Abstract:
The present invention features apparatus and a method for controlling luminance emitted from a back light module for a large, tiled, flat-panel, liquid crystal display (LCD). A mechanism is provided for achieving luminance stability of fluorescent lamps, high efficiency and long life through controlling and maintaining lamp tube wall temperatures. Air flow variations are controlled by a microprocessor and a network of electric fans. The lamps may also be dimmed selectively to obtain optimum lamp tube wall temperatures. There is also provided a mechanism for safely preventing overheating of the back light and display due to high ambient temperatures or component failures. Further, the controls can be used in conjunction with ambient light sensing or manual controls to maintain appropriate output light intensity.

Description:
FIELD OF THE INVENTION 
     This invention pertains to large tiled, flat-panel displays and, more particularly, to an apparatus for controlling the temperature and thereby maintaining a bright, uniform illumination in a back light system for large, tiled, flat panel displays with intense lighting capability in the range of 50,000 to 150,000 nits. 
     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 contains an array of fluorescent lamps. FPDs of this type have been increasing in size by about 1 to 2 inches diagonal yearly. The median size in 1999 for use in desk top PCs is 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 included as reference. However, tiling, as described in U.S. Pat. No. 5,661,531 and also included as 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 (candellas/square meter) 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 a bright and uniform illumination of the display over its entire active area is very difficult to do. The intensity required for some applications, in particular that required for a large, tiled, flat-panel LCD display as described in U.S. Pat. Nos. 5,661,531, entitled CONSTRUCTION AND SEALING OF TILED, FLAT-PANEL DISPLAYS, and 5,867,236, entitled CONSTRUCTION AND SEALING OF TILES, FLAT-PANEL DISPLAYS, causes the lamps to produce a significant amount of heat in order to meet predetermined brightness specifications. In addition, fluorescent lamps are designed to run most efficiently at predetermined elevated temperature. It is desirable to run them at their ideal design temperature, which is usually between approximately 50 and 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 preferable to illuminate the area with an array of fluorescent lamps. The optimum number of lamps required depends on the size of the area to be illuminated and the display brightness specifications preferably in the range of 300 nits to the viewer as described in co-pending patent applications, Ser. Nos. 09/406,977 and 09/407,620 filed concurrently herewith and hereby incorporated by reference. A large area display requires multiple lamps to illuminate it to the desired level 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 there are fewer lamp cathodes. The resultant designs have lamp tubes placed horizontally, one above the other. This produces a chimney effect, where the upper lamps receive heated air from the lamps below. The temperature differential from top to bottom can become severe. The lamp tube temperature differences can cause significant variations in the luminance of the back light as well as decreased life expectancy. 
     This invention provides unique means for controlling the temperatures to be substantially the same for all lamp tubes and allows for maintaining a desired predetermined level of maximum brightness by keeping all of the lamp tubes at or very near optimum temperature. This then provides for uniform brightness at peak efficiencies and the longest possible lamp life. The controls for the back light keep the lamps individually or in groups at the optimum temperature, regardless of the ambient temperature. One component of the control is provided through the combined use of variable speed cooling fans, temperature sensors, and control logic. 
     This invention further provides a second unique means of predetermined accurate control by the addition of dimming ballasts that can be used in the previously mentioned combined control system to very accurately control the temperature profile in the hierarchy of lamps and add the capability for safety operation (controlled shut down) in the event of gross overheating. With the individual control over lamp input power, each lamp can be dimmed such that it stays at the optimum temperature. This can be done by characterizing the temperature distribution in the back light, or by providing individual sensors on each lamp. 
     Additionally, the dimming ballasts along with the control logic can be used for keeping the lamps off until the display system is activated. The dimming feature is also used for manual brightness control or ambient light sensing control. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to back light systems for tiled, flat-panel displays which require unusually high levels of illumination in the range of 50,000 to 150,000 nits. The invention reflects the useful discovery that a substantially constant and uniform luminance output of the back light module is readily obtained if the lamp tube wall temperature is kept substantially constant. This invention provides unique means for achieving this through control of the lamp temperature using a combination of air flow variations and lamp dimming means. Further, the controls can be used in conjunction with ambient light sensing or manual controls to maintain predetermined output light intensity. In addition, the back light module controls can be used to avoid overheating and loss of the display due to such overheating. Another benefit of the control system is enhanced life expectancy, due to minimizing overheating and providing a lower time-averaged lamp temperature. 
     Methods are disclosed herein for controlling luminance emitted 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 efficiency and also lamp life. This invention provides means for achieving luminance stability, high efficiency and long life through controlling and maintaining lamp tube wall temperatures. In another embodiment of this invention, means are disclosed to selectively dim the lamps to obtain optimum lamp tube temperatures. Actual practice of this invention has resulted in luminance output over a large area (½ square meter) exceeding 50,000 nits and ability to hold this luminance level within 2 percent over days of continuous operation. 
     In accordance with the present invention, there is also provided means for safely preventing overheating of the back light and display due to high ambient temperatures or component failures. 
    
    
     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 graphically illustrates the temperature characteristics of a fluorescent lamp; 
     FIG. 2 a  is a side, cross sectional view of a multiple lamp back light and a display in accordance with the present invention; 
     FIG. 2 b  illustrates a plan view of a multiple lamp back light; 
     FIG. 3 graphically illustrates the lamp temperature in an uncontrolled back light assembly; 
     FIG. 4 a  graphically depicts the air flow needed to optimally cool a back light assembly; 
     FIG. 4 b  graphically depicts air flow effect on lamp temperatures; 
     FIG. 5 graphically illustrates the temperature distribution across the back light; 
     FIG. 6 schematically depicts a back light with multiple lamps, multiple fans, individual lamp temperature sensors and dimming ballasts, in accordance with the invention; 
     FIG. 7 is an electrical schematic diagram illustrating the fan speed control logic of the present invention; 
     FIG. 8 is an electrical schematic diagram illustrating the dimming ballast control logic; 
     FIG. 9 graphically illustrates temperature control vs. resultant power consumption over time; and 
     FIG. 10 graphically illustrates temperature control operation characteristics of the back light control of the present invention. 
    
    
     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 of a large area back light for a flat-panel display that requires high luminance levels. In addition, the invention features apparatus and a method for controlling the back light for optimized efficiency, lamp life and safe operation. 
     Now referring to FIG. 1, there is shown a graph of the temperature characteristics of a fluorescent lamp. A typical fluorescent lamp, not shown, is designed to operate most efficiently at a predetermined lamp tube wall temperature. Maximum brightness occurs near the point of maximum efficiency  11 . The ideal temperature then is said to be T 0  12. The ideal temperature is determined by the lamp construction and its parameters (e.g., phosphors, mercury vapor pressure, etc.). 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 allowing easier ignition of the gas. 
     Now referring to FIG. 2 a , a side, cross sectional view of a flat-panel display  20  and its back light assembly  21  is shown. The back light assembly  21  comprises a light box cavity  22 , an array of fluorescent lamps  23 , and a light diffuser  24 . Cooling fans  29  are attached to the cavity  22 . Some display applications require additional optics  28  to enhance certain characteristics of the exiting light. An example is the previously mentioned tiled, flat-panel LCD display, which uses highly collimated light. The additional optics  28  required to perform this enhancement can be inefficient, and therefore necessitate that a high luminance be produced by the back light  21 . 
     FIG. 2 b  shows a front view of the back light assembly  21 . 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  26  supplies high frequency (usually 20-30 KHz) AC power to the lamps  23 . 
     FIG. 3 is a graph showing typical thermal profiles of the lamps  23  in the back light module  21 , when operated with only natural convection cooling means. The temperature of the lowest lamp  34  is the coolest, increasing successively for lamps  33 ,  32  and achieving the greatest increase in the topmost lamp  31 . The cathode areas (or ends)  36  of the lamps  23  are at a higher temperature than are intermediate regions of the lamps  23 , due to the power consumption of the respective cathodes  36 . 
     Also shown is the effect of the thermal chimney on the temperature of the center of the lamps, depicted on the graph by reference numeral  35 , as air passes over the lamps. Lamp  31  is heated not only by the power supplied it, but also by the rising warm air from all of the lamps  32 ,  33 ,  34  disposed below it. The resultant operating lamp temperature range  37  is quite large. The object of this invention is to provide means for reducing this temperature range  37  to near zero. 
     FIG. 4 a  depicts a graph of the air flow profile  40  required to attempt to keep the lamps  23  (FIG. 2 a ) at a more uniform temperature over the range of luminance values selected for operation. FIG. 4 b  depicts a graph of temperature profiles of the top and bottom lamps  23  before and after this air flow profile  40  is applied to the back light module  21 . The upper lamp reaches a temperature profile referred to as reference numeral  31 , without air flow; and referred to as reference numeral  41  with air flow. The lower lamp reaches a temperature profile shown as  34  without air flow; and referred to as reference numeral  44  with air flow. The resultant lamp operating temperature range in the back light module  21  reduces from the convection value  37  to the forced air value  47 . 
     Now referring to FIG. 5, a graphical depiction of the temperature of the central portion of each lamp tube  23  is shown for various fan speeds. When no air is driven through the back light module  21 , the vertical temperature distribution is shown by curve  51 , and has a large temperature range, T 0  37. Curve  52  shows the temperature distribution with the fans at half speed. Curve  53  shows a minimum temperature range  47  and an average operating temperature near the ideal temperature  12 . 
     FIG. 6 schematically shows the relationship of the cooling fans  29 , lamps  23 , dimming ballasts  26 , and temperature sensors  63  and  64 , which are mounted in intimate contact with the lamps  23 . 
     FIG. 7 is an electrical schematic diagram that depicts a closed loop circuit for controlling fan speeds. One type of temperature sensor  71  in this embodiment is a thermistor forming part of a voltage divider network with fixed resistors  73  and held between a reference voltage  72  and ground  74 . The divided voltage  76  is fed into a microprocessor  70  via analog-to-digital converters  75 . This temperature sensor  71  in this embodiment can be used as sensors  63 ,  64 . 
     A microprocessor  70  uses the digital temperature data  78  to adjust fan speeds. The digital output  79  of the microprocessor  70  is fed into the motor drive amplifiers  77  via digital-to-analog converters  76 . In this embodiment, the motor drive amplifiers  77  then supply a DC voltage to the fans  29 . 
     The simplest form of a control algorithm is to adjust all the speeds of the fans to be the same, based on the value of one sensor S 1 . Air flow is uniform across the lamps  23 . This is the most cost efficient control scheme. The adjustment to the microprocessor output  79  to changes in the input  78  is accomplished using a simple lookup table, not shown. The lookup table is empirically developed by actual test results. Notice that only one sensor and one motor drive amplifier is needed for this simplest of controls. 
     A two cooling zone air flow control system can be accomplished in two ways. The simplest is to thermally profile the unit during actual testing and determine the air speed ratios desired between the two cooling zones. A more complex method is to use two sensors  63  and  64 , of the type  71  for example, to independently control the air flow (a) up through the center of the back light assembly  21  and (b) for the sides of the back light assembly  21 . Additional sensors and motor drive amplifiers, not shown, can be added to attempt to control the temperature distribution more accurately within the back light assembly  21 . It has been found that a dual cooling zone with one sensor is adequate for most applications. 
     FIG. 8 shows the control system used for dimming the lamps individually or in groups. The control again is through lookup tables in the microprocessor  70 . Lamp temperature digital data  78  is fed to the microprocessor  70 , as previously shown. The ballasts  26  have a dimming feature such that the output of a ballast  26  is proportional to a DC input voltage  84 . The digital output  82  of the microprocessor  70  is converted to the appropriate ballast voltage  84  via a digital-to-analog converter  83 . Each lamp may be driven by one ballast  26  or the lamps  23  may be ganged so that one ballast  26  can drive several lamps 
     In simplest form, the ballasts  26  are all given the same dimming voltage  84 . The dimming voltage  84  is controlled by one sensor  71  (the same one used for fan control) and the external brightness command  81 . Dimming voltage  82  and fan speed voltage  79  are determined from a lookup table, the inputs for which are temperature sensor data  78  and brightness setting  81 . 
     Brightness increases based on input  81 , as long as the average maximum temperature does not exceed the ideal  12 . Brightness can be decreased by external input. Microprocessor output  82  to the ballasts is decreased accordingly. In addition, fan speed data  79  is lowered to a predetermined level based on a new lower ideal temperature that has been empirically determined by actual testing. 
     Referring now to FIG. 9, the most expensive and most controllable system includes a lamp temperature sensor  71  and a dimming ballast  26  for each lamp. The lamp temperature and lamp power for the upper  31  and lower  34  lamp are shown. The temperature range  47  can further be reduced by using the dimming feature of the ballasts  26 . Temperature range can be brought to near zero with individual or multiple cooling zone lamp dimming capability. Input may be multiple lamp sensors  71  or by the use of a predetermined thermal profile imbedded in the dimming output lookup table in the microprocessor  70 . 
     Referring now to FIG. 10, a normal operation of the back light  21  is shown along with a safe mode operation sequence of events. The normal operation of the back light module  21  is to initially turn it on. Fan speeds and dimming output data are set at predetermined initialization levels. As the unit heats up, lamp temperature follows curve  104  towards the preset brightness level  102  and upper operating temperature level  103 . 
     As the temperature level  103  is reached, power is reduced incrementally in steps to the lamps  23  via the dimming output data. When temperature reaches an acceptable lower operating temperature, the fan speed is incrementally increased. This area of control on the curve is the normal operation area, depicted by reference numeral  105 . In the event of an over temperature condition  106 , the lamp power is reduced via the dimming output data level to a predetermined safe power (brightness) level  101 . The lamp temperature then drops, following path  107 . When the temperature is in a safe zone, the lamp power is again increased following curve  108  towards the normal operating area  105 . If this over temperature condition reoccurs a predetermined number of times, a shut down will occur. 
     Since other control configurations can be formulated to fit particular operating requirements and environments, it will be apparent to those skilled in the art that the invention is not considered limited to the examples chosen for purposes of disclosure, and covers all changes and modifications which do 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.