Abstract:
The present invention features a system for uniformly distributing luminance and a high degree of collimation from a back light module for a flat-panel, liquid crystal display (LCD). A constant and uniform luminance output of the back light module is obtained through appropriate selection of lamps, geometry and optical components. An appropriate balance of lamps, lamp spacing, reflective light back plane, and diffuser and collimating optics are chosen to produce a high brightness back light module with very high intensity output over very large surfaces. Variations in intensity over the illuminated area are minimized using light recycling in conjunction with the collimating optics. Optimum geometries are determined for the purpose of maximizing light output at high efficiencies, while minimizing luminance gradients across the display. Finally, a precise collimator eliminates light beyond a defined angle, as required in a tiled, flat-panel LCD.

Description:
FIELD OF THE INVENTION 
     This invention pertains to back light systems for flat-panel displays and, more particularly, to a back light system that produces high intensity, collimated light for very large 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 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 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 difficult to do. The intensity required for some applications and, in particular, that required for a large, tiled, seamless flat-panel LCD display causes the lamps to produce a significant amount of heat. In addition, fluorescent lamps are designed to run most efficiently at an elevated temperature, so it is desirable to operate them at their ideal design temperature, which is usually about 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 requires 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 less lamp cathodes are present. The resultant proffered designs orient lamp tubes horizontally, one above the other with predetermined preferred angular and spacing relationships for increasing reflective efficiency of the back wall of the cavity. 
     The present invention provides a mechanism for using an array of high output and efficient fluorescent lamps for producing maximum brightness. Additionally, the back light assembly cavity of the inventive apparatus is treated with a highly diffuse and efficient reflective surface. Also added are commercially available optics, such as Brightness Enhancing Films (BEFs) and a diffuser for maximizing the output of the BEFs, reflector, and back light geometry. 
     The invention also provides for a very uniform light field across the back light exit surface. 
     The invention further provides means for incorporating a sharp cut-off collimator, as described in U.S. Pat. No. 5,903,328, hereby incorporated by reference. 
     Additionally, when used with the invention described in copending U.S. patent applications, Ser. Nos. 09/407,619 and 09/406,977, both filed concurrently herewith and also hereby incorporated by reference, the apparatus of this invention provides a very uniform, high luminance back light system capable of maintaining display brightness under a wide range of environments over long periods of time. It is particularly suited for illuminating a large tiled, seamless flat-panel LCD. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a system 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 and also lamp life. This invention provides means for achieving luminance uniformity and a high degree of collimation. 
     A highly efficient and diffuse reflective surface treatment is disclosed. Reflection efficiency of this invention is significantly higher than other available treatments for large areas. In particular, a constant and uniform luminance output of the back light module is obtained through appropriate selection of lamps, geometry and optical components. A preferred balance of lamps, lamp spacing, reflective light back plane, and diffuser and collimating optics are chosen to produce a high brightness back light module with very high intensity output over very large surfaces. The variations in intensity over the illuminated area are minimized using light recycling in conjunction with the collimating optics. Variations are further reduced by incorporating the invention disclosed in patent application Ser. No. 09/406,977. 
     This invention provides means for achieving this goal through selection of combinations of components and appropriately designed geometry. A particular application is a large, tiled, flat-panel display having visually imperceptible seams as described in the aforementioned U.S. patent application, Ser. Nos. 08/652,032, 09/368,291, and U.S. Pat. No. 5,903,328. The back light module system, with thermal enhancements such as those disclosed in Ser. No. 09/406,977 and applicable controls, such as those disclosed in Ser. No. 09/407,619 provides for an efficient, reliable, large area, high intensity light source for flat-panel displays. 
     Additionally, optimum geometries are determined for the purpose of maximizing light output at high efficiencies, while minimizing luminance gradients across the display. These optimum geometries are also determined for maximizing light output using BEFs and light recycling. 
     Finally, a precise collimator such as that disclosed in Ser. No. 09/024,481 is added which eliminates light beyond a defined angle, as required in a tiled flat-panel LCD. 
    
    
     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  illustrates a side view of a multiple lamp back light and a display in accordance with the present invention; 
     FIG. 2 b  illustrates a planar view of the multiple lamp back light depicted in FIG. 2 a;    
     FIG. 3 a  is a schematic diagram illustrating lamp and reflector spacing relationships; 
     FIG. 3 b  graphically depicts light output as a function of lamp spacing; 
     FIG. 4 is a graph depicting light output as a function of the number of lamps; 
     FIG. 5 is a schematic view of a high efficiency reflective surface treatment; 
     FIG. 6 depicts a back light design with display, in accordance with the present invention; 
     FIG. 7 graphically illustrates the collimation attributes of the optics; 
     FIG. 8 shows a schematic, cross-sectional view of a tiled, color display having invisible seams; 
     FIG. 9 depicts a heat sink used to cool the lamp ends, in accordance with the present invention; 
     FIGS. 10 a  and  10   b  depict a back light cavity back plane with louvers; 
     FIG. 11 is an electrical schematic diagram illustrating the fan speed control logic of the present invention; 
     FIG. 12 is an electrical schematic diagram illustrating the dimming ballast control logic; and 
     FIG. 13 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 uniformity and collimation of light exiting a large area back light for a flat-panel display. A back light for a large tiled, flat-panel display requires high luminance levels and a precise predetermined degree of collimation. In addition, the invention provides an optimum design for the efficiency, cooling, luminance and image quality desired in a large, flat-panel display, particularly a tiled LCD. 
     Now referring to FIG. 1, a typical fluorescent lamp (not shown in the FIGURE) 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 o    12 . The ideal temperature  12  is determined by the lamp construction and its 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 . Lamps are cooled by fans  29 . 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, which uses highly collimated light. The additional optics  28  required to collimate the light are somewhat inefficient. This necessitates 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 the 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 a  illustrates an arrangement of lamps  23  and the reflecting back plane  30  of the back light cavity  22 . Lamps  23  have a diameter D and are spaced apart by dimension S. The lamps  23  are positioned at a distance H from the back plane  30 . 
     FIG. 3 b  shows the effect of changing the ratio of S to H. The light output  31  can be calculated easily by assuming that the back plane surface  30  is 100% reflective, while the lamp tubes  23  are 100% absorbing. For a given diameter D of a lamp  23  and lamp space S, there is an optimum distance  32  for the back plane surface  30  to lamp tube  23  space H. 
     A first approximation analysis can easily be obtained through a consideration of the geometry in FIG. 3 a.  Light leaving the lamp  23  exits forward toward the display, is absorbed by neighboring lamps, or is sent back to the back plane  30 . It is desired to have as much light possible to reflect off the back plane. A first approximation is to assume that the back plane is a mirror; in reality it is a diffusive reflector. The lamp is assumed to be a line source. 
     Light rays leaving the rear of the lamp will reflect back into the lamp if they leave the lamp at angles smaller than B. If the exiting angle is larger than A, the light will be absorbed by neighboring lamps. Light rays exiting the rear of lamp  23  that have exit angles between A and B will escape forward through the interlamp space S. A first approximation of angle A is        A   =       tan     -   1                         (     D   /   4     )       (     H   +     D   /   2       )                                
     A first approximation for the angle B is        B   =       tan     -   1                         (     S   +     D   /   2       )       (     H   +     D   /   2       )                                
     The escape angle is then 
     
       
         
           E=A−B 
         
       
     
     There is a value H, given S and D, that maximizes the light escape angle E. The maximum is found by setting the differential equal to zero. That is               E          H       =     0   =              A          H       -          B          H         =               H            {       tan     -   1                       D     2        (       2      H     +   D     )                           tan     -   1            (         2      S     +   D         2      H     +   D       )         }                                  
     FIG. 4 illustrates the results of an analysis to determine the number of lamps  23  to be used in a back light assembly  21  having a predetermined size. The assumptions are the same as used to generate FIG. 3 b.  In addition, the optimum lamp  23  to reflective back plane  30  space H was chosen for calculation. The curve of total light output from the back light cavity  42  is shown as a function of the number of lamps installed. The desired light level  40  is also presented. It will be noted that, as the number of lamps increase, the light output increases until a maximum illumination  43  occurs prior to reaching the point of maximum lamp capacity  44 . 
     The lamps  23  block light reflected from the reflector surface  30 , from the rear half of the lamp tube. Also, as more lamps are used, spaced closer together, they block light from each other. The number of lamps  41  corresponding to the desired light output  40  is also shown. 
     A good approximation of the total light output of the back light assembly, without considering collimation and related light recirculation, can be obtained by considering the geometry. A lamp tube  23  produces light rays uniformly over 360 degrees. The light exits forward toward the display, is absorbed by neighboring lamps or it exits rearward and hits the reflective back plane  30 . The light reflecting off the back plane  30  either exits the back light through space S or is absorbed by a lamp. 
     The light absorbed by a neighboring lamp can be expressed by the angle of light rays leaving the lamp. Or          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 the forward, but the light then reflected out of the back light cavity from the back plane is          φ   back     =       S     D   +   S                       φ   forward                              
     The total light exiting from the back light assembly is L:        L   =       Nl   360                     {       φ   forward     +     φ   back       }                              
     where l is the total light output of one lamp. The results are plotted in FIG.  4 . 
     Since the power consumed by each lamp  23  is constant, efficiency is related to light output and the number of lamps. The curve  42  is nearly linear until the number of lamps approaches 50% 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  41  is shown in FIG.  4 . 
     FIG. 5 shows a unique surface treatment for the back plane  30  of the back light cavity  22  of the back light assembly  21 . The back light cavity  22  is constructed of aluminum with a moderately high gloss finish  50 . A somewhat reflective white powder coat of paint  51  is applied to the aluminum back plane  30 . The surface texture finish of the paint  51  is chosen through experiment to best reflect diffuse light. 
     The texture features of peak-to-peak roughness and off-planar angularity of the microsurfaces are chosen to reflect and disperse light without imaging shadows of the texture details. Next, a white Teflon sheet is applied to the back plane  30  using an optically clear adhesive. The Teflon sheet is a commercially available product with a high loading of titanium dioxide powder filler. The film is sufficiently thick to maximize the reflected light. Specific designs use a 0.05 mm thick paint  51  and 0.25 mm of Teflon material. 
     Now referring to FIG. 6, there is shown a cross sectional view of a back light assembly  21  with additional optics  28  and flat-panel display  20 . The back light assembly  21  consists of a back light cavity  22  with reflecting back plane lamps  23  and a glass cover plate  61 . A diffuser is added to complete the back light assembly  21 . 
     Collimating optics consist of crossed BEFs  63  and  64  and a collimator  65 . The diffuser and collimating optics are sandwiched between two glass plates  61  and  62 . These plates  61  and  62  may be any optically clear, with enough stiffness to support the film optics over the expanse needed. A flat-panel display  20  is placed in front of the optics assembly  28  by a distance F, leaving an air space  66 . This air space  66  is vented to ambient air to allow for further cooling of the display  20 . 
     As aforementioned, the collimating optics makes use of BEFs. A BEF accepts light at high angles of incidence and sends light at near normal angles of incidence back to 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. Specifically, 10% fewer lamps are used. The coupling of light into the BEFs  63  and  64  is also affected by the distance B that they are placed from the lamps  23 . 
     The luminance output of the BEFs increases with proximity to the lamps, but luminance uniformity decreases with closeness to the lamps. For practical reasons a reasonable space is required between the lamps and the glass optics holder  61  for air flow to cool the cavity  22 . 
     The preferred diffuser  24  is a high transmission holographic type diffuser which is chosen to have a near Lambertian distribution in order to couple a maximum amount of light into the BEFs  63  and  64  and to permit a maximum amount of recycling in the back light cavity  22 . The diffuser  24  need not be of the holographic type, but is must have high transmission efficiency and produce a Lambertian distribution of light. The lamps are not 100% absorbing and the reflective back plane is not 100% reflecting, although reflectivity is greater than 95%. Accordingly, fine tuning is necessary in the design parameters of lamp spacing, back plane space, and BEF spacing to the lamps. 
     The collimator  65 , also disclosed in the aforementioned U.S. Pat. 5,903,328, consists 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. Untiled, large displays do not require a sharp cut-off collimator. Unfortunately, the collimator, having a physical structure, creates a shadow image which can be seen on the display. To prevent imaging of the collimator, the display is placed further away so that cell images overlap, or are defocused, and therefore are not visible to the viewer. 
     FIG. 7 depicts the degree of collimation or angular distribution of light emitted from each of the optical components. The diffuser  24  emits a Lambertian distribution  71 , as stated hereinabove. The BEFs focus light forward in a distribution  72  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  72  has a significant amount of light energy remaining beyond the cut-off angle (˜30° in the preferred embodiment) desired for a seamless, tiled, flat-panel display. 
     The collimator eliminates such unwanted light by cutting off light beyond the collimation angle, as shown by its emission distribution  73 . 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 industry capability have been achieved. Luminance values exceeding 100,000 nits (candellas/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 commercially available unit even at lower brightness levels. 
     Now referring to FIG. 8, one embodiment of a seamless, tiled display is illustrated in cross-sectional view. The seamless display  150  comprises an image source plane  151  comprising a color filter layer  152  and lightvalve layer aperture areas  153 . It should be understood that the image source plane  151  can be disposed anywhere between the viewer and the source. The tiles are presented by the glass layers  154 , which are separated by a gap  155 . This gap  155  and the areas between the lightvalve areas  56  are covered by a mask  157 , in order to make the image source plane uniform. An overlaid screen surface  158  is used to project the image source plane into the image view plane. A lens surface may be used, instead of the screen surface  158 , for generating the image view plane. 
     When the seam  155  is blocked from the backlight source, the seam is still noticeable because of ambient light and scattered light from the sides of the tiles. However, when the seam  155  is blocked directly from above, using a mask  157 , which is aligned to the tiles and lightvalves of the display, then the seam is not perceptible when viewed directly along the surface normal. However, for sufficiently large viewing angles away from the surface normal, the seam  155  is no longer shadowed by the mask  157 , and thus becomes visible. If the view angle range for seamless appearance is unacceptably small, it can be enhanced through the use of a microlens array. The closer the screen  158  can be placed to the mask  157 , the larger the view angle range becomes for seamless appearance. The mask reduces the transmitted light flux significantly. A thin polarizer layer  159  can be placed between the image source plane  151  and the screen  158 . 
     FIG. 9 is an exploded view of a cathode heat sink assembly  240  in accordance with the invention. The heat sink assembly  240  serves as a lamp holder (not shown) as well. The heat sink assembly  240  covers the cathode area of the fluorescent lamps  23  (FIG. 2 b ). The heat sink assembly  240  consists of two mating parts: the heat sink body  241  and the heat sink cap  245 . Both of these two parts  241  and  245  have respective, essentially semicircular cavities  242  for receiving lamps  23 . The two mating parts  241  and  245  are held together by fasteners  248 . 
     Prior to placing the lamps  23  into the heat sink cavities  242 , thermally conductive elastomeric tape  246  is placed around the lamps  23  in the cathode area. The thermal tape  246  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  246 . 
     A thermal sensor  244  is mounted in the heat sink body  241  using thermal adhesive. The heat sink temperature is uniform across the lamps  23 . The temperature at the top of the heat sink  240  is the most indicative of the lamp temperatures in the back light cavity  22  (FIG. 2 b ). The temperature at the sensor  244  represents 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 cooling fans (not shown). The use of fans to cool a light box, of course, is well known to those skilled in the art. 
     The heat sink assembly  240  is mounted in the back light cavity  22  with cooling fins  247  protruding from the rear of the cavity  22 . This allows cool ambient air to flow convectively over the heat sink fins  247 . This additionally allows the heat sink  240  to be at a near uniform temperature. The sensor  244  is located at an optimum thermal location for use in a temperature control system. 
     Referring now to FIG. 10 a,  there is shown an array of louvers, or open slots, dispersed behind the lamps  23 . Different sized louvers  261 ,  262  and  263  are used for thermal balancing. The louvers  261 ,  262  and  263  are punched into the back plane of the back light cavity  22 . This plane is a highly efficient, diffusive reflector; the louver surface is reflective as well. The louvers  261 ,  262  and  263  present no visible slot to the viewer, due to the diffusive reflectivity characteristic of the back plane. 
     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 (not shown) to enter the back light cavity  22  through louvers  261 ,  262  and  263  placed behind the lamps  23 . A filter  264  is placed behind the back light cavity  22 , as shown in FIG. 10 b.    
     The height H and width W of the louvers  261 ,  262  and  263  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 disposed 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. 11 is an electrical schematic diagram that depicts a closed loop circuit for controlling fan speeds. One type of temperature sensor  371  in this embodiment is a thermistor forming part of a voltage divider network with fixed resistors  373  and held between a reference voltage  372  and ground  374 . The divided voltage  376  is fed into a microprocessor  370  via analog-to-digital converters  375 . The temperature sensor  371  in this embodiment can be used as sensors  363 ,  364 . 
     A microprocessor  370  uses digital temperature data  378  to adjust fan speeds. The digital output  379  of the microprocessor  370  is fed into the motor drive amplifiers  377  via digital-to-analog converters  376 . In this embodiment, motor drive amplifiers  377  then supply a DC voltage to the fans (not shown). 
     The simplest form of control algorithm adjusts the speed of all 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  379  to changes in the input  378  is accomplished using a simple lookup table, not shown, which is empirically developed by actual test results. Only one sensor and one motor drive amplifier is needed for this simplest of controls. 
     A two 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 zones. A more complex method is to use two sensors  363  and  364  of the type  371  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 control the temperature distribution more accurately within the back light assembly  21 . It has been found that a dual zone with one sensor is adequate for most applications. 
     FIG. 12 shows the control system used for dimming the lamps individually or in groups. The control again is through lookup tables in the microprocessor  370 . Lamp temperature digital data  378  is fed to the microprocessor  370 , as previously shown. Ballasts  26  have a dimming feature such that the output of a ballast  26  is proportional to a DC input voltage  384 . The digital output  382  of the microprocessor  370  is converted to the appropriate ballast voltage  384  via a digital-to-analog converter  383 . Each lamp  23  may be driven by one ballast  26 . Alternatively, the lamps  23  may be ganged, so that one ballast  26  can drive several lamps  23 . 
     In simplest form, the ballasts  26  are all given the same dimming voltage  384 . The dimming voltage  384  is controlled by one sensor  371  (the same one used for fan control) and the external brightness command  381 . Dimming voltage  382  and fan speed voltage  379  are determined from a lookup table, the inputs for which are temperature sensor data  378  and brightness setting  381 . Brightness increases based on input  381 , as long as the average maximum temperature does not exceed the ideal. Brightness can be decreased by external input. Microprocessor output  382  to the ballasts is decreased accordingly. In addition, fan speed data  379  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. 13, 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  begins when initially turned on. Fan speeds and dimming output data are set at predetermined initialization levels. As the unit heats up, lamp temperature follows curve  404  towards the preset brightness level  402  and upper operating temperature level  403 . 
     As the temperature level  403  is reached, power to the lamps  23  is reduced incrementally in steps 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  405 . In the event of an over temperature condition  406 , the lamp power is reduced via the dimming output data level to a predetermined safe power (brightness) level  401 . The lamp temperature then drops, following path  407 . When the temperature is in a safe zone, the lamp power is again increased, following curve  408  towards the normal operating area  405 . If this over temperature condition reoccurs a predetermined number of times, a shut down occurs. 
     Since other optical configurations can be formulated 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.