AUTOMOTIVE PIXEL LIGHTING BASED ON PROJECTORS

Innovation speed in automotive lighting has been increased considerably within the last decade. Due to that there is a lack of time to do scientific research, especially in consideration of the effects of the lighting technologies on the human driver. At the Light Technology Institute (LTI) in Karlsruhe an automotive research head lamp is developed by which on the one hand there will be a safe of costs and time proving new head lamp technologies and on the other hand it will be possible to do comprehensive research studies at the human-machine interface (HMI).

AUTHORS

DIPL.-ING. STEFFEN MICHENFELDER

works as Scientific Assistant at the Chair

of Applied Lighting Technology, Department Optical Technologies in the Automobile (OTIA), at the KIT in Karlsruhe

(Germany).

PROF. DR. RER. NAT. CORNELIUS NEUMANN

is Head of the Department Optical Technologies

in the Automobile (OTIA) and General Lighting

Technology (LTI) at the KIT in Karlsruhe

(Germany).

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PEER REVIEWRECEIVED 2013-05-23REVIEWED 2013-07-14ACCEPTED 2013-08-13

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1 INTRODUCTION

The fundamental aim of automotive head lighting systems is pro-viding an appropriate sufficient illuminating of the traffic space during night-time driving. That is on the one hand a sufficient luminance to generate the contrast levels that are necessary for recognising people and relevant objects. And on the other hand it is indispensable to avoid causing glare effects on the other traffic participants. In this context, one speaks of the prevention of physi-ological as well as psychological glare.

Driving with the high beam being permanently on is, out of built-up areas, the best solution for the driver. But it’s obvious, that due to the high traffic density nowadays such a drive is not possible. To protect the opposing traffic as well as cars driving ahead an asymmetric cut-off line was established in 1957.

The adaption level of automotive head lamps has been increased by functionalities like bending light or AFS. Nowadays the head-lamps adapt to surrounding conditions like brightness, rain, fog as well as to the changing street geometries (urban road, rural road or highway). Current developments want to make the light distri-butions of the head lamps as similar as possible to the high beam, while preventing glare. With the help of camera technique other participants of the traffic are detected and either the light distri-bution is darkened at the participants’ position (vertical cut-off line) or the cut-off line is as close to the detected object as pos-sible (adaptive cut-off line).

The potential effects of the methods, which are supposed to lead to an increasing efficiency and power of automotive head lamps, on the human driver have to be analysed scientifically. In consideration of the increasing innovation speed, time and research funds are getting less and less. In the following paper the development and technical implementation of a research head lamp is described, which is supposed to be a multifaceted tool for rapid prototyping and physiological studies at the HMI. The latter could be e.g. a variation of the luminance or homoge-neity level in the near field and its effect on the visual ability of the driver.

2 AIMING AND METHODS

The research head lamp Propix continues the thesis of Rechentin [1], who built up the LED-head lamp Voxellight, consisting of 128 single modules, at the LTI in 2006. The LEDs of Voxellight could be operated with PWM and independently from each other. But the modules had to be arranged manually to create a certain light distribution. And the luminous flux of 3000 lm (comparable with halogen headlamps) cannot meet the conditions of current research projects. As a further stage six micro-mirror arrays are used in Propix, which provide a resolution of respectively two mil-lion pixels. The Propix head lamp system has been developed regarding the following aims: : Dividing the traffic space into a pixel array, being able to exam-

ine every possible light distribution. : The amount of pixels should be much higher than current LED

matrix beam head lights (that is to say 25 pixel in the Audi A8, round about 100 pixel in current research).

: Luminous flux higher than the one of every currently used light distribution (6600 lm at HID lamps) and the required light inten-sity has to be realised.

: Car sensor data (such as steering angle, velocity or push of a button) as well as camera data capable of being integrated.

: Light distributions of common formats being importable easily and quickly into the research head lamp system.

3 TECHNICAL IMPLEMENTATION

Due to the high amount of pixels and the concomitant degrees of freedom projectors with a full HD resolution have been used for the Propix system. Thereby two million pixels per projector can be switched. It has been assumed that for modelling current light distributions a maximum illuminance of 180 lx (at 25 m distance) is necessary [5]. The projectors were chosen due to the require-ments for the illuminance and spatial distribution (see 3.2) as well as due to ecological and logistic aspects. For the Propix system six projectors have been used, providing a respective light inten-sity of 30.000 cd. Totally approximately 30.000 lm (system out-put) can be used to generate light distributions. The projectors use UHP (ultra high pressure) lamps as light sources.

3.1 LIGHT SOURCESFor this application two principle light guiding techniques can be used. On the one hand the LCD and on the other hand the DLP digital projector. The unbearable characteristics of the LCD tech-nology are its high sensitivity for dust and dirt, its insufficient con-trast values and the memory effect. The latter means a burning in of the liquid crystals when using static images. A problem of the 1-Chip DLP technology occurs due to the rotation of the colour wheel which is used to generate colour images. If an observer is moving relatively to the projector picture a subjectively disturbing rainbow effect is observable. Due to this fact the colour wheels have been removed, whereby the possibility of analysing different spectral characteristics of the light distributions has been lost. 3-Chip DLP, that don’t have this colour effect, aren’t used due to their high weight, power consumption and costs.

3.2 SPATIAL ARRANGEMENTArranging the six projectors on a rack in front of the test car, 1, several issues have to be taken into account:

1 INTRODUCTION

2 AIMING AND METHODS

3 TECHNICAL IMPLEMENTATION

4 SUMMARY AND OUTLOOK

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: All relevant objects in traffic space have to be illuminated by the projectors. Vertically the area from the near field to the over-head signs of the motorways/Autobahn has to be covered, hori-zontally an angle of ± 45° is demanded [6], the Propix system has due to construction reasons only ± 32°.

: The illuminance values have to be high enough to display every common light distribution [5] and to still have further upside potential.

: There has to be a certain overlap of the projectors’ light emis-sion to calibrate the whole system.

The six projectors are arranged in a 2 × 3 array, two vertical and three horizontal levels, ①. The areas of overlapping are shown in ➋. The upper three projectors are mainly supposed to make a high beam or a marking light in the far field while the lower ones are responsible for low beam and light functions nearer to the car. In the hotspot (HV0) and around it the illuminance on a 25 m wall in front of the car can be set to a maximum of 259 lx, the offset illuminance is less than 0.18 lx. Thereby all requirements for ECE according light distributions can be fulfilled.

3.3 POWER SUPPLYProviding the energy, which is required by the Propix system, in the test car is a serious problem. The projectors respectively have

a power consumption of 370 W. Together with the control com-puter, displays and several electronic devices the total need of power is approximately 2.5 kW. To generate such an amount of energy a second light generator has been installed instead of the climate compressor. At 1500 rpm 2.4 kW are generated. The remaining amount of energy is provided by the generator that is installed natively in the test car. For running the projectors need a voltage of 230 V AC, wherefore the 12 V DC voltage of the light generators has to be transformed by converters. Due to electro-magnetic compatibility special devices are used, which have a with Emark classification. The CAN-bus of the test car has been ana-lysed and the results have been shown no diminishing of the sig-nal quality. ➌ shows the electrical arrangement and the installa-tion of the converters, batteries, contactors, switches, fuses and so on.

3.4 CONSTRUCTION OF THE ARRANGEMENTThe projectors have been positioned on an arrangement in front of the car to connect them fixedly to the test car. The cables for energy supply and image transmission via HDMI are in a special arranged car trench at the bottom of the car. Designing the rack the following issues have to be regarded: : The weight should be as low as possible to counteract vibrations

of the structure. : The construction has to be inflexible/stiff since the calibration

of the projector is very susceptible to the smallest displacements or rotations [2].

: The projectors have to be in the right position with the intended turn and tilt angles, so that the radiation pattern is given in accordance with ②.

: The projectors have to be protected from dust and wetness and the lenses have to be specially secured against falling rocks.

The basic framework has been made of aluminium profiles which are connected to each other via screws, brackets etc. The housing is made of thin stainless steel in which cut-outs for the electrical plugs, HDMI plugs, temperature sensors and ventilation grids have been integrated. The front covering of the rack has to protect the projectors from stone impacts but must not influence the light radiance. Common security and acrylic glass cannot be used since

➊ Schematic setting of the Propix test car: six projectors in front of the car, two data processors in the back and connections to the CAN bus of the car and to a scene camera for the detection of objects on the road

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➋ Emission of the projectors and their overlapping shown by different grey levels

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they are too brittle, leading to cracks in the material during a drive. The material of choice is a special polycarbonate which has a spec-tral non-selective transmission rate of 89 % and which is very resistant against distortion, ➍.

The projectors have been fixed via defined bore holes on a pro-file. Providing a stable connection the original covering of the pro-jectors had to be redesigned. All bearing devices, made of plastic in the original, have been replaced by aluminium components, ➎. Thereby especially the running conditions of the UHP lamp had to be taken into regard. If the surrounding temperature differed too much from the provided one, the lamp could take a severe dam-age. Therefore the temperature of the original as well as of the redesigned housing has been measured. Results show that the difference between both housings is within 5 %, which is consid-ered as acceptable. A thermal simulation of the whole rack includ-ing the projectors also showed that temperature conditions are kept within the rack, ➏. Confirming measurements at the real rack haven’t been done at this point of time.

3.5 CALIBRATIONWhile modelling light distributions it is essential that the behav-iour of a light ray is similar to the simulation. For this purpose the position of the projectors both relative to each other and to the test car has to be well-known. Due to the sharp image of the pro-jection system small deviations of the radiation angle or the spa-tial position leads to a clearly remarkable shifting of the projector images. In a first step a static calibration of the projectors has been developed. On a wall in a certain distance points are dis-played by the projectors. First of all, the vertical position of the points is regarded. The imaginary lines between the centre of the three projector lenses and the corresponding points on the wall have to been parallel to the ground, ➐. The vertical deviations from this line are determined by a laser system and integrated into the Propix software.

The horizontal deviation is determined in the following way: Again imaginary lines between the centre of the three projector lenses and the points on the wall are generated. These three lines are both parallel to the ground and parallel to each other. The dis-tance between the centres of the projectors (for example B1L and B1M in ➑) is the reference and is determined as well as the dis-tance of the points on the wall. Thereby the horizontal deviation of the projectors is corrected. The whole calibration is done for the first row as well as for the second row.

The advantage of the static calibration is the high precision of the calibration. The accurity is one pixel on the calibration wall, according to a angle accurity of 0.02°. But the calibration process is very time-consuming and it is not sure that the position of the projectors relative to each other doesn’t change while driving. Therefore at the moment research on an online calibration of the projector system is done. One possible solution is to use a mono camera in the test car which records certain test patterns, which are generated by the projectors, on the street. According to that the radiation of the projectors is modified dynamically.

3.6 IMAGE PROCESSINGCurrently, common head lamp systems use two light distributions, mostly identical ones. As displayed in ②, the whole picture of the Propix system consists of six single pictures, which overlap partly. To generate one continuous light distribution, every projector has

➌ Electrical installation in the back of the test car (left the fuse box with RCD circuit breakers, in the middle three inverters with contactors and fuses, right the battery and switch panel)

➍ Design CAD data of the rack

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to have an own picture. Therefore it was the task to develop an algorithm that converts the conventional light distribution into the six corresponding projector pictures. The algorithm has to take into regard for example the resolution of the input and the output system, the natural light distribution of the projectors and their grey alue characteristics. For more detailed information [2-4].

4 SUMMARY AND OUTLOOK

An automotive research headlamp has been built up, which is a multifunctional tool for doing research on automotive lighting. With the help of its pixel accurate, high resolution and the software based control it is possible to display every light distribution on the street – within the limitations of the Propix system.

Besides the online calibration, mentioned above, intensive research is done to integrate sensor data like the velocity, the steering angel and the image of a mono camera into the control software. Parallel to that, work is done with the aim of a real-time generation and manipulation of the images of the projectors.

REFERENCES[1] Rechentin, J.-M.: Systemdesign eines Kfz-LED-Scheinwerfers. Karlsruhe, Universität, Diplomarbeit, 2006[2] Michenfelder, S.; Neumann, C.: Entwicklung eines Projektor-basierten Forschungsscheinwerfers zur Erprobung neuartiger Lichtverteilungen. 5. VDI Fachtagung Optische Technologien, Karlsruhe, 2012[3] Michenfelder, S.; Neumeyer, M.; Neumann, C.: Conversion Algorithm for Automotive Lighting Research Head Lamp. 11. Internationales Forum für den lichttechnischen Nachwuchs (Lux Junior), Dörnfeld/Ilm, 2013[4] Michenfelder, S.; Neumann, C.: Pixel Lighting – An Automotive Lighting Research Head Lamp. International Symposium of Automotive Lighting (ISAL), Darmstadt, 2013[5] Huhn, Wolfgang: Anforderungen an eine adaptive Lichtverteilung für Kraft-fahrzeugscheinwerfer im Rahmen der ECE-Regelungen. Herbert Utz Verlag, München, 1999[6] Dahlem, Thomas: Methoden zur Bewertung von Kraftfahrzeugscheinwerfern. Herbert Utz Verlag, München, 2000

➎ Remodelling of the supporting parts of the projector housing made of aluminium instead of plastic previously used ➏ Simulating temperature flows inside of the rack

B1

B2 ∆b2

∆b1

Vertical

➐ Calibration of the vertical angle with the help of the distance of an imaginary line parallel to the ground

B1L

B1R

B1M

∆b1

∆b2

P

Horizontal

➑ Calibration of the horizontal angle with the help of the deviation of the distance of the points on the wall from the distance of the centres of the projector

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