United States Patent [191
`Johnson
`
`3,774,021
`[111
`[45] Nov. 20, 1973
`
`[54] LIGHT EMITTING DEVICE
`[75] Inventor: Bertrand Harold Johnson, Murray
`Hill, NJ.
`[73] Assignee: Bell Telephone Laboratories
`Incorporated, Murray Hill, NJ.
`May 25, 1972
`[22] Filed:
`[21] Appl. No.: 256,800
`
`[52] US. Cl .............. .. 240/2.1, 240/1 EL, 240/2.17,
`240/8.16, 313/108 D
`[51] Int. Cl. ......................................... .. G011! 11/28
`[58] Field of Search ................ .. 240/1 EL, 2.1, 2.17,
`240/41.1, 8.16; 313/108 D, 113
`
`[56]
`
`3,676,668
`2,538,475
`
`References Cited
`UNITED STATES PATENTS
`7/1972 Collins et a1 ................. .. 240/8.l6 X
`l/195l
`Skrastin, Jr. ................ .. 240/1 EL X
`
`2,835,789
`2,724,766
`3,596,136
`
`5/1958 Roper ........................ .. 339/125 L X
`11/1955 l-lawley et a]. .1
`........ .. 240/1 EL
`7/1971
`Fischer ......... ..
`313/108 D X
`
`3,450,870
`
`6/1969 Curl . . . . . . . . .
`
`. . . . . . . . . .. 240/8.16
`
`3,638,009
`
`l/1972 Strianese ...................... .. 240/8.16 X
`
`Primary Examiner—Richard M. Sheer
`Att0rney--W. L. Keefauver et a1,
`
`ABSTRACT
`[57 ]
`A light emitting diode module with a dome encapsu
`lant in which the geometry is designed to maximize
`the light output in the plane of the junction. The mod
`ule is adapted to couple light into a planar light guide,
`such as a telephone dial faceplate, so that discrete re
`mote regions of the faceplate can be illuminated. The
`objective generally is to illuminate several such re
`gions with a lesser number of light emitting elements.
`
`7 Claims, 7 Drawing Figures
`
`VIZIO 1007
`
`

`
`PAIENIEUnuv 20 1915
`
`SHEET 10F 3
`
`3.774.021
`
`VIZIO 1007
`
`

`
`v PAIENIEDuuvzo 1915
`
`SHEET 20F 3
`
`3,774,021
`’
`
`FIG. 3A
`
`VIZIO 1007
`
`

`
` mzmsnunveo ms
`
`3.'774;021
`
`SHEET 30F 3
`
`FIG 4
`
`
`
`HQ 5
`
`
`
`
`|/////A*€////// AIIIHWM
`
`'\\V
`
`VIZIO 1007
`
`VIZIO 1007
`
`

`
`1
`LIGHT EMITTING DEVICE
`
`3,774,021
`
`5
`
`There is currently a good deal of interest in illuminat
`ing dials and the like with so-called solid-state light
`emitting elements. In many cases, it is only essential to
`illuminate discrete regions of a relatively large area. If
`there are several such regions, too many to include a
`separate light emitting device in each, it is desirable to
`have a convenient way of coupling light from one, or
`a few, light emitting devices to several such regions.
`A speci?c application with this requirement is the il
`lumination of pushbuttons in telephone dials. Typi
`cally, there are 12 buttons, spaced apart in a 3 X 4
`matrix. Telephone line powering of the illuminating
`means is compatible with the use of only a few light
`emitting diodes. An ef?cient means of coupling the
`light from one or a few diodes to all 12 buttons is the
`speci?c objective of this invention. In its broader as
`pects, it proposes a means for coupling light between
`a number of discrete regions in a dial display or the like
`and a lesser number of light emitting devices.
`The coupling means is an essentially planar light
`guide that intersects the regions to be illuminated. The
`light emitting devices are designed according to princi
`ples of internal reflection so that the predominant por
`tion of the light produced in each diode is emitted in a
`confined plane and with a uniform flux over 360°. The
`light can thereby be coupled efficiently into the light
`guide.
`These and other aspects of the invention will become
`more apparent from the following detailed description.
`In the drawing:
`FIG. 1 is a front-sectional view of a light emitting de
`vice module combining the usual diode chip with an en
`capsulating dome that directs a predominant portion of
`the light emitted from the diode into the horizontal
`plane normal to the section;
`FIG. 2 is a plan view of the module of FIG. 1;
`FIGS. 3A, 3B and 3C are schematic diagrams show
`ing by simple ray optics the behavior of light in the de
`vice of the invention;
`FIG. 4 is a plan view of the planar light guide which,
`for the particular application illustrated, is a faceplate
`for a pushbutton telephone dial; and
`FIG. 5 is a front section through 5—5 of FIG. 4 show
`ing the interface coupling the light from the light emit
`ting device into the light guide and from the light guide
`into a pushbutton to be illuminated.
`Referring ?rst to FIG. 1, the light emitting module 10
`includes the usual type of light emitting device. In this
`particular device, there are two light emitting diodes 1 l
`and 12, so that at least one diode will be illuminated re
`gardless of the polarity of the DC bias. This feature is
`a consequence of the particular system that powers the
`light emitting elements and is in no way essential to the
`invention. Two diodes connected back-to-back may
`also ?nd use with A.C. power. The diodes 11 and 12
`are mounted on a lead frame that comprises tabs 13
`and 14. The tabs can be connected to an appropriate
`power source for activating the light emitting devices.
`The bonded surface of the diodes forms one electrical
`connection with the other provided by bonded wires 15
`and 16.
`v
`The dome encapsulant 17 is molded around the lead
`frame and the diodes in a conventional manner. The
`encapsulant can be any of several optically transparent
`materials, such as'a silicone, epoxy or acrylic resin. The
`
`2
`shape of the dome is an important feature of the inven
`tion. The sidewall 18 can be cylindrical or conically
`shaped but in either case shaped so that all light ema
`nating from the diodes that is incident on the wall on
`the first pass intersects at an angle less than the critical
`angle and thus escapes the sidewall. While it is evident
`that light leaving the dome through this surface will
`have a large angular spread, this is acceptable within
`certain boundaries as will be seen when the dome is
`coupled to the light guide. It is pertinent to note that
`the angular spread is a function of the height of this
`sidewall and the distance separating the sidewall from
`the light emitting device. A decrease in the former or
`an increase in the latter will produce a narrower angu
`lar spread in the first pass output beam through the
`sidewall, but will result in greater losses through the
`ceiling of the dome. The invention is largely aimed at
`reducing the latter, although it is evident that various
`tradeoffs occur. For the purpose of de?ning the inven
`tion, it is sufficient at this point to require that all first
`pass rays emanating from the light emitting devices and
`incident on the sidewall meet the sidewall at an angle
`less than the critical angle.
`The refraction and re?ection of first pass rays inci
`25'
`dent on the ceiling of the dome is somewhat more com
`plex. The objective of this dome design is to minimize
`loss of light through this surface. Light re?ected from
`this surface has a high probability of exiting the dome
`through the sidewall 18, the ultimate goal.
`The behavior of light rays incident on the ceiling of
`the dome can be analyzed readily if the shape of the
`ceiling is assumed to be an inverted cone. The devia
`tion from the conical shape that is evident in the dome
`shown in FIG. 1 results from various fabrication consid
`erations, most notably, the susceptability of sharp
`edges or points in plastic casting dies to wear, and the
`consequences in molded bodies of the presence of
`acute local strains induced by geometrical extremes. In
`a simple ray optics analysis, there are two relevant vari
`ables in this structure —- the distance of the apex of the
`inverted cone from the light emitting surface, and the
`slope or conicity of the conical surface. Given a height
`and position of the sidewall 18 with respect to the light
`emitting surface, only one of the parameters is variable.
`The height of the sidewall corresponds, as will be seen,
`to the thickness of the faceplate,iand, therefore, is a
`likely parameter to be fixed by non-optical design crite
`ria. However, we have already noted that a maximum
`height exists for the sidewall to allow exit for all rays in
`cident on the ?rst pass. With this as a starting point,
`consider the simple model shown in FIG. 3A. The side
`wall 18 is assigned dimension y and the distance sepa
`rating the base of the sidewall and the light emitting de
`vice 11 is x. The angle 92 of the sidewall is picked to
`be 90°, a convenient end point. (The sidewall can as
`sume a negative slope with an analysis similar to the fol
`lowing. However, no advantage is seen in that struc
`ture.) The angle 61 is the maximum angle of incidence
`from a ?rst pass ray r,. The limiting condition for this
`60
`case is simply:
`61 > 90°—¢= tan_‘(X/y)
`where d) is the critical angle.
`It is quickly evident that y can exceed the length
`shown as‘ 92 decreases. The limiting case is'that in
`vwhich 62 = 90°— (1). By symmetry, the limiting case is
`that in which the sidewall is normal to the ray r. The
`
`50
`
`30
`
`35
`
`45
`
`55
`
`65 '
`
`VIZIO 1007
`
`

`
`3,774,021
`
`4
`the buttons are electrically active through capacitive
`coupling. In the specific structure shown, the faceplate
`is provided with holes 22 that accommodate pushbut
`tons, only one of which is shown at 24. The pushbut
`' tons are constructed of translucent plastic, which col
`lect the light from the waveguide and, through disper
`sion of the light within the translucent plastic, e?‘ec
`tively illuminate numerals formed on the surface of the
`buttons. The numerals indicate telephone dial ‘num
`
`bers.
`
`>
`
`I
`
`'
`
`20
`
`3
`important fact is that, as the sidewall is sloped and
`more of the output (in an angular sense) of the light
`emitting element exits through the sidewall, more of it
`also exits in directions approaching normal to the plane
`of the light guide and this is contrary to our assigned
`goal.
`At this point, it is helpful to consider the slope of the
`sidewall with reference to the light guide. An exem
`plary combination is schematically shown in FIG. 3B.
`The relevant portion of the light guide is indicated by
`20 and comprises a part of an essentially planar sheet
`of plastic (from the standpoint of optics, the guide will
`tolerate considerable curvature) or other appropriate
`transparent material with a beveled edge 21 adapted to
`mate approximately with sidewall 18. The interface be
`tween these elements will result in some optical loss de
`pending upon the care in matching the geometry of
`these surfaces.
`Referring to FIG. 3B and assuming that the critical
`angle = 92, a series of light rays subtended by the arc
`designated 9L will be lost through the surface of the
`light guide 20 because they are incident along the re
`gion of 20 indicated by the dimension “L3? This is a
`geometric consequence of the slope of the sidewall 18.
`(The dimension x remains fixed.) There are at least
`three ways of overcoming this loss. An obvious one is
`to reduce the thickness dimension y so that the top
`edge, where the light guide and the dome intersect, oc
`curs at the point of incidence of ray r2. However, the
`thickness dimension may be in?exible in which case the
`geometry may be adjusted as in FIG. 3C. Now the prob
`lem rays are incident on the dome of the structure and
`can be coupled into the guide 20 as shown. It is evident
`that a third alternative lies in adjusting the dimension
`x to achieve a similar result. Indeed, the three parame
`ters just discussed can be varied within reasonable lim
`its to obtain the desired result. That result is most, easily
`expressed as ensuring that all rays, emanating from the
`light emitting element, strike the top edge of the light
`guide, where it interfaces with the internally re?ecting
`dome of the light emitting element, at an angle greater
`than the critical angle and are thereby re?ected on the
`first pass. Any rays that are re?ected on the first pass
`within the light guide have a high degree of probability
`of being coupled into the region in which illumination
`is desired.
`The light guide, which in the preferred application
`serves as'the faceplate of a pushbutton dial, is shown in’
`plan view in FIG. 4, with the section 5—5 shown in
`50
`FIG. 5. This-faceplate has 12 holes 22 to accommodate
`the pushbuttons. The side edges of the faceplate are
`provided with re-entrant portions 23 so that light rays
`incident along this edge will be reflected and retained
`within the guide. It will be appreciated that, if the mate
`rial of the faceplate has a normal index of refraction
`characteristic of transparent plastic materials, i.e.,
`>>1,then virtually every ray incident on a major sur
`face of the light guide will be internally reflected. Ef?
`cient coupling of the light into the guide from the light
`emitting elements 17 was discussed in connection with
`FIGS. 2 and 3. Coupling of the light from the guide into
`the regions to be illuminated depends on the objective.
`For example, the regions indicated at'22 could consist
`of inserts of a material with high-light scattering. These
`could be regions of the plastic faceplate roughened by
`chemical etching or mechanical abrasion. Either of
`these alternatives would be appropriate, for instance, if
`
`The arrangement of the light emitting elements with
`respect to the 12 regions to be illuminated is particu
`larly effective. The light emitting elements are located
`approximately at the center of the two squares formed
`by interconnecting the nearest neighbors in the middle
`two rows of the 3 X 4 matrix. Light emitted from the
`elements 17 will either be internally reflected from the
`edge of the guide or will be incident on one of the push
`buttons 24 on the first pass. It is evident that each of the
`openings 22 will be exposed to substantial light.
`It will be evident from FIG. 3A that the light emitting
`module and the opening in the faceplate into which it
`extends can be cylindrical rather than conical.
`It is significant to recognize that light can be coupled
`25"
`readily between discrete regions, such as those de
`scribed, by using re?ective coatings. However, the
`structure described here achieves equivalent results
`without the expense of such coatings.
`Various additional modifications and extensions of '
`30'
`this invention will become apparent to those skilled in
`the art. All such variations and deviations which basi
`cally rely on the teachings through which this invention
`has advanced the art are properly considered to be
`within the spirit and scope of this invention.
`35
`What is claimed is:
`1. An illuminated faceplate in ‘which a number'of dis
`crete regions of the faceplate are illuminated with light
`from a lesser number of light emitting elements com
`, prising:
`a ‘substantially planar sheet of transparent plastic
`having a plurality of spaced-apart regions to be illu
`minated, the sheet having at least one approxi
`mately circular opening through its thickness,
`at least one light emitting device ?tted within each
`opening of the sheet, each light emitting device
`comprising!
`at‘lea'st one light emitting diode,
`electrical leads for contacting the diode,v
`a transparent plastic dome encapsulating the diode
`and a portion of the electrical leads while leaving
`a portion of the ‘electrical leads exposed,
`the plastic dome having a peripheral geometry corre
`sponding approximately to that of the opening in
`the sheet, and when inserted within the opening
`having a top surface exposed through the opening
`in the surface of the sheet, the top surface having
`a depression therein extending toward the light
`emitting diode to form an approximately conically
`shaped, internally reflecting surface with respect to
`the light emitting diode, the conically shaped sur
`face having an apex angle such that all of the light
`incident on the dome except that essentially atthe
`apex will be internally reflected and coupled into
`the sheet and the periphery of the plastic dome
`having a geometry with respect to the light emitting
`diode such that substantially all the light incident
`on the periphery is coupled into the sheet.
`
`45"
`
`60
`
`65
`
`VIZIO 1007
`
`

`
`3,774,021
`
`6
`
`5
`2. The faceplate of claim 1 in which the transparent
`plastic dome encapsulating the diode has a cylindrical
`shape with the depression formed into the top surface.
`3. The faceplate of claim 1 in which the transparent
`plastic dome encapsulating the diode has the shape of 5
`a truncated cone with the depression formed into the
`top surface.
`4. The faceplate of claim 1 in which the discrete re
`gions of the faceplate to be illuminated comprise holes
`in the faceplate with pushbuttons extending through
`the holes.
`5. The faceplate of claim 4 including two light emit
`
`ting elements.
`6. The faceplate of claim 5 having 12 holes arranged
`in a 3 X 4 matrix with the two light emitting elements
`coupled to the sheet approximately at the center of the
`two squares formed by connecting nearest neighbors of
`the two middle rows of three holes in the matrix.
`7. The illuminated faceplate of claim 1 in which the
`edges of the sheet are provided with re-entrant portions
`that internally re?ect light incident on these portions
`and reduce light loss.
`* * * i1
`
`*
`
`l5
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`VIZIO 1007