Abstract:
A dimmable arc lamp assembly comprises a lamp enclosure comprising a chamber enclosing a light-emitting material, first and second electrodes extending into the chamber of the lamp enclosure, and a heating element proximate the chamber configured to heat at least a portion of the lamp enclosure to a temperature greater than the boiling point of the light-emitting material such that the light-emitting material remains in a gaseous state. Because the light-emitting material remains above its boiling point during lamp operation, dimming is not susceptible to control issues that can result from condensation of the light-producing material. Such lamps may be used in various applications such as in flat panel displays.

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to high-pressure arc lamps, and more particularly relates to techniques and structures for managing the dimming of high pressure arc lamp assemblies such as those used in liquid crystal displays. 
     BACKGROUND 
     An arc lamp is any light source in which an electric arc produces visible light. Typically, arc lamps include a glass or plastic tube that is filled with light-emitting materials such as argon, mercury, sodium or other inert gas. When an electric potential is applied between two electrodes inserted into the tube, the resultant electric arc breaks down the gaseous materials and produces an ongoing plasma discharge that results in visible light. 
     Arc lamps have provided lighting in numerous home, business and industrial settings for many years. More recently, arc lamps have been used as backlights in liquid crystal displays such as those used in computer displays, cockpit avionics, flat panel televisions and the like. Such displays typically include any number of pixels arrayed in front of a relatively flat light source. By controlling the light passing from the backlight through each pixel, color or monochrome images can be produced in a manner that is relatively efficient in terms of physical space and electrical power consumption. 
     Despite the widespread adoption of displays and other products that incorporate arc light sources, however, designers continually aspire to improve the performance of the light source, as well as the overall performance of the display. In particular, the nature of many arc lamps can lead to difficulties in controlling a dimmable display. As a result, various techniques for controllable dimming the light produced from an arc lamp have been attempted, with varying degrees of success. 
     Accordingly, it is desirable to provide devices and techniques for effectively and efficiently controlling the brightness of various arc lamps and arc lamp displays. Other desirable features and characteristics will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY 
     Numerous lamp assemblies, displays and techniques are described herein. Various embodiments, for example, provide a dimmable arc lamp assembly that comprises a lamp enclosure comprising a chamber enclosing a light-emitting material, first and second electrodes extending into the chamber of the lamp enclosure, and a heating element proximate the chamber configured to heat at least a portion of the lamp enclosure to a temperature greater than the boiling point of the light-emitting material such that the light-emitting material remains in a gaseous state. Because the light-emitting material remains above its boiling point during lamp operation, dimming is not susceptible to control issues that can result from condensation of the light-producing material. Such lamps may be used in various applications, such as in flat panel displays. 
     In other exemplary embodiments, a method of operating an arc lamp suitably comprises a lamp enclosure housing a light-producing material in contact with first and second electrodes. Various embodiments of the exemplary method comprise the steps of: heating the lamp enclosure to a lamp temperature equal to at least a boiling temperature of the light-producing material; providing an electric potential across the first and second electrodes to thereby produce an amount of light from the light-producing material; and adjusting the electric potential across the first and second electrodes to adjust the amount of light produced by the light-producing material. 
     Other embodiments include other lamps or displays incorporating structures and/or techniques described herein. Additional detail about various example embodiments is set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is an exemplary pressure versus temperature plot for an exemplary arc lamp; 
         FIG. 2  is a diagram of an exemplary arc lamp with an exemplary heating element; 
         FIG. 3  is a diagram of an exemplary arc lamp with an alternate embodiment of a heating element; and 
         FIG. 4  is a block diagram of an exemplary display assembly having a heated arc lamp assembly. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the invention is merely example in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     According to various exemplary embodiments, an arc lamp is provided with a heat source (such as a resistive or radiant heat source) that maintains the light-producing material at or above its boiling point so that the material remains in the gaseous state during lamp operation. Because the material in the lamp bulb remains in the gaseous state, it generally behaves with substantial linearity during the dimming process, thereby improving the ease with which the lamp can be dimmed. Any source of heat can be used, such as a simple resistive wire wrapped around the bulb, or a radiant heat source that simply warms the environment surrounding the bulb. Any other source of heat may be used in any number of equivalent embodiments. 
     With initial reference to  FIG. 1 , a general principal of operation can be described with respect to plot  100 , which shows the typical pressure versus temperature behavior of the light-producing materials found within the arc lamp bulb. Generally speaking, the light-producing materials behave as an ideal gas when the temperature is in excess of the material&#39;s boiling point. This portion of plot  100  is shown as section  104  to the right of transition point  105 . To the left of transition point  105 , the temperature is below the boiling point of the light-producing material, meaning that condensation can take place and droplets of liquid can form within the bulb. When condensation occurs, the pressure generally decreases exponentially with temperature, and this is shown as region  102  of plot  100 . Point  105  on plot  100 , then, corresponds to the point at which the light-producing material found within the internal chamber of the bulb remains in the gaseous state. Operation to the right of transition point  105  (corresponding to temperature greater than T E ) generally exhibits a linear pressure curve, whereas operation to the left of transition point  105  (corresponding to a temperature less than T E ) generally exhibits an exponential pressure curve. 
     The linearity of region  104  in plot  100  can be derived from the well-known ideal gas law, which is shown in Equation (1) below: 
                   P   =       nR   V     ⁢   T             (   1   )               
wherein P is pressure (e.g. in Pa), n is the number of moles of ideal gas, R is a constant (e.g. the gas constant of 8.314472 m 3 ·Pa/K·mol), V is the volume (e.g. in cubic meters) and T is the temperature (e.g. in degrees Kelvin). The particular constants and units of measure will vary depending upon the embodiment and the desired system of measurement applied. Nevertheless, since the volume of the arc bulb chamber is typically unchanging, it can be readily stated that the pressure of an ideal gas contained within the arc lamp bulb is linearly related to the temperature. Hence, region  104  of plot  100  is shown as a line with relatively constant slope.
 
     In a typical arc lamp, dimming of the light produced is carried out by reducing the voltage applied to the electrodes of the lamp. As the voltage (and the associated electrical current) in the bulb is reduced, however, the temperature of the bulb typically decreases, thereby producing changes in the bulb pressure that are consistent with plot  100 . In most conventional lamps, reducing the brightness (i.e. dimming the lamp) is made significantly more complicated when the temperature of the lamp decreases below transition point  105  and the pressure curve becomes non-linear. Difficulties can arise, for example, from the complexity of driving a plasma with non-linear behavior. To prevent this from occurring, the temperature of the lamp can be maintained above T E  by a separate heat source, thereby providing pressure linearity regardless of the voltage applied to the lamp electrodes. 
     Referring now to  FIG. 2 , an exemplary arc lamp  200  suitably includes a bulb or other lamp enclosure  202 . Lamp enclosure  202  may be formed, for example, from glass, plastic or other appropriate material. In various embodiments, enclosure  202  contains an internal chamber  204  filled with light-producing material such as argon, neon, xenon, metal halide, sodium, mercury and/or any other material as appropriate. A first electrode  206  and a second electrode  208  suitably extend into chamber  204  of lamp  200  to provide an electric potential  212  that can produce visible light from the light-producing material. Typically, potential  212  is produced from a conventional ballast or power supply circuit as appropriate. 
     To prevent the lamp  200  from cooling below the transition point (e.g. point  105  in  FIG. 1 ), a supplemental heat source  210  is provided proximate to lamp chamber  204 . “Proximate to” in this sense simply means that the heat source  210  is placed in a position that is close enough and oriented such that heat is provided from source  210  to chamber  204 . This allows for the temperature in chamber  204  to remain above T E  ( FIG. 1 ) and the light-emitting materials in chamber  204  remain in the gaseous state to provide linear pressure response. When the light emitting materials remain in the gaseous state, the electric potential  212  across electrodes  206  and  208  can be varied to increase or decrease the brightness of lamp  200  without concern that the pressure inside chamber  204  will change in a non-linear manner. 
     Heat source  210  is any supplemental source of heat energy capable of warming chamber  204  to a temperature greater than T E . In various embodiments, heat source  210  is a simple wire (e.g. a ni-chrome wire) that produces heat from electrical resistance. In other embodiments, heat source  210  can produce heat from any sort of resistive, radiant or other manner.  FIG. 3 , for example, shows an exemplary enclosure  202  that is warmed by an external radiant heater  302 . Again, heat can be provided to enclosure  202  and/or chamber  204  in any manner, using any currently-known or subsequently developed technique for generating heat. 
     Turning now to  FIG. 4 , an exemplary display assembly  400  suitably includes an arc lamp  200  that provides light to a waveguide  402 . Typically, light emitted from the opposite end of waveguide  402  is collimated (e.g. by collimator  404 ) or otherwise directed toward a liquid crystal or other display  406  to produce a viewable image. Control electronics  405  suitably provide appropriate control signals  408 ,  410  and/or  412  to direct the production and transmission of light from lamp  200  to display  406 . An example of one type of arc lamp display may be found in U.S. Pat. No. 6,775,460 (which is assigned the same assignee as the present disclosure), although the concepts set forth herein may be used with any type of arc lamp display or other arc lamp application. 
     Control electronics  406  include any integrated, discrete and/or other electronic components that are useful for controlling the operation of any part of display assembly  400 . In various embodiments, control electronics  406  include a digital microcontroller capable of executing instructions in digital object code form to execute the logical processes of controlling the display assembly. Such instructions may be encoded and stored in any manner, such as in any type of read-only, random access and/or flash memory, and/or may be embedded as firmware, microcode or the like. In other embodiments, control electronics  406  are implemented with programmable arrays and/or discrete logic capable of providing similar functionality. 
     The particular operation of the display assembly  400  varies from embodiment to embodiment. In one example, control electronics  405  activate a power supply  212  associated with heat source  210  via a control signal  410 . Signal  410  may be used to activate the source prior to or during lamp operation, and may be modified as appropriate. In various embodiments, heat source  210  is only activated while the temperature of the lamp  202  (or chamber  204  of lamp  202 ) drops below the transition temperature or boiling point of the light-emitting materials residing in chamber  204 . In other embodiments, heat source  210  remains active through operation of the display. In such cases, the operation of heat source  210  may be modulated or otherwise adjusted as appropriate to maintain the desired temperature of lamp  202  and/or chamber  204 . In still other embodiments, heat source  210  may be activated only when the brightness of lamp  202  is dimmed to a level that would otherwise result in operation to the left of point  105  in plot  100 . In such embodiments, control (e.g. via signal  412 ) of ballast or other power supply  401  provides primary dimming control by varying the electrical potential applied across the electrodes (e.g. electrodes  206 ,  208 ) of lamp  202 , while control via signal  410  activates heat source  210  to ensure that lamp temperature does not drop below the transition point T E  ( FIG. 1 ). Control signal  408 , in turn (which typically represents many separate or multiplexed control signals) is provided to control the liquid crystal or other pixel elements of display  406  to pass, filter, block and/or otherwise affect light transmitted to display  406  from lamp assembly  200 . Many other equivalent operating techniques could be formulated without departing from the concepts set forth herein. 
     While at least one example embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the example embodiment or example embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an example embodiment of the invention. It should be understood that various changes may be made in the function and arrangement of elements described in an example embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.