WO2008129487A2

WO2008129487A2 - High pressure discharge lamp and vehicle headlight

Info

Publication number: WO2008129487A2
Application number: PCT/IB2008/051493
Authority: WO
Inventors: Marcus J. H. Kessels; Lingli Wang
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-04-24
Filing date: 2008-04-18
Publication date: 2008-10-30
Also published as: WO2008129487A3

Abstract

The invention relates to a high pressure discharge lamp (10) and a vehicle headlight (100) comprising the high pressure discharge lamp. The high pressure discharge lamp according to the invention comprises a discharge vessel (12) comprising electrodes (16, 18) for creating a discharge arc (20) and for defining a longitudinal axis (30) connecting the electrodes. The discharge arc is displaced with respect to the longitudinal axis at a distance on a displacement axis (32) arranged perpendicular to the longitudinal axis. The high pressure discharge lamp further comprises a reflective surface (40; 42; 44; 46; 50; 52) for reflecting light emitted by a discharge arc towards a light-transmission window (60; 62) of the discharge vessel while substantially preventing the reflected light to pass through the discharge arc. The effect of the measures according to the invention is that the reflective surface is arranged to redirect the reflected light away from the discharge arc towards the light-transmission window, thus improving an efficiency of the high pressure discharge lamp.

Description

High pressure discharge lamp and vehicle headlight

FIELD OF THE INVENTION

The invention relates to a high pressure discharge lamp. The invention also relates to a vehicle headlight comprising the high pressure discharge lamp.

BACKGROUND OF THE INVENTION

High pressure discharge lamps are known per se. They are used, inter alia, as light source in headlights for vehicles, such as, a car or a motorcycle. High pressure discharge lamps are relatively standard in headlights for vehicles due to their relatively high efficacy and due to the spectral composition of the emitted light which is relatively similar to that of daylight. The high pressure discharge lamps are, for example, arranged in a paraboloidal reflector in which light emitted by the known high pressure discharge lamps is blocked over a circumferential angle of 180° - 15° to create an asymmetrical light beam. This asymmetrical light beam is required for right-hand or left-hand traffic and avoids to blind approaching traffic.

Such high pressure discharge lamp is, for example, known from US-A-6 445 129. Here, a discharge lamp, in particular for motor vehicle headlamps is described. The discharge vessel is made of glass or the like. Two electrodes extend into the discharge vessel and form an arc gap. The discharge vessel is shown to have elliptical shape in longitudinal section, and the discharge formed between the electrodes has arcuate shape. A coating applied on the inside or outside of the discharge vessel may have reflecting properties and can be used as a light reflector. The metallic coating extends in longitudinal direction over the central part of the discharge vessel, and in circumferential direction extends over the lower half thereof for reflecting light that would otherwise be lost. A drawback of the known illumination system is that an efficiency of the known high pressure discharge lamps is not optimal. SUMMARY OF THE INVENTION

It is an object of the invention to provide a high pressure discharge lamp having increased efficiency.

According to a first aspect of the invention the object is achieved with a high pressure discharge lamp as claimed in claim 1. According to a second aspect of the invention, the object is achieved with a vehicle headlight as claimed in claim 12 and comprising the high pressure discharge lamp as claimed in claim 1. The high pressure discharge according to the invention comprises a discharge vessel enclosing a discharge space being provided with an ionisable filling comprising one or more halides. The discharge space comprises electrodes arranged opposite of each other for creating, in operation, a discharge arc. The electrodes further define a longitudinal axis which connects the electrodes of the high pressure discharge lamp. In operation, the discharge arc is displaced with respect to the longitudinal axis at a distance on a displacement axis arranged perpendicular to the longitudinal axis. The high pressure discharge lamp further comprises a reflective surface which reflects light from the discharge arc towards a light-transmission window of the discharge vessel for increasing a light flux through the light-transmission window while substantially preventing the reflected light to pass through the discharge arc. The longitudinal axis is part of an imaginary plane arranged perpendicular to the displacement axis and which divides the discharge vessel into two parts. The reflective surface and the discharge arc are arranged on opposite sides of this imaginary plane. The reflective surface is arranged to redirect the reflected light away from the discharge arc towards the light-transmission window

The effect of the high pressure discharge lamp according to the invention is that the arrangement of the reflective surface enables the reflected light to illuminate the light-transmission window while substantially preventing the reflected light to pass through the discharge arc. In the known high pressure discharge lamps, a reflective surface is arranged to reflect the light from the reflected surface substantially back to the discharge arc. The reason is that substantially reflecting the light back to the discharge arc prevents multiple light sources in the discharge vessel which prevents the occurrence of ghost images. However, as a result of the reflection of the light back to the discharge arc is that part of the reflected light will be absorbed by the discharge arc and thus will be lost, reducing the efficiency of the high pressure discharge lamp. In the high pressure discharge lamp according to the invention, the reflected surface is arranged to reflect the light away from the discharge arc and towards the light-transmission window. The arrangement of the reflective surface in the high pressure discharge lamp according to the invention renders a major part of the reflected light to pass by the discharge arc after reflection and not pass through the discharge arc and thus counteracts light absorption of the reflected light by the discharge arc. Due to the reduction of the light absorption of the reflected light the efficiency of the high pressure discharge lamp is increased.

High pressure discharge lamps according to the invention are often used in vehicle headlights. The distribution of the light emitted by the headlight of a vehicle is a predefined light distribution which must be such that the road up to approximately 75 meters in front of the vehicle together with a verge of the road is well illuminated while avoiding blinding approaching traffic. The requirements for these predefined light distributions of vehicle headlights are, for example, defined in regulations of the United Nations Economic Commission for Europe (UNECE), for example, in E/ECE/324 E/ECE/TRANS/505 Rev.l/Add.97/Rev.l and Rev.l/Add.98/Rev.l, respectively, which are known to the person skilled in the art, and which can be found at http://www.unece.org. To obtain the above described predefined light distribution via a headlight having a paraboloidal reflector, light emitted by the known high pressure discharge lamps is typically blocked over a circumferential angle of 180° - 15°. In the high pressure discharge lamp according to the invention the reflective surface is beneficially arranged in the part of the high pressure discharge lamp which is typically blocked such that the light reflected by the reflective surface is used to improve the efficiency of the high pressure discharge lamp in the vehicle headlight.

Alternatively, the known high pressure discharge lamps may, instead of blocking the light over the circumferential angle of 180° - 15°, reflect the light over the circumferential angle of 180° - 15° back to the discharge arc. However, part of the reflected light in the known high pressure discharge lamps will be absorbed by the discharge arc and still will be lost. In the high pressure discharge lamp according to the invention the reflected light is redirected by the reflective surface away from the discharge arc which reduces the absorption of the light by the discharge arc and which further improves the efficiency.

Thus the use of the high pressure discharge lamp according to the invention in a vehicle headlight would either require less power to achieve substantially the same illumination of the predefined light distribution compared to known high pressure discharge lamps, or would result in improved visibility for the driver of the vehicle while maintaining the power consumption of the headlight at the same level compared to the known high pressure discharge lamps. In an embodiment of the high pressure discharge lamp, the high pressure discharge lamp comprises a further reflective surface for reflecting light from the discharge arc to a further light-transmission window of the discharge vessel for increasing a light flux emitted from the discharge vessel through the further light-transmission window while substantially preventing the reflected light to pass through the discharge arc, the further reflective surface and the discharge arc being arranged on opposite sides of the imaginary plane, and the further reflective surface being arranged to redirect the reflected light away from the discharge arc towards the further light-transmission window. A benefit of this embodiment is that both the reflective surface and the further reflective surface are arranged on the opposite side of the imaginary plane compared to the discharge arc, both at a part of the high pressure discharge lamp which typically is blocked to obtain the predefined light distribution required in vehicle headlights. Thus the use the high pressure discharge lamp having both reflective surfaces, further increases the efficiency of the high pressure discharge lamp and would either require even less power to achieve substantially the same illumination of the predefined light distribution compared to known high pressure discharge lamps, or would result in further improved visibility for the driver of the vehicle while maintaining the power consumption of the headlight at the same level compared to the known high pressure discharge lamps.

In an embodiment of the high pressure discharge lamp the light-transmission window and/or the further light transmission window is arranged in a vicinity of an intersection between the imaginary plane and the discharge vessel, the light-transmission window and/or the further light-transmission window each cover a transmission circumferential angle around the longitudinal axis of less than 50 degrees.

The inventors have realized that an increase of the light flux emitted by the high pressure discharge lamp near the intersection between the imaginary plane and the discharge vessel improves a distribution of the light emitted by a vehicle headlight comprising a paraboloidal mirror within the predefined light distribution. In the predefined light distribution as indicated above, a center part should obtain the highest light flux from the vehicle headlight to obtain good visibility for the driver of the vehicle. In the predefined light distribution, the center part is indicated with the letters 5OR, 50V and 75R. Without being bound by any theoretical theory, the inventors have found that increasing the light flux through a light-transmission window arranged in the vicinity of the intersection between the imaginary plane and the discharge vessel mainly improves the light flux at the center part of the predefined light distribution. The high pressure discharge lamp according to the invention reflects the light away from the discharge toward the light-transmission window arranged in the vicinity of the intersection between the imaginary plane and the discharge vessel, resulting in a substantial improvement of the visibility for the driver of the vehicle. Because the reflected light is redirected such that the reflected light is prevented to pass through the discharge arc, the efficiency of the reflected light is relatively high.

In an embodiment of the high pressure discharge lamp the reflective surface and/or the further reflective surface are arranged at an outer wall of the discharge vessel. A benefit of this embodiment is that the high pressure discharge lamp can be made very compact while improving the efficiency of the high pressure discharge lamp compared to the known high pressure discharge lamps.

In an embodiment of the high pressure discharge lamp the reflective surface and/or the further reflective surface are inclined with respect to the discharge vessel via a protrusion from the outer wall or via an indentation in the outer wall. A benefit of this embodiment is that it enables a relatively simple method of manufacturing the reflective surface and/or the further reflective surface, which may be manufactured during the manufacturing of the discharge vessel. The discharge vessel is typically formed of a translucent material such as quartz or aluminum-oxide. When the discharge vessel is, for example, made of quartz, the quartz is heated after which the quartz is formed into the discharge vessel in a mould. When the discharge vessel is, for example, made of aluminum- oxide, the discharge vessel is first formed in the mould after which the shape of the discharge vessel is fixed in a sintering process step. The mould used for the shaping of the quartz discharge vessel or for the shaping of the aluminum-oxide discharge vessel may, for example, simultaneously produce the protrusions and/or indentations of the outer wall of the discharge vessel A further benefit when using this mould would be that the reproduction of the reflective surface and/or further reflective surface would be relatively high because a same mould would be used to produce a plurality of these discharge vessels for a plurality of these high pressure discharge lamps.

Alternatively, the protrusion may, for example, be produced via additional material applied to the outer wall of the discharge vessel. The additional material may, for example, be translucent material having a predetermined shape to form the reflective surface and/or the further reflective surface. This additional material may, for example, be glued to the outer surface of the discharge vessel, or may, for example, be melted or sintered to the outer surface of the discharge vessel. Local melting of the additional material or of the outer wall of the discharge vessel may, for example, be obtained via laser-melting which is a well known technique to the person skilled in the art.

In an embodiment of the high pressure discharge lamp a cross-section of the discharge vessel perpendicular to the longitudinal axis being a substantially ellipsoidal cross- section of the discharge vessel, wherein a curvature of the reflective surface is different from a curvature of the ellipsoidal cross-section of the discharge vessel. A benefit of this embodiment is that by changing the curvature of the reflective surface, the reflected light may be focused, for example, towards the light transmission window. The cross-section of the discharge vessel may, for example, be a circular cross-section. In an embodiment of the high pressure discharge lamp the reflective surface covers a first reflective circumferential angle CC around the longitudinal axis, the first reflective circumferential angle CC being arranged between a first angle CC 1 and a second angle cc2 with respect to the imaginary plane, wherein

0 degrees < ccl < 35 degrees, and 55 degrees < cc2 < 65 degrees, and wherein the further reflective surface covers a second reflective circumferential angle β around the longitudinal axis, the second reflective circumferential angle β being arranged between a third angle βl and a fourth angle β2 with respect to the imaginary plane, wherein

125 degrees < βl < 135 degrees, and 155 degrees < β2 < 165 degrees. A benefit of this embodiment is that the position of the outer wall of the discharge vessel with respect to the discharge is relatively well defined. When applying the reflective surface on the outer wall of the discharge vessel, the reproducibility and/or accuracy of the applied reflective surface with respect to the discharge is increased. This increased accuracy and/or reproducibility, for example, avoids fine-tuning of the high pressure discharge lamp inside the vehicle headlight.

In an embodiment of the high pressure discharge lamp the high pressure discharge lamp comprises an outer envelope, wherein the reflective surface and/or the further reflective surface are arranged at the outer envelope. A benefit of this embodiment is that the outer envelope is further away from the discharge and, in operation, has a lower temperature compared to the discharge vessel. Consequently, the reflective surface and/or further reflective surface are at a lower temperature compared to the discharge vessel. Because the reflective surface and/or the further reflective surface typically comprise a reflective layer for reflecting the light, the reduction in temperature of the reflecting layer improves the life-time of the reflective layer. In addition, the reduction of the temperature of the reflective layer typically increases a choice of materials suitable for use as reflective layer.

In an embodiment of the high pressure discharge lamp the reflective surface and/or the further reflective surface are inclined with respect to the outer envelope via a protrusion from the outer envelope or via an indentation in the outer envelope. A benefit of this embodiment is that the reflective surface and/or the further reflective surface may be manufactured during the manufacturing of the outer envelope. For example, using a mould would result in a relatively high reproducibility of the reflective surface and/or the further reflective surface. The protrusion may, for example, be produced via additional material applied to the outer wall of the discharge vessel, the additional material, for example, being translucent material.

In an embodiment of the high pressure discharge lamp a cross-section of the outer envelope perpendicular to the longitudinal axis being a substantially ellipsoidal cross- section of the outer envelope, wherein a curvature of the reflective surface is different from a curvature of the ellipsoidal cross-section of the outer envelope. Again, a benefit of this embodiment is that by changing the curvature of the reflective surface, the reflected light may be focused, for example, towards the light transmission window. The cross-section of the outer envelope may, for example, be a circular cross-section. In an embodiment of the high pressure discharge lamp the reflective surface covers a third reflective circumferential angle γ around the longitudinal axis , the third reflective circumferential angle γ being arranged between a fifth angle γl and a sixth angle γ2 with respect to the imaginary plane, wherein

0 degrees < γl < 35 degrees, and 55 degrees < γ2 < 65 degrees, and wherein the further reflective surface covers a fourth reflective circumferential angle δ around the longitudinal axis, the fourth reflective circumferential angle δ being arranged between a seventh angle δl and a eight angle δ2 with respect to the imaginary plane, wherein

125 degrees < δl < 135 degrees, and 155 degrees < δ2 < 165 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings:

Fig. 1 shows a cross-sectional view along a longitudinal axis of a high pressure discharge lamp according to the invention,

Figs. 2 A to 2D show cross-sectional views perpendicular to the longitudinal axis of the high pressure discharge lamp according to the invention, in which the reflective surface is arranged at an outer surface of the discharge vessel,

Figs. 3A and 3B show a further cross-sectional views perpendicular to the longitudinal axis of the high pressure discharge lamp according to the invention, in which the reflective surface is arranged at the outer envelope of the high pressure discharge lamp, and Fig. 4 shows a cross-sectional view of a vehicle headlight according to the invention.

The figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the figures are denoted by the same reference numerals as much as possible.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a cross-sectional view along a longitudinal axis 30 of a high pressure discharge lamp 10 according to the invention. The high pressure discharge lamp 10 comprises a discharge vessel 12 which encloses a discharge space 14. The discharge space 14 comprises one or more halides and comprises electrodes 16, 18 arranged opposite of each other for creating, in operation, a discharge arc 20. The high pressure discharge lamp 10 comprises a connector 74 for connecting the high pressure discharge lamp 10 in a reflector, for example, of a vehicle headlight. A first electrode 16 is directly connected to the connector 74 and a second electrode 18 is directed back to the connector 74 via an insulating pipe 72. In operation, a discharge arc 20 is present between the first and second electrodes 16, 18. The discharge arc 20 is displaced with respect to the longitudinal axis 30 in a direction of a displacement axis 32. The displacement axis 32 is indicated in Fig. 1 with a dash-dotted line 32. In the embodiment of the high pressure discharge lamp 10 shown in Fig. 1, the high pressure discharge lamp 10 further comprises an outer envelope 70 which surrounds the discharge vessel 12 and protects the discharge vessel 12. This outer envelope 70 is not essential to all embodiments of the high pressure discharge lamp 10 according to the invention.

The high pressure discharge lamp 10 according to the invention further comprises a reflective surface 40 for reflecting light from the discharge arc 20 towards a light-transmission window 60, 62 (see Figs. 2A to 3B) while preventing all or a majority of the reflected light to pass through the discharge arc 20. The reflective surface 40 of the high pressure discharge lamp 10 according to the invention is arranged to reflect the light away from the discharge arc 20 towards the light-transmission window 60, 62. Because the reflective surface of the high pressure discharge lamp 10 prevents the reflected light to pass through the discharge arc 20, an efficiency of the high pressure discharge lamp 10 is improved. Typically, the discharge arc 20, next to emitting light, also absorbs light. In the known high pressure discharge lamps the reflected light typically is reflected back to the discharge arc to prevent the occurrence of ghost images. However, part of the reflected light is absorbed and thus is lost. Due to the arrangement of the reflective surface 40 which prevents the reflected light to pass through the discharge arc 20, the efficiency of the high pressure discharge lamp according to the invention is improved compared to the known high pressure discharge lamps.

Figs. 2 A to 2D show cross-sectional views perpendicular to the longitudinal axis of the high pressure discharge lamp according to the invention, in which the reflective surface is arranged at an outer surface of the discharge vessel.

Fig. 2A shows a first embodiment of the high pressure discharge lamp 10. The cross-section of the discharge vessel 12 comprises an ellipsoidal shape and in the embodiment shown in Fig. 2A the ellipsoidal shape is substantially circular. The discharge vessel 12 comprises a discharge arc 20 which is indicated with a grey ellipse 20 which is displace with respect to the longitudinal axis 30 along a displacement axis 32. The cross- section of the discharge arc 20 is a purely diagrammatical representation of the discharge arc 20, and in operation, the cross-section of the discharge arc 20 may not be smooth and may not have an ellipse shape. The reflective surface 40 is arranged on the outer wall 11 of the discharge vessel 12. The reflective surface 40 is inclined with respect to the outer wall 11 of the discharge vessel via an indentation 41. This indentation in the outer wall 11 of the discharge vessel 12 can relatively easily be formed using, for example, the same mould which is used to manufacture the discharge vessel 12. A benefit of this embodiment is that the production of the indentation 41 is relatively simple and that the use of the same mould improves the reproducibility of the shape of the indentation 41. The inclination of the reflective surface 40 enables the reflected light to be redirected by the reflective surface 40 towards the light-transmission window 60 while preventing all or a majority of the reflected light to pass through the discharge arc 20. The reflective surface 40 is not only inclined with respect of the outer wall 11 of the discharge vessel 12, but a curvature of the reflective surface 40 with respect to the longitudinal axis 30 may also be different compared to the curvature of the outer wall 11 of the discharge vessel 12. By adapting the curvature of the reflective surface 40, the illumination of the light-transmission window 60 may be adapted, for example, to fully illuminate the light-transmission window 60 (as shown in Fig. 2A). Or, alternatively, the curvature of the reflective surface 40 may be chosen to only illuminate part of the light- transmission window 60, for example, near the intersection 35 between the imaginary plane

34 and the discharge vessel 12. The inventors have found that increasing the light flux through the light-transmission window 60 which arranged in the vicinity of the intersection

35 between the imaginary plane 34 and the discharge vessel 12, the light flux at the center part of the predefined light distribution of a vehicle headlight as defined in the regulations of the United Nations Economic Commission for Europe (UNECE) is improved. This improves, for example, the visibility for the driver of the vehicle. The light-transmission window 60, in the embodiment shown in Fig. 2A, is arranged near the intersection 35 between the imaginary plane 34 and the discharge vessel 12. In the embodiment shown in Fig. 2A the light-transmission window 60 overlaps the intersection 35 by an angle of approximately 15 degrees with the imaginary plane 34 at the side of the imaginary plane 34 where also the reflective surface 40 is located. This may, for example, used to obtain the predefined light distribution of the vehicle headlight in which 180° -15° degrees is blocked.

Fig. 2B shows a cross-sectional view of a second embodiment of the high pressure discharge lamp 10 according to the invention. Again, the high pressure discharge lamp 10 comprises a discharge vessel 12 and the discharge arc 20 displaced from the longitudinal axis 30 along the displacement axis 32. The reflective surface 42 again is arranged at an opposite side of the imaginary plane 34 compared to the discharge arc 30 and is arranged to reflect light from the discharge arc 30 towards the light-transmission window 62. In contrast with the cross-sectional view as shown in Fig. 2A in which the light- transmission window 60 is arranged at a first intersection 35, now the light transmission window 62 is arranged at a second intersection 34 between the imaginary plane 34 and the discharge vessel 12. Again, the reflective surface 42, for example, is inclined compared to the discharge vessel 12 and may, for example, comprise a different curvature compared to the substantially circular cross-section of the discharge vessel 12 to optimally illuminate the light-transmission window 62. Furthermore, the light-transmission window 62 as shown in Fig. 2B is fully arranged at the same side with respect to the imaginary plane 34 as the discharge arc 20. The inventors have found that also an increase in the light flux through the light-transmission window as indicated in Fig. 2B would improve the light flux at the center part of the predefined light distribution of a vehicle headlight which improves the visibility to the driver.

Fig. 2C shows a cross-sectional view of the third embodiment of the high pressure discharge lamp 10 according to the invention. In the embodiment shown in Fig. 2C the discharge vessel comprises the reflective surface 40 and comprises the further reflective surface 42. Both the reflective surface 40 and the further reflective surface may each be inclined with respect to the discharge vessel 12 in which the inclination of the reflective surface 40 and the further reflective surface 42 relative to the discharge vessel 12 may be different from each other. Also the curvature of the reflective surface 40 and of the further reflective surface 42 may be different than the curvature of the circular cross-section of the discharge vessel 12 in which the curvature of the reflective surface 40 may, for example, also be different from the curvature of the further reflective surface 42. As is shown in Fig. 2C the reflective surface 40 is arranged to reflect the light from the discharge arc 20 away from the discharge arc 20 towards the light-transmission window 60 and the further reflective surface 42 is arranged to reflect the light from the discharge arc 20 away from the discharge arc 20 towards the further light-transmission window 62. The light-transmission window (60; 62) and/or the further light-transmission window (62) each cover a transmission circumferential angle (φl, φ2) around the longitudinal axis (30) of less than 50 degrees. Both the reflective surface 40 and the further reflective surface 42 are arrange to prevent all or a major part of the reflected light to pass through the discharge arc 20 thus improving an efficiency of the high pressure discharge lamp 10 according to the invention. Fig. 2C shows the first angle CC 1 and the second angle cc2 defining the borders of the first reflective circumferential angle CC around the longitudinal axis 30 of the reflective surface 40. The first angle ccl may lie in a range between 0 degrees and 35 degrees. The second angle cc2 may lie in a range between 55 degrees and 65 degrees. Fig. 2C also shows the third angle βl and the fourth angle β2 defining the borders of the second reflective circumferential angle β around the longitudinal axis 30 of the further reflective surface 42. The third angel βl may lie in a range between 125 degrees and 135 degrees. The fourth angle β2 may lie in a range between 155 degrees and 165 degrees. Fig. 2D shows a fourth embodiment of the high pressure discharge lamp 10 according to the invention in which the reflective surface 44 and the further reflective surface 46 are inclined compared to the discharge vessel 12 via protrusions 45 from the outer wall 11 of the discharge vessel 12. These protrusions 45 may be produced by attaching additional material, for example, translucent material to the outer surface 11 of the discharge vessel 12, for example, via gluing or, for example, via locally melting the discharge vessel 12 or the protruding material. Alternatively, the protrusions may be produced, for example, by the same method for shaping the discharge vessel 12, for example, using a mould. A benefit of using a mould is that the production of the protrusion 45 is relatively simple and that the use of the same mould improves the reproducibility of the shape of the protrusion 45. The inclination of the reflective surface 44 and of the further reflective surface 46 enables the reflected light to be redirected towards the light-transmission window 60 and the further light transmission window 62, respectively, while preventing all or a majority of the reflected light to pass through the discharge arc 20. Also the curvature of the reflective surface 44 and of the further reflective surface 46 may be different than the curvature of the circular cross-section of the discharge vessel 12 in which the curvature of the reflective surface 44 may, for example, also be different from the curvature of the further reflective surface 46.

In the embodiment shown in Fig. 2D the high pressure discharge lamp 10 comprises both the reflective surface 44 and the further reflective surface 46 to increase the light flux through the light-transmission window 60 and the further light transmission window 62, respectively. However, also the use of only the reflective surface 44 or only the further reflective surface 46 would already improve the efficiency of the light emitted by the high pressure discharge lamp 10 and would result in a high pressure discharge lamp 10 according to the invention. In embodiments of the high pressure discharge lamp 10 as shown in Figs. 2A to 2D the outer envelope of the high pressure discharge lamp has been omitted. In alternative embodiments of the high pressure discharge lamp 10 according to the invention the high pressure discharge lamp 10 may have an outer envelope 70 (see Figs. 3 A and 3B). This outer envelope 70, for example, is applied as an insulating shield to avoid excessive heat loss and to keep the discharge vessel 12 at a relatively high temperature while minimizing the power required to maintain this relatively high temperature.

Figs. 3A and 3B show a further cross-sectional views perpendicular to the longitudinal axis 30 of the high pressure discharge lamp 10 according to the invention, in which the reflective surface 50, 52 is arranged at the outer envelope 70 of the high pressure discharge lamp 10. The high pressure discharge lamps 10 again comprise the discharge vessel 12 and the discharge arc 20 displaced with respect to the longitudinal axis 30 along the displacement axis 32. The high pressure discharge lamps 10 as shown in Figs. 3 A and 3B also show the light-transmission window 60 and the further light-transmission window 62, respectively. The reflective surface 50 and the further reflective surface 52 are arranged at an outer wall of the outer envelope 70 and are inclined with respect to the circular cross-section of the outer envelope 70 to reflect the light from the discharge arc 20 away from the discharge arc 20 towards the light-transmission surface 60 and the further light-transmission surface 62, respectively. Alternatively, the reflective surface 50 and/or the further reflective surface 52 may be arranged at an inner wall of the outer envelope 70 (not shown).

Furthermore, the inclination between the reflective surface 50 and the outer envelope 70 and between the further reflective surface 52 and the outer envelope 70 may, for example, provided using an indentation in the outer surface 70 (not shown).

Fig. 3A shows the fifth angle γl and the sixth angle γ2 defining the borders of the third reflective circumferential angle γ around the longitudinal axis 30 of the reflective surface 50. The fifth angle γl may lie in a range between 0 degrees and 35 degrees. The sixth angle γ2 may lie in a range between 55 degrees and 65 degrees.

Fig. 3 B shows the seventh angle δl and the eighth angle δ2 defining the borders of the fourth reflective circumferential angle δ around the longitudinal axis 30 of the further reflective surface 52. The seventh angel δl may lie in a range between 125 degrees and 135 degrees. The eighth angle δ2 may lie in a range between 155 degrees and 165 degrees.

In an alternatively embodiment (not shown) of the high pressure discharge lamp 10 according to the invention, the high pressure discharge lamp 10 comprises both the reflective surface 50 and the further reflective surface 52, comparable to the embodiment shown in Fig. 2C. In a further alternative embodiment, the reflective surface 40 may be arranged at the outer surface 11 of the discharge vessel 12 and the further reflective surface 52 may be arranged at the outer envelope 70, or vice versa.

Fig. 4 shows a cross-sectional view of a vehicle headlight 100 according to the invention. The vehicle headlight 100 comprises a paraboloidal mirror 102 for reflecting the light from the high pressure discharge lamp 10 onto the road in front of the vehicle. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A high pressure discharge lamp (10) comprising a discharge vessel (12) enclosing a discharge space (14) being provided with an ionisable filling comprising one or more halides, the discharge space (14) comprising electrodes (16, 18) arranged opposite of each other for creating, in operation, a discharge arc (20) and for defining a longitudinal axis (30) connecting the electrodes (16, 18), the discharge arc (20) being displaced with respect to the longitudinal axis (30) at a distance on a displacement axis (32) perpendicular to the longitudinal axis (30), the high pressure discharge lamp (10) further comprising a reflective surface (40; 42; 44; 46; 50; 52) for reflecting light from the discharge arc (20) towards a light- transmission window (60; 62) of the discharge vessel (12) for increasing a light flux emitted from the discharge vessel (12) through the light-transmission window (60; 62) while substantially preventing the reflected light to pass through the discharge arc (20), the longitudinal axis (30) being part of an imaginary plane (34) arranged perpendicular to the displacement axis (32) and dividing the discharge vessel (12), the reflective surface (40; 42; 44; 46; 50; 52) and the discharge arc (20) being arranged on opposite sides of the imaginary plane (34), and the reflective surface (40; 42; 44; 46; 50; 52) being arranged to redirect the reflected light away from the discharge arc (20) towards the light-transmission window (60; 62).

2. A high pressure discharge lamp (10) as claimed in claim 1, wherein the high pressure discharge lamp (10) comprises a further reflective surface (42; 46; 52) for reflecting light from the discharge arc (20) to a further light-transmission window (62) of the discharge vessel (12) for increasing a light flux emitted from the discharge vessel (12) through the further light-transmission window (62) while substantially preventing the reflected light to pass through the discharge arc (20), the further reflective surface (42; 46; 52) and the discharge arc (20) being arranged on opposite sides of the imaginary plane (34), and the further reflective surface (42; 46; 52) being arranged to redirect the reflected light away from the discharge arc (20) towards the further light-transmission window (62).

3. A high pressure discharge lamp (10) as claimed in claim 1 or 2, wherein the light-transmission window (60; 62) and/or the further light transmission window (62) is arranged in a vicinity of an intersection (35, 36) between the imaginary plane (34) and the discharge vessel (12), the light-transmission window (60; 62) and/or the further light- transmission window (62) each cover a transmission circumferential angle (φl, φ2) around the longitudinal axis (30) of less than 50 degrees.

4. A high pressure discharge lamp (10) as claimed in claim 1, 2 or 3, wherein the reflective surface (40; 44) and/or the further reflective surface (42; 46) are arranged at an outer wall (11) of the discharge vessel (12).

5. A high pressure discharge lamp (10) as claimed in claim 4, wherein the reflective surface (40; 44) and/or the further reflective surface (42; 46) are inclined with respect to the discharge vessel (12) via a protrusion (45) from the outer wall (11) or via an indentation (41 ) in the outer wall (11).

6. A high pressure discharge lamp (10) as claimed in claim 4 or 5, a cross-section of the discharge vessel (12) perpendicular to the longitudinal axis being a substantially ellipsoidal cross-section of the discharge vessel (12), wherein a curvature of the reflective surface is different from a curvature of the ellipsoidal cross-section of the discharge vessel (12).

7. A high pressure discharge lamp (10) as claimed in claim 4, 5 or 6, wherein the reflective surface (40; 44) covers a first reflective circumferential angle (α) around the longitudinal axis (30), the first reflective circumferential angle (α) being arranged between a first angle (αl) and a second angle (cc2) with respect to the imaginary plane (34), wherein 0 degrees < αl < 35 degrees, and 55 degrees < cc2 < 65 degrees, and wherein the further reflective surface (42; 46) covers a second reflective circumferential angle (β) around the longitudinal axis (30), the second reflective circumferential angle (β) being arranged between a third angle (βl) and a fourth angle (β2) with respect to the imaginary plane (34), wherein

125 degrees < βl < 135 degrees, and 155 degrees < β2 < 165 degrees.

8. A high pressure discharge lamp (10) as claimed in claim 1, 2 or 3, the high pressure discharge lamp comprises an outer envelope (70), wherein the reflective surface (50) and/or the further reflective surface (52) are arranged at the outer envelope (70).

9. A high pressure discharge lamp (10) as claimed in claim 8, wherein the reflective surface (50) and/or the further reflective surface (52) are inclined with respect to the outer envelope (70) via a protrusion (72) from the outer envelope (70) or via an indentation in the outer envelope (70).

10. A high pressure discharge lamp (10) as claimed in claim 8 or 9, a cross-section of the outer envelope (70) perpendicular to the longitudinal axis (30) being a substantially ellipsoidal cross-section of the outer envelope (70), wherein a curvature of the reflective surface is different from a curvature of the ellipsoidal cross-section of the outer envelope (70).

11. A high pressure discharge lamp (10) as claimed in claim 8, 9 or 10, wherein the reflective surface (50) covers a third reflective circumferential angle (γ) around the longitudinal axis (30), the third reflective circumferential angle (γ) being arranged between a fifth angle (γl) and a sixth angle (γ2) with respect to the imaginary plane (34), wherein 0 degrees < γl < 35 degrees, and 55 degrees < γ2 < 65 degrees, and wherein the further reflective surface (52) covers a fourth reflective circumferential angle (δ) around the longitudinal axis (30), the fourth reflective circumferential angle (δ) being arranged between a seventh angle (δl) and a eight angle (δ2) with respect to the imaginary plane (34), wherein 125 degrees < δl < 135 degrees, and 155 degrees < δ2 < 165 degrees.

12. Vehicle headlight (100) comprising a high pressure discharge lamp (10) as claimed in claims 1 or 2.