Arc lamp with external magnetic means

The present invention provides an arc lamp for producing a high intensity point source of light by magnetically compressing the light emitting element of the arc lamp. Arc lamp point light source 30 includes bulb 32 having two electrode 34, 36 spaced apart to have gap 35 therebetween and hermetically sealed within bulb envelope 44 filled with ionizing gas. Connector 46 is provided for applying a voltage potential across electrodes 34, 36 causing electric current to pass therebetween to generate arc plasma 42. Annular magnet 48 and bar magnet 50 located outboard of bulb envelope 44 generate a magnetic field around arc plasma 42 to compress the volume thereof between electrodes 34, 36.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a point or near point light source for use 
in a projection display application, and more particularly to a xenon arc 
bulb having a magnetic field generated around the bulb envelope to 
compress the arc plasma therein. 
2. Description of Related Art 
Arc lamps are a type of electric-discharge lamp in which an electric 
current flows between two electrodes which are placed in a gas or vapor 
environment. The light emitted from these lamps is produced from the 
luminescence of the gas resulting from the increased energy state caused 
by the current passing therethrough. This energized gas between the 
electrodes is referred to as the arc plasma. A special type of arc lamp is 
the xenon arc lamp which typically incorporates two electrodes enclosed in 
a fused quartz bulb filled with xenon gas at a pressure above atmospheric 
pressure. The light emitted from a xenon arc bulb is substantially 
continuous throughout the visible spectrum and approximates daylight in 
color. Another advantage of xenon arc bulbs is that they are capable of 
producing a high intensity light. For these reasons xenon arc bulbs are 
used to artificially illuminate objects in light valve-based and 
film-based projection display systems, fiber optics networks, as well as 
solar simulation systems. 
However, as electronics and optics have become increasingly smaller, the 
existing arc lamps have proven to be bulky and/or inefficient. In 
addition, much of the light generated by these light sources cannot be 
directed into or collected by the smaller components because of their 
size; instead it must be absorbed by the bulb or surrounding components 
where it generates unwanted heat. In addressing these problems a series of 
optical components have been employed to focus, direct and collimate the 
light. However, these additional components are counterproductive to the 
miniaturization of these systems since they add size and cost of the 
systems. Accordingly, there is a need to provide a smaller light source 
which does not sacrifice brightness or intensity. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention an arc lamp is 
provided that produces a high intensity source of light by reducing or 
compressing the arc plasma, the light emitting element of the arc lamp. 
The present invention includes a bulb having two electrodes spaced apart 
and defining a gap therebetween. The electrodes are hermetically sealed 
within a bulb envelope which is filled with ionizing gas. A connector is 
provided for applying a voltage potential across the electrodes to enable 
electric current to pass between the electrodes and generate an arc 
plasma. The present invention further includes a magnetic field means 
located outboard of the bulb envelope for generating a magnetic field 
around the arc plasma. This magnetic field acts on the arc plasma to 
compress the volume thereof in the inter-electrode region. As a result of 
the reduction in volume of the arc plasma, a higher intensity light source 
is produced than a bulb of equal power having an uncompressed arc plasma. 
The high intensity of light from the compressed arc plasma affords greater 
collection efficiency of the energy emitted. As a result the point light 
source enhances performance for projection display systems which 
incorporate the light source independent from the object source, fiber 
optic networks and solar simulation systems.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It should be understood from the outset that the present invention will be 
described in connection with a specific embodiment which illustrates the 
best mode of practicing the invention known at the time that this 
application was filed. However, various modifications will become apparent 
to those skilled in the art after having the benefit of studying the text, 
drawings and claims which follow. With that caveat in mind, the attention 
of the reader should now turn to the drawings, with particular reference 
to FIG. 1. 
In accordance with the preferred teachings of this invention, projection 
display system 10 is provided for generating and displaying an image. 
Projection display system 10 includes power supply 28 for energizing 
projection display system 10 coupled to point light source 30 and optical 
elements 12 disposed along an optical axis 26. In this embodiment, light 
is generated and emitted from point light source 30 and projected along 
optical axis 26 where it encounters a plurality of optical elements 12 
which format the light emitted from point light source 30 to generate and 
display an image. 
More particularly, point light source 30 includes bulb 32 having two 
electrodes, cathode 34 and anode 36, disposed and hermetically sealed 
within bulb envelope 44. Connector 46 provides an electrical connection 
between power supply 28 and electrodes 34, 36 such that a voltage 
potential may be applied across electrodes 34, 36 without disrupting the 
environment within bulb envelope 44. While the embodiment displayed in 
FIG. 1 and described herein contemplates utilizing a direct current power 
supply source, one skilled in the art would readily appreciate that an 
alternating current power supply could be substituted therefor without 
deviating from the scope of the present invention. 
Point light source 30 further includes annular magnet 48 located about 
optical axis 26 and partially surrounding bulb 32 opposite optical 
elements 12. Annular magnet 48 is also located so as to minimize its light 
blocking affects and magnetic interference with surrounding components. A 
pair of bar magnets 50, 50' are producing magnetic fields 122, 122', 
respectively located within the vicinity of annular magnet 48 to adjust 
and refine the magnetic field 123 generated by annular magnet 48 to the 
desired configuration. As illustrated in FIG. 1 a permanent magnet has 
been utilized for annular magnet 48 and magnets 50, 50'. However, an 
electromagnet or any other means for producing the desired magnetic field 
could be incorporated into the present invention to achieve the desired 
result. 
Referring now to FIGS. 2 and 3, a portion of bulb 32 is shown enlarged 
including cathode 34 and anode 36 spaced apart and defining gap 35 
therebetween gap 35 having a width W1. Bulb envelope 44 encloses cathode 
34 and anode 36 to provide a hermetically sealed environment around these 
electrodes and is filled with ionizing gas, typically xenon. 
In operation, a voltage potential generated by power supply 28 is applied 
across electrodes 34 and 36, thereby causing a current flow across gap 35 
having width W1. This current flow charges the ionizing gas particles in 
gap 35 between these electrodes, thus increasing their energy state and 
generating arc plasma 42. These energized ions generate and emit light 
from bulb 32. The volume of arc plasma 42 is defined longitudinally by 
current emitting area 38, where the electrical current arc originates from 
cathode 34, and current collecting area 40, where the electrical current 
arc terminates at anode 36, and radially by the boundary where the 
ionizing gas particles are in an unenergized state as shown by 
iso-brightness line 110. Arc plasma centroid 43 is located at the center 
of mass of arc plasma 42 near current emitting area 38. The uncompressed 
volume of arc plasma 42 is best illustrated in FIGS. 2 and 3 by the 
iso-brightness contour lines 102, 104, 106, 108 and 110, iso-brightness 
contour line 102 being the brightest contour line and iso-brightness 
contour line 110 being the dimmest contour line and the outer boundary of 
arc plasma 42. FIG. 2 illustrates a standard arc lamp, such as the 
commercially available, 2500 watt xenon arc lamp made by Hanovia, part no. 
995C0010, in which the arc plasma between electrodes 34 and 36 is in an 
uncompressed state. 
Referring now to FIGS. 4 and 5, a presently preferred bulb is shown which 
has been modified from the standard bulb shown in FIG. 2 for the present 
invention. Point light source 30 is shown wherein a magnetic field acts 
upon arc plasma 42 to compress the volume thereof. The magnetic field as 
represented by magnetic lines of force 120 generated by annular magnet 48 
and magnets 50 shown in FIG. 1. Magnetic lines of force 120 act upon arc 
plasma 42 in a substantially radial direction as can be best seen in FIG. 
5 and magnetic lines of force 120 act upon arc plasma 42 in a 
substantially longitudinal direction as can best be seen in FIG. 4. 
Furthermore, the magnetic field may be generated such that magnetic lines 
of force 120 converge on arc plasma centroid 43 to compress arc plasma 42 
thereto. 
Arc plasma 42 includes current emitting area 38 which is in contact with 
cathode 34 and current collecting area 40 which is in contact with anode 
36. The maximum possible current capable of flowing between electrodes 34 
and 36 short of melting them is determined by size of these areas. To 
preserve the integrity of electrodes 34 and 36, current emitting area 38 
and current collecting area 40 should not be reduced in size when arc 
plasma 42 is compressed. Thus, the magnetic fields imposed on arc plasma 
42 must not compress or quench current emitting area 38 or current 
collecting area 40. 
In its preferred embodiment, the present invention contemplates compressing 
the overall volume of arc plasma 42 of bulb 32 to approximately 50% of the 
uncompressed arc plasma volume. In order to achieve an overall 50% 
compression of arc plasma 42 and still maintain the above-described area 
requirements, the volume of arc plasma 42 in gap 35 having width W2 may be 
locally reduced by as much as fourfold. 
As previously described, it is desirable to incorporate a bulb which has 
been modified for arc plasma compression. Bulb 32 may include a modified 
electrode gap spacing, electrode shape and surface contour to facilitate 
the generation of a compressed arc plasma. As shown in FIG. 4, electrodes 
34 and 36 are located substantially closer together than electrodes 34 and 
36 shown in FIG. 2. The width W2 is considerably smaller than the width 
W1. The physical size of bulb envelope 44 may be reduced as a result of 
the compressed volume of arc plasma 42 and the closer spacing W2 of 
electrodes 34, 36. In addition, a concave or focusing electrode shape 
could be employed to facilitate arc plasma compression. Furthermore, the 
electrode surface could be shaped to ensure that the flux lines of the 
magnetic field intersect the electrode surface in a substantially 
perpendicular manner. 
Additional modifications to bulb 32 could include a modified ionizing gas 
constituent and pressure, as well as a unique bulb envelope geometry to 
locate the magnets around the arc plasma. The Hanovia bulb identified 
above is filled with xenon gas so that its cold fill pressure is 
approximately three (3) atmospheres. In a preferred embodiment the fill 
pressure of bulb 32 would be determined on the basis of the ease of arc 
ignition, the desired temperature of the arc near bulb envelope 44, 
expected bulb lifespan, and efficacy of radiation of arc plasma 42. 
Compression of arc plasma 42 and the accompanying increase in the current 
density will change the energy balance in gap 35. If the temperature 
inside bulb envelope 44 increases, the opacity will be different. Thus, a 
different fill pressure may be required to achieve the desired radiation. 
Similarly, the radiation from bulb 32 is a function of the ionizing gas 
used to fill bulb envelope 44. The selection of the gas has a direct 
effect on the spectral characteristics, brightness, bulb lifespan and 
threshold ignition voltage for bulb 32. Accordingly, it may be desirable 
to add other constituents to the presently preferred xenon gas fill which 
will alter these characteristics. For example, xenon gas fills which 
contain a proper doping may be incorporated to achieve the desired bulb 
characteristic, such as mercury, krypton or other constituents presently 
used in xenon arc lamps. 
Referring again to FIG. 1, a variety of optical elements 12 are 
incorporated into projection display system 10 to collect light emitted 
from point light source 30 and direct it towards image plane 20. More 
particularly, concave mirror 14 which may be parabolic, conic or concave 
asphere in shape is a reflective element which collects light not emitted 
directly towards image plane 20 and directs it thereto. In a preferred 
embodiment, concave mirror 14 is fabricated out of electroformed nickel 
which is coated with a highly reflective material such as aluminum or 
custom dichroic material as is commonly utilized in arc lamp illumination 
systems. Light which is projected down optical axis 26 via point light 
source 30 or concave mirror 14 impinges on stop 16 to appropriately format 
the light. 
The light which is transmitted along optical axis 26 and within the 
dimensions of aperture 17 in stop 16 is transmitted therethrough to the 
remaining optical elements. The balance of the light transmitted to stop 
16 is blocked from further transmission along optical axis 26. Lenses 18 
and 19 are interposed along optical axis 26 to further format the light. 
For example, biconvex lens 18 is employed to magnify the light transmitted 
through aperture 17, while collimating lens 19 formats the light such that 
it is transmitted substantially parallel to optical axis 26 towards image 
plane 20. Lenses 18 and 19 further act to filter out undesirable energy. 
For example, in a liquid crystal light valve based projection display 
system, these elements would filter out ultraviolet energy and 
heat-generating infrared energy which could damage the liquid crystal 
material contained in the light valve. Similarly, prism 22 and light valve 
24 are located along optical axis 26 and serve to appropriately format the 
light transmitted thereto for displaying an image. 
Projection display system 10 has been described in general terms. Detailed 
descriptions of various projection display systems can be found in the 
following patents, including U.S. Pat. No. 4,650,286 entitled "Liquid 
Crystal Light Valve Color Projector" issued on Mar. 17, 1987 to Koda et 
al.; and U.S. Pat. No. 4,127,322 entitled "High Brightness Full Color 
Image Light Valve Projection System" issued Nov. 28, 1978 to Jacobson, et 
al. which are incorporated herein in their entirety by reference. 
One skilled in the art would readily recognize that the xenon arc lamp 
point light source of the present invention could be readily adapted into 
most projection display systems incorporating a separate light source from 
the object source. In addition, while projection display system 10 
described above and illustrated in FIG. 1 shows point light source 30 
disposed parallel to optical axis 26, one skilled in the art would readily 
recognize that in some embodiments it may be preferred to orient point 
light source 30 perpendicular to optical axis 26. Thus, the present 
invention contemplates an embodiment which enables the light source to be 
oriented in an optimal orientation relative to optical axis 26. 
Furthermore, one skilled in the art would readily appreciate that the 
present invention is not limited to use in the above described systems but 
may be incorporated into any existing projection system which uses a 
separate light source, including light valve based and film based 
projector systems. Commercial examples of such systems are the Hughes HJT 
projectors and the Hughes-Fullerton large screen projectors. 
From the foregoing, those skilled in the art should realize that the 
present invention provides a high intensity point light source by 
utilizing magnetic elements to generate a magnetic field which acts upon 
the arc plasma within an arc lamp to compress the volume of the arc 
plasma. As noted from the outset, the invention has been described in 
connection with a few particular examples. However, various modifications 
and other applications will become apparent to those skilled in the art 
after having the benefit of studying the specification, drawings and the 
following claims.