Lamp and headlight unit for a motor vehicle
The invention relates to a lamp comprising a lamp vessel with light-emitting means, which vessel is mounted on a lamp base. The invention further relates to a headlight for a motor vehicle with a reflector and a lamp arranged in the reflector.
Besides incandescent lamps, which have been in use in the automotive field for a long time, discharge lamps are being increasingly used for motor vehicle lighting because of their substantially improved luminous efficacy as compared with incandescent lamps. In known discharge lamps, a gas discharge is generated in a sealed discharge vessel between two electrodes, thus emitting a very intense light. A discharge lamp usually has a lamp base with a lamp cap, a burner comprising a discharge vessel, and an outer bulb. The cap serves for retaining the burner and for positioning as well as for electrically contacting the lamp in a headlight. The actual light generation takes place inside the discharge vessel. An outer bulb made of glass is usually arranged around the discharge vessel. This bulb serves to filter out UV radiation from the emitted light.
There is a problem, however, especially in mercury- free xenon lamps, i.e. that no optimum illumination can be achieved with conventional dual projection optical systems in particular for the passing or low beam because of the different luminance distribution as compared with the mercury-containing xenon lamps. A development of optimized dual projection systems specially aimed at mercury-free xenon lamps is very expensive, in particular also in view of the ever increasing miniaturization demanded for motor vehicle parts. The incandescent lamps also used in the automotive field also have a lamp cap that is connected to a lamp vessel. At least one incandescent coil serving to emit light is arranged inside the lamp vessel. A motor vehicle headlight comprises a reflector and a lamp projecting through an opening into the interior of the reflector. The lamp is exactly positioned with its cap in a holder of the reflector such that the lamp vessel is held in an exactly known position. The reflector surface is illuminated by the emitted light. A headlight beam is thus formed in dependence on the shape of the reflector surface and is projected in forward direction.
Increasingly, dual projection systems have come into use in recent times, which are capable of performing two lighting functions, i.e. forming both the low beam and the high beam. The use of a movable diaphragm in the imaging plane of the dual projection optical system achieves a switch-over between low beam and high beam. Xenon lamps as well as halogen lamps may be used for this. In addition, further lighting functions such as, for example, highway beam, bad weather light, or city light may be set through further modifications of the movable diaphragm as well as its arrangement on a rotating cylinder.
The authorities have issued legally binding specifications for the construction of such lamps in order to comply with the requirements of good road illumination and to avoid nuisance to oncoming traffic caused by glare. Thus it has been laid down, for example, that the electrodes of a discharge lamp must be arranged on the centerline of the discharge vessel, with a tolerance range of +/- 0.25 mm being permitted. This specification follows the until now unanimous opinion that a central arrangement of the electrodes is most in accordance with the requirements imposed on the low and the high beam. The problem arises, however, in particular with mercury- free lamps, that no optimum illumination can be achieved in the low beam area. Especially against the background of the miniaturization of motor vehicle parts that is increasingly demanded, the lamps available at present are found to be highly vulnerable as regards their geometric manufacturing tolerances.
This is where the invention comes to the fore. The invention has for its object to provide a lamp which is less sensitive to manufacturing tolerances as the component part dimensions are reduced and which also renders possible a better illumination by the low beam. According to the invention, this object is achieved in that the light-emitting means are arranged at least partly in a position away from the central axis of the lamp cap.
The invention provides a lamp which is less sensitive to geometric manufacturing tolerances arising from specific production circumstances in the case of a reduction in the component dimensions and which in addition renders possible an improved illumination with the low beam. The light-emitting means are preferably two electrodes connected to current supply leads for generating an electric discharge, which electrodes are arranged in a gastight manner in mutual opposition in a discharge vessel. Alternatively, the light-emitting means are formed as at least one incandescent coil.
In a further embodiment of the invention, at least one electrode is arranged away from the central axis. Preferably, said at least one electrode is arranged shifted
horizontally to the right when viewed from the lamp cap along the lamp. The center of gravity of the illumination is shifted towards the right-hand street edge thereby. The same effect is achieved when the lamp vessel is alternatively or additionally similarly shifted with respect to the central axis. In an embodiment of the invention, said at least one electrode is arranged shifted vertically downwards when viewed from the lamp cap along the lamp. The center of gravity of the illumination is shifted towards the light-dark cut-off thereby. The same effect is achieved when the lamp vessel is alternatively or additionally similarly shifted with respect to the central axis. Advantageously, the distance of the at least one shifted electrode to the central axis of the lamp cap in the horizontal and/or vertical shifting direction is between 0.1 and 0.2 mm. It was found that, given such a dimensioning, very good results are obtained for the low beam. At the same time the legally binding tolerance limits of 0.25 mm to the central axis of the lamp cap are not exceeded in this case. Preferably, the lamp is constructed without mercury. This takes into account requirement regarding the environment.
The invention further has for its object to provide a headlight unit for motor vehicles wherein the illumination is improved while observing the legal specifications, also in the case of increasing miniaturization. According to the invention, this object is achieved in that a lamp as claimed in one of the claims 1 to 9 is arranged in the reflector.
The invention thus offers a headlight unit for a motor vehicle wherein the illumination is improved, while the legal specifications are observed also in the case of an increasing miniaturization.
Further embodiments and modifications of the invention are defined in the dependent claims. An example of an embodiment of the invention is shown in the drawings and will be described in more detail below. In the drawings:
Fig. 1 is a diagrammatic picture of a discharge lamp;
Fig. 2 is a diagrammatic picture of a discharge lamp with electrodes in a shifted arrangement a) in plan view, b) in side elevation, c) with the relevant light distribution diagram, and
Fig. 3 is a diagrammatic picture of an optical system for dual projection.
The xenon discharge lamp 10 chosen as an example here is constructed without mercury and comprises a lamp cap 12, a burner 14 with a discharge vessel 16, and supply leads 18. Electrical conductors are arranged in the supply leads 18 and are connected via sealing foils 23 to electrodes 22 in the inner volume of the discharge vessel 16. The discharge vessel 16 is sealed off at its two mutually opposed ends by means of respective press seals. The outer supply leads 18 are electrically connected to the electrodes 22 inside the discharge vessel 16 in a known manner by means of sealing foils 23. When a voltage is applied to the supply leads 18 thus connected to the electrodes 22, a gas discharge can be generated inside the discharge vessel 16 of the lamp. Such discharge lamps are known to those skilled in the art in various modifications, which is why the known construction and operation thereof will not be described in any more detail here.
It was surprisingly found that the illumination of the low beam can be significantly enhanced when the electrodes are shifted to the right over 0.1 to 0.2 mm, viewed from the lamp cap along the lamp, and at the same time shifted in downward direction over the same distance relative to the central axis M of the discharge vessel 16. The central axis M of the discharge vessel is identical to the central axis M of the lamp cap 12 in this embodiment. The sensitivity of the low beam light values to geometric manufacturing tolerances of the electrodes 22 is significantly reduced at the same time, in particular in lamps having small dimensions. The shift of 0.1 to 0.2 mm lies within the legally prescribed tolerance range. Any additional shift of the electrodes 22 over more than 0.2 mm leads to a too strong shift in the position of the high beam maximum such that legal limits are exceeded. In countries with left- hand traffic, where the required lighting geometry is veered to the left, the horizontal shift is to be correspondingly performed to the left. In the embodiment, the electrodes 22 are shifted 0.15 mm to the right and 0.15 mm downward relative to the central axis M of the discharge vessel 16, viewed from the lamp cap 12 along the lamp 10.
Fig. 2 shows the electrode shift d and the illuminance created thereby on a vertical wall. Compared with lamps according to the prior art, in which the electrodes are positioned on the central axis of the lamp cap, there arises a significant shift towards the road edge in the illumination.
The optical projection system 40 consists of an elliptical mirror 42, a movable diaphragm 44, a projection lens 46, and a lamp 10 which projects through a central opening of the elliptical mirror 42 into the optical system 40 for dual projection (Fig. 3). The lamp 10 is
centrally positioned in a predefined location inside the reflector housing. For this purpose, the lamp cap 12 is held in a holder (not shown in any detail) of the reflector housing. The light source Q (a light arc formed between the electrodes 22 in the discharge vessel 16 in the embodiment) is imaged by the elliptical mirror 42 in the plane of the movable diaphragm 44 and from there through the projection lens 46 into the traffic space. The image of the diaphragm edge here forms the light-dark cut-off HD, separating the illuminated traffic space from the non-illuminated traffic space. Headlight beams for various traffic situations can be formed by means of differently shaped diaphragm edges. A, lowering or shifting of the light source Q relative to the optical axis of the headlight improves the illumination in the region of the light-dark cut-off and at the right-hand driving lane edge.
The lowering and/or shifting of the light source Q relative to the optical axis may alternatively be realized through modification (shifting) of the lamp cap holder of the headlight unit. This solution has the same effect as a lamp vessel 20 that is shifted relative to the central axis M of the lamp cap 12 while the lamp cap itself is not shifted. The shifting of the electrodes 22 relative to the central axis M of the lamp cap
12 may lead to a reduction in the distance covered by the high (driving or main) beam. This effect can be compensated for, for example, in that the entire headlight unit is pivoted slightly upwards by means of the level control, which is often present anyway, when the high beam is activated.