EP0324652A1

EP0324652A1 - Vehicle headlamp with controlled beam position

Info

Publication number: EP0324652A1
Application number: EP89300340A
Authority: EP
Inventors: Harold L. Jr. Intellectual Dept. Rothwell; George J. Intellectual Prop. Dept. English
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1988-01-14
Filing date: 1989-01-13
Publication date: 1989-07-19
Also published as: JPH01215639A

Abstract

A high intensity arc discharge light source includes an arc lamp (20) for producing a high intensity arc (21) between a pair of electrodes, means (40) for producing a magnetic field for deflecting the arc, and means (44) for controlling the deflection of the arc by controlling the magnetic field. The deflection of the arc can be utilized to control the intensity of a light beam, the focus of a light beam or the direction of a light beam when the arc lamp is utilized in an optical system. A variable intensity projection system includes an arc lamp (20), an aperture plate (26) having an aperture (28), a lens (24) for focusing an image of the arc on the aperture and means (44,40) for applying a controlled magnetic field to the arc so as to vary the position of the arc and thereby vary the intensity of the beam passing through the aperture (28). A vehicle headlamp includes an arc lamp (80) positioned at the focal point (83) of a reflector (84) a lens (88) and a coil (92) for producing a magnetic field in the region of the arc. The magnetic field deflects the arc relative to the focal point (83) so as to change the direction of the headlamp light beam. The beam can be controlled by the magnetic field between high and low beam positions or can be varied continuously between upper and lower limits. An electric field can also be utilized for deflection of the arc.

Description

This invention relates to vehicle headlamps utilizing high intensity arc discharge lamps and, more particularly, to a vehicle headlamp wherein the beam position is controlled by a magnetic or an electric field applied to the arc.
High intensity arc discharge lamps, such as metal halide lamps, mercury vapor lamps and high pressure sodium lamps, are widely used for general lighting purposes, but are also utilized in numerous applications wherein the light output from the lamp passes through an optical system and forms a light beam. Examples of such systems include projection systems, vehicle headlamps, spotlights and the like. In many of those applications, it is desirable to control the light intensity, the focus or the beam direction. For example, in projection systems, it is often desirable to vary the light intensity, and in vehicle headlamps, it is necessary to provide high and low beam positions.
Arc discharge lamps include a sealed arc tube, a pair of spaced-apart electrodes within the arc tube and a fill material that produces a luminous arc discharge when suitably energized. In operation, an arc discharge,consisting of a plasma of charged particles and carrying an arc current, is sustained between the electrodes. Some lamp types are designed for operation with an a.c. power source while others are designed for operation with a d.c. power source.
Usually it is not desirable to dim arc discharge lamps by reducing the power input, since this changes the spectral power distribution. Therefore, dimning of optical systems utilizing metal halide lamps is normally accomplished by mechanical devices such as irises, shutters, devices for moving the lamp with respect to a focal point, etc. Such mechanical devices are relatively slow and unreliable.
In the case of vehicle headlamps, high and low beam positions are normally provided by separate lamps, one for a high position and one for a low position, or by a single lamp assembly with two filaments, one for high beam and one for low beam. In either case, the cost is increased by the necessity for separate high and low beam elements. Recently, it has been proposed to utilize arc discharge lamps in vehicle headlamps because they have a longer operating life than tungsten filament headlamps and do not include a filament that is subject to breakage resulting from shock.
It is known that when a high pressure arc discharge lamp is operated with the arc axis in a horizontal orientation, the arc arches upwardly between the electrodes due to convection currents within the arc lamp. The convection currents occur because of the tendency of the hot gases in the arc to rise. It is further known in the prior art that the position of an arc can be stabilized by the application of a fixed magnetic field. Magnetic arc stabilization is disclosed in U.S. Patent No. 3,113,234 issued December 3, 1963 to Schlegel and in U.S. Patent No. 4,001,626 issued January 4, 1977 to Drop et al. The use of a fixed magnetic field for spreading the arc within an arc tube so as to increase the luminous output and the operating efficiency, is disclosed in U.S. Patent No. 4,443,734 issued April 17, 1984 to Gross et al. In U.S. Patent No. 4,434,385 issued February 28, 1984 to Touho et al, magnets are utilized to bias the discharge zone, and therefore the light intensity pattern, in a fluorescent lamp.
It is a general object of the present invention to provide improved vehicle headlamps.
It is another object of the present invention to provide vehicle headlamps utilizing high intensity arc discharge lamps.
It is a further object of the present invention to provide a vehicle headlamp having a single arc lamp for producing high and low beam positions.
It is a further object of the present invention to provide a vehicle headlamp having a continuously variable beam position.
It is yet another object of the present invention to provide a vehicle headlamp wherein beam position is controlled by a magnetic or an electric field.
It is another object of the present invention to provide a vehicle headlamp having long operating life and high reliability.
It is another object of the present invention to provide a vehicle headlamp wherein the beam position can be electronically controlled from a selected sensor or switch.
According to the present invention, these and other objects and advantages are achieved in a vehicle headlamp comprising an arc lamp for producing a high intensity arc between a pair of electrodes, the arc in normal use having a horizontal orientation, optical means positioned to direct a portion of the light from the arc lamp for forming a light beam, magnetic means for producing a magnetic field in the region of the arc, the magnetic field having at least a component that is horizontal and perpendicular to the arc and means for controlling the magnetic field so that a portion of the arc is deflected upwardly or downwardly by the magnetic field, whereby the light beam is deflected upwardly or downwardly.
Preferably, the optical means comprises a reflector, and the arc lamp is located at or near the focal point of the reflector. The control means can be utilized for switching the magnetic field so as to switch the light beam between the high and low beam positions. In addition, the control means can include means for continuously varying the magnetic field so as to raise or lower the light beam.
The magnetic field that controls the beam direction can be controlled by a hand switch, a foot switch or a photocell for sensing oncoming vehicle headlamps. The magnetic field can also be controlled in response to the output of a level sensor mounted on the vehicle so that the beam direction is varied as the vehicle goes up or down hills or tips when rounding corners.
The magnetic means can include a conductive coil positioned adjacent to the arc and means for supplying an electrical current through the coil. The arc lamp can be positioned at the focal point of the reflector and oriented perpendicular to the reflector axis. In this case, the coil is positioned to generate a magnetic field parallel to the reflector axis. Alternatively, the arc lamp can be positioned at the focal point of the reflector and oriented parallel to the reflector axis. In this case, the coil is positioned to generate a magnetic field perpendicular to the reflector axis.
When the arc lamp is energized by an a.c. power source of a predetermined frequency, the magnetic means includes means for producing an a.c. magnetic field of the same predetermined frequency, and the magnetic field and the a.c. power source have a fixed phase relationship. When the arc lamp is energized by a d.c. power source, the magnetic means includes means for producing a d.c. magnetic field.
According to another aspect of the invention, a vehicle headlamp includes an arc lamp, optical means for directing light from the arc lamp and forming a light beam, and means for producing an electromagnetic field for deflection of the arc from its normal path so as to deflect the light beam. The electromagnetic field can be a magnetic field as described above or an electric field, typically produced between deflection plates positioned on opposite sides of the arc.

Brief Description of the Drawings

For a better understanding of the present invention together with other and further objects, advantages and capabilities thereof, reference is made to the accompanying drawings which are incorporated herein by reference and in which:

FIG. 1A is a schematic diagram illustrating the upward arching of an arc due to convection currents;
FIGS. 1B-1D are schematic diagrams illustrating the deflection of an arc by a magnetic field;
FIG. 2 is a schematic diagram of a high intensity arc discharge light source having controllable intensity;
FIG. 3 is a schematic diagram of a high intensity arc discharge light source having controllable intensity;
FIG. 4 is a side elevation view of a vehicle headlamp having a magnetically-controlled beam direction;
FIG. 5 is a plan view of the vehicle headlamp of FIG. 4 with the reflector removed;
FIG. 6 is a perspective view, partially cut away, of a vehicle headlamp having a magnetically-controlled beam direction;
FIGS. 7A and 7B are side elevation views of the vehicle headlamp of FIG. 4 illustrating high and low beam positions;
FIG. 8 is a simplified perspective view of a vehicle headlamp with an arc lamp aligned with the reflector axis;
FIG. 9 is a schematic diagram of a spotlight having magnetically-controlled focus; and
FIG. 10 is a schematic diagram illustrating deflection of an arc by an electric field.

Detailed Description of the Invention

In accordance with the present invention, a high intensity arc discharge lamp is controlled by the application of a magnetic field to the arc within the arc discharge lamp. By varying the magnetic field in a controlled manner, the position of the arc within the arc lamp is controlled. Control of an arc with a magnetic field is described with reference to FIGS. 1A-1D. In each of FIGS. 1A-1D, a luminous arc discharge is formed between a pair of electrodes 10 and 12. It will be understood that the arc is formed within an arc tube containing a fill material which ionizes to form an arc discharge upon application of a suitable operating voltage. A magnetic field can be used to control the position of an arc in a metal halide lamp, a mercury vapor lamp, a high pressure sodium lamp or any other arc discharge lamp in which a well-defined arc is formed between a pair of electrodes. The details of the arc lamp construction are omitted for simplicity, since they are well-known in the art and form no part of the present invention.
An arc 14 with no applied magnetic field is illustrated in FIG. 1A. An arc axis 16 is defined as a straight line between electrodes 10 and 12. When the arc lamp is operated with the arc axis 16 in a horizontal orientation, the arc 14 arches upwardly as a result of convection currents within the arc tube.
Referring now to FIG. 1B, a magnetic field B is applied to the arc 14, causing it to be deflected from the position shown in FIG. 1A. The arc 14 consists of a plasma of charged particles and has an associated current, i, between electrodes 10 and 12. The magnetic field B interacts with the current ii to produce a force F_B on the charges in the arc 14. The resulting force F_B is perpendicular to the direction of the current, i, and to the magnetic field B. In general, the magnetic field can have any desired direction, but must have at least a component perpendicular to the arc 14, since the perpendicular component determines the force, F_B, on arc 14. In addition, the magnetic field must be disposed in the region of the arc 14 so that it interacts with the moving charged particles in the arc. In accordance with well-known electromagnetic principles, the force F_B is given by
F_B = ilBsin ϑ (1)
where i is the plasma current, l is the arc length between electrodes 10 and 12, B is the applied magnetic field and ϑ is the angle between the current, i, and the magnetic field, B. As shown in FIG. 1B, the arc 14 is deflected downwardly to coincide with arc axis 16. In FIG. 1C, the magnitude of the magnetic field is increased so that the arc 14 is deflected below arc axis 16. It will be understood that the ends of the arc 14 which terminate on electrodes 10 and 12 are not deflected by the magnetic field. The portion of the arc 14 between its ends is the part that is deflected.
While the applied magnetic field, B, has been illustrated in FIGS. 1B and 1C as opposing the upward curvature of the arc due to convection currents, it will be understood that the arc tube can have any desired orientation and that the applied magnetic field can likewise have any desired orientation relative to the arc, provided that the magnetic field has at least a component perpendicular to the arc 14. By increasing or decreasing the magnitude of the magnetic field B, the deflection of the arc 14 from its normal position is controlled in a manner described more fully hereinafter.
In order for the magnetic field to produce a fixed deflection of the arc in an arc lamp, it is necessary for the magnetic field and the arc current to have a fixed, instantaneous directional relationship. This is easily accomplished in the case of a d.c. lamp where both the arc current direction and the magnetic field direction remain fixed, except when movement of the arc is desired. In the case of an a.c. arc lamp, the current direction is continuously varying, and it is necessary to apply a magnetic field having the same frequency and a fixed phase relative to the arc current. Thus, the magnetic field and arc current track each other to produce a fixed deflection. When this relationship is not maintained, then an effective spreading of the beam is produced.
In an arc lamp which operates from a d.c. power source, the arc current, i, has a constant magnitude and direction. In this case, the application of a d.c. magnetic field causes a fixed deflection of the arc 14. The direction of the deflection is perpendicular to the directions of the arc current and the applied magnetic field. The magnitude of the deflection is a function of the magnitude of the applied magnetic field.
In the case of an arc lamp that is operated from an a.c. power source, the arc current varies in a sinusoidal manner, and, a d.c. magnetic field causes a sinusoidally varying force to be applied to the arc. The effect is that the arc rapidly oscillates and appears to the human eye to be widened or spread out. In order to obtain a fixed deflection of an arc having an a.c. arc current, it is necessary to apply a magnetic field that has the same time variation as the arc current. For example, when the arc current varies sinusoidally, a magnetic field having the same frequency as the arc current and locked in phase to the arc current produces a deflection of constant direction and magnitude. This can be accomplished in practice by energizing the lamp and the magnetic field from the same a.c. source. The arc deflection can be varied by changing the magnitude of the a.c. magnetic field while maintaining the same frequency and a fixed phase relative to the arc current. The direction of the arc deflection can be reversed by reversing the phase of the magnetic field relative to the arc current.

In some cases, it is desirable to spread the arc 14 so that it is wider than normal in order to vary the focus of a light beam, as described hereinafter. The spreading of the arc 14 can be accomplished as shown in FIG. 1D by applying an a.c. magnetic field to an arc having a d.c. current, or by applying a d.c. magnetic field to an arc having an a.c. current. Alternatively, an a.c. arc can be spread by an a.c. magnetic field having a frequency that is different from the frequency of the arc current. In each case, the resulting force on the beam is time varying and causes a time varying arc deflection which appears as an arc spreading. The required magnetic fields for a.c. and d.c. arc currents are summarized in Table 1.

TABLE 1

Arc Current	Magnetic Field For Fixed Deflection	Magnetic Field For Arc Spreading
a.c.	a.c., same frequency and fixed phase relative to arc current	d.c., or a.c. of different frequency from arc current frequency
d.c.	d.c.	a.c.

An optical system utilizing an arc lamp having a magnetically-deflected arc is illustrated in FIG. 2. An arc lamp 20, such as a metal halide arc lamp, is oriented with its arc axis 22 horizontal. The arc.lamp 20 can be a metal halide lamp. The power source for arc lamp 20 is omitted from FIG. 2 for simplicity, since arc lamp power sources are well-known in the art. An image of an arc 21 in arc lamp 20 is focused by a lens 24 on an aperture plate 26 having an aperture 28. In one illustrative example, the system of FIG. 2 can be a projection system in which a projection lens 29 projects an image of aperture 28 so that a beam pattern 30 is formed on a projection screen (not shown). An optical axis 34 passes through arc 21, lens 24, aperture 28 and projection lens 29.
A magnetic apparatus 32 generates a magnetic field parallel to optical axis 34. The magnetic field causes the arc 21 to be deflected upwardly or downwardly in a direction perpendicular to optical axis 34. As a result, the image of arc 21 is deflected relative to aperture 28, causing the intensity of beam pattern 30 to vary.
In a preferred embodiment, the arc lamp 20 is energized by a 60 Hz a.c. power source. The magnetic apparatus 32 includes an electrically-conductive coil 40 oriented in a plane perpendicular to optical axis 34 and positioned in close proximity to arc 21. The coil 40 includes multiple turns of an electrical conductor coupled through a transformer 42 to a control unit 44. The control unit 44 receives 60 Hz a.c. power and supplies to coil 40 a current that is controlled by a control signal. The magnitude of the magnetic field generated by coil 40 is a function of the a.c. current supplied through it. Accordingly, the control signal directly controls the position of the arc 21 and the intensity of the beam pattern 30. The control unit 44 can be a variable transformer or any other suitable device for controlling the current supplied to coil 40.
Another optical system utilizing an arc lamp having an arc that is controllably deflected by a magnetic field is illustrated in FIG. 3. An arc lamp 50 is oriented so that its arc axis coincides with an optical axis 52. The arc lamp 50 is positioned so that an arc 54 within arc lamp 50 is at the focal point of a reflector 56, typically an eliptical reflector. The reflector 56 reflects a portion of the light from lamp 50 so as to form a light beam 58 in the direction of optical axis 52. Light beam 58 illuminates an aperture plate 60 having an aperture 62. A portion of light beam 58 passes through aperture 62 and is focused by a projection lens 64. The focused light beam can be used for projection.
A magnetic apparatus 68, which generates a magnetic field perpendicular to optical axis 52, includes an electrically conductive coil 70 positioned adjacent to arc 54. Preferably, the plane of coil 70 is perpendicular to optical axis 52. Coil 70 includes multiple turns of an electrical conductor and is coupled through a transformer 72 to a control unit 74. The control unit 74 receives a.c. power from a suitable source and supplies current to coil 70 under control of a control signal. The current supplied to coil 70 causes generation in the region of arc 54 of a magnetic field which causes arc 54 to be deflected relative to the focal point of reflector 56. The deflection of arc 54, in turn, causes beam 58 to change direction and to be shifted relative to aperture 62. As a result, the intensity of beam pattern 64 is varied as a function of the magnetic field applied to arc 54.
A vehicle headlamp incorporating a high intensity arc discharge lamp and means for deflection of the arc with a controllable magnetic field is shown in FIGS. 4-6. An arc lamp 80 is positioned at a focal point 83 of a reflector 84. The arc lamp 80 is positioned such that during normal use an arc 86 within lamp 80 is horizontal and is perpendicular to an axis 82 of reflector 84. A headlamp lens 88 is positioned in front of reflector 84 on axis 82. When the reflector 84 is a parabolic reflector, the combination of arc lamp 80, reflector 84 and lens 88 produce a substantially collimated light beam 90 that is suitable for illuminating the way in front of a vehicle.
A coil 92 is mounted within reflector 84 in close proximity to arc lamp 80. The coil 92 comprises multiple turns of an electrical conductor oriented to produce a magnetic field parallel to axis 82 and passing through arc lamp 80. When the arc lamp 80 is energized by a.c. power, the coil 92 is supplied through a transformer 94 with a.c. current from a control unit 96. As described above, the a.c. current supplied to coil 92 should be of the same frequency and fixed in phase relative to the arc current in lamp 80. By varying the current supplied to coil 92, the arc 86 is deflected upwardly or downwardly relative to focal point 83 and causes beam 90 direction to be raised or lowered as described hereinafter. The control unit 96 receives a.c. power from a suitable power source, usually a d.c.-to-a.c. converter in a vehicle, and controls the current through coil 92 in response to a control input from a sensor or a switch 98.
In a preferred embodiment, the arc lamp 80 is an oval shaped 45 watt metal halide arc lamp. This lamp has an overall length of about four centimeters, an arc length of about five millimeters and requires 90 volts a.c. and 0.5 amperes for operation. A preferred coil 92 utilizes approximately 30 turns of 24 gauge .022 inch(0.56mm) diameter enamel wire, wound around an insulating sleeve 99. With reference to FIG. 6, lamp leads 80a and 80b and coil leads 92a and 92b pass through sleeve 99 and reflector 84, and are connected to suitable power sources.
Operation of the vehicle headlamp shown in FIGS. 4-6 to produce high and low beams is illustrated with reference to FIGS. 7A and 7B. In FIG. 7A, arc 86 is located on axis 82 at focal point 83, and light beam 90 is directed by reflector 84 parallel to axis 82. In the present example, the arc 86 is maintained at focal point 83 by a magnetic field, B, supplied from coil 92. Referring now to FIG. 7B, the arc 86 is displaced upwardly relative to focal point 83. In the present example, the upward displacement results from the removal, or turning off, of the applied magnetic field. The light beam 90 is now directed by reflector 84 at a downward angle relative to axis 82. Thus, there is provided a high beam position and a low beam position controlled by the magnetic field, B, from coil 92.
It will be understood that the magnetic fields selected for producing high and low positions are a matter of design choice. As an alternative to the above-described configuration, an applied magnetic field can position arc 86 above focal point 83, as shown in FIG. 7B, while the removal of the magnetic field can position the arc 86 at the focal point 83, as illustrated in FIG. 7A. Furthermore, the high and low positions can be established by two different magnetic field magnitudes rather than on and off conditions.
Another important feature is that the vehicle headlamp disclosed herein is not limited to high and low beam positions. Instead, the angle between light beam 90 and axis 82 can be varied continuously between upper and lower limits, depending on the desired operation. In one application, the beam 90 is controlled between high and low beam positions by a hand switch, a foot switch or a photocell for sensing oncoming vehicle headlamps. In another embodiment, the direction of light beam 90 is controlled by a level sensor mounted on the vehicle. The level sensor provides an out-of-level indicator signal when the vehicle goes up or down hills, or tips from side to side when rounding a corner. The level indicator signal can be provided to the control unit 96 so as to adjust the direction of the light beam 90 as desired. It will be understood that any other desired sensor, switch or control device can be utilized to control the direction of light beam 90.
The reflector 84 is not necessarily parabolic, but can be elliptical, spherical or any other suitable shape. As shown in FIG. 8, an arc lamp 91 can be oriented so that the arc axis coincides with an axis 93 of a reflector 95. In this case, the coil is positioned either to the left or to the right of arc lamp 91 in order to obtain a magnetic field perpendicular to axis 93 and an arc deflection in the upward or downward direction. Alternatively, two (or more than two) coils 100, 101 can be used for deflection of the arc. As shown in FIG. 8, the coils 100 and 101 are positioned on opposite sides of arc lamp 91. The coils 100, 101 are rectangular or otherwise shaped to minimize blockage of the light path between the arc lamp 91 and the reflector 95. The same or separate power sources can be utilized to energize coils 100, 101. However, the above-described relationships between the arc current and the magnetic field must be maintained. Typically, the size and shape of the reflector are selected to complement the size, shape and orientation of the arc lamp.
According to another feature of the invention, a magnetically-controlled arc lamp can be utilized to focus or defocus a light beam, as illustrated in FIG. 9. An arc lamp 102 is positioned on an axis 104 at the focal point 106 of a reflector 108. An arc 110 within arc lamp 102 is oriented perpendicular to axis 104. With no applied magnetic field, the arc 110 is relatively small and compact and produces a focused light beam 112. To defocus and spread the light beam, a magnetic field is applied to the arc 110, as shown in FIG. lD and described hereinabove. In order to spread the beam rather than deflect it in a fixed direction, the magnetic field must be time varying in relation to the arc current. When a d.c. arc lamp is utilized, the magnetic field is a.c. and, conversely, when an a.c. arc lamp is utilized, the magnetic field is d.c. or a.c. of a different frequency from the arc current. The required magnetic field is provided by a coil 114 which is positioned on the axis 104 so as to produce a magnetic field along axis 104 through arc 110. The magnetic field causes a spreading of the arc as indicated at 116, and a defocused, spread-out light beam 118 is produced.
Arc deflection has been described thus far in connection with the force applied by a magnetic field to the moving charged particles in the arc. Arc deflection can also be accomplished by an applied electric field as shown in FIG. 10. An arc 130 is formed between a pair of electrodes 132, 134. An electric field E, having at least a component perpendicular to arc 130, is applied in the region of the arc 130 by deflection plates 136 and 138 positioned on opposite sides of arc 130. When a voltage is applied between plates 136 and 138, the electric field E is produced in the region between them. The deflection plates 136, 138 can have any convenient size or shape for producing the desired electric field E in the region of arc 130. The electric field E deflects the arc 130 in the same manner as an applied magnetic field, except that the force in arc 130 is parallel to the electric field E, whereas the force is perpendicular to an applied magnetic field B. The arc deflection arrangement shown in FIG. 10 can be utilized in the optical systems shown in FIGS. 2-9 and described hereinabove.
While there has been shown and described what is at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

1. A vehicle headlamp characterised in that it comprises:
an arc lamp (80,91) for producing a high intensity arc between a pair of electrodes, said arc in normal use having a substantially horizontal orientation;
optical means (84,95) positioned to direct a portion of the light from said arc lamp for forming a light beam; and
means (92,100:101,136:138) for producing an electric or magnetic field in the region of said arc for interaction with and deflection of said arc from its normal path so as to deflect said light beam upwardly or downwardly.

2. A vehicle headlamp as claimed in Claim 1 characterised in that said means (92,100:101) for producing said field produces a magnetic field having at least a component perpendicular to said arc so that said arc is deflected upwardly or downwardly.

3. A vehicle headlamp as claimed in Claim 1 or 2 characterised in that said optical means (84,95) comprises a reflector.

4. A vehicle headlamp as claimed in Claim 3 characterised in that said arc lamp (80) is located at or near the focal point (83) of said reflector (84).

5. A vehicle headlamp as claimed in Claim 2, or Claim 3 or 4 as appended to Claim 2, characterised in that said arc (80,91) is energized by an a.c. power source of a predetermined frequency and that said means for producing said field includes means for producing an a.c. magnetic field of said predetermined frequency, said magnetic field and said a.c. power source having a fixed phase relationship.

6. A vehicle headlamp as claimed in any one of Claims 1-5, in combination with control means (96,98) for switching said field so as to switch said light beam between a high position and a low position.

7. A vehicle headlamp as claimed in any one of Claims 1-5, in combination with control means (96) for continuously varying said field so as to raise or lower said light beam.

8. A combination as claimed in Claim 7 characterised in that it further includes level sensing means (98) responsive to the orientation of a vehicle on which said headlamp may be mounted and that said control means (96) is adapted to respond to said sensing means to vary said field and to control the vertical beam direction in accordand with the vehicle orientation.

9. A vehicle headlamp as defined in Claims 2 and 3 or any one of Claims 4-8 as appended thereto characterised in that said reflector (84) has a reflector axis, that said arc lamp (80) is positioned at the focal point (83) of the reflector (84) and is oriented perpendicular to said reflector axis (82), that said means for producing said magnetic field includes a conductive coil (92) positioned adjacent to said arc and means (94) for supplying an electrical current through said coil (92) and that said coil (92) is positioned to generate said magnetic field parallel to said reflector axis (82).

10. A vehicle headlamp as claimed in Claim 3 or any one of Claims 4-9 as appended thereto, characterised in that said reflector (84,95) has a substantially parabolic shape.