveguide along
`a path extending from the light source towards an optical
`diverter in the waveguide. The optical diverter in the path
`redirects light rays away from the path towards the periphery
`of the waveguide.
`Yet another aspect of the invention relates to a lighting
`apparatus comprising a top surface, a bottom surface in
`spaced relationship to the top surface and cooperating with
`the top surface to form a waveguide having a thickness
`defined by the distance between the top and bottom surfaces,
`and at least one solid state point light source mounted to
`input light into the waveguide between the surfaces. One of
`the surfaces has a curvature relative to the other surface
`which yields a substantial variation in the thickness of the
`waveguide in a selected region of the waveguide. The
`variation follows a geometric contour selected to redirect
`light propagating between the surfaces of the waveguide so
`that
`the redirected
`light exits the
`top surface of
`the
`waveguide.
`
`Another aspect of the invention relates to a lighting
`apparatus comprising a waveguide having top and bottom
`surfaces and a peripheral edge. The waveguide has a thick(cid:173)
`ness defined by the distance between the top and bottom
`surfaces. The thickness at the peripheral edge is substantially
`different than the thickness in a region intermediate oppos(cid:173)
`ing sides of the peripheral edge. The thickness has a geom(cid:173)
`etry selected to enhance ejection of light from the top surface
`intermediate the opposing sides. At least one light source is
`disposed proximate to the peripheral edge to introduce light
`into the waveguide between the top and bottom surfaces.
`Yet another aspect of the invention relates to a lighting
`apparatus comprising a waveguide of solid material, the
`waveguide having a top surface, a bottom surface and a side
`surface. A light source is mounted to input light into the
`waveguide and reflective material is juxtaposed with one of
`the top and bottom surfaces wherein at least a portion of one
`of the top and bottom surfaces has a pattern of elongate
`structures that generally increase in density with distance
`from the light source.
`In yet another aspect of the invention, there is disclosed
`an illumination and display device comprising an optical
`waveguiding layer and an illumination coupler embedded in
`an
`interior region of
`the waveguiding
`layer. In one
`embodiment, the illumination coupler includes one or more
`semiconductor
`light emitting devices. A portion of
`the
`optical waveguiding layer has a pair of symmetric (a)
`nonplanar, curved surfaces, or (b) a plurality of flat, planar
`surfaces approximating the nonplanar, curved surface. The
`pair of symmetric surfaces form a cusp lying on the axis of
`the one or more semiconductor light emitting devices to
`produce total internal reflection of light from the one or more
`semiconductor light emitting devices into the waveguiding
`layer. Display elements are formed on surfaces of
`the
`waveguiding layer to cause light to be emitted from the
`waveguiding layer.
`
`Another aspect of the invention relates to an illumination
`and display device, comprising an optical waveguiding
`layer, with an illumination coupler embedded in an interior
`region of the waveguiding layer, wherein the illumination
`
`LGD_000509
`
`

`
`US 6,473,554 Bl
`
`coupler includes one or more semiconductor light emitting
`devices. Display elements formed on the surface of the
`waveguiding
`layer cause light to be emitted from
`the
`waveguiding layer.
`Yet another aspect relates to an illumination and display 5
`device, comprising an optical waveguiding layer with an
`illumination coupler embedded in an interior region of the
`waveguiding layer. In one embodiment, the illumination
`coupler includes one or more semiconductor light emitting
`devices, each of the one or more semiconductor light emit- 1 0
`ting devices having a longitudinal axis that is parallel to the
`surface of the optical waveguiding layer. A hole or recess
`may be formed in the interior region of the waveguiding
`layer where the one or more semiconductor light emitting
`devices is placed. The device also may comprise display
`elements formed on the surface of the waveguiding layer to
`cause light to be emitted from the waveguiding layer.
`
`1 5
`
`mined range of angles, and mounting the optical cavity to
`illuminate at least a portion of a room.
`In another aspect of the invention, there is disclosed a
`method of manufacturing a lighting apparatus. One embodi(cid:173)
`ment of the method comprises wrapping a flexible sheet of
`reflective material around one side of a tubular light source,
`juxtaposing a member forming an optical cavity with
`another side of the tubular light source so that light from the
`source is introduced into the optical cavity, and attaching the
`flexible sheet to the member such that the sheet retains the
`tubular source in juxtaposition with the member.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`These and other features of the invention will now be
`described with reference to the drawings of a preferred
`embodiment, which are intended to illustrate and not to limit
`the invention, and in which:
`A further aspect of the invention is directed to an illumi(cid:173)
`FIG. 1 is a perspective view of wristwatch incorporating
`nation device comprising a waveguide having an illumina(cid:173)
`one embodiment of a lighting apparatus;
`tion coupler embedded in an interior region thereof. The 20
`waveguide has generally parallel top and bottom surfaces
`FIG. 1A is a cross-sectional view of the watch of FIG. 1
`outside of the interior region such that light is guided
`taken along line 1A—1A;
`therebetween. The illumination coupler comprises a refrac(cid:173)
`FIG. 2 is a top plan view of the lighting apparatus used to
`tive index interface configured to capture light rays propa(cid:173)
`illuminate the watch of FIG. 1;
`gating along a line that forms less than the critical angle of 25
`FIG. 3 is a cross-sectional side view of the lighting
`total internal reflection with respect to at least one of the top
`apparatus of FIG. 2 taken along the line 3—3;
`and bottom surfaces, such that the captured light rays are
`FIG. 4 is a cross-sectional view of the lighting apparatus
`injected therebetween for propagation outside of the interior
`of FIG. 2 taken along the line 4—4;
`region. In one embodiment, the illumination coupler com(cid:173)
`FIG. 4A is an enlarged view of a portion of FIG. 4;
`prises a surface configured for total internal reflection of 30
`FIG. 4B is a cross-sectional view similar to that of FIG.
`light incident thereon. The illumination coupler of
`this
`embodiment is integrally formed with the waveguide from a
`4, but with elongate structures on the top surface;
`single piece of transparent material, and the reflecting sur(cid:173)
`FIG. 4C is a cross-sectional view similar to that of FIG.
`face is uncoated. A point source of light is disposed at least
`4, but with reflective material surrounding
`the entire
`partially, preferably fully, within a cavity formed in the 35
`waveguide;
`waveguide adjacent
`the total internal reflecting surface.
`FIG. 5 is a top plan view of an alternative embodiment of
`Display elements may be included on at least one of the
`a waveguide for use in the lighting apparatus of FIG. 2;
`surfaces for ejecting light from the waveguide. Additionally,
`FIG. 6 is a top plan view of yet another embodiment of a
`diffusive reflective material may be included on at least one
`waveguide for use in the lighting apparatus of FIG. 2;
`of the top and bottom surfaces.
`FIG. 7 is a top plan view of yet another embodiment of a
`waveguide for use in a lighting apparatus;
`FIG. 8 is a top plan view of a waveguide showing another
`embodiment of an optical diverter;
`FIG. 9 is a cross-sectional view of a lighting apparatus
`used with a light enhancing structure;
`FIG. 10 is a top plan view of a waveguide having a dimple
`for redirecting light rays;
`FIG. 11 is a cross-sectional side view of the lighting
`apparatus of FIG. 10 taken along the line 11—11;
`FIG. 12 is a perspective view of an alternate embodiment
`of a lighting apparatus;
`FIG. 13 is a cross-sectional view of the lighting apparatus
`of FIG. 12;
`FIG. 14 is a perspective view of a housing used with the
`lighting apparatus of FIG. 12;
`FIG. 15 is a perspective view of a lighting apparatus
`including a total internal reflection region;
`FIG. 16 is cross-section view of the lighting apparatus of
`FIG. 15 taken along the line 16—16;
`FIG. 16A is an enlarged view of a portion of FIG. 16;
`FIG. 17 is a perspective view of an alternative embodi(cid:173)
`ment of a lighting apparatus including a total
`internal
`reflection region;
`FIG. 18 is a cross-sectional view of the lighting apparatus
`of FIG. 17 taken along the line 18—18;
`
`40
`
`In yet another aspect of the invention, there is disclosed
`a lighting apparatus, comprising a device that emits light and
`an optical cavity that is formed by diffusive
`reflective
`surfaces, the cavity having an output region through which
`light from the cavity passes. The light emitting device is 45
`mounted to supply light to the cavity while being hidden
`from direct view through the output region. The cavity has
`a diffusely reflective surface area and the output region also
`has an area. The ratio of the area of the output region to the
`sum of (i) the area of the output region and (ii) the surface 50
`area of the cavity is at least 0.05. Additionally, the cavity has
`a depth and the output region has an edge to edge bisector
`dimension, the ratio of the depth to the bisector dimension
`being significantly less than 0.1. The lighting apparatus
`additionally comprises an angular spectrum restrictor posi- 55
`tioned to restrict output illumination through the output
`region to a predetermined range of angles.
`Another aspect of the invention involves a method of
`lighting. The method comprises producing
`illumination
`from an optical cavity formed by diffusely reflecting sur- 60
`faces and outputting illumination from the cavity through an
`output illumination region. Producing of the illumination
`comprises directing light rays from a source of illumination
`into the cavity such that the source of the illumination is not
`visible through the output illumination region. The method 65
`further comprises restricting the angular spectrum of illu(cid:173)
`mination from the output illumination region to a predeter(cid:173)
`
`LGD_000510
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`US 6,473,554 Bl
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`FIG. 19 is a schematic side view of a prior art "bullet lens"
`LED;
`FIG. 20 is a schematic side view of a prior art "bare"
`LED;
`FIG. 21 is a perspective view of an exit sign incorporating
`an alternative embodiment of a lighting apparatus;
`FIG. 22 is a rear perspective view of an automobile
`having taillights that incorporate a lighting apparatus;
`FIG. 23 is a top view of a wrist watch incorporating an
`alternative embodiment of a lighting apparatus;
`FIG. 24 is a cross-sectional side view of the exit sign of
`FIG. 21 taken along the line 24—24;
`FIG. 25 is a perspective view of an exit sign incorporating
`yet another embodiment of a lighting apparatus;
`FIG. 26 is a side view of an exit sign
`incorporating
`extractive display elements;
`FIG. 27 is a side view of an extractive display element;
`FIG. 28 is a side view of an alternative embodiment of an 20
`extractive display element;
`FIG. 29 is a perspective view of an exit sign incorporating
`circular grooves for extracting light; and
`FIG. 30 is a cross-sectional side view of the exit sign of
`FIG. 29.
`
`w
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`25
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`Referring to FIGS. 1 and 1A, one embodiment of the
`
`30
`
`shapes may be utilized for various applications. The light
`source 44 is mounted within a triangular or V-shaped notch
`50 in the peripheral edge of the waveguide 42. The notch 50
`serves as an optical diverter which redirects light transmitted
`by the light source 44, utilizing refractive index differences
`at the interface 52 formed by the sides of the notch 50. In a
`preferred embodiment, the V-shaped optical diverter 50 is
`symmetrical such that a line passing through its apex and the
`center of the waveguide 42 bisects the V-shaped notch.
`In a preferred embodiment, the light source is a single,
`solid state, point source of light, such as a light emitting
`diode (LED) 44 mounted on a carrier or circuit board (not
`shown). The carrier on which the LED 44 is mounted is
`attached to the waveguide at the back of the notch 50 (i.e.,
`the open end opposite the apex) utilizing an adhesive (not
`shown). The LED 44, which is typically in the form of a
`cube of solid state material that emits light from each of
`multiple faces (i.e., its top surface and four sides), is spaced
`from the sides of the notch 50 with air therebetween. The
`difference in index of refraction between the waveguide and
`the air creates the refractive index interface 52 that causes
`light to refract as the light passes into the waveguide 42. In
`accordance with this advantageous feature, a substantial
`fraction of the light passing through the interface 52 is
`refracted toward opposite sides of the waveguide 42 (i.e., the
`sides generally adjacent to and on opposite sides of the light
`source 44). In this manner, regions of the waveguide 42 that
`are located to the sides of the light source 44 are efEciently
`illuminated, and the diverter 50 thereby contributes to uni(cid:173)
`form illumination.
`
`The waveguide 42 is preferably comprised of a material
`that is transparent to light produced by the LED 44, such as
`a transparent polymeric material, and may be manufactured
`by various well-known methods, such as machining or
`injection molding. Preferred materials for the waveguide 42
`are acrylic, polycarbonate, and silicone. Acrylic, which has
`an index of refraction of approximately 1.5, is scratch
`resistant and has a lower cost relative to polycarbonate.
`Polycarbonate, which has an index of refraction of approxi(cid:173)
`mately 1.59, has higher
`temperature capabilities
`than
`acrylic. Polycarbonate also has improved mechanical capa(cid:173)
`bilities over acrylic. Silicone has a refractive
`index of
`approximately 1.43. The refractive index of air is 1.0. The
`dimensions of the waveguide 42 may vary, although the
`waveguide 42 is desirably very thin relative to its length so
`as to provide a low profile. The distance between the top
`surface 56 and the bottom surface 58 is preferably on the
`order of 1-3 mm, while the diameter of the waveguide 42 is
`typically at least 2.0 mm.
`
`As shown in FIG. 2, a plurality of display elements
`comprising elongate structures 54 extend across
`the
`waveguide 42 for redirecting light propagating within the
`waveguide 42. Although illustrated as lines, the elongate
`structures 54 have a three-dimensional shape, as described in
`detail below. In the illustrated embodiment, the elongated
`elongate structures 54 are arranged in a pattern consisting of
`intersecting lines that extend radially outward from a com(cid:173)
`mon point at the center of the waveguide 42 through which
`the stem 38 passes. The elongate structures 54 are preferably
`spaced apart by an equal angular distance, such as 0.3
`degrees. It has been found that the pattern of radial grooves
`utilized in the watch 20 are highly effective in eliminating
`"shadows" that would otherwise be cast by the watch stem
`38.
`As illustrated in FIG. 3 the waveguide 42 includes a top
`surface 56 and an opposed bottom surface 58, which are
`substantially parallel to each other. A side or edge surface 60
`
`,„
`
`45
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`5 0
`
`present invention is utilized as a back lighting apparatus 26
`for a wristwatch 20. Although the wristwatch 20 is shown
`having a generally circular shape, it will be appreciated that
`the wristwatch may have any of a variety of shapes.
`An outer housing 22 of the wristwatch 20 encloses a thin
`disk 30 of substantially transparent material that is spaced
`below a watch crystal 24. The disk 30 has an upper display
`surface 32 and an opposed bottom surface 34. The display
`surface 32 includes indicia 35 (FIG. 1), such as numerals for
`indicating time, although other indicia could also be dis-
`played. Two hands 36 of the watch 20 are mounted for
`rotation about a stem 38 that extends through the disk 30.
`The stem is driven by a watch mechanism 40 in a well
`known manner. Alternatively, the disk 30 may comprise a
`liquid crystal display in which indicia, such as the hands and
`numerals, are electrically generated.
`Referring to FIG. 1A, the lighting apparatus 26 is inter(cid:173)
`posed between the disk 30 and watch mechanism 40 for
`illuminating the disk 30. The lighting apparatus 26, which is
`preferably in contact with the disk 30, includes a waveguide
`42 and a light source 44 positioned along a peripheral edge
`of the waveguide 42.
`In the illustrated embodiment, the housing 22 supports the
`disk 30 and the lighting apparatus 26 in juxtaposed relation(cid:173)
`ship to each other. The disk 30 is mounted so that its 55
`periphery is supported by a first annular lip 46 which
`partially covers the display surface 32 to define a viewing
`area or illumination region of the display surface 32. The
`waveguide 42, which has a diameter slightly larger than that
`of the disk 30, is mounted so that its periphery is supported go
`by a second annular lip 48 in the housing 22. The second
`annular lip 48 is sized to shield the light source 44 from
`being directly visible through the illumination region of the
`display surface 32. By way of example, the diameter of the
`illumination region may be 27 mm.
`As shown in FIG. 2, the waveguide 42 has a circular shape
`optimal for illuminating a circular watch, although other
`
`65
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`US 6,473,554 Bl
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`5
`
`extends between the top and bottom surfaces 56, 58 along
`the periphery of the waveguide 42. While the surfaces 56,58
`are typically flat for backlight applications, the surfaces 56,
`58 may also be formed as curved surfaces, such as when the
`waveguide is utilized as a taillight for an automobile.
`A diffusive reflective material 62 is positioned adjacent
`the bottom surface 58 and side surface 60 of the waveguide
`42, with the material 62 also preferably covering a periph(cid:173)
`eral strip 61 on the top surface 56. In the embodiment
`illustrated, the peripheral strip 61 is sufficiently wide to 10
`cover the notch 50 so that the top, the bottom, and the
`outside edge of the notch 50 are covered by the reflective
`material 62. The peripheral strip is also preferably suffi(cid:173)
`ciently wide that the LED 44 cannot be viewed directly from
`viewing angles of 75 degrees or less (it being understood 15
`that the viewing angle is measured from a line normal to the
`top surface 56). By way of example, the peripheral strip may
`be equal in width to the width of the second annular lip 48
`of the watch 20 (FIG. 1).
`The diffusive reflective material 62, which has a reflec- 20
`tivity of at least 88% may comprise a single layer or multiple
`layers of diffusely reflective tape, such as DRP™ Backlight
`Reflector, manufactured by W. L. Gore & Associates.
`DRP™ Backlight Reflector has a reflectivity of approxi(cid:173)
`mately 97%-99.5%, depending on its thickness and the 25
`wavelength of the light. Alternatively, the reflective material
`62 could comprise a paint or coating that is applied to the
`surfaces 58 and 60, such as white house paint or a more
`exotic material, such as the Labsphere Corporation's Spec-
`traflect paint. Spectraflect paint's reflectivity is considerably 30
`higher than house paint, roughly 98%, while the reflectivity
`of a good white house paint is approximately 90%.
`Because the reflective material 62 covers the bottom and
`side surfaces 58, 60, as well as the peripheral strip on the top
`surface 56, light will be emitted from the waveguide in an 35
`output illumination region or aperture 65 comprising the
`central uncovered portion of the top surface 56 (i.e., the
`portion of the surface 56 interior to the peripheral strip 61).
`It will be appreciated that light rays incident on the top
`surface 56 at an angle of incidence (i.e., the angle of the ray 40
`relative to a line normal to the surface) at least equal to a
`critical angle will be totally internally reflected toward the
`bottom surface 58. That is, the top surface 56 will reflect all
`of such light back into the waveguide 42. Light rays having
`an angle of incidence less than the critical angle are trans- 45
`mitted through the top surface 56. T