The invention relates to a halogen lamp for use in a headlight, particularly an automobile headlight, as well as to a headlight with such a halogen lamp.

A typical headlight halogen lamp has a gas-filled envelope, which as a rule comprises quartz glass, and a filament arranged in the envelope. The envelope of the halogen lamp is connected on one side to a base, with which the halogen lamp can be inserted into a reflector of the headlight. The reflector and the halogen lamp together with its base are so dimensioned that the filament is located in a defined area in the reflector, mostly in the area of the focal point of the reflector, so that the light emitted in operation by the filament is radiated, via the reflector, from the reflector opening - that is from the headlight - in the area to be illuminated in a defined way. Usually, the reflector opening is covered by a headlight glass operating as a lens.

By the design and defined arrangement of the different components of such a headlight system, it is ensured that in an automobile headlight with a low beam function, the light is emitted on the area of the road to be illuminated in such a way that the oncoming traffic is not dazzled. In order to avoid that direct sight of the filament causes dazzling of oncoming traffic, the envelope of the lamp can be provided, on the side facing in the direction of the reflector opening, that is as a rule on its dome, with an anti-glare cap coating, which absorbs the light incident on this coating as completely as possible. A typical example of such halogen lamps used in automobiles are H7 and H4 lamps. In this case, the dome covering is produced as a rule by means of black dip paint. For this purpose, the lamp is immersed head first in an immersion basin containing black paint, so that the anti-glare cap is formed on the outside of the envelope. Unfortunately, however, the light, which is emitted by the coil in the direction of the black dome and absorbed by it, is no longer available for lighting purposes. Therefore, the efficiency of the lamp is reduced by the anti-glare cap. Measurements on conventional H7 lamps have shown that the loss of luminous flux caused by the anti-glare cap coating amounts to approximately 8 % of the initial value without such a black dome.

For this reason, it is proposed in US 5,660,462, to design such an anti-glare cap in the form of a specularly reflecting coating. The reflecting coating and the filament are designed and arranged in such a way that the specularly reflecting coating reflects the light radiation emitted by the filament back to the filament. It is expressly pointed out in this document that the reflecting coating should reflect the light specularly, as a diffuse reflection of the light is not concentrated on the filament and would therefore lead to unacceptable glare by the headlight. It is obvious that the engineering freedom for the design of the lamp is substantially limited by the described requirements regarding the special arrangement of the filament and the design and arrangement of the anti-glare cap coating. Other technical requirements like thermal load capacity, easy manufacture of the lamp etcetera, must take second place to the compiled conditions. Furthermore, particularly high precision is necessary for the production of these lamps. Therefore, the production of the lamps involves higher costs. Particularly, it is not possible to use this method with the H4 and H7 lamps as presently used in a majority of the vehicles.

It is an object of the present invention to further develop a halogen lamp of the type mentioned in the opening paragraph to the effect that the luminous flux loss is reduced with simple and economical means, without this resulting in impermissibly increased glare.

This object is achieved by a lamp in accordance with claim 1.

In accordance with the invention, the anti-glare cap coating is designed so as to be essentially diffusely reflecting at least on a side (hereinafter referred to as "inner side" of the anti-glare cap coating) facing the filament inside the envelope. The term "essentially diffusely reflecting" is to be understood to mean here that light radiation - unlike in the case of, for example, a metallic surface - is reflected non-directionally to a large extent.

It has been found in several series of experiments that, if the inside of the antiglare cap coating is designed so as to be essentially diffusely reflecting, the light losses can be reduced by approximately 50% compared to a black coating. It was also determined that the resultant increase in glare of the automobile headlights equipped with the lamps is only very slight and lies far below the permitted limiting values. By using such a diffusely reflecting layer it is guaranteed namely- as opposed to a specularly reflecting layer - that the light is radiated homogeneously non-directionally from the relatively broad cap. Therefore,

unlike reflecting coatings, it does not have to be ensured that the reflection back to the coil takes place in order to definitely avoid glare. For this, it is not necessary that the halogen lamp and the coil have a special geometry. Particularly, it is possible to provide conventional H4 or H7 lamps easily and economically with an anti-glare cap coating in accordance with the invention instead of the classic black coating, and thus increase the light efficiency of these lamps.

As a result of the gain in luminous flux obtained from the anti-glare cap coating in accordance with the invention, the same luminous flux of the lamp in operation requires a smaller current, as a result of which the life span of the lamp is extended. Alternatively, at the same electrical power and thus the same life span, the increased luminous flux can also be used to attain, for example, a higher color temperature. For this purpose, a denser optical filter can be used, for example, at or in the envelope, which overall leads to the desired higher color temperature, with the light losses caused by the denser filters being compensated by the higher luminous flux due to the anti-glare cap coating in accordance with the invention.

The dependent claims each comprise particularly favorable embodiments and further embodiments of the invention.

Preferably, such an anti-glare cap coating in accordance with the invention has a white layer at least on the side facing the inside of the envelope. Thereby, a preferred, essentially completely diffusely reflecting surface or layer is achieved on the inside of the anti-glare cap coating. In addition, a white layer is highly reflecting having a reflectivity of, as a rule, above 90%. As a result, the light efficiency of the lamp is particularly increased.

Such a white layer can be produced very easily from, for example, zirconium oxide, which material is also sufficiently heat-resistant for use as a coating on such a lamp. However, for this purpose, in principle, another sufficiently high temperature-resistant white paint, particularly dipping varnish or the like, can also be applied, which forms a suitable, preferably matte white layer.

If less reflectivity is sufficient or even desired, and in order to obtain any specific color effects, bright, colored coatings, for example light yellow, can also be used. In a further preferred variant, the anti-glare cap coating comprises several layers - that is an inner diffusely reflecting layer and an outer opaque layer. The opaque layer ensures that any residual light passing through the diffusely reflecting layer is absorbed reliably, so that the anti-glare cap does not allow any light to pass into the impermissible

spectral region. A dark, preferably black layer can be used, for example, as outer layer, as it is also used hitherto for the generation of the anti-glare cap coating of halogen lamps.

As already mentioned, the halogen lamp can, in principle, have any form, that is, it is not necessary that the anti-glare cap is formed in such a way that the light is predominantly reflected back towards the filament.

In a preferred example of embodiment, the envelope has, as with a classic halogen lamp of the H4 or H7 type, an essentially cylindrical form, wherein, at a rear face the base for assembly in the headlight reflector is arranged and, at an opposite front face, the anti-glare cap coating is arranged. Preferably, the invention is used with already known halogen lamps of the H4 or H7 type .

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter, though the invention should not be considered as limited to these. In this case, like reference numerals refer to like parts.

In the drawings:

Fig. 1 shows a diagrammatic partial cross-sectional view of a H7 halogen lamp with an anti-glare cap coating in accordance with the invention, according to a first example of embodiment,

Fig. 2 shows a diagrammatic partial cross-sectional view of a H7 halogen lamp with an anti-glare cap coating in accordance with the invention, according to a second example of embodiment,

Fig. 3 shows a sectional view through a reflector with a halogen lamp accommodated therein, in accordance with Fig. 2,

Fig. 4 shows a table with test results, which show the reduction of the light loss by the application of an anti-glare cap coating in accordance with the invention, instead of the usual black anti-glare cap coating on a

H7 lamp,

Fig. 5 shows a table with test results, which show the glare values produced in a headlight by lamps provided with different anti-glare cap coatings in accordance with the invention.

In Figs. 1 and 2, what is called a H7 lamp 1 is represented diagrammatically.

This lamp has an essentially cylindrical gas envelope 2, which is attached at its rear end to a base 5. At an end face of this base 5, plug contacts 7, 8 are arranged via which the lamp 1 is connected to the electrical system of the automobile.

Within the envelope 2 there is a filament 3, which is electrically contacted via rigid conductor wires 4, which also hold the coil 3 in the appropriate position within the envelope 2. These conductor wires 4 pass through the base 5 to the plug contacts 7, 8.

The lamp 1 can be nested in a reflector 13 of the automobile (see Fig. 3) by means of a flange 6 at the base 5. For this purpose, a lamp opening 15 is provided in the rear part of the reflector 13 (viewed against the main radiation direction L of the reflector 13), through which opening the lamp 1 is inserted into the reflector 13. At the lamp opening 15 there is a base support plate 14, into which the flange 6 can be inserted with a specific orientation, so that the filament 3 is in a defined position within the reflector 13, usually in the area around the focal point.

On the side opposite the base 5, that is on the side of the lamp 1 facing the reflector opening 16 situated in the direction of propagation of the main radiation L, the lamp 1 is provided with an anti-glare cap coating 9. Here, this anti-glare cap coating 9 comprises a white or bright, colored layer 10, by means of which the light radiated by the coil 3 in the direction of the anti-glare cap coating 9 is diffusely reflected back into the envelope 2.

Fig. 2 shows an alternative example of embodiment in which the anti-glare cap coating 9 comprises two layers, i.e. an inner layer 11 and an outer layer 12. Here, the inner layer 10 is a white matte layer, which is formed of zirconium oxide. The application of the zirconium oxide takes place in a dipping process with subsequent curing of the layer remaining on the envelope's surface. After the curing of the white layer 10, a customary matte black layer 12 is likewise formed in a classic dipping process. This matte black layer 12 ensures that any light passing through the white layer 11 is surely absorbed.

As can be seen from Fig. 3, the use of such an anti-glare cap coating 9 has the advantage that, on the one hand, direct sight of the coil 3, viewed against the main radiation direction L, that is into the reflector opening 16 of the reflector 13, is not possible, as the antiglare cap coating 9 located on the side of envelope 2 facing towards the reflector opening 16

shields the coil 3 completely. On the other hand, however, the light radiated from the inside by the coil 3 onto the anti-glare cap coating 9 is reflected back again into the area of the lamp 1 and thus increases the integral luminous flux of the lamp 1.

In order to be able to quantitatively determine the effect of the anti-glare cap coating 9 in accordance with the invention, different measurements were carried out. For this purpose, several H7 lamps were first measured photometrically without any coating and the total luminous flux emitted by the relevant lamp was measured. Subsequently, half of the lamps were provided with a black dome in the classic way and again measured photometrically. The other half of the lamps was first provided with an inner white layer of zirconium oxide. The usual black layer was then formed on this white inner layer, as is represented in Fig. 2.

In the table in Fig. 4, results of these measurements are represented for ten of the lamps, wherein the first to the fifth lamp were each provided with the black anti-glare cap coating (hereinafter also referred to as "dome " for short) and the sixth to tenth lamp were provided with a dome which is white on the inside and black on the outside. The first column comprises the number of the lamp during the test. The second column comprises the measured integral luminous flux values without anti-glare cap coating. The third column comprises the respective integral luminous flux values of the first five lamps after they have been coated with a conventional black dome. The light loss through the black dome in percent is listed in the fourth column. The fifth column comprises the respective values of the integrally measured luminous flux of the lamps 6 to 10, which were first provided with a white/black dome in accordance with the invention. The last column comprises the percentage light loss caused by the white/black dome.

As can be inferred from this Table, the light loss through the black anti-glare cap is on average 8.32%, whereas the light loss through the white anti-glare cap is only

4.36%. The light loss with a classic H7 lamp is thus almost halved by the design of the antiglare cap coating in accordance with the invention.

In order to check whether the lamps coated in accordance with the invention lead to too high glare values in the headlight during operation, further different measurements were carried out.

Fig. 5 shows, as an example, a table of measurements, which measurements were carried out in a test headlight of the type VW Golf 4 ®. Here, again H7 lamps were used in each case.

The first column comprises a serial number for identification of the lamps in the test. The second column indicates the color in which the anti-glare cap and/or at least the inside of the anti-glare cap is designed. All the coatings concerned are matte coatings.

In the third and fourth column, the respective glare values are stated in lux at the measuring points EHV and EB50L determined according to the ECE.

The first two lamps are comparison lamps with a usual black anti-glare cap. In the row below the two measured value rows, the average value of the two measurements on the comparison lamps is stated in the third and fourth column, respectively. Here, the EHV glare value is on average approximately 0.56 lux and the EB50L glare value is approximately 0.30 lux.

The lamps 3 and 4 are each provided with an inner white coating (as represented in Fig. 2). Here, on average, the EHV glare value is approximately 0.60 lux and the EB50L glare value is approximately 0.36 lux. Thus, the glare values are in fact slightly increased, but still far below the permitted limiting values. These limiting values are indicated at the bottom of the Table in the third and fourth column. Here, it concerns the maximum in accordance with the ECE standard.

For comparison, measurements on lamps with three different, partly matte silver coatings were carried out. In the Table in Fig. 5, only the respective average values of these measurements are indicated. It is shown that the glare values are strongly increased with all tested silver coatings and are above the limiting values. This is because of the fact that these coatings simply do not reflect in an essentially diffuse manner, but at best within an angle of emergence that is widened compared with a mirror surface.

These results show that, in the very simple way in accordance with the invention, lamps can be produced, which make better use of the light produced by the filament, without the risk of a too strong increase of the glare values.

Finally, it is pointed out once again that the systems and methods represented in the Figures and the description merely are examples of embodiments, which can be varied substantially by the expert without leaving the framework of the invention. Thus, notably the anti-glare cap coating can be attached not only to the outside of the envelope, but a corresponding coating can also be applied to the inside of the envelope. Likewise, it is possible to apply the coating to the envelope in very different ways. Thus, depending upon coating material and structure, besides the dipping process already mentioned, for example, spraying methods or so-termed sputtering or vacuum deposition methods can be used for the generation of the anti-glare cap coating.

In addition, for the sake of completeness, it is pointed out that the use of the indefinite articles "a" or "an" does not exclude a plurality of the relevant features.