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`‘ Automotive
`Engineer
`
`Vol17N05 October/November1992
`
`THE LIBRARV OF THE
`
`OCT 2 9 1992
`
`UNIVERSITY OF |LL|NOIS
`URBANA-CHA' ”RIGN
`
`
`
`Page 1 0f 3
`
`TOYOTA EXHIBIT 1011
`
`Page 1 of 3
`
`TOYOTA EXHIBIT 1011
`
`

`

`DESIGN PERSPECTIVE
`
`LEDs for exterior lighting
`
`
`
`Until recently international regulations have mandated
`against the use of Light Emitting Diodes for exterior light-
`lng on vehicles. A terminology change recently ratified
`would appear to remove this barrier. Does this now mean
`that LEDs will become as commonplace on the outside of
`vehicles as they currently are on the Inside? Hewlett-
`Packard is a leading supplier ot optoelectronic devices
`and Martin Lister ot 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-
`tion. traditionally only able to emit non-
`visble Infra-ted light, to emit light in the
`red area of the visible spectrum. This
`development led Hewlett-Packard to
`lmroduce, In 1968. the lirst commercially
`available Light Emitting Diode (LED)
`display and so launch the era cl a com-
`pletely new generation of lighting prod-
`ucts.
`It seems that ever since the LED
`was lirst introduced there has been
`speculation on it and when it would be
`used In automotive lighting applications.
`Within the driver's compartment devel-
`opments have certainly been made,
`however. the use or LEDs for exterior
`' applications has been less prevalent.
`This Is not primarily due to the lack of
`technological development in LEDs as
`high-brightness devices now exist In a
`number 01 ditterent colour combinations
`whose perlormance clearly lit them tor
`exterior lighting applications. The bigger
`challenge to the take-up of LEDs has
`been regulatory. The lighting on vehi-
`cles Is governed by a large body of
`lntemational legislation. Up until recently
`this legislation has been specific in its
`recommendations for exterior lighting; it
`has unequivocally specified that incan-
`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 tor the use oi LEDs on the
`outside cl vehicles.
`However. over the past 12 months
`there has been a change to the termi-
`nology used in this legislation that
`appears to have removed the regulatory
`barriers. In amendments to the legisla-
`tion. the terminology has been changed
`Irom ‘incandescent lamp' to ‘Iight
`source'. This now means that lighting
`devices other than filament bulbs can be
`used in exterior applications and this
`obviously Includes devices such as
`LEDs. So it 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 lirst
`appropriate to review the benefits to the
`vehicle designer of using these devices.
`The lirst and overwhelming advan-
`tage of course is that of reliability. LEDs
`generate their light output Irom a solid
`state device as opposed to a white hot
`glowing filament. lntrinsically this implies
`greater reliability and this is borne out by
`the data. LEDs are typically rated tor
`over 50 000 hours of operating lite com
`pared to a law thousand at best for
`Incandescent lamps. In addition cars
`and trucks provide a very extreme envi-
`ronment Ior any electrical device.
`Resistance to vibration and shock and
`extremes of temperature and humidity
`are essential tor good reliability. The
`solid state LED easily outperforms the
`incandescent lamp with its fragile fila-
`ment In this environment.
`One of the benefits to the vehicle
`manufacturer that result from this relia-
`bility advantage is greater Ireedom in
`styling. LED lighting-panels can be
`located on areas subject to shock and
`vibration where traditionally incandes-
`cent lights would not be appropriate.
`
`
`
`Fig I. Critical reflectance angles to! conical
`straight walled reflector cavity
`
`
`
`The reliability of the devices -: ..
`that they can be incorporated ‘
`vehicle on a sealed-Ior-Ilie v
`
`.
`designers do not have to
`
`sion for access to the device ..
`J~
`ing. It can thus be integrated
`
`body panels making tor a-‘s ?
`
`.
`more aesthetically-pleasing
`
`since LED lamps for vehicles.
`7 ,
`pie light sources designers cat .
`
`ment with lamp appearances" -,
`crate ‘polnts' at light to evenly]
`
`tor cosmetic lights or appliques
`On a direct part-lor-part .. '
`
`.- -.
`a LED light source is higher -
`its incandescent counterpart.
`'.
`
`price is not necessarily the best
`
`tor comparison. LEDs promote ..
`
`ings in other areas. This tell
`
`reduces design complexity. .
`reduces sheet metal, additional ,
`
`.
`.
`parts. labour and other costs.
`significantly less power. up to one
`
`of that used by Incandescent
`This gives the potential for lower a
`
`lower weight electrical systems. 7
`
`generate lower levels of heat
`incandescent lamps. ellrnlnatl
`i:
`
`need for specialised high tem- ,,
`
`resistant epoxies in the lamp -
`Finally. oi course. a sealed for lite
`.
`
`ing system can significantly reduce
`ranty costs and Improve customer -_
`
`faction.
`There is one other adv
`appears to be given by the use cl
`.-
`
`in exterior lighting and this is ,. ’:
`not necessarily expected orlgl 2‘
`their manulacturers'. they turn on v’
`or than incandescent lamps. Tests 0
`shown that when used in a step
`they give an earlier warning to w '
`vers. The dltterence ls between I
`I:
`200 ms. At motorway speeds
`equates to a car's length in stqapig
`lance. This can contribute dir
`salety and has the potential to :-'
`the probability and severity 01 rest
`collisions.
`
`’
`
`Page 2 0f 3
`
`n-...s. AUA'AIM“
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`Page 2 of 3
`
`

`

`Recent technology advances
`Light is emitted from an Light Emitting
`Diode due to the recombination of elec-
`trons and holes Inside the on Iunction
`Each recombination results in the
`release of a photon of light The materi-
`als whose properties are well suited to
`exhibiting this phenomenon are those
`found in the third and fifth columns of
`the Periodic Table, the Ill-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-
`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 ls Intro-
`duced in the packaging oi the LED.
`It
`occurs when light is reflected back at
`the interface of two materials whose
`index of refraction differ. The third muse
`oi loss is Critical Angle loss. The etiect
`of this is seen when an observer moves
`in relation to the LED. When the observ-
`er moves off the axis of the device the
`light output decreases markedly. Since
`the phenomena that result In these loss-
`es cannot be overcome LED develop-
`ment has focused on making advances
`in the production of the lll-V materials
`and on the packaging used. For exam-
`ple. Hewlett-Packard has developed
`devices using Aluminium Gallium
`Arsenide (AlGaAs) in which the sub-
`strate is etched away to eliminate pho-
`ton absorptlon internally. The result Is a
`Transparent Substrate device designat-
`ed TS-AIGaAs. Using transparent sub-
`strate technology HP has produced
`LEDs that give a light intensity 100
`times brighter than the traditional LED.
`Also. because the opticd llux emitted
`by a LED is fairly low. LED manufactur-
`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 sui-
`flcient optical flux to be used In high
`brightness applications but unfortunately
`their cost mandated against them. An
`alternative approach used by the com-
`panytopackagetheslngledielsaSpin
`DIP. Four of the pins are then used to
`conduct heat away from the cathode of
`the device allowing the lamp to operate
`atuptoiOOmaofdrivecurrentwithout
`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-
`ventional packaging techniques. The
`lamps use a deeper reflector clsh allow-
`ingthemtocatchthe lightemittedfrom
`the sides of the diode and direct It
`upwards. where a convex lens further
`«I
`
`Page 3 0f 3
`
`
`
`
`
`receptor
`thtoufl
`cover
`'Sweeping' turn-signal Indicator pro-
`Fig 2.
`posal by Stanley Electric Ivith. 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-
`tional indicators using the technology.
`The use of LEDs 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
`optical 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-
`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-
`cal ilux emitted by a light source. the
`cavity redirects the light emitted at wide
`angles so that It strikes the rear of the
`legend. The periormance of the cavity
`depends upon Its geometry and equa-
`tions can be derived to calculate the
`amount of light output exiting the hous-
`Ing alter 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 0., light rays emitted from the
`light source will not interact with the cav»
`ity at all. Some rays will strike the cavity
`wall and reflect at zero degrees; this
`angle 0,- is twice the cavity wall angle.
`Ci, At one angle 0, 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 Ob“. light will be
`reflected downwards. Finally some light.
`at angles greater than 0,”. 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" I.
`
`merits a cavity was developed
`'
`a high mount brake light
`
`comprises a white. r «r:
`.
`moulded cavity made from.
`
`dioxide tiled polycarbonate.
`had straight walls with a 19 1
`
`angle. The exit aperture was
`.
`15 mm and the entrance was "
`
`square. The performance of
`
`was characterised using a
`LED lamps each measured will
`of drive current. For all lam"
`
`sured. luminous intensities in
`
`1000 candela/m2 were recorded
`
`and centre with 300 to 400 ell/fl)2 .
`corners.
`‘.I.
`The combination of do --
`
`the materials. die packag'llil. and
`
`of the optical cavity together
`LEDs can now provide the
`required for exterior lighting
`
`tions. Colour potential has".
`
`changed; originally LEDs only _
`1'
`light in the red area of the :-~_.
`
`Developments made in thet .
`=
`have meant that today devlfll .
`
`available in red. high eliicieiicy
`
`orange. amber. yellow and-I
`Further developments being indie
`promise the availability of blue
`
`-' ..
`giving the potential. when
`with the other primary colours. i
`state indicator that emits whle -
`
`
`Exterior applications
`Virtually all of the major vehicle
`
`-
`lecturers now have projects
`today that are either investi--
`
`developing exterior lighting
`:-
`
`based on LEDs. One of the firsts“
`
`is for highmount stop lights: ital;
`
`size. resistance to shock and loft
`
`output (reducing nuisance to red?
`
`sengers) favouring their use here. 1
`convertible cars there is no rear
`'
`
`screen in which to mount the v ?
`
`resistance of diode lamps to shear.
`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 aw '
`investigated is for side markers
`heavy goods vehicles. a harsh
`ment for lighting products As as
`life units LEDs outlast incand
`lights. keeping vehicles on the 3.:-
`related application'Is for direction
`tor repeaters on cars. Ford In the
`has recently announced their » w _
`to use LEDs on the new Thu -:.-
`model: in this case for cosmetic« i.
`es. as applique lighting made . -. "
`through the use of the low profile.
`videsanexampleof howthe use‘ii
`id state devices can expand the .‘.
`tions of lighting on vehicles andj
`LEDsneednotbeviewedas .....,
`for incandescent lights. Enter l69= ’
`
`October/Novunliei“: .
`
`Page 3 of 3
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