Lighting devices of this type are arranged in the interior and exterior regions of motor vehicles and serve as interior lighting, reading lamps and lamps which are arranged in the region of the doors and which illuminate the ground around the vehicle in darkness in order to enhance the safety and the comfort of the users of the vehicle in the process of entry. By means of these lighting devices which are known as such, fouled areas and, in certain circumstances, sources of danger located beneath the vehicle can be recognized more easily in addition, particularly in a dark environment, the subjective well being of the users of the vehicle is enhanced by the illumination of the environment of the vehicle. During the process of parking in an unilluminated environment the exterior lighting can be utilized as a parking aid.
With the known lighting devices, in the vast majority of cases incandescent lamps and, relatively rarely, also gas discharge lamps are employed as electrical lighting means. These two types of lighting means have major disadvantages for the stated purpose of illuminating definite regions inside or outside a vehicle.
The principal disadvantage with the use of incandescent lamps is their very high incandescent-coil temperatures which are made necessary by the system for operation and the high thermal loads that occur as a result, concentrated in one place. In addition, the incandescent lamps themselves are relatively large and offer acceptable light efficiency in operation only when they are installed together with an appropriate reflector. The high thermal losses, the size of the incandescent lamps and the use of reflectors give rise to a minimum size of such lighting devices, which cannot be reduced. This minimum size renders difficult the integration of this lighting device into subassemblies that are present on the vehicle, such as, for instance, the outside mirror, the inside rear-view mirror, vehicle sheets (panels), bumpers, trim strips, covers, consoles, shelves, glove compartments and ashtrays.
For this reason, in many cases the lighting devices have to be arranged in places that are unfavorable for the emission of light, since only in such places is the requisite space available for mounting of the lighting device. Since the size proportions are different in every vehicle, no universally applicable lighting devices can be employed that are economical to manufacture on an industrial scale, but rather a vehicle-specific lighting device has to be developed and manufactured for each type of vehicle. overall, the creative scope as regards the design of lighting devices of this type is severely limited.
By virtue of the thermal losses of incandescent lamps which occur concentrated in one place, temperature problems arise in the region of the mounting location of the lighting device. As a result of the waste heat that arises, the materials of the surrounding subassemblies are very severely loaded thermally. In order to counteract damage to these materials, use is made of expensive, temperature resistant materials, and elaborate measures have to be provided for the dissipation of heat. Since even these measures would not be sufficient in the case of continuous loading, the maximum amount of waste heat has to be limited, to which end timing devices and/or thermal protection devices have to be provided, in order to switch off the lighting prior to the overheating of components. Besides, after the maximum thermal load has been attained, a long recovery interval is required with a view to cooling the components.
On account of the high temperature in the lighting coil of incandescent lamps the latter are susceptible to shock and vibration, and the service life of incandescent lamps lies distinctly below the service life of the vehicle. The lighting devices, particularly where use is made of elaborate optical systems, therefore have to be designed in such a way that the incandescent lamp can be exchanged. Since the lighting device has to be opened each time the incandescent lamp is exchanged, in the case of lighting devices arranged in the exterior region of the motor vehicle special means have to be provided for sealing. In addition, a lighting device that is capable of being disassembled gives rise to ledges or gaps in the surface of the subassemblies into which the lighting device is integrated, which can lead to wind noises or to vibration problems.
The spatial extent of the incandescent coil of incandescent lamps, together with the large exit aperture of the reflector to be used, results in a large divergence angle when light is emitted from the lighting device. A large divergence angle makes it difficult to exert influence on the beam path and hence to exercise deliberate control of the lighting characteristic, particularly also with regard to undesirable scattered light. For this reason the irradiated fields of illumination can only be adapted with difficulty to the optimal requirements of the lighting task by influencing ihe lighting characteristic. Optimization of the luminous-intensity distribution is also generally impossible for this reason, since, as a rule, only one incandescent lamp is used by way of light source. If extreme demands are nevertheless made as regards the shape of the luminous-intensity distribution, elaborate and expensive optical systems have to be employed, for instance special reflectors or asymmetric, nested Fresnel lens systems. The development and manufacture of such systems is time-consuming and expensive. Through the use of gas-discharge lamps only the problems caused by the thermal losses of incandescent lamps are neutralized. On the other hand, gas-discharge lamps require a still larger space for installation, and their lighting characteristic can barely be influenced, even through the use of special optical systems.