Abstract:
A high-intensity arc lamp comprises a glass envelope with a pressurized gas atmosphere. A cathode and an anode structure are disposed within. A pointed tip of the cathode is juxtaposed by a central hole in a face of the anode and a small gap between them. Such central hole is vented away from the arc down inside the anode structure. During operation, heat from the arc at the entrance to the central hole drives a wind of xenon gas down through such vents in the anode structure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to electric arc lamps, and more specifically to pressurized gas types that can operate at power levels in excess of a kilowatt and have a very small, stable point of radiation.  
           [0003]    2. Description of the Prior Art  
           [0004]    Texas Instruments (Austin, Tex.) introduced the digital micromirror device (DMD) in 1987. The DMD is a micro-electro-mechanical systems (MEMS) that digitally controls thousands of tiny mirrors in an array on a semiconductor chip. The MEMS structure is fabricated by complementary metal oxide semiconductor (CMOS) technology processes over a CMOS memory. Each light switch has an aluminum mirror that reflects light in one of two directions, depending on the electronic state of the underlying memory cell. Video images can be projected by shining a powerful light on the DMD and electronically tilting individual mirrors to form whole images on the array. Bursts of digital light pulses with various durations are interpreted by the eye as shades of gray. Color filters are used in combination to create full-color projected images.  
           [0005]    Digital light projection (DLP) systems include image processing, memory, a light source, and optics. A typical DLP system is capable of projecting large, bright, seamless, high-contrast color image with better color fidelity and consistency than traditional types of displays.  
           [0006]    When a DMD memory cell is in the 1-state, the mirror rotates to its +10 degree position (relative to zenith). In the 0-state, the mirror rotates to its −10 degree position. In the DMD, a suitable light source and projection optics are arranged so the mirror can reflect incident light either in or away from the pupil of a projection lens. Typically, the 1-state of the mirror produces a pixel that appears bright on the screen, and the 0-state of the mirror appears dark. Gray scale is achieved by binary pulse-width modulation, e.g., tilting the mirror to the 1-state for different time durations according to the brightness shade needed. Color pixels can be generated by using stationary or rotating color filters, in combination with one, two, or three DMD chips.  
           [0007]    The DMD light switch is a MEMS structure consisting of a mirror that is rigidly connected to an underlying yoke. The yoke in turn is connected by two thin, mechanically compliant torsion hinges to support posts that are attached to the underlying substrate. Electrostatic fields developed between the underlying memory cell and the yoke and mirror cause rotation in the positive or negative rotation direction. The rotation is limited by mechanical stops to +10 or −10 degrees. The fabrication of the DMD superstructure begins with a completed CMOS memory circuit. Through the use of six photomask layers, the superstructure is formed with alternating layers of aluminum for the address electrode, hinge, yoke, and mirror layers and hardened photoresist for the sacrificial layers that form the two air gaps. The aluminum is sputter-deposited and plasma-etched. The sacrificial layers are plasma-ashed to form the air gaps.  
           [0008]    Texas Instruments is actively pursuing two broad business opportunities for DLP, projection displays and continuous-tone color printing. Projection displays are needed for large audiences, portable business uses, and consumer/home appliances.  
           [0009]    Digital display engines (DDE&#39;s) are now being marketed that include a DLP subsystem ready for integration with a video interface, a power supply, a sound sub-system, controls, and a cabinet Texas Instruments is manufacturing DDE&#39;s for business projectors with VGA resolution (640×480). SXGA resolution (1280×1024) have also been demonstrated.  
           [0010]    Unfortunately, the prior art in high-intensity lamps lack the particular kind of light source needed for good DPL systems. The arc needs to be short and very stable. But in conventional lamps, the arcs are relatively long and jitter. This makes less than all the light produced available to the DMD and the image on the display screen.  
           [0011]    Prior art attempts at very-short arc lamps with solid cathodes and anodes have resulted in the two expanding and colliding under the heat of operation.  
         SUMMARY OF THE PRESENT INVENTION  
         [0012]    It is therefore an object of the present invention to provide a high intensity lamp suitable for DLP systems.  
           [0013]    It is another object of the present invention to provide an arc lamp with a small, stable arc during operation.  
           [0014]    It is a further object of the present invention to provide an arc lamp that pumps its internal pressurized atmosphere through the arc and out vents in its anode structure.  
           [0015]    Briefly, a high-intensity arc lamp embodiment of the present invention comprises a glass envelope with a pressurized gas atmosphere. A cathode and an anode structure are disposed within. A pointed tip of the cathode is juxtaposed by a central hole in a face of the anode and a gap between them is on the order of 0.050 inches. Such central hole is vented away from the arc down inside the anode structure. Furthermore, the central vent hole prevents a physical-contact collision between the anode and cathode during the heat of operation. During operation, heat from the arc at the entrance to the central hole drives a wind of xenon gas down through such vents in the anode structure.  
           [0016]    An advantage of the present invention is that an arc lamp is provided that can operate at kilowatt power levels and has a long operational life.  
           [0017]    Another advantage of the present invention is that a light source is provided which is suitable for DLP systems.  
           [0018]    A further advantage of the present invention is that an arc lamp is provided with a very small point of light source that is also very stable during operation.  
           [0019]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.  
       
    
    
     IN THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a cross-sectional diagram of an improved very-short arc lamp embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a cross-sectional diagram of a cathode and anode assembly embodiment of the present invention similar to that shown in FIG. 1;  
         [0022]    [0022]FIG. 3 is a schematic diagram of a digital light projection embodiment of the present invention that depends on a lamp like that shown in FIGS. 1 and 2; and  
         [0023]    [0023]FIG. 4 represents an arc-lamp electrode assembly embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    [0024]FIG. 1 illustrates a xenon arc lamp embodiment of the present invention, referred to herein by the general reference numeral  100 . The arc lamp  100  comprises a quartz glass envelope  102 , a cathode  104 , an anode  106 , and a pressurized xenon atmosphere  108 . A cathode stem  110  is supported by a metal rod  112  in a lamp holder, as is an anode stem  114  by another metal rod  116 . The electrical connection to the cathode  104  is made by a number of metal foils  118  to a cathode connection ring  120 . Similarly, the electrical connection to the anode  106  is made by a number of metal foils  122  to an anode connection ring  124 .  
         [0025]    [0025]FIG. 2 illustrates an arc lamp anode assembly  200  which is like anode  106  shown in FIG. 1. The arc lamp electrode assembly  200  faces a cathode  202  and includes a head  204  with a central vent hole  206  in its cathode-facing surface  207 . The head  204  is supported by and electrically connected to an anode stem  208 . In a typical embodiment, the cathode  202  is thoriated tungsten 2%, the anode head  204  is pure tungsten, and the remaining metal parts are made of molybdenum.  
         [0026]    During operation, a wind  210  develops as a convection current of xenon gas is blown away from the central vent hole  206  by a plasma arc  212 . The sharp tip of cathode  202  is placed unusually close to the anode&#39;s cathode-facing surface  207 , and the plasma arc  212  actually develops as a small point-like arc immediately in front of the ring of the central hole  206 . As the lamp heats from a cold start, the distance the plasma arc  212  jumps between the cathode and anode decreases. This is due to axial thermal expansion of the cathode and the anode structure. In a prototype that was tested, the arc spacing was about 0.050 inches cold, decreasing to about 0.040 inches when the lamp was hot.  
         [0027]    The wind  210  assists in preventing the plasma arc  212  from shorting out, and preventing destructive holes from being burned through in the tungsten of the cathode and/or anode. Such vent hole placement and resulting gas circulation also improves heat distribution throughout the anode.  
         [0028]    Experiments on prototype devices have demonstrated a stable plasma arc  212  that produces a very small but brilliant point of light. The arc ran at 16.5 volts and drew eighty amps direct current (DC). The small, stable arc characteristics make embodiments of the present invention very good choices in digital light projection (DLP) applications. Such prototypes had holes about 0.125 inches and three exit vents about 0.075 inches in diameter. The anode head  204  had an outside diameter of about 0.625 inches. In a first prototype lamp that was tested, the anode&#39;s cathode-facing surface  207  was convex and bullet-nosed. Such will be appropriate in some applications.  
         [0029]    All embodiments of the present invention include the sharp tip cathode  202  placed very close to a central hole  206  in an anode, and all pump the chimney-like wind  210  out through vents in the stem end of the anode head  204 . However, some embodiments of the present invention further include a mirror finish on the anode&#39;s cathode-facing surface  207 .  
         [0030]    The cathode-facing surface  207  in FIG. 2 is shown having a concave cross section that acts as a mirror lens. Tests have shown that the mirror-like surface does not tarnish or blacken during many hours of operation, and can be depended upon to help collect light from the plasma arc  212  and direct it out in light rays  214 . Further, external reflectors can be added to direct all the radiated light into an optics system or on to a DMD in a DLP system.  
         [0031]    [0031]FIG. 3 represents a digital light projection (DLP) system, and is referred to herein by the general reference numeral  300 . The DLP system  300  comprises a light source  302 , a first optics system  304 , a digital micromirror device (DMD)  306 , a projection output optics system  308 , and a display screen  310 . Systems for color video may further include multiple DMD&#39;s and color filters. The light source critically includes a point-like arc lamp  312  like those shown in FIGS. 1 and 2, and a mirror reflector  314  for collecting the lamp&#39;s light output and bringing it to a focus. The mirror reflector  314  is preferably a parabolic or elliptical type that has been electroformed from rhodium and nickel on a mandrel.  
         [0032]    During operation, a kilowatt or more of electrical power is input to the lamp  312 . A digital video signal is applied to the DMD  306 . An image represented by the video will appear on the display  310 . The typical power input to lamp  312  is 16-18 volts at 60-80 amps DC. The DMD  306  may be implemented with good results with commercial devices marketed by Texas Instruments (Austin, Tex.).  
         [0033]    Lamp embodiments of the present invention do not necessarily depend on xenon atmospheres, other rare gases can be used such as argon, krypton, etc. A mercury vapor lamp constructed as described herein will also benefit from the unique anode illustrated. However xenon produces a white light at 55000 Kelvin, which is a close match for natural sunlight which is very desirable in DLP applications.  
         [0034]    [0034]FIG. 4 represents an arc-lamp electrode assembly embodiment of the present invention, and is referred to herein by the general reference numeral  400 . Such electrode assembly  400  is a substitute for those shown in FIGS.  1 - 3 . The electrode assembly  400  also produces a point-like arc  401  between a cathode  402  and an anode  404 . A stem  406  supports the whole of anode  404 . A cathode-facing hole  408  is provided for entry of a stabilizing gas flow. Such travels axially down a central shaft  410  and radially out a number of exhaust manifolds  412  and exhaust ports  414 .  
         [0035]    Arrows  416  represent the gas flow which enters cathode-facing hole  408  after first passing through the arc  401 . Differences in heating of the envelope gases generate such circulation, e.g. like in a chimney. Arrow  418  represents the exiting flows of gases returning to the main pool of pressurized inert gas inside the lamp&#39;s envelope.  
         [0036]    The anode  404  differs from those shown in FIGS.  1 - 3  by its bull-nose contour. Light gathering from the point-like arc  401  may be totally left to an external reflector system. The mass of material around the central shaft is preferably comprised of pure tungsten, and thick enough to prevent liquid puddles of metal to form on the face due to excessive thermal resistance.  
         [0037]    The gap bridged by the point-like arc  401  is preferably on the order of 0.050 to 0.040 inches, cold to hot. The flow of gases into the central shaft  410  through the arc help stabilize the arc. The placement of hole  408  relative to the tip of cathode  401  provides for reduced gap changes between cold-start and hot operation.  
         [0038]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.