Patent Document

TECHNICAL FIELD 
   The present invention generally relates to fluorescent lamps, and more particularly relates to techniques and structures for improving the life and/or efficiency of fluorescent lamps such as those used in liquid crystal displays. 
   BACKGROUND 
   A fluorescent lamp is any light source in which a fluorescent material transforms ultraviolet or other energy into visible light. Typically, a fluorescent lamp includes a glass tube that is filled with argon or other inert gas, along with mercury vapor or the like. When an electrical current is provided to the contents of the tube, the resulting arc causes the mercury gas within the tube to emit ultraviolet radiation, which in turn excites phosphors located inside the lamp wall to produce visible light. Fluorescent lamps have provided lighting for numerous home, business and industrial settings for many years. 
   More recently, fluorescent lamps have been used as backlights in liquid crystal displays such as those used in computer displays, cockpit avionics, night vision (NVIS) applications and the like. Such displays typically include any number of pixels arrayed in front of a relatively flat fluorescent 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 fluorescent light sources, however, designers continually aspire to improve the amount of light produced by the light source, to extend the life of the light source, and/or to otherwise enhance the performance of the light source, as well as the overall performance of the display. In the NVIS arena, in particular, there is a need to reduce power consumption while also improving the displayed view presented to the user. 
   Accordingly, it is desirable to provide a fluorescent lamp and associated methods of building and/or operating the lamp that improve the performance of the lamp. 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 
   In various embodiments, methods and apparatus are provided for improving the efficiency of a fluorescent lamp suitable for use as a backlight in an avionics or other liquid crystal display (LCD). An exemplary apparatus includes a channel configured confine a vaporous material that produces an ultra-violet light when electrically excited. A first electrode and a second electrode assembly disposed within the channel and configured to apply an electrical potential across at least a portion of the channel to electrically excite the vaporous material. Control circuitry is configured to provide control signals to the first and second electrodes to apply the electrical potential in a manner that produces a mean electron energy that substantially maximizes probabilities of collisions between electrons and particles that that produce desirable emissions. For example, the electron energy can be configured to produce more emissions in the light-producing channel at wavelengths less than about 400 nm than emissions having wavelengths greater than about 800 nm, or so, although the particular wavelengths emphasized may vary in other embodiments. Additional detail about various exemplary 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 exploded perspective view of an exemplary flat panel display; 
       FIG. 2  is a block diagram that shows additional detail of an exemplary fluorescent bulb and the control electronics of an exemplary fluorescent lamp; 
       FIG. 3  is a plot of an exemplary spectral emission for an exemplary vaporous material present within a fluorescent lamp cavity; and 
       FIG. 4  is a plot showing exemplary collision probabilities for various electron energies. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description of the invention is merely exemplary 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. 
   Various techniques for improving the efficiency, luminescence and/or other performance aspect of a fluorescent light source are described herein. Each of the various techniques and structures described herein may be independently applied to any and all types of fluorescent light sources, including so-called “aperture lamps”, “flat lamps”, fluorescent bulbs, and the like. 
   According to various exemplary embodiments, the fluorescent light source in a display is driven in a manner that emphasizes emissions (e.g. mercury emissions) that stimulate light in the visible spectrum by exciting phosphors, over higher wavelength emissions (e.g. Argon emissions). In night vision (NVIS) applications, in particular, high wavelength emissions can be difficult to filter from the visible display, and in fact can be amplified in some embodiments. Reducing the amount of high-wavelength emissions in a display therefore improves the display presented to the user while conserving energy used to drive the display. 
   Turning now to the drawing figures and with initial reference to  FIG. 1 , an exemplary flat panel display  100  suitably includes a backlight assembly with a substrate  104  and a faceplate  106  confining appropriate materials for producing visible light within one or more channels  108 . Typically, materials present within channel(s)  108  include argon (or another relatively inert gas), mercury and/or the like. To operate the lamp, an electrical potential is created across the channel  108  (e.g. by coupling electrodes  102 ,  103  to suitable voltage sources and/or driver circuitry), the gaseous mercury is excited to a higher energy state, resulting in the release of a photon that typically has a wavelength in the ultraviolet light range. This ultraviolet light, in turn, provides “pump” energy to phosphor compounds and/or other light-emitting materials located in the channel to produce light in the visible spectrum that propagates outwardly through faceplate  106  toward pixel array  110 . 
   The light that is produced by backlight assembly  104 / 106  is appropriately blocked or passed through each of the various pixels of array  110  to produce desired imagery on the display  100 . Conventionally, display  100  includes two polarizing plates or films, each located on opposite sides of pixel array  110 , with axes of polarization that are twisted at an angle of approximately ninety degrees from each other. As light passes from the backlight through the first polarization layer, it takes on a polarization that would ordinarily be blocked by the opposing film. Each liquid crystal, however, is capable of adjusting the polarization of the light passing through the pixel in response to an applied electrical potential. By controlling the electrical voltages applied to each pixel, then, the polarization of the light passing through the pixel can be “twisted” to align with the second polarization layer, thereby allowing for control over the amounts and locations of light passing from backlight assembly  104 / 106  through pixel array  110 . Most displays  100  incorporate control electronics  105  to activate, deactivate and/or adjust the electrical parameters  109  applied to each pixel. Control electronics  105  may also provide control signals  107  to activate, deactivate or otherwise control the backlight of the display. The backlight may be controlled, for example, by a switched connection between electrodes  102 ,  103  and appropriate power sources. While the particular operating scheme and layout shown in  FIG. 1  may be modified significantly in some embodiments, the basic principals of fluorescent backlighting are applied in many types of flat panel displays  100 , including those suitable for use in avionics, desktop or portable computing, audio/video entertainment and/or many other applications. 
   Fluorescent lamp assembly  104 / 106  may be formed from any suitable materials and may be assembled in any manner. Substrate  104 , for example, is any material capable of at least partially confining the light-producing materials present within channel  108 . In various embodiments, substrate  104  is formed from ceramic, plastic, glass and/or the like. The general shape of substrate  104  may be fashioned using conventional techniques, including sawing, routing, molding and/or the like. Further, and as described more fully below, channel  108  may be formed and/or refined within substrate  104  by sandblasting in some embodiments. 
   Channel  108  is any cavity, indentation or other space formed within or around substrate  104  that allows for partial or entire confinement of light-producing materials. In various embodiments, lamp assembly  104 / 108  may be fashioned with any number of channels, each of which may be laid out in any manner. Serpentine patterns, for example, have been widely adopted to maximize the surface area of substrate  104  used to produce useful light. U.S. Pat. No. 6,876,139, for example, provides several examples of relatively complicated serpentine patterns for channel  108 , although other patterns that are more or less elaborate could be adopted in many alternate embodiments. 
   Channel  108  is appropriately formed in substrate  104  by milling, molding or the like, and light-emitting material is applied though spraying or any other conventional technique. Light-emitting material found within channel  108  is typically a phosphorescent compound capable of producing visible light in response to “pump” energy (e.g. ultraviolet light) emitted by vaporous materials confined within channel  108 . Various phosphors used in fluorescent lamps include any presently known or subsequently developed light-emitting materials, which may be individually or collectively employed in a wide array of alternate embodiments. Light emitting materials may be applied or otherwise formed in channel  108  using any technique, such as conventional spraying or the like. In various embodiments, an optional protective layer may be provided to prevent argon, mercury or other vapor molecules from diffusing into the light-emitting material. When used, such a protective layer may be made up of any conventional coating material such as aluminum oxide or the like. Alternatively, various embodiments could include a protective layer that includes fused silica (“quartz glass”) or a similar material to prevent mercury penetration. 
   Cover  106  is typically made of glass, ceramic glass or plastic, and is suitably attached to substrate  104  by glass fritting or the like in a manner that seals the vaporous materials within channel  108 . 
   Turning now to  FIG. 2 , an exemplary light source system  600  suitably includes a fluorescent lamp  602 , a driver circuit  630 , and optional control circuitry  620 . In various embodiments, control circuitry  620  senses and/or controls the temperature, pressure and/or other characteristics of lamp  602 , and further provides one or more control signals  626  to driver circuit  630  to produce desired operation of system  600 . Driver circuit  630  is typically implemented using any conventional analog and/or digital circuitry to apply any number of control signals  623 A-B,  634 A-B to produce light in lamp  602 . In various embodiments, driver circuit  630  and control circuitry  620  are incorporated within a single device or circuit, and may be further combined with control electronics  105  for display  100  as described above. 
   Lamp  602  is any bulb or other light source capable of producing fluorescent light resulting from electrical excitation of vaporous materials residing within channel  108 , as described above. In various embodiments, lamp  602  suitably includes two or more electrode assemblies  604 A-B that provide an interface between external sources of electrical energy and the gas or plasma residing within channel  108 . In a conventional implementation, electrode assemblies  604 A-B each include two or more electrodes  612 A-B,  614 A-B interconnected by one or more filaments  610 A-B. In the exemplary embodiment of  FIG. 2 , for example, one assembly  604 A includes two electrodes  606 A and  608 A interconnected by filament  610 A, and the other assembly  604 B includes electrodes  606 A and  608 B interconnected by filament  610 B. Driver circuit  630  provides appropriate electrical signals  623  A-B,  634 A-B that can be applied to electrodes  606 A-B,  608 A-B (respectively) to produce light. In a conventional embodiment, an alternating current is applied across each filament  610 A-B, while a voltage difference is applied across channel  108  (e.g. a difference in charge is created between filament  610  and filament  610 B) to allow electrons to migrate across the charged plasma within channel  108  from one end to the other. Signals  623 A-B and  634 A-B may be generated and applied in any manner to implement a wide array of equivalent operating techniques. 
   Various techniques of operating control electronics  620  and/or driver circuitry  630  can further improve the performance of lamp  602 . By providing suitable drive signals  623 ,  634  to the lamp, for example, light output can frequently be improved, often with a decrease in applied drive power. Referring now to  FIG. 3 , a simplified emission spectral plot  800  for an exemplary plasma residing within a light source channel  108  suitably exhibits peak emissions at various wavelengths. Peak  804  (which may be centered around a wavelength of approximately 285 nm or so), for example, reflects the presence of mercury (Hg) within the plasma, and peaks  806  and  810  (which may be centered around wavelengths of approximately 810 and 840 nm, respectively) reflects the presence of argon. Generally speaking, it is desirable to maximize emissions in the ultraviolet range (shown by region  802  in  FIG. 3 ) to create a higher level of UV “pump” radiation in channel  108  that, in turn, causes phosphor or other light-emitting material in channel  108  to produce visible light (e.g. light within region  802 ) for the display. The mercury emissions that are maximized along peak  804 , for example, lie within the desired wavelength range for such emissions. It is therefore desirable in many embodiments to maximize mercury emissions  804  (and/or other emissions with similar wavelengths) to increase the amount of beneficial UV radiation produced by the plasma. 
   Conversely, the emissions peaks  806 ,  810  typically associated with argon lie outside the useful range of radiated emission. Not only are such emissions incapable of providing adequate “pump” radiation to phosphors or other light emitting materials within channel  108 , but such emissions can actually interfere with operation of infra-red sensitive equipment used in close proximity to the display. In particular, emissions at relatively high wavelengths (e.g. above 750 nm or so) can be highly undesirable in certain displays, particularly those relating to night vision (NVIS) applications. Such infra-red sensitive equipment typically includes automatic gain control (AGC) circuitry that amplifies radiation with wavelengths higher than the visible range (e.g. infrared radiation), as indicated by region  803  in  FIG. 3 . Emissions produced in range  803  by the display itself can therefore significantly degrade NVIS performance. As a result, many NVIS and other displays currently incorporate expensive filtering to remove such emissions above a particular wavelength (shown as λ N  in  FIG. 2 ). By removing the source of emissions lying within region  803 , however, the need for such filtering is significantly reduced and/or eliminated. 
     FIG. 4  shows an exemplary plot  900  of the collision probabilities for mercury (curve  902 ) and for argon (curve  904 ) as functions of applied electron energy. In practice, most conventional displays simply maximize the amount of electrical power used to drive lamp  602 , resulting in operation toward the rightward edge of  FIG. 4 . As can be appreciated from  FIG. 4 , operation at relatively high electron energies (corresponding to a relatively high applied potential between the ends of lamp  602 ) tends to increase undesirable argon collisions  904  while reducing beneficial mercury collisions  902 . 
   To improve efficiency and reduce the amount of undesired emissions, control circuitry  620  can be used to maintain the voltage produced by driver circuit  630  at a level that increases such beneficial mercury emissions while avoiding detrimental argon emissions. Stated another way, control circuitry  620  maintains the voltage across lamp  602  in such a way that produces electron energies in the range of curve  902  in  FIG. 4  rather than in the right-hand portion of curve  904 . By optimizing the voltage of pulses applied across lamp  602 , the amount of beneficial UV light produced is increased while the amount of undesired infrared or near-infrared emissions can be significantly decreased (e.g. often by an order of magnitude or more). Exemplary embodiments therefore drive the plasma using pulses or other electrical signals  623 ,  634  in a manner that gives mean electron energies that maximize probabilities of collisions with particles that produce light in the ultraviolet range, rather than in the infrared/NVIS range  803  ( FIG. 3 ) 
   Because peak  902  for mercury emission is relatively narrow compared with the curve  904  representing argon emissions, however, it may be desirable in certain embodiments to carefully control not only the voltages and/or currents applied to each electrode (e.g. with signals  623 A-B and  634 A-B), but also to either monitor or control the pressure and/or temperature of lamp  602  as appropriate. That is, the operating characteristics of lamp  602  typically change with respect to temperature and pressure. To respond to fluctuations in conditions while maintaining operation within the limits of curve  902 , it may be desirable in some embodiments to sense the temperature  622  and pressure  624  using any type of suitable sensors and to correspondingly adjust the electrical signals  623 A-B,  634 A-B using any algorithm, lookup table and/or other technique. Alternatively, temperature  622  and/or pressure  624  may be controlled (using, e.g., a thermoelectric heater or the like) by control electronics  620  using any conventional techniques. 
   While at least one exemplary 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 exemplary embodiment or exemplary 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 exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.

Technology Category: 5