EP0501668A2

EP0501668A2 - Improved light source design using ellipsoidal reflector

Info

Publication number: EP0501668A2
Application number: EP92301360A
Authority: EP
Inventors: John Martin Davenport; William Walter Finch; Richard Lowell Hansler
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1991-02-25
Filing date: 1992-02-19
Publication date: 1992-09-02
Also published as: EP0501668A3; JPH0582100A

Abstract

An improved light source and reflector arrangement particularly suited for use with a metal halide arc tube (28) wherein the reflector (20) is an ellipsoidal reflector and is disposed in a downward facing direction with the metal halide arc tube (28) being positioned vertically within the reflector (20). The light source and reflector arrangement, when used in an automotive lighting application requiring DC operation, has a thick diameter anode electrode (30) at the lower position and a thinner cathode electrode (32) at the upper position. The metal halide arc tube (28) is secured within the reflector (20) such that only the portion of the arc tube free of the presence of molten metal halide is disposed at the first optical focal point of the reflector.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

U.S. Patent Application Serial No. (to be assigned) entitled Improved Remote Vehicle Lighting System, filed herewith and assigned to the same assignee as the present invention, is related to the present invention.

FIELD OF THE INVENTION

This invention relates to an improved design for a light source using an ellipsoidal reflector. More particularly, this invention relates to such a light source as utilizes a gas discharge type of light source such as a metal halide arc tube, in conjunction with an ellipsoidal reflector and efficient optical fiber coupling of the light source output to a remote position.

BACKGROUND OF THE INVENTION

Because of the high intensity, color temperature, durability and efficiency characteristics exhibited by a gas discharge light source such as a metal halide arc tube, such a light source has found widespread acceptance in a number of fields including theatrical special effects, photography, medicine, industrial and commercial settings as well as vehicle lighting. In the field of vehicle lighting, the use of a metal halide arc tube in conjunction with a parabolic reflector for each of the forward lighting positions, which it is understood can be as many as four, has proven advantageous over the previously utilized incandescent type of lighting arrangement because of the above-listed gas discharge source characteristics. As applied to the field of vehicle lighting, the metal halide arc tube has been a part of the overall design effort directed at reducing the dimensions and numbers of components at the forward end of the vehicle. It has been well recognized and documented that if designers could reduce the profile of the vehicle front end, the effect would be to improve the aerodynamic performance and hence fuel efficiency of such vehicle.
With respect to the contribution of reducing the space for vehicle forward lighting, it should be recognized that present designs require a separate light source for each of the necessary lighting positions. In other words, since the vehicle must have both high and low beam capabilities on the passenger and driver side and such lighting requires individual light sources as well as reflectors to achieve the necessary spread and focussed portions of the standard pattern of road illumination, there is a significant amount of space presently utilized for vehicle forward lighting. One technique utilized to reduce this overall space requirement has been the introduction of a single light source whereby, the light generated by such single source is distributed to the various lighting positions by means of a light guide arrangement which typically employs a bundle of fiber optic cables broken out and distributed as necessary. In this manner, there is a substantial reduction in the amount of space needed to achieve proper vehicle forward illumination since there is no longer a need to have individual light source assemblies for up to the four light source positions. An example of a lighting design which provides such a single light source and distribution arrangement can be found in U.S. Patent Nos. 4,811,172 and 4,958,623 issued to Davenport et al. and which is assigned to the same assignee as the present invention.
As discussed above, to satisfy the necessary light distribution requirements, typically a bundled fiber optic arrangement is utilized. It is well known that optical fibers provide an efficient means for transmitting light from one location to another. The goal in coupling such fiber optics to the light source is to find the most efficient arrangement that results in the least amount of light lost between the light source output and the entry point of the grouping of optical fibers that will then be distributed throughout the vehicle. A known method of efficiently coupling the light source to the fiber optics is to provide an ellipsoidal reflector with the light source placed at one focus and the fiber optics placed at the other. Still another approach is to provide a collimating lens arrangement which focuses the light output from the source such that it can be more efficiently gathered at the entry point of the bundle of fiber optic cables. An example of the latter approach can be found in U.S. Patent No. 4,887,190 issued on December 12, 1989 to Sadumune et al. It should be noted however that this approach is directed to the field of providing a light source for theatrical special effects. Accordingly, there is not the same space restrictions that would affect an application for vehicle lighting in which the use of the additional components necessary to achieve the collimating function would be exactly contrary to the intended purpose of reducing the space needed for the lighting function.
Further to the goal of achieving the most efficient coupling between the light source and the fiber optics, there has been particular attention paid to the orientation of the light source with respect to the reflector used and the fiber optic end in which the reflected light is gathered. It can be appreciated that when a metal halide arc tube is utilized as the light source, there are certain characteristics of the metal halide lamp that must be taken into consideration. As a result of the cooling of the lower part of the bulb by the convection currents, the metal halide liquid tends to collect on this lower part of the arc tube. Therefore, in collecting light from a metal halide discharge lamp by means of an ellipsoidal mirror, the molten halide on the wall of the arc tube may block some of the light. If the lamp is operated horizontally, some of the light which would strike the lower part of the horizontally pointing reflector will be absorbed by the metal halide thereby resulting in an undesirable loss of intensity as well as the undesirable effect of producing a yellow color of light for that amount of light that has passed through the halide. Examples of gas discharge light sources disposed in a horizontal relation to the reflector can be found in U.S. Patent Nos. 4,860,172 issued to Schlager et al., 4,594,529 issued to de Vrijer as well as the previously discussed U.S. Patent No. 4,887,190.
As an alternative to the horizontal disposition of the light source with respect to the reflector, it is possible to orient the light source in a vertical relation to the reflector. An example of such an arrangement is can be seen in figure 10 of U.S. Patent No. 4,523,806 where the light source is illustrated as being centered at the optical axis of the reflector. Though such an arrangement overcomes the problem of having the molten metal halide residing in the bottom of the bulb portion of the arc tube such that the area in which the gas discharge occurs is free from the influence of the molten metal halide, such molten metal halide will still be present at the bottom end portion of the envelope and still adversely affects the light transmissive properties of the light source.
Though it is known that by disposing the metal halide arc tube vertically with respect to the elliptical reflector, advantages are provided over the horizontal disposition, the use of such an arrangement in a vehicle forward lighting arrangement requires further considerations. For instance, since the power source for the vehicle lighting is provided by means of a DC battery, consideration must be given to the effects of cataphoresis, which effects result in the driving of positive ions toward the cathode. Therefore, it would be advantageous if a light source were to overcome the effects of cataphoresis by enhancing the convection that keeps the halide distributed in the lamp. Additionally, a typical high intensity discharge lamp requires a warm-up period that generally precludes the ability to generate instantaneous light. In the field of vehicle forward lighting, it is essential that the high intensity light output be provided substantially at the instant at which it was demanded. Accordingly, it is necessary that an invention which provides an efficient coupling between the single source light output and the fiber optic light guide arrangement also provide the capability of attaining the high intensity output in a time which approaches instantaneous. To this end, it has been found that the introduction of one of the rare gases will aid in the starting operation. An example of a metal halide arc tube having a starting gas from the group of rare gases can be found in the previously cited U.S. Patent No. 4,594,529 issued to de Vrijer.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved light design for a gas discharge light source which is utilized in conjunction with a reflector and a light distribution arrangement which typically includes a bundle of optical fibers.
It is a further object of the invention to utilize a metal halide arc tube as the light source in such a manner that interference in light output by molten metal halide is substantially eliminated.
It is yet another object of the invention to provide such an improved light source as the single source of light for a vehicle forward lighting application in which the single light source output is efficiently coupled to a fiber optic light guide arrangement for distribution to the various forward lighting positions.
In accordance with the principles of the present invention, there is provided, an improved light source and coupling arrangement which utilizes a metal halide arc tube energized by means of a DC voltage as the source of light. The metal halide arc tube is disposed in a vertical manner within an ellipsoidal reflector such that the anode connection to the arc tube is disposed at the bottom of the arc tube as opposed to the topmost disposed cathode connection. Additionally, with the ellipsoidal reflector oriented in a downward pointing direction, the arc tube is disposed within the reflector in a manner which is offset from the optical focal point of the reflector. In this manner, the portion of the vertically oriented metal halide arc tube in which the molten liquid halide collects, does not interfere with the reflective properties of the light source within the ellipsoidal reflector.
The improved light source and optical coupling arrangement may further contain a mirrored element to reflect light into the bundle of optical fibers. Additionally, the improved light source and optical coupling arrangement may provide for a thermal isolation between the ballast used to drive the light source and the light source/coupling arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an elevational view partly in block diagram form of an improved light source and optical coupling arrangement constructed in accordance with the present invention.
Figure 2 is an elevational view partly in section detailing the light source and reflector portion of the present invention.

DESCRIPTION AND OPERATION

As seen in figure 1, an improved light source and optical coupling arrangement 10, constructed in accordance with the present invention, includes a first segment 12 which contains the lighting and optics components, and a second segment 14 which contains the voltage supply circuitry for energizing the lighting components. The output of the first segment 12 is illustrated as including a number of optical fibers 16 which extend to the various lighting positions associated with the vehicle (not shown). For a discussion of the ballast circuit shown in fig. 1 in block diagram form, U.S. Patent No. 4,904,907 issued to Allison et al. and assigned to the same assignee as the present invention, is referred to and is hereby incorporated by reference. The circuitry which comprises the ballast electronics of the second segment 14 is disposed within a separate container 18 which provides thermal isolation of the electronics contained therein from any heat generated by the components disposed in the first segment 12. The output of the ballast circuit of second segment 12 is coupled to a lamp device disposed within a reflector element 20, such lamp device to be described hereinafter in further detail.
The reflector device 20 shown as part of the first segment 12, is an ellipsoidal reflector oriented in a downward facing direction with respect to the actual orientation of the light output from the second segment 12 which is channelled to the appropriate lighting positions in a horizontal orientation. The output of the ellipsoidal reflector 20 is directed to the surface of a mirror element 22 which is oriented at approximately a 45 degree angle such that the transposition of the downward directed light output to a horizontal plane is achieved. The mirror element 22 can be a cold mirror; that is, the portions of the light output from the lamp that would result in the generation of heat would be removed by means of a filter arrangement. For this purpose, the mirror element 22 may be coated with multiple layers of a dielectric coating to reduce the amount of infrared and ultraviolet light incident on an optical coupling shown in fig. 1 in block diagram form as reference 24. The inner surface of reflector element 20 can be similarly coated for the same purpose. Of course, it is understood that a hot mirror could be used in conjunction with the cold mirror without departing from the scope of the present invention.
The optical coupling device 24 shown in fig. 1 performs the function of gathering at the input ends of the optical fibers 16, the highest percentage of input light as possible. For the present invention, the optical coupling can be affected merely by positioning the input ends (not shown) of the optical fibers at the focussed light spot shown here as reference (a) and occurring at the righthandmost side of the optical coupling device 24. It should be understood that the optical coupling, in its simplest form merely involves positioning of the optical fiber input ends at the light focus point (a), however, it is contemplated that the present invention encompasses additional coupling arrangements as for instance, a collimating lens arrangement instead of the mirror element 22 disclosed herein. Additionally, it should be understood that although reference has been made to the use of optical fibers, the present invention applies to systems utilizing other light transmissive means and, in fact, the use of the term "optical fibers" is intended to include an optical light guide which may have a larger diameter than a fiber and/or may be hollow in construction.
Similar to the enclosure 18 for the ballast electronics of the second segment 14, the first segment 12 includes a housing 26 in which the lighting and optical components are disposed in thermal separation from the ballast electronics of the second segment 14. It is also possible to dispose both the first and second segments 12 and 14 in a common housing to preserve space. In this approach, the thermal separation between the two segments could be achieved by means of a metal or thermal partition between the segments that would prevent the heat generated by the lighting devices from adversely affecting the electronics of the second segment 14.
The optimization of the light output for the first segment 12 is achieved by placement of the lamp 28 within the reflector 20 as shown in fig. 2. The lamp 28 is a metal halide arc tube vertically disposed within the downward facing reflector 20. As further seen in fig. 2, this invention is primarily directed to an automotive lighting application, which because such application relies on a DC source of energy, typically results in the metal halide lamp 28 having an anode electrode 30 disposed in the bottom end of a lamp envelope 34 and a cathode electrode 32, disposed at the upper end of the lamp envelope 34. It should be understood however that the DC output of the automotive battery could be modulated so that it would be possible to utilize a lamp operable by means of an AC source. The anode electrode 30 is disposed in the lower position to balance the otherwise asymmetric heating caused by the natural convection currents within the lamp. Since more heat must be dissipated by the hotter running anode electrode 30, for a lamp operating from a DC source, by disposing the anode electrode 30 in the lower position, a more symmetric balance of convection heating can be achieved than would otherwise be available. In operating a low wattage metal halide arc tube, which in this instance can be on the order of 35 watts, it is necessary to consider the effects of cataphoresis. Under the effects of cataphoresis, metal halide ions are influenced by the electric fields caused by either a DC power source or by a low frequency AC source such that the ions are driven towards one or both of the ends of the lamp envelope. It can be appreciated that under such an influence, these driven halides would not contribute to the amount of halides occurring between the electrodes and therefore, do not contribute to the desired illumination.
In DC operation particularly, the effect of cataphoresis would be to drive the halide ions toward the cathode electrode 32. By placing the anode electrode 30 at the bottom end of the vertically disposed metal halide lamp 28, there will be a more efficient overall convection within the lamp envelope 34 thereby resulting in a more even distribution of the metal halide and an enhanced lamp efficacy.
As further seen in fig. 2, the metal halide lamp 28 is constructed having the anode electrode 30 of a substantially greater thickness than that of the cathode electrode 32. In practice, one way of achieving the optimum convection symmetry is to provide the cathode electrode 32 constructed of approximately 9 mil tungsten rod whereas the anode electrode 30 would be constructed of approximately 40 mil bullet shaped tungsten rod. On the cathode end of the lamp envelope 34, an additional molybdenum outer lead 36 is disposed. Additionally, the end of the lamp envelope 34 in which the cathode electrode 32 is disposed is longer than the end for the anode electrode 30. The elongated portion 34a of the lamp envelope 34 serves the purpose of providing means for securing the metal halide lamp 28 at the optimum position within the reflector 20 as will hereinafter be discussed. Electrical connectors 38 and 40 provide the source of energy for the metal halide lamp 28 in a conventional manner.
Regarding the construction of the metal halide lamp 28, in addition to other known arrangements, lamp 28 can contain 1.9 mg of 19:1 molar ratio of sodium scandium iodide as well as 1.4 mg of Mercury and six atmospheres of Xenon. Additionally, metal halide lamp 28 can be constructed with an arc gap of 2.4 mm. With the addition of the multiple atmospheres of Xenon, light will be produced in an essentially instantaneous manner when a high current is passed through the discharge during starting.
As previously discussed, disposition of the hotter running anode electrode 30 in the vertically lowermost position results in the optimum convection symmetry within the metal halide lamp 28. It can be appreciated however that molten metal halide will still collect at the bottom of the lamp envelope 34. In a typical metal halide arc tube, the metal halide liquid tends to collect on the lower part of the arc tube as a result of the cooling of the lower portion caused by the convection currents. If this arc tube is operated horizontally, some of the light which would strike the lower part of a horizontally pointing reflector would be absorbed by the liquid metal halide. Should light pass through such molten halide to be reflected by this reflector, there would be an undesirable loss of intensity as well as a light output which would have an undesirable yellow content.
To avoid these undesirable characteristics, optimum positioning of the metal halide lamp 28 within the ellipsoidal reflector 20 is proposed. By specific positioning, the vertically oriented metal halide lamp 28 can be disposed within the downward facing ellipsoidal reflector 20 such that light collected by the ellipsoidal reflector 20 will come through only the upper portion of the metal halide lamp 28 which is free from molten metal halide. If the metal halide lamp 28 is operated vertically, the molten metal halide will tend to be restricted to no more than the lower 1/3 of the lamp envelope 34. With the ellipsoidal reflector 20 pointing downward and the metal halide lamp 28 disposed vertically with the anode electrode 30 at the lower position, the desired effect of avoiding light passing through the molten metal halide can be achieved by positioning the metal halide lamp 28 within the ellipsoidal reflector 20 in a manner offset from the first optical focal point of the reflector 20. Since the molten metal halide will reside in approximately the lower 1/3 of the metal halide lamp 28, positioning of the lamp 28 should be such that only the upper 2/3 of the metal halide lamp 28 is located at the first optical focal point of the ellipsoidal reflector 20. In other words, the arc is central at the first focal point and moved down until only light passing through the upper 2/3 of the bulb is intercepted by the ellipsoidal reflector.
In practice, once the metal halide lamp 28 is positioned as discussed above, the metal halide lamp 28 will be secured in the ellipsoidal reflector 20 such that there can be no deviation from such position. One way of securing the metal halide lamp 28 in the reflector 20 is by the use of a cement (not shown). The cement can be placed in the neck portion 20a of the ellipsoidal reflector 20 and allowed to first air dry for a short period of time. Following this air drying step, the cement can be heated for another period of time, which initial heating can be provided merely by operating the metal halide lamp 28. Finally, the cement can be further cured by a short period of high temperature firing (approx.400 degrees C).
Although the hereinabove described embodiment constitutes a preferred embodiment of the invention, it should be understood that modifications can be made thereto without departing from the scope of the invention as set forth in the appended claims. For instance, though the present invention is described in terms of a DC source of power, it should be appreciated that the principles of the present invention would apply equally as well for operation of a metal halide lamp using a low frequency (60 hz) AC source. Additionally, the light source need not be a DC type exclusively just because of operation by means of the automotive battery system. It would be possible to modulate the output of the DC source so as to work in conjunction with an AC light source and still be within the scope of the present invention. Additionally, one could enjoy the benefits of this invention by alternate means of securing the vertically disposed metal halide lamp within a downward facing ellipsoidal reflector in an offset manner relative to the optical focal point of such reflector.

Claims

A light source apparatus comprising:
a gas discharge lamp having an anode and a cathode electrode disposed at opposite ends of a lamp envelope;
a curved reflector having an optical focal point associated therewith;
said gas discharge lamp being disposed within said curved reflector in a vertical manner; and
said gas discharge lamp further being disposed within said curved reflector in a manner offset from said optical focal point such that one of said ends of said lamp envelope is outside of said optical focal point.
A light source apparatus as set forth in claim 1 wherein said curved reflector is an ellipsoidal reflector.
A light source as set forth in claim 2 wherein said ellipsoidal reflector is disposed in a downward facing direction.
A light source as set forth in claim 1 wherein said gas discharge lamp is a metal halide arc tube.
A light source as set forth in claim 4 wherein said metal halide arc tube is disposed within said curved reflector in a manner whereby said anode electrode is located at the bottommost position in relation to said cathode electrode.
A light source as set forth in claim 1 wherein said anode electrode is substantially thicker in dimension than said cathode electrode.
A light souce as set forth in claim 1 further comprising power supply means for providing a DC voltage to said gas discharge lamp.
A light source as set forth in claim 7 wherein said power supply means is disposed in a thermally isolated manner with respect to said light source.
A light source as set forth in claim 1 wherein such disposition of said gas discharge lamp within said curved reflector is such that more than half of said gas discharge lamp is disposed at said optical focal point of said curved reflector.
A light source as set forth in claim 3 wherein said gas discharge lamp is a metal halide arc tube having an anode electrode which is substantially thicker in dimension than a cathode electrode of said metal halide arc tube, said anode electrode being disposed in a vertically lower position relative to said cathode electrode.
A light source as set forth in claim 4 wherein said metal halide arc tube contains a fill of high pressure xenon.
A light source as set forth in claim 5 wherein the upper end of said metal halide arc tube in which said,cathode electrode is located is secured within said curved reflector so as to prevent alteration of said offset disposition of said metal halide arc tube relative to said optical focal point of said reflector.
A light source apparatus comprising:
a gas discharge lamp having a first and a second electrode disposed at opposite ends of a lamp envelope;
an ellipsoidal reflector having an optical focal point associated therewith, said ellipsoidal reflector being oriented in a downward facing direction;
said gas discharge lamp being disposed within said ellipsoidal reflector in a vertical manner;
said gas discharge lamp further being disposed within said ellipsoidal reflector in a manner offset from said optical focal point so that one of said ends of said lamp envelope is disposed outside of said optical focal point; and
wherein said second electrode is disposed in said end disposed outside of said optical focal point and said second electrode is substantially larger in diameter than said first electrode.