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Vol 17 No 5 October/November 1992 THE LIBRARY OF T~E.
`OCT 2 91~~2
`
`Page 1 of 3
`
`TOYOTA EXHIBIT 1011
`
`

`

`LEOs for exterior lighting
`
`Until recently International regulations have mandated
`against the use of Light Emitting Diodes for exterior light(cid:173)
`Ing on vehicles. A terminology change recently ratified
`would appear to remove this barrier. Does this now mean
`that LEOs will become as commonplace on the outside of
`vehicles as they currently are on the Inside? Hewlett(cid:173)
`Packard is a leading supplier of optoelectronic devices
`and Martin Lister of the company's Components Group
`reviews the developments made In LED technology and
`their applications In automotive engineering.
`
`In the 1960's production processes
`were developed that allowed a p-n junc(cid:173)
`tion, traditionally only able to emit non(cid:173)
`visible Infra-red light, to emit light in the
`red area of the visible spectrum. This
`development led Hewlett-Packard to
`Introduce, In 1968, the first commercially
`available Light Emitting Diode (LED)
`display and so launch the era of a com(cid:173)
`pletely new generation of lighting prod(cid:173)
`ucts. It seems that ever since the LED
`was first introduced there has been
`speculation on if and when it would be
`used In automotive lighting applications.
`Within the driver's compartment devel(cid:173)
`opments have certainly been made,
`however, the use of LEOs for exterior
`applications has been less prevalent.
`This Is oot primarily due to the lack of
`technological development in LEOs as
`high-brightness devices now exist In a
`number of different colour combinations
`whose performance clearly fit them for
`exterior lighting applications. The bigger
`challenge to the take-up of LEOs has
`been regulatory. The lighting on vehi(cid:173)
`cles is governed by a large body of
`international legislation. Up until recently
`this legislation has been specific In its
`recommendations for exterior lighting; it
`has unequivocally specified that Incan(cid:173)
`descent lamps should be used. With this
`terminology In place it seemed that
`despite the technological developments
`taking place there would never be an
`opportunity for the use of LEOs on the
`outside of vehicles.
`However, over the past 12 months
`there has been a change to the termi(cid:173)
`.nology used in this legislation that
`appears to have removed the regulatory
`barriers. In amendments to the legisla(cid:173)
`tion, the terminology has been changed
`from 'incandescent lamp' to 'light
`source'. This now means that lighting
`devices other than filament bulbs can be
`used In exterior applications and this
`obviously Includes devices such as
`LEOs. So If the regulatory barriers have
`
`now been removed does this mean
`there will be a rush to design in solid
`state lighting on all new cars? To
`answer this question It is perhaps first
`appropriate to review the benefits to the
`vehicle designer of using these devices.
`The first and overwhelming advan(cid:173)
`tage of course is that of reliability. LEOs
`generate their light output from a solid
`state device as opposed to a white hot
`glowing filament. Intrinsically this implies
`greater reliability and this Is borne out by
`the data. LEOs are typically rated for
`over 50 000 hours of operating life com(cid:173)
`pared to a few thousand at best for
`incandescent lamps. In addition cars
`and trucks provide a very extreme envi(cid:173)
`ronment for any electrical device .
`Resistance to vibration and shock and
`extremes of temperature and humidity
`are essential for good reliability. The
`solid state LED easily outperforms the
`incandescent lamp with its fragile fila(cid:173)
`ment in this environment.
`One of the benefits to the vehicle
`manufacturer that result from this relia(cid:173)
`bility advantage is greater freedom In
`styling . LED lighting ·panels can be
`located on areas subject to shock and
`vibration where traditionally incandes(cid:173)
`cent lights would not be appropriate.
`
`Fig t. Critical re"ectance angles for conical
`straight walled reflector cavity
`
`The reliability of the devices also
`that they can be incorporated
`vehicle on a sealed-for-life
`designers do not have to
`slon for access to the device
`ing. It can thus be integrated
`body panels making for a
`more aesthetically-pleasing
`since LED lamps for vehicles
`pie light sources designers cal
`ment with lamp appearances
`crete 'points' of light to evenly lit
`for cosmetic lights or appliqu8s.
`On a direct part-for-part
`a LED light source is higher
`Its Incandescent
`price is not necessarily the best
`for comparison. LEOs promote
`lngs in other areas . This
`reduces design complexity.
`reduces sheet metal, additional
`parts, labour and other costs.
`significantly less power, up to
`of that used by Incandescent
`This gives the potential for lower
`lower weight electrical systems.
`generate lower levels of heat
`incandescent lamps, elim
`need for specialised high ... .,.,,...~,.
`resistant epoxies In the lamp
`Finally, of course, a sealed for
`lng system can significantly
`ranty costs and improve cust011l8(
`faction.
`There is one other advantage
`appears to be given by the use of
`In exterior lighting and this is
`not necessarily expected orklinalllf
`their manufacturers', they
`er than incandescent lamps.
`shown that when used in a
`they give an earlier warning to
`vers. The difference is between 130
`200 ms. At motor\Yay speeds
`equates to a car's length in
`tance. This can contribute
`safety and has the potential to
`the probability and severity of reat
`collisions.
`
`Page 2 of 3
`
`

`

`Recent technology advances
`Light is emitted from an light Emitting
`Diode due to the recombination of elec(cid:173)
`trons and holes inside the p-n junction.
`Each recombination results in the
`release of a photon of light. The materi(cid:173)
`als whose properties a.re well suited to
`exhibiting this phenomenon are those
`found in the third and fifth columns of
`the Periodic Table, the 111-V materials.
`Not all of the light released is seen by
`the observer, losses occur, typically in
`three areas. The first of these is loss
`due to absorption within the LED materi(cid:173)
`al. The photons released are emitted in
`all directions. If the substrate on which
`the junction is grown is opaque (as with
`Gallium Arsenide) only those photons
`emitted upwards and within a critical
`angle can be utilised as light output.
`The second cause of loss Is referred
`to as Fresnel loss. Fresnel loss is intro(cid:173)
`duced in the packaging of the LED. It
`occurs when light is reflected back at
`the interface of two materials whose
`index of refraction differ. The third cause
`of loss is Critical Angle loss. The effect
`of this is seen when an observer moves
`in relation to the LED. When the observ(cid:173)
`er moves off the axis of the device the
`light output decreases markedly. Since
`the phenomena that result In these loss(cid:173)
`es cannot be overcome LED develop(cid:173)
`ment has focused on making advances
`in the production of the 111-V materials
`and on the packaging used. For exam(cid:173)
`ple, Hewlett-Packard has developed
`devices using Aluminium Gallium
`Arsenide (AIGaAs) in which the sub(cid:173)
`strate Is etched away to eliminate pho(cid:173)
`ton absorption internally. The result is a
`Transparent Substrate device designat(cid:173)
`ed TS-AIGaAs. Using transparent sub(cid:173)
`strate technology HP has produced
`LEOs that give a light intensity 1 00
`times brighter than the traditional LED.
`Also, because the optical flux emitted
`by a LED is fairly low, LED manufactur(cid:173)
`ers in the past have introduced multiple
`LED die in a single package. Devices
`with 2 die have been common and
`some years ago HP introduced a device
`with 4 die. These• devices produced suf(cid:173)
`ficient optical flux to be used in high
`brightness applications but unfortunately
`their cost mandated against them. An
`alternative approach used by the com(cid:173)
`pany to package the single die is a 5 pin
`DIP.. Four of the pins are then used to
`conduct heat away from the cathode of
`the device allowing the lamp to operate
`at up to 100 rna of drive current without
`exceeding its power rating. In these
`series of Brewster lamps the LED die is
`mounted in an optical reflector providing
`improved optical performance over con(cid:173)
`ventional packaging techniques. The
`tamps use a deeper reflector dish allow(cid:173)
`ing them to catch the light emitted from
`the sides of the diode and direct It
`upwards, where a convex lens further
`
`:.ysrem
`
`:>)'Stem4_
`:system 2
`System 3
`~ c:... c.. .:==J
`•
`•
`•
`•
`~ ~ ~ c::a::J
`•
`•
`•
`•
`~ c:::::a -=a c=I:l
`0110
`
`Light
`source cover
`
`~·· c:::d
`
`receptor~
`Photorecepto
`cover
`Fig 2. 'Sweeping' turn-signal indicator pro(cid:173)
`posal by Stanley Electric with, below, their
`proposal for a combined LED rear lamp and
`proximity sensor
`collimates the light. The resulting optical
`output and radiation pattern provides
`significant improvements over conven(cid:173)
`tional indicators using the technology.
`The use of LEOs in high brightness
`applications requires good optical
`design principles to be followed in the
`design of the housing surrounding the
`lamps. The housing is more critical for
`diodes than for incandescent bulbs for a
`number of reasons. Firstly the lower
`optica.l output of these devices requires
`that none of this be wasted, care is
`need to ensure that the maximum
`amount of light from the unit strikes the
`legend area. Secondly incandescent
`lamps tend to have a radiation pattern in
`which light is emitted equally in all direc(cid:173)
`tions whereas, as noted above, the
`luminous intensity tends to peak in one
`direction.
`A properly designed white reflective
`optical cavity can utilise most of the opti(cid:173)
`cal flux emitted by a light source, the
`cavity redirects the light emitted at wide
`angles so that it strikes the rear of the
`legend. The performance of the cavity
`depends upon its geometry and equa(cid:173)
`tions can be derived to calculate the
`amount of light output exiting the hous(cid:173)
`ing after reflection from the cavity walls.
`For any optical cavity critical angles
`exist and these are shown in Fig 1 for a
`conical straight walled cavity. At angles
`less than Qd light rays emitted from the
`light source will not interact with the cav(cid:173)
`ity at all. Some rays will strike the cavity
`wall and reflect at zero degrees; this
`angle 0; is twice the cavity wall angle,
`Ow· At one angle 0 1 light rays will strike
`one cavity wall and graze the top of the
`other side of the cavity. Some fraction of
`the light will be trapped by the cavity, for
`angles greater than 0 1055, light will be
`reflected downwards. Finally some light,
`at angles greater than Om, will miss the
`entrance to the cavity altogether.
`Hewlett-Packard has conducted
`experiments with a number of cavity
`geometries Including round and square
`
`conical. As a result of these
`ments a cavity was developed
`a high mount brake light. The
`comprises a white, rectangular
`moulded cavity made from
`dioxide filled polycarbonate ..
`had straight walls with a 19
`angle. The exit aperture was 1
`15 mm and the entrance was
`square. The performance of
`was characterised using a
`LED lamps each measured with
`of drive current. For all
`sured, luminous intensities in
`1 000 candelaJm2 were recorded at
`end centre with 300 to 400 cdlffil al
`corners.
`The combination of ~'~"'''"'lni'\IN"*j
`the materials, die packaging,
`of the optical cavity together
`LEOs can now provide the nt~rfnrrMIIil
`required for exterior
`tions. Colour potent
`changed; originally LEOs only
`light in the red area of the
`Developments made in the
`have meant that today devices
`available in red, high efficiency
`orange, amber, yellow and
`Further developments being
`promise the availability of
`giving the potential, when
`with the other prfmary colours, of a
`state indicator that emits white lighl
`Exterior applications
`Virtually all of the major vehicle
`facturers now have
`today that are either 1nvest1gat1ng
`developing exterior lighting
`based on LEOs. One of the first of
`is for high-mount stop lights: their
`size, resistance to shock and low
`output (reducing nuisance to rear
`sengers) favouring their use here.
`convertible cars there is no rear
`screen in which to mount the
`resistance of diode lamps to shock
`vibration means that they can be
`ed on the boot lid and several
`turers, for example, are currently
`oping lights incorporated into the
`spoiler.
`Another application being
`investigated is for side m
`heavy goods vehicles, a harsh
`ment for lighting products. As
`life units LEOs outlast
`lights, keeping vehicles on the
`related application is for nir~~N~tm
`tor repeaters on cars. Ford in
`has recently announced their
`to use LEOs on the new Th1uncleltllnl
`model: in this case for cosmetic
`es, as applique lighting made
`through the use of the low
`vides an example of how the
`id state devices can expand the
`tions of lighting on vehicles
`LEOs need not be viewed as rAni-1WIIII
`for incandescent lights. Enter 169
`
`Page 3 of 3
`
`

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