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SAE TECHNICALPAPER SERIES 980008

A New Concept of Headlight: LightGuide Headlamp

Alban L’HerminéValeo Lighting Systems

Reprinted From: Automotive Lighting Technology(SP-1323)

International Congress and ExpositionDetroit, Michigan

February 23-26, 1998

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ISSN 0148-7191Copyright 1998 Society of Automotive Engineers, Inc.

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980008

A New Concept of Headlight: Light Guide Headlamp

Alban L’HerminéValeo Lighting Systems

Copyright © 1998 Society of Automotive Engineers, Inc.

ABSTRACT

A new headlamp concept is under development whichbrings innovation and style flexibility to automotive light-ing in addition to savings in overall volume.

Thanks to this technology named « Baroptic » (from theEuropean Research Program « Eureka »), the low beamfunction has been reduced to a light pipe 45 mm high,100 mm deep and 190 mm long. This volume is consider-ably reduced compared to a traditional low beam unit.

The «Baroptic» can be positioned vertically, horizontallyor inclined, depending on styling preference.

The flux emitted by the light source (halogen or HID) isnot reflected on the surface of the reflector (as with aComplex Shape or parabolic reflector) nor projecteddirectly on the road (such as with sealed beam or ellipti-cal projector). It is transmitted by a light pipe and thenprojected through lenses which are positioned along thepipe. These lenses in combination with shields, definethe characteristics of the beam, including width, length,distribution, cut off and homogeneity.

The results achieved, with either a European or Americanbeam match 70 mm height headlamps ( from traditionalconcepts). If the flux given by the comparable models areidentical (300 lm minimum), the new technology opti-mizes the distribution of the emitted light.

INTRODUCTION

Generally, headlamps are considered as a styling aspectof the vehicle. Car manufacturers’ stylists continuouslylook for innovation and differentiation.

Simultaneously, headlamp volume constraint and acces-sibility are becoming all the more important as sizereduction is required.

Those requirements have to be added to headlamp mainlighting functions which impact on safety and comfort forthe driver.

Therefore, the research associated with to headlampdesign leads to develop technologies that optimize thebalance between style, volume and of course lightingperformance.

As an example, low profile headlamps required by carmanufacturers cannot be designed with traditional con-cept without drastic and so far unacceptable photometri-cal losses.

This first low profile headlamp (45 mm useful height)uses a light guide concept compatible with halogen orHigh intensity discharge lamps to give a high qualitybeam distribution.

Figure 1. Useful aperture comparison

This new concept brings a new shape appearance bothlit and unlit, and offers the opportunity to be located in dif-ferent positions (slanted, horizontal...) in the vehicle.

LIGHT GUIDE CONCEPT

GENERAL – Traditional reflector concepts use parabolicshape reflectors for collection and projection. Then a lensspreads the light in front of it.

In Elliptical headlamps, collection is done by an ellipticalreflector and projection is done by the lens.

The light guide concept such as an optical fiber systemseparates collection and projection. Light is first collectedand injected in the light guide. After transport, it is thenprojected on the road from the light guide output. Onemajor critical issue is that fiber bundle flux coupling effi-ciency is low and not compatible with halogen lamp use.

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Figure 2. Global overview

The «Baroptic» concept can be described as follow :

Light is collected and coupled into the light guide. Thefunction is performed by a small elliptical mirror.

Light is then guided through large (relative to fiber) lightpipe and delivered at different outputs which can be con-sidered as secondary sources.

Finally light delivered by each sub output is projected onthe road through lenses.

LIGHT COUPLING – Derived elliptical shape reflectoroptimizes flux coupling from the light source into the lightguide.

The hot spot (or high radiance levels) have to be main-tained for long range performance.

TOTAL REFLECTION IN LIGHT GUIDE – The light guideis fabricated from a high transmission quality opticalmaterial. It’s refractive index of around 1.5 makes thatrays injected in the guide and arriving on the guide borderwith an angle over 42 degree reflect totally on it.

Light is guided thanks to total reflection in the guidewhich means minimum loss of light transport.

The light guide behaves as a device which averages radi-ance and increases uniformity.

The guide basic material is glass but, in some cases, canbe plastic.

LIGHT GUIDE INPUT OR OUTPUT OPTICALINTERFACE – Three kinds of guide interfaces can beconsidered, depending on deviation angle from the guidedirection to input or output

Simple refraction on material interface can be used forsmall angles.

Figure 3. Simple refraction guide interface

Refraction plus total reflection can be used for intermedi-ate angles.

Figure 4. Refraction+Total reflection on guide interface

Refraction plus reflection on coating can be used forlarge angles.

Figure 5. Refraction + Reflection on coating

The higher the angle, the lower is the coupling efficiency.

LIGHT PROJECTION AND BEAM DISTRIBUTION –The light provided by each guide output is projectedthanks to lenses or a mirror.

Different kinds of refractive elements can be used.

• Thick lenses .

• Fresnel lenses ( linearized or not )

• Reflectors are derived from parabolic reflectors.

Optical quality has to meet two main requirements

1. Give a maximum intensity for the hot spot.

2. Shield imaging with a high contrast quality (glare andhot spot close to each other )

BEAM DISTRIBUTION – Beam distribution is given bythe superposition of each lens projection.

Bulb+

reflector axis

Light GuideBeam

Bulb+

reflector axis

Light Guide

Bulb+

reflector axis

Light Guide

3

Figure 6. Beam distribution

DESIGN

FRESNEL LENSES – Fresnel lenses are often used inthe automotive industry, but mostly in rear lamps. Themain constraint in low beam headlamp functions is to becompatible with glare specifications.

Traditional Fresnel lenses are designed with a revolutionaxis.

For this reason we have developed a software dedicatedto calculate the shape of high quality Fresnel lenses, nolonger axis symmetric, but which can be tilted. Thosetilted Fresnel lenses have a promising future in this con-cept.

SIMULATION – Multi-reflection which occurs in the lightguide makes it difficult to have a first order evaluation ofthe photometrical quality.

Multi-reflection requires simulations. But one shallchoose a non sequential ray-tracing as many reflectionsoccur in the guide.

In addition the software has to be compatible with com-plex shape design.

The impact is that such software is all the more time con-suming as the complexity of the system increases. Forthis reason step by step simulations are used in the opti-cal design phases.

On one hand simulations are used to quantify the averag-ing effect on max intensity due to multi réflexion. Thispoint is a driving parameter in order to organize the beamwith a good max intensity. On the other hand it indicatesthe lens minimum dimension in order to collect all thepower emitted from the guide ouputs.

Modification of in house traditional headlamp design soft-ware has been done in order to fit with this need. Com-mercial software is becoming more and more efficientand likely to meet those new requirements in the future.

Figure 7. Non sequential ray tracing simulation

MATERIALS & TEMPERATURE – The light guide con-cept headlamp implies high radiance levels in the guide.High radiance implies high temperatures.

In the case of H7 lamp use it is necessary to have glassfor the light guide.

Molded glass doesn’t fulfill our high geometric accuracyrequirements. This is due to its high viscosity.

For this reason we use « float » glass for the guide.

High temperature resistant plastics are also necessaryfor Fresnel lenses in spite of their lower transmission.

Integrated sources such as halogen bulbs glued indichroïc reflectors will have a good impact on tempera-ture reduction on Fresnel lenses. In addition theirreduced volume will be an interesting advantage. Thosesources are under development for automotive applica-tions

For HID configurations thermal constraints are com-pletely different. Indeed arc lamps do not have the hugeinfra red emission ratio like halogen lamp ( close to black-body emission ).

FLEXIBILITY

CONFIGURATION FLEXIBILITY – The increased num-ber of elements brings more styling flexibility.

The light guide function is to bring light from the source toa remote position, before projection. This remote positioncan be horizontal or vertical.

This flexibility can be used to find the best fit with modernvehicle aerodynamic lines.

Light guide orientation

Figure 8. Horizontal configuration (Top view)

Figure 9. Horizontal slanted configuration (Top view)

Lamps can be either halogen or HID.

The high flux and radiance given by HID lamps allows toreach an additional step in headlamp challenged design.

Front

Side

Side

Front

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Figure 10. Vertical configuration ( Side view)

Figure 11. Modified lamp position (Top view)

The light source can be on the guide axis, or on an angle.

Unlike traditional concepts, lamp access can be donefrom the top. This allows a depth reduction.

Figure 12. Lamp replacement.

BEAM DISTRIBUTION FLEXIBILITY – Light distributiondepends on the way the different lens projections areorganized. This includes the amount of light, position andsize.

Global distribution and balance between maximum inten-sity and beam width can be adjusted through each imagepattern and position.

For example vertical translation of the reflector in front ofthe guides changes priority between hot spot and beamwidth.

STYLE FLEXIBILITY – Associated to the light guide con-figuration, the style can be modified in a wide range, giv-ing different night signature.

Fresnel lenses or thick lenses also give lot of styling free-dom.

Other variations can also depend on the geometry of theemission area that can be rectangular or following irregu-lar patterns.

In the validation plan, the horizontal and 30° slanted con-figuration has been considered as the first stage. It corre-sponds to one of the most used standard configuration.

RESULTS

STYLE – Different prototypes have already been evalu-ated and the ratio length to width is very much welcomedby designers and also by the common driver who neversaw such emission geometry during the night... and suchappearance during the day.

Figure 13. Concept car using technical aspects

Vehicle Stylists have decided, in the case shown on Fig-ure 4, to emphasize High Tech style and differentiationinstead of volume and low profile.

Figure 14. Vertical configuration

Figure 15. Horizontal configuration.

Other different styles can be considered regarding lensdesign required.

Front

Top

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PERFORMANCE

HID raw data correspond to measurements done on thesame configuration after changing H7 bulb to HID lamp.These large photometrical margins increase flexibility onother parameters.

Figure 16. Isolux measured on prototype

INSTALLATION – Depending on car manufacturer con-straints, priorities can be modified.

The following examples can be given.

Thickness reduction: Removing lamps by the top allowsus to benefit from the 100 mm depth. Without spare roomfor removal, this can be of high interest when short frontoverhang is required (Van or small town car.)

Vertical configuration: when installation it along thefender.

CONCLUSION

The availability of such a new concept in the styling tool-box, will bring innovation for the next generation of cars.

And further investigations on the concept enable to us toconsider:

• Compact lighting systems with both functions (lowand high beam) generated by only one HID lamp.

• Capacity for intelligent lighting bringing additionalfunctions like beams adapted to curves.

Performance table

Lamp Flux lm Max I Lux

H7 310 30

HID 900 70