Abstract:
The present invention features a system for uniformly distributing luminance and a high degree of collimation from a back light module for flat-panel, liquid crystal displays (LCDs) simultaneously. A constant and uniform luminance output of the back light module in two directions is obtained through appropriate selection of lamps, geometry and optical components. An appropriate balance of lamps, lamp spacing, diffusers and light collimating optics are chosen to produce a high brightness back light module with very high intensity output over two very large surfaces. Variations in intensity over the illuminated area are minimized using light recycling in conjunction with the reflective diffusers and collimating optics. Precision collimators eliminate light beyond a defined angle, as required in tiled or monolithic flat-panel LCDs with predetermined display specifications.

Description:
This application is a divisional reissue application of application Ser. No. 11/154,995 filed Jun. 17, 2005 (now U.S. Pat. No. Re. 40,355), which is a reissue application of U.S. Pat. No. 6 578,985. 
     This divisional reissue application is one of three related reissue applications for the reissue of U.S. Pat. No. 6,578,985. The reissue applications are 11/154,995 (the issued parent reissue), 11/882,394 (the present reissue), and 11/882,393 (another divisional reissue of 11/154,995, that was filed on the same date as the present reissue application). 
    
    
     This application is related to U.S. patent application Ser. No. 09/368,921 filed Aug. 6, 1999 (now U.S. Pat. No. 6,657,698); U.S. patent application Ser. No. 09/406,977, filed Sep. 28, 1999 (now U.S. Pat. No. 6,417,832); U.S. patent application Ser. No. 09/407,619, filed Sep. 28, 1999 (now U.S. Pat. No. 6,447,146); U.S. patent application Ser. No. 09/407,620, filed Sep. 28, 1999 (now U.S. Pat. No. 6,341,879); and U.S. patent application Ser. No. 09/490,776, filed Jan. 24, 2000 (now U.S. Pat. No. 6,680,761), all of which are included herein by reference. In addition, this application is related to U.S. Pat. Nos. 5,661,531, 5,867,236 and 5,903,328, all of which are also included herein by reference. These copending applications and issued patents are all commonly assigned to the assignee of the present application. 
     FIELD OF THE INVENTION 
     This invention pertains to back light assemblies for flat-panel displays and, more particularly, to a back light module with a single array of lamps that produces high intensity, collimated light in two directions suitable for use with large, back-to-back, tiled flat-panel displays. 
     BACKGROUND OF THE INVENTION 
     Flat-panel displays (FPDs) made in accordance with known active matrix (e.g., TFT, etc.) liquid crystal display technologies (e.g., AMLCD) are typically mounted in front of a back light module which contains an array of fluorescent lamps. AMLCD flat-panel displays of this type have been increasing in size by about 1 to 2 inches diagonal, yearly. The median size in 1999 for use in desktop PCs was about 15 inches diagonal viewing area. A few very large displays are made in the range of 20 to 28 inches diagonal. Tiled AMLCD FPDs may be made in the range of 40 inches diagonal, as described in copending U.S. patent applications Ser. Nos. 09/368,921 1999 (now U.S. Pat. No. 6,657,698) and 09/490,776 (now U.S. Pat. No. 6,680,761). Tiled FPDs, as described in U.S. Pat. No. 5,661,531, require extremely intense back light sources with highly collimated light, masked optical stacks, and pixel apertures that may have low emitted light efficiency. Thus, lighting with unusually high intensity ranges of 50,000 to 150,000 nits is desirable. Also, intensity uniformity over the very large areas of tiled FPDs is very important. Unique back light designs, including temperature control features, are necessary to achieve such high intensities at reasonable power consumption. 
     Maintaining bright (i.e., high intensity) and uniform illumination of the display over its entire active area is difficult to do. The intensity required for some applications and, in particular, that required for large, tiled, seamless flat-panel LCD displays, causes the lamps to produce a significant amount of heat. In addition, since fluorescent lamps are designed to run most efficiently at an elevated temperature, it is desirable to operate them at or near their ideal design temperature, which is usually about 50 to 60 degrees Centigrade. 
     Small, edge-lit back light modules, such as those used in notebook or laptop PCs, do not produce sufficient brightness for use in a large area display, nor are they capable of illuminating that large an area uniformly. Thus, it is necessary preferable to illuminate these larger areas with an array of large fluorescent lamps. The number of lamps required depends on the size of the area to be illuminated and the display brightness requirements. A large area display generally requires multiple lamps to illuminate it properly. A large area display that can be viewed from two sides (i.e., a back-to-back display) requires proportionally more lamps, as well as unique design features to achieve the desired intensities and maintain optimized lamp efficiency through temperature control of the lamps. 
     Since most displays are designed to be wider than they are tall, it is advantageous, from a reliability and power perspective, to place the lamps in a horizontal orientation. This typically results in the use of fewer lamps and, consequently, lower power consumption, since fewer lamp cathodes are present. The resulting preferred designs orient lamp tubes horizontally, one above the other with predetermined, preferred spacing relationships to each other and to each of the back-to-back displays, one disposed on each side of the lamp array. 
     It is, therefore, a principal object of the invention to provide a back light module designed to illuminate back-to-back displays. 
     It is an additional object of the invention to provide a back light module for use with large flat panel displays, either monolithic or tiled. 
     It is another object of the invention to provide a back light module designed to provide a high intensity light output. 
     It is a further object of the invention to provide a back light module capable of delivering highly collimated light. 
     It is an additional object of the invention to provide a back light module having a very high operating efficiency. 
     It is a still further object of the invention to provide a back light module having a cooling structure to maintain a substantially uniform operating temperature. 
     It is yet another object of the invention to provide a back light module utilizing an array of horizontally-mounted fluorescent tubes. 
     It is an additional object of the invention to provide a back light module incorporating a cavity to maximize and control light recirculation. 
     It is another object of the invention to provide a back light assembly incorporating diffusers, collimators and brightness-enhancing films (BEFs). 
     It is a further object of the invention to provide a back light assembly suitable for illuminating large, back-to-back, tiled flat-panel displays having visually imperceptible seams. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a back light module which uniformly distributes luminance to back-to-back flat-panel, liquid crystal displays (LCDs) simultaneously. Fluorescent lamps are used due to their high efficiency. However, luminance, efficiency, and lamp life of fluorescent lamps are all functions of lamp tube temperature. The present invention provides an apparatus and method for achieving luminance uniformity and a high degree of light collimation in back-to-back displays with one single back light module source. 
     In particular, a constant and uniform luminance output of the back light module is obtained through appropriate selection of lamps, optimization of back light module geometry and use of additional optical components. A preferred balance of lamps, lamp spacing, diffuser and collimating optics is chosen to produce a high brightness back light module with very high, uniform intensity output over very large surface areas. Light is recycled from one display module to the other as the light is reflected from each of the display&#39;s optical stacks. The optical stacks of the two display modules typically include polarizers, masks, diffusers etc. In addition, light is reflected from the light collimating optics and the light enhancing and diffusing films also typically present in the optical stacks. 
     This invention provides a method for achieving this goal through selection of combinations of components and appropriate design geometries. A particular application of the inventive back light module is for use in integrating two large, tiled, flat-panel displays having visually imperceptible seams as described in the aforementioned U.S. patent application Ser. Nos. 08/652,032 (now U.S. Pat. No. 5,867,236), and 09/368,291 09/368,921 (now U.S. Pat. No. 6,657,698), and U.S. Pat. No. 5,903,328. The back light module system, with thermal enhancements such as those disclosed in U.S. patent application Ser. No. 09/406,977 (now U.S. Pat. No. 6,417,832) and applicable controls, such as those disclosed in U.S. patent application Ser. No. 09/407,619 (now U.S. Pat. No. 6,447,146), provides for an efficient, reliable, large area, high intensity light source usable with back-to-back flat-panel displays. 
     Additionally, optimum geometries are determined for the purpose of maximizing light output at high efficiencies, while minimizing luminance gradients across the two displays. These optimum geometries are also determined for maximizing light output using brightness enhancing films (BEFs) and light recycling. 
     Finally, precise collimators such as that disclosed in U.S. patent application Ser. Nos. 09/024,481 (now U.S. Pat. No. 6,152,580) and 60/177,447 (now U.S. Pat. No. 6,654,449), eliminate light beyond a defined cut-off angle for each flat panel display, as required in a tiled flat-panel LCD. 
     It will be obvious that while the back light assembly of the invention is optimized for use with tiled, AMLCD flat-panel displays, it may also be used with monolithic and monolithic-like displays. 
    
    
     
       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 graph of luminance vs. temperature in a typical fluorescent lamp; 
         FIG. 2a  is a schematic, cross-sectional view of a multiple lamp back light simultaneously illuminating back-to-back displays; 
         FIG. 2b  is a plan view of the multiple lamp back light shown in  FIG. 2a ; 
         FIG. 3  is a schematic diagram illustrating lamp and diffuser spacing relationships; 
         FIG. 4  is a graph showing light output as a function of the number of lamps installed; 
         FIG. 5  is a schematic, sectional view of a back light assembly in use with back-to-back flat panel displays in accordance with the present invention; 
         FIG. 6  is a graph showing luminance as a function of deviation from a normal caused by the collimation attributes of the optics; and 
         FIG. 7  is a ray diagram showing typical reflections of light rays between diffusers and light collimating (i.e. brightness enhancing) films. 
     
    
    
     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 an apparatus and a method for controlling the luminance level, luminance uniformity and collimation of light exiting a large area back light suitable for use with back-to-back flat-panel displays. The back light assembly is suitable for use with large, tiled, flat-panel displays which require high luminance levels and a precise, predetermined degree of collimation. In addition, the present invention provides an optimum design taking into account efficiency, cooling, luminance and image quality for use in integrating back-to-back flat-panel displays with a single light source. The design is useful with tiled flat-panel displays and large monolithic or monolithic-like LCD displays. 
     Referring first to  FIG. 1 , there is shown a graph  100  of the light output (i.e., luminance) and efficiency (i.e., efficacy) of a typical fluorescent lamp as a function of temperature. Fluorescent lamps generally operate most efficiently at a predetermined, optimum lamp tube wall temperature. Maximum brightness usually occurs near the point  102  of maximum efficacy. 
     The ideal temperature T o    104  may then be determined from the temperature axis of graph  100 . The ideal temperature  104  is determined by the lamp construction, particularly dependent on such parameters as the phosphor, cathode construction and the mercury vapor pressure. The most efficient lamps  128  are generally the class of fluorescent lamps of the hot cathode type. Hot cathode lamps have a preheat cycle during which the cathodes are heated, thereby causing easier ignition (i.e., striking) of the gas within the lamp. 
     Now referring to  FIG. 2a , there is shown a side view  120  of back-to-back flat-panel displays  122  and its back light assembly  124 . The back light assembly  124  consists of a light box cavity  126 , an array of fluorescent lamps  128 , and light diffusers  130 . Lamps  128  are cooled by fans (not shown). 
     Some display applications require additional optical components  132  to enhance certain characteristics of the exiting light. For example, tiled, flat-panel LCD displays require highly collimated light. The additional optical components  132  required to collimate the light may be somewhat inefficient. This necessitates that a high luminance be produced by the back light assembly  124 . 
     Referring now also to  FIG. 2b , there is shown a front view of the back light assembly  124  of  FIG. 2a . The lamps  128  are held in the light box cavity  126  by lamp holders  134 . The lamps  128  are wired to the ballast  136  by a wiring harness  138 . The ballast  136  supplies high frequency (usually 20-30 KHz) AC power to the lamps  128 . Efficient, high-frequency electronic ballasts are well known to those skilled in the art and any suitable unit may be chosen for use with the instant invention, the ballast forming no part thereof. 
     It will be obvious that temperature sensing devices, fan speed control circuitry, lamp dimming controls, heat sinks and other such temperature control devices and methods which are known to those skilled in the art could be used in conjunction with the back light of the present invention to help control the surface temperatures of the lamps  128 . As an example, the lamp holder  134  can be a heat sink with an attached thermistor (not shown) to measure lamp temperature and its output used to regulate the voltage to one or more fans thereby regulating fan speed, or the voltage may be used to regulate the output of dimming ballast  136 . 
     Referring now to  FIG. 3 , there is shown a schematic diagram  140  of a portion of a back light assembly where certain critical dimensions and/or distances are identified. Two lamps  128 , each having a diameter D  142 , are arranged adjacent one another, spaced apart a distance S  144 . Lamps  128  are positioned a distance H  146  away from the diffusers  130 . These dimensions may be used in design calculations in manners well known to those skilled in the art. 
     If lamps  128  are assumed to be line sources, luminance may be calculated according to the equation: 
     
       
         
           
             A 
             = 
             
               
                 tan 
                 
                   - 
                   1 
                 
               
               ⁢ 
               
                 
                   D 
                   4 
                 
                 
                   H 
                   + 
                   
                     D 
                     2 
                   
                 
               
             
           
         
       
     
     Assuming that the required luminance A is known, the number of lamps may readily be calculated. 
     Referring now to  FIG. 4 , there is shown a graph  160  illustrating the effect of varying the S  144  and H  146  dimensions on the light output from a back light assembly. Having this information, the required number of lamps  128  of a predetermined size (diameter) D  142  required to produce the necessary luminance may be calculated. 
     The curve of total light output from the back light cavity  126  is a function of the number of lamps  168  installed. The desired light level  162  is also shown. It will be noted that, as the number of lamps increases, the light output increases until a maximum illumination  164  occurs prior to reaching the point of maximum lamp capacity  166 . Also, as more lamps  168  are used, or the lamps are spaced closer together, they block light from each other. The number of lamps  168  corresponding to the desired light output  162  is also shown. 
     It is also necessary that the diffusers  130  be highly efficient, but not of high transmissivity. One diffuser  130  behaves as a diffuser for the display on its side of the lamps  128 . However, the same diffuser  130  behaves as a reflector for the opposite display. Since the collimating films  182  &amp;  184  (BEFs) require recirculating light in order to be efficient, the diffusers  130  must both transmit and reflect light. A transmission of 50-75% has been found to be effective in this application. 
     Now referring to  FIG. 7 , several light rays are traced to explain the interaction between the diffusers  130  and the collimating films  182  and  184 . The efficiency of these collimating films  182 ,  184  are conditional on a good optical coupling with their reflective surfaces. Consider a light ray that emanates from the lamp  128  and strikes the upper diffuser  130  at a point A. If the diffuser  130  is, for example, 60% transmissive, then 60% of the light will be transmitted through the diffuser  130  and result in a “Lambertian” distribution (i.e., be uniformly distributed at all angles relative to the surface of diffuser  130 ) of light aimed at the collimating film  182 . However, 40% of the light is reflected (also in a Lambertian distribution) toward the lower diffuser  130 . One light ray from the transmitted light at point A heads toward point B on the collimating film. The angle of incidence (for example, less than 60° from normal) of this light ray is such that it is reflected back toward the diffuser at point C. At point C, 40% of the light ray is reflected back toward the collimating film  182 . This type of reflection is highly efficient compared to light that re-enters the lamp cavity. Light which enters the light cavity must cross two diffuser/air interfaces (thus losing light) and some may be absorbed or scattered by the lamps. 
     Consider now another light ray reflected from point C, and is directed toward point D. This light ray has a favorable angle of incidence (for example, 60-85°) and is sent forward to the next collimating film  184  ( FIG. 5 ) and eventually the LCD tile  194  ( FIG. 5 ). Some of the reflected light from point C is sent to the lower diffuser  130  at point E. Some of this light will end up in the lower display and some will be reflected from the lower diffuser  130  and be sent toward point F and some of this light will make it to point G on the collimating film  182  and be sent forward to the upper display. As can be seen, light rays will continue to be reflected between the elements  130  and  182 . 
     The nature of the efficient coupling of reflected light between the collimating film  182  and the adjacent diffuser  130  improves the forward gain of the collimated light output. The key to the collimation efficiency is the highly efficient, but relatively low transmission diffuser. 
     A good approximation of the total light output of the back light assembly, without considering collimation and related light re-circulation, can be obtained by considering the geometry. A lamp tube  128  produces light rays substantially uniformly over 360 degrees. The light exits forward toward a first display, is absorbed by neighboring lamps or exits rearward and hits the alternative display. The light reflecting off one display either exits through the lamp array and into the second display or is absorbed into the array of fluorescent lamps. 
     The light absorbed by a neighboring lamp can be expressed by the angle of light rays leaving the lamp: 
     
       
         
           
             
               ϕ 
               1 
             
             = 
             
               
                 sin 
                 
                   - 
                   1 
                 
               
               ⁡ 
               
                 ( 
                 
                   D 
                   
                     S 
                     + 
                     D 
                   
                 
                 ) 
               
             
           
         
       
     
     The space S is given by the number of lamps N housed in the width W of the back light cavity, and is: 
     
       
         
           
             S 
             = 
             
               
                 W 
                 - 
                 ND 
               
               
                 N 
                 - 
                 1 
               
             
           
         
       
     
     The light exiting forward is given by its angle:
 
φ forward =180−2φ 1  
 
     The light exiting rearward is the same as that exiting forward; thus, the total light exiting from the back light assembly is: 
             L   =         N   ⁢   l     360     ⁢     {       ϕ   forward     ⁢     ϕ   back       }             
where [ 1 ]  l  is the total light output of one lamp. The results are plotted in  FIG. 4 .
 
     Since the power consumed by each lamp  128  is constant, efficiency is related to light output and the number of lamps. The curve  170  is nearly linear until the number of lamps approaches one-half of the maximum that can be installed in the allotted space. It is desirable then to choose a light output design point near this inflection point. Thus, an optimum number of lamps  168  are shown in  FIG. 4 . 
     Referring now to  FIG. 5 , there is shown a schematic, cross-sectional view  180  of the inventive back light assembly with back-to-back displays. Many optical components typically used in both single and back-to-back configurations are shown. 
     Light collimating optics  132  consist of crossed BEFs  182  and  184  and a collimator  186 . The diffusers and collimating optics  132  are sandwiched between glass plates  188  and  190 . These plates  188  and  190  may be optically clear, with enough stiffness to support the film optics over the expanse needed. Flat-panel displays  122  are placed in front of the optics assemblies  192  and separated by a distance F, leaving air spaces  194 . These air spaces  194  are vented to ambient air to allow for further cooling of the displays  122 . 
     As was previously stated, the collimating optics use BEFs which accept light at high angles of incidence and send light at near normal angles of incidence back towards the back light assembly for recycling. It is desirable to have as much reflective area available as possible for the BEFs. However, more lamps produce more light output. The first pass design choice for lamp spacing S is increased slightly. It has been found that increasing lamp spacing such that the number of lamps is reduced by approximately 10% provides satisfactory results. The coupling of light into the BEFs  182  and  184  is also affected by the distance B that they are placed from the lamps  128 . 
     The luminance output of the BEFs increases with proximity to the lamps, but luminance uniformity decreases with proximity to the lamps. For practical purposes, a reasonable space H  146  is required between the lamps  128  and the glass optics holder for air flow to cool the cavity  126  ( FIG. 2a ). 
     The preferred diffuser  130  is a high efficiency, low transmission diffuser which is chosen to have a near Lambertian distribution in order to couple a maximum amount of light into the BEFs  182  and  184  and to permit a maximum amount of recycling in the back light cavity  126 . The diffuser  130  must efficiently reflect light, it must have high transmission efficiency, and it must produce a Lambertian distribution of light. Additionally, the lamps are not 100% absorbing. Consequently, fine tuning is necessary in the design parameters of lamp spacing, back plane space, and BEF spacing to the lamps. 
     The collimators  186 , also described in detail in the aforementioned U.S. Pat. No. 5,903,328, consist of open hexagonal cells in a honey comb configuration, coated with a highly light-absorbing paint. The aspect ratio of cell width to cell depth determines the cut-off angle or collimation angle. 
     The use of a sharp cut-off collimator is preferred in a seamless, tiled, flat-panel display. Non-tiled, large monolithic or monolithic-like displays do not require cut-off angles as sharp as those for tiled displays. A more efficient collimator design which may be applied is disclosed in United States Provisional Patent Application Serial No. 60/177,447. Unfortunately, collimators, having a physical structure, create a shadow image which can be seen on the display. To prevent imaging of the collimator, the display is placed a predetermined distance F away so that cell images overlap, or are defocused, and therefore are not visible to the viewer. 
       FIG. 6  depicts the degree of collimation or angular distribution of light emitted from each of the optical components. The diffuser  130  emits a Lambertian distribution  200 , as stated hereinabove. The BEFs  182 ,  184  focus light forward in a distribution  202  that has a theoretical forward gain of 2.2 for the type used herein. Actual achieved forward gain is about 1.9. The BEF distribution  202  has a significant amount of light energy remaining beyond the cut-off angle (˜301 in the preferred embodiment) that is undesirable for use with seamless, tiled, flat-panel displays. 
     The collimator  186  eliminates such unwanted light by cutting off light beyond the collimation angle, as shown by its emission distribution  204 . The surface absorption of the collimator cell must be sufficient to prevent luminance of more than 1% of normal luminance beyond the collimation angle. 
     Brightness levels far exceeding existing industry capability have been achieved with the inventive design. Luminance values exceeding 100,000 nits (candelas/square meter) have been reached. Reasonable designs with exceptional efficiency have been prototyped with luminance output exceeding 50,000 nits, a uniformity of luminance of 10% at an efficiency better than any currently available commercial back light unit, even those achieving lower brightness levels. 
     Since other modifications such as in optical configurations can be made to fit particular operating specifications and requirements, 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.