One light source per headlight
The new BMW 5 Series headlights have a ten watt light bulb as the light source, located in the back of the headlight. Four flexible glass fiber light guides transmit the light through the headlight housing to the light rings. The rings are thin light guide rods into which the light is fed from opposite directions through two input points, to achieve an even illumination all around.
A demanding challenge
Before this design could be converted into reliable technology, a whole range of demanding challenges had to be faced up to. Without the use of glass fiber light guide technology it would have been impossible to achieve success. As the temperature in the headlight housing gets as high as 150°C, the light source could not be located directly on the ring. As plastic light guides would not stand up to the high temperatures involved either, only glass fiber light guides could be considered to connect the light source and the ring. These will resist temperatures of up to 350°C for many years without any problems or adverse effect on their optical properties.
The conventional sheathing used to protect the glass fiber bundles proved to be less resistant and would not have been able to withstand the extreme conditions in the headlight housing. Working jointly with the Hella specialists, SCHOTT looked for a suitable alternative material that not only had to meet the technical requirements but that was also acceptable from a cost point of view. The new sheath material chosen was a thermoplastic elastomer (TPE) which met both conditions excellently. In addition TPE like glass fiber is chemically resistant to motor oil and battery acid, two corrosive liquids which the materials could come into contact with at any time in the engine compartment.
The high flexibility of the glass fiber light guides proved to be a further benefit in design and manufacture. Glass fibers can be manufactured considerably thinner than plastic which results in greater flexibility, a property which is especially useful in the extremely tightly configured space in the headlight housings. As headlights also have to be adjustable, many of the components in the housing, such as the reflector for example, require a certain amount of “play”, which increases the problem of space even more. “The enormous flexibility of the glass fiber light guides gave our designers extra design freedom”, says Hella’s Ralf Appelbaum, explaining one of the decisive benefits of glass for this project.
Joint development work