Abstract:
The present invention features apparatus for uniformly distributing luminance from a back light module for a flat panel, liquid crystal display (LCD). Luminance uniformity, high efficiency and long lamp life are achieved by distributing the lamp cathode thermal energy and maintaining uniform lamp wall temperatures. A heat sink is attached to the fluorescent lamps in the cathode areas, providing cooler operating temperatures at the lamp ends. A thermal sensor is also mounted in the heat sink body. In addition, open louver slots positioned behind the lamps allow for cool air to enter behind each lamp. The size, shape and position of these louvers can be selected so that the lamp temperatures are essentially constant over their entire length.

Description:
FIELD OF THE INVENTION 
     This invention pertains to apparatus for producing uniform, high luminance light and, more particularly, to a system for producing uniform, high luminance light in a large area, back light system for flat panel displays. 
     BACKGROUND OF THE INVENTION 
     Large flat-panel displays made in accordance with known active matrix (or TFT) liquid crystal display technologies are typically mounted in front of a back light module which L contains an array of fluorescent lamps. FPDs of this type have been increasing in size annually by about 1 to 2 inches, diagonally. The median size in 1999 for use in desktop PCs was about 15 inches diagonal view area. A few very large displays are made in the range of 20 to 25 inches diagonal. Tiled AMLCD FPDs may be made in the range of 40 inches diagonal, as described in copending U.S. patent application Ser. No. 09/368,921, assigned to the common assignee and hereby incorporated by reference. 
     However, tiling, as described in U.S. Pat. No. 5,661,531, and also included by reference, requires extremely intense light sources with substantially collimated lighting, masked optical stacks, and pixel apertures that have very low emitted light efficiency. Thus, lighting with unusually high intensity ranges of 50,000 to 150,000 nits is desirable with uniformity over very large FPD areas. Unique designs and control features are necessary to achieve such high intensities at reasonable wattages for consumer or business applications. 
     Maintaining such a bright illumination uniformly over the entire active area of the display is difficult to do. The intensity required for some applications, and in particular, that required for a large tiled flat panel LCD display as described in U.S. Pat. No. 5,867,236, issued Feb. 2, 1999, entitled CONSTRUCTION AND SEALING OF TILED, FLAT-PANEL DISPLAYS, causes the lamps to produce a significant amount of heat. Moreover, fluorescent lamps are designed to run most efficiently at an elevated temperature, so it is desirable to operate them at a predetermined ideal design temperature, which is usually in the range of 50 to 60 degrees Centigrade. 
     Small, edge-lit, back light modules used in notebook or laptop PCs do not produce sufficient brightness for a large area display, nor are they capable of illuminating a large area uniformly. Thus it is necessary to illuminate the area with an array of fluorescent lamps. The number of lamps required depends on the size of the area to be illuminated and the display brightness specifications. A large area display needs multiple lamps to illuminate it properly. 
     Since most displays are designed to be wider than they are tall, it is advantageous from a reliability and power perspective to use horizontal lamps. This results in fewer lamps and less power, since fewer lamp cathodes are required. The resultant designs use lamp tubes placed horizontally, one above the other. This produces a chimney effect, the upper lamps receiving heated air from the lamps below. As expected, the temperature differential from top to bottom can become severe. Unfortunately, lamp tube temperature differences cause significant variations in the luminance of the back light and contribute to decreased life expectancy. 
     Fluorescent lamps, particularly high efficiency hot cathode types, operate with a significant amount of the power consumption at the ends (cathodes). This naturally produces high temperatures at the cathodes of the lamp tube. A typical lamp operates in open air with a tube wall temperature preferably at about 55 degrees Centigrade, while the end may be higher than 85 degrees. 
     This invention provides a unique conduction cooling structure means for uniformly distributing the heat generated by the lamp tube cathodes, thus helping to maintain maximum brightness by keeping all of the lamp tube ends at or very near a uniform temperature. The temperature of the lamp ends is kept near the temperature of the central section of the lamp tube, preferably about 55° C., which provides for uniform brightness along the lamp tube within a few percent at peak efficiencies and ensures the longest possible lamp life. 
     This invention further provides unique means for directing cool fresh air to impinge on predetermined portions of lamp tubes so as to develop cooling means and uniform temperature distributions in the stack of bulbs. The invention is also capable of providing a more uniform temperature distribution across the array of lamp tubes in a high luminance output back light module for a large area flat panel display. 
     Additionally, when used in combination with the invention disclosed in copending U.S. patent application Ser. No. 09/407,619 (RDI-125), filed Sep. 28, 1999, hereby incorporated by reference, the present invention provides a very uniform, high luminance back light system capable of maintaining brightness within a few percent over periods of days under a wide range of environments. It is particularly suited for the application of a back light system for a large tiled, flat panel LCD. Such an application is disclosed in copending U.S. patent applications, Ser. No. 09/409,620 (RDI-127), filed Sep. 28, 1999 and Ser. No. 09/368,291, filed Aug. 6, 1999, both also incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided apparatus for uniformly distributing luminance from a back light module for a flat panel, liquid crystal display (LCD). Fluorescent lamps are commonly used in back light modules for LCDs due to their high efficiency. Luminance from fluorescent lamps is a function of lamp tube temperature, as is the efficacy of the lamp and the operating life thereof. This invention provides means for achieving luminance uniformity, high efficiency and long life by distributing the lamp cathode thermal energy and maintaining uniform lamp wall temperatures. 
     A unique heat sink attachment conduction cools the cathode areas of the fluorescent lamps. Cooler operating temperatures are achieved at the lamp ends, which has two significant benefits. First, the lower operating temperature of the cathode increases the lamp life, and second, provides for more even distribution of temperature and, therefore, uniform lamp luminance output in the range of a few percent over the length of the tube. A thermal sensor is also mounted in the heat sink body. In addition, open louver slots positioned behind the lamps allow for cool air to enter behind each lamp. The size, shape and position of these louvers can be selected so that the lamp temperatures are essentially constant over their entire length. 
     A constant and uniform luminance output of the back light module is further obtained through appropriate selection of lamps, reflective back light cavity and light diffuser. This invention provides means for achieving very high and uniform luminance output, 35,000 to 150,000 nits, over a very large surface area at minimal power consumption through appropriate design of the cathode heat sinks in conjunction with a set of specific air inlet louvers. 
     The cathode heat sinks also provide an optimum location for locating a temperature sensor. The sensor can be used in a control system, such as that described in the aforementioned patent application, Ser. No. 09/407,619, to efficiently manage the back light output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which: 
     FIG. 1 is a graphical illustration of the temperature characteristics of a fluorescent lamp; 
     FIG. 2 a  illustrates a side view of a multiple lamp back light and a display; 
     FIG. 2 b  illustrates a planar view of the multiple lamp back light depicted in FIG. 2 a;    
     FIG. 3 graphically illustrates the thermal profiles of lamps in a back light module when operated with only natural convection cooling in an uncontrolled back light; 
     FIG. 4 depicts a heat sink used to cool the lamp ends, in accordance with the present invention; 
     FIG. 5 graphically illustrates the temperature distribution with the heat sink; 
     FIG. 6 depicts a back light cavity back plane with louvers; and 
     FIG. 7 graphically illustrates the temperature distribution with louvers. 
    
    
     For purposes of both clarity and brevity, like elements and components will bear the same designations and numbering throughout the figures. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, the invention features apparatus and a method for controlling the luminance uniformity of a large area back light for a large, tiled, flat panel display that requires high luminance levels. In addition, the invention provides an optimum location for a temperature sensor for controlling the back light for optimized efficiency, lamp life and safe operation. 
     Now referring to FIG. 1, a typical fluorescent lamp is designed to operate most efficiently at a predetermined lamp tube wall temperature. Maximum brightness occurs near the point of maximum efficacy  11 . The ideal temperature then is said to be T 0   12 . The ideal temperature  12  is determined by the construction of the lamp (not shown in this FIGURE) and its components and parameters, such as phosphors and mercury vapor pressure. The most efficient lamps are those referred to as hot cathode lamps. These lamps have a preheat cycle during which the cathodes are heated, thereby causing easier ignition of the gas. 
     Now referring to FIG. 2 a,  a side view of a flat panel display  20  and its back light assembly  21  is shown. The back light assembly  21  consists of a light box cavity  22 , an array of fluorescent lamps  23 , and a light diffuser  24 . One or more fans  29  are mounted to the lamp enclosure to cool the assembly. Some display applications require additional optics  28  to enhance certain characteristics of the exiting light. An example is the aforementioned tiled, flat panel LCD display, for which highly collimated light is required. The additional optics  28  required to perform this collimating function is relatively inefficient; therefore, it is necessary for high luminance to be produced by the back light  21 . 
     FIG. 2 b  shows a front view of the back light assembly  21  depicted in FIG. 2 a.  The lamps  23  are held in the light box cavity  22  by lamp holders  25 . The lamps  23  are wired to a ballast  26  by a wiring harness  27 . The ballast supplies high frequency (usually 20-30 Khz) AC power to the lamps  23 . 
     Referring now also to FIG. 3, illustrated are typical thermal profiles of the lamps in the back light module  21  when operated with only natural convection cooling. The temperature of the lowermost lamp  34  is the lowest, the temperature increasing for lamps  33 ,  32  and the topmost lamp  31 . The cathode areas  36  or ends of the lamps  23 , shown at the extreme positions along the X-axis of the graph, have higher temperatures due to the power consumption of the cathodes  36 . The cathode area  36  of a high efficiency, hot cathode, fluorescent lamp  23  usually operates at a significantly higher temperature than does the rest of the lamp tube. 
     Also shown is the effect of the thermal chimney on the temperature of the center of the lamps  35  as air passes over the lamps  23 . Lamp  31  is heated not only by the power supplied it, for example, but also by the rising warm air from all of the lamps  32 ,  33 ,  34  below it. The resultant operating lamp temperature range  37  is quite large. The object of this invention is to provide two different, yet complimentary, means for reducing this temperature range  37 . 
     FIG. 4 is an exploded view of a cathode heat sink assembly  40  in accordance with the invention. The heat sink assembly  40  serves as a lamp holder  25  as well. The heat sink assembly  40  covers the cathode area  36  of the fluorescent lamps  23 . The heat sink assembly  40  consists of two mating parts: the heat sink body  41  and the heat sink cap  45 . Both of these two parts  41  and  45  have respective, “essentially” semicircular cavities  42  for receiving lamps  23 . The two mating parts  41  and  45  are held together by fasteners  48 . 
     Prior to placing the lamps  23  into the heat sink cavities  42 , thermally conductive elastomeric tape  46  is placed around the lamps  23  in the cathode area  36 . The thermal tape  46  provides compliance so that the lamp tubes  23  are not overly stressed during assembly. High viscosity thermal grease can be used in conjunction with the tape. 
     A thermal sensor  44  is mounted in the heat sink body  41  using thermal adhesive. The heat sink temperature is uniform across the lamps  23  and is an excellent mounting surface for the sensor  44 . The temperature at the top of the heat sink  40  is the most indicative of the lamp temperatures in the back light cavity  22 . The temperature at the sensor  44  represents all of the lamp cathode heat plus some of the heat produced in the chimney of the lamp array  23 . The output of the sensor can be used to regulate the speed of fans  29 . 
     The heat sink assembly  40  is mounted in the back light cavity  22  with cooling fins  47  protruding from the rear of the cavity  22 . This provides for cool ambient air to convectively flow over the heat sink fins  47 . This additionally allows the heat sink  40  to be at a near uniform temperature. The sensor  44  is located at an optimum thermal location for use in a temperature control system. 
     Now referring also to FIG. 5, temperature profiles along the lamp tubes  23  are shown for the top lamp  31  and bottom lamp  34  in the back light assembly  21 . The central portions of the lamps  35  have an elevated temperature  51  due to the chimney effect. The addition of the heat sink assembly  40  in the cathode areas  36  of the lamps  23  does not change the temperature  51  in the central area of the lamp  35 . The addition of heat sinks  40  on the lamp end temperatures  52 ,  53  is depicted on this graph. The top lamp  31  has a temperature  36  near the lamp ends or cathode areas, prior to installing heat sink  40 . The heat sink  40  reduces the lamp end temperature  52  near to that at the bulk of the lamp. The bottom or coolest lamp  34  in the array  23  shows that the cathode area temperature  36  may be slightly overcooled to a temperature  53 . 
     The remaining problem in obtaining lamp temperatures along the lamp tube length is the elevated temperatures  51  at the central portion  35  of the uppermost lamps  31  and  32 . As mentioned hereinabove, this phenomenon is a result of the previously mentioned chimney effect. A heat sink cannot be attached to the central portion of these lamps, since it would be in the field of view and would present an objectionable optical artifact. A solution would be to inject cool air into the cavity  22  near the upper lamps  31  and  32 . Of course, the mechanism to perform this cool air injection process must not be visible to the user. 
     Referring now to FIG. 6 a,  there is shown an array of louvers, or open slots, dispersed behind the lamps  23 . Different size louvers  61 ,  62  and  63  are used for thermal balancing. The louvers  61 ,  62  and  63  are punched into the back plane of the back light cavity  22 . This plane is a highly efficient, diffusive reflector and requires that the louver surface be reflective as well. The louvers  61 ,  62  and  63  present no visible slot to the viewer. The diffusive reflectivity characteristic of the back plane allows this to be viable. 
     In summary, the lamp tubes  23  can be made to operate at a uniform temperature along their entire length by allowing cool ambient air pulled by fans  29  to enter the back light cavity  22  through louvers  61 ,  62  and  63  placed behind the lamps  23 . A filter  64  is placed behind the back light cavity  22 , as shown in FIG. 6 b.    
     The height H and width W of the louvers  61 ,  62  and  63  can be determined experimentally, guided by analysis. It is desired that the air temperature and flow rate be constant along the lamp tube length. To counterbalance the chimney effect, larger and more numerous louvers are needed at the top of the lamp array  23  and near the horizontal center. The objective is to maintain each lamp at a uniform temperature along its length, but not necessarily to maintain the same temperature from lamp to lamp. 
     FIG. 7 shows the result of incorporating an appropriate combination of louvers  61 ,  62  and  63  in a back light cavity  22 . The louvers  61 ,  62  and  63  have little effect on the lower lamp  34  and essentially no effect in lamp end temperatures  36  versus non-louvered lamps shown as reference numeral  76  on the lower lamp  34 . The temperature of the upper lamp  31  at the center region  35 , prior to the introduction of louvers  61 ,  62  and  63 , is shown-at reference numeral  75 . After allowing fresh air to impinge on lamp  31  by louver  61  and by reducing the air temperature reaching lamp  31  by the effects of louvers  62  and  63  placed below lamp  31 , the temperature of lamp  31  is reduced to a lower temperature  71 . The lamp temperature gradient in the back light  21  reduces from a high range  37  to a new lower range  77 . 
     The combination of heat sink assemblies  40  and non-visible back plane air inlet louvers  61 ,  62  and  63  permits the construction of a back light assembly  21  in which the lamp temperature, and therefore lamp luminance, is very uniform. Additionally, a thermally stable and optimum location for a temperature sensor  44  is provided for use in a temperature control system. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, this invention is not considered limited to the example chosen for purposes of this disclosure, and covers all changes and modifications which does not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.