Patent Publication Number: US-2007115686-A1

Title: Lighting assembly, backlight assembly, display panel, and methods of temperature control

Description:
FIELD OF THE INVENTION  
      This invention relates to lighting panels and display panels.  
     BACKGROUND  
      For flat-panel display applications, it may be desired to obtain a lighting assembly that provides a substantially uniform distribution across a plane. For example, such an assembly may be used for backside illumination of a transmissive or transreflective display panel such as a liquid crystal display (LCD).  
      An LCD device generally includes a glass LCD panel and a backlight system. The display may also include circuitry such as lamp driver electronics, panel driver electronics, and an interface card to convert an analog or digital video signal (such as digital video interface or DVI) into another form such as low-voltage differential signaling (LVDS). Typical advantages of LCD technology over cathode-ray tube (CRT) technology include a smaller size and less weight for a similar display area.  
      Backlight systems include edge-light type and direct type backlights. A direct-type backlight typically can provide a higher light intensity than an edge-light type, and thus a direct-type backlight is typically more suitable for large-sized display panels.  
      Operating environments for LCD displays may be limited in temperature due to the nature of the LCD technology. Above a particular temperature, the LCD molecules become randomly oriented, rather than being aligned according to the applied voltage. At high temperatures, an LCD display may become opaque, yielding a black display regardless of the driving signal. This phenomenon, called “clearing” of the panel, is temporary and nondestructive, but it limits use of the panel to within certain temperature limits. High temperatures may also cause reduced efficiency and lifetime of the light sources and/or circuitry.  
     SUMMARY  
      A lighting assembly according to one embodiment includes a light source; a backplate having a reflecting surface arranged to reflect light of the light source; and a heat transfer substrate disposed between the light source and the reflecting surface and arranged to transfer heat between the light source and the backplate. The heat transfer substrate is substantially transparent to light of the light source and has a thermal conductivity greater than that of air. The heat transfer substrate includes an interface in contact with the light source, which interface is substantially transparent to light of the light source and has a thermal conductivity greater than that of air.  
      A lighting assembly according to another embodiment includes a plurality of light sources disposed in a substantially planar arrangement; a backplate having a reflecting surface arranged to reflect light of the plurality of light sources; and a heat transfer substrate disposed between the plurality of light sources and the reflecting surface and arranged to transfer heat between the plurality of light sources and the backplate. The heat transfer substrate is generally planar, is substantially transparent to light of the plurality of light sources, and has a thermal conductivity greater than that of air. The heat transfer substrate includes a plurality of interfaces, the plurality of interfaces being substantially transparent to light of said light sources and having thermal conductivities greater than that of air. Each of the plurality of interfaces is in contact with a corresponding one of the plurality of light sources.  
      Embodiments also include methods of controlling a temperature of a lighting assembly, such as a lighting assembly according to one of the other embodiments. One such method includes receiving an indication of at least one of (A) a luminance of the light source and (B) a temperature of at least one among the light source, the backplate, and the heat transfer substrate; and controlling at least one among a cooling device and a heating device to change a temperature of the backplate. The act of controlling is based at least in part on the received indication. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A-1C  show cross-sections of representative portions of assemblies according to different embodiments.  
       FIGS. 2-7  show top views of assemblies according to different embodiments.  
       FIGS. 8A-8C  show cross-sections of representative portions of assemblies according to different embodiments.  
       FIG. 9  shows a top view, two sectional views, and two side views of an assembly according to an embodiment.  
       FIGS. 10A and 10B  show cross-sections of representative portion of assemblies according to different embodiments.  
       FIGS. 10C and 10D  show cross-sections of implementations of backplate  400 .  
       FIG. 11  shows a perspective representation of an assembly according to an embodiment.  
       FIG. 12  shows a perspective representation of an assembly including an embodiment as shown in  FIG. 11  including a collector.  
       FIGS. 13A-13C  show cross-sections of representative portion of assemblies according to different embodiments.  
       FIGS. 14A and 14B  show cross-sections of representative portion of assemblies according to different embodiments.  
       FIG. 15  shows an example of a relation between luminance and temperature.  
       FIGS. 16A-18  show examples of sensor placements.  
       FIGS. 19-22  show examples of methods of temperature control according to different embodiments.  
       FIG. 23  shows a cross-section of an edge-lit backlight assembly according to an embodiment. 
    
    
     DETAILED DESCRIPTION  
      Fluorescent tubes are an efficient and mature lighting technology. For high-brightness applications, fluorescent lamps are typically more economical than light-emitting diodes (LEDs). Fluorescent lamps are currently the technology of choice for backlight assemblies for LCD panels. An LCD panel typically transmits only seven percent of the illuminating light, however. A color LCD panel typically has lower light transmission than a monochrome panel, and it may be desirable to obtain a comparable brightness to a monochrome (grayscale) display. For example, it may be desired to achieve a display brightness of 500 cd/m 2 . Such a display brightness requires a very bright backlight.  
      A direct-type backlight assembly includes one or more lamps within a box, with the LCD panel on one side of the lamps and a reflector on the other side of the lamps. Display brightness may be increased by increasing the light intensity of the backlight: for example, by including more lamps and/or raising the lamp driving current. However, such solutions may lead to increased heat generation. The efficiency of fluorescent lamps decreases at high temperatures, and high temperatures may also lead to clearing of the LCD panel. Internal ventilation of the backlight may not be feasible, as it may be desirable to keep out dust.  
      Embodiments include embedding lamps in a thermally conductive and optically transmissive medium for improved heat distribution, possible thermal control. The light source is at least partially embedded in a heat transfer element, which passively transfers heat between the lamps and a backplate. The backplate may be actively cooled (for example, forced-air cooling by a fan).  
       FIG. 1A  shows a cross-section of light sources  100   a,b  partially embedded in a heat transfer substrate  200 . Light sources  100   a,b  may be different light sources or different parts of the same source (e.g. a U-shaped lamp as described below). In other embodiments, the light source may have a different shape in cross-section, such as rectangular or elliptical, with the embedding being along either axis.  
      Light source  100  may be implemented as an elongated tube. In examples as described herein, light source  100  is a cold-cathode fluorescent lamp (CCFL). Such lamps are typically driven at a frequency of tens of kHz, typically 20-100 kHz, and a voltage of 900-1500 volts, and they may have an operating lifetime of 20,000 hours or more. The lamp holder is typically made of silicone rubber or plastic, and it may be desirable for the lamp holder to have a low dielectric constant to minimize losses (for example, the lamp holder may be porous). In other implementations, light source  100  may be a hot-cathode fluorescent lamp, an LED, or another lighting technology.  
      Heat transfer substrate  200  is a solid that has a thermal conductivity greater than air (i.e. greater than 0.025 W/(m·K)). Heat transfer substrate  200  is also substantially transparent or translucent to visible light (or to light of the light source  100  that is desired for the particular application). In the examples described herein, heat transfer substrate  200  is made of polymethyl methacrylate (PMMA), which has a thermal conductivity of 0.187 W/(m·K), about seven times greater than that of air. In other implementations, glass may be used (thermal conductivity of 1.1-1.2 W/(m·K)), although PMMA transmits more visible light (92% transmission) than glass. Especially in applications where light source  100  includes fluorescent lamps (or where the desired application includes ultraviolet illumination), it may be desirable for heat transfer substrate  200  to be resistant to clouding from exposure to ultraviolet radiation.  
      Heat transfer substrate  200  may have any dimensions desired for the particular application, although it may be desired to limit the thickness of the substrate to reduce absorption of light from the light source, to limit the quantity of heat stored in the substrate, and/or to increase the rate of heat transfer to a backplate. In the particular examples described herein, heat transfer substrate  200  is a sheet about four millimeters thick. It may also be desirable for heat transfer substrate  200  to be at least as large as an LCD panel to be illuminated.  
      Heat transfer substrate  200  is thermally coupled to light source  100  via an interface  300 , which also has a thermal conductivity greater than air and is substantially transparent or translucent to visible light (or to light of the light source  100  that is desired for the particular application). For example, interface  300  may be optically clear. The thickness of interface  300  may be only a few tenths of a millimeter. In one example, light sources  100  are tubes of diameter 4.6 mm, embedded in channels of heat transfer substrate  200  that have diameter 5 mm, such that the intervening spaces along the lengths of the channels are filled by respective interfaces  300 . It may also be desirable for interface  300  to have an index of refraction η similar to that of heat transfer substrate  200  and/or light source  100 . Using materials having similar indices of refraction may help to reduce internal reflections at their interface. For PMMA, the index of refraction η=1.49, and it may be desirable for interface  300  to have an index of refraction not less than 1.39 and not greater than 1.59. In the examples as described herein, interface  300  is a layer of a silicone polymer.  
      In other implementations, heat transfer substrate  200  may be made of a soft, deformable, or nonrigid solid (such as a silicone polymer) having the specified thermal and optical properties. In such cases, the material of heat transfer substrate  200  may be capable of forming a good thermal and optical bond to light source  100 , and interface  300  may be indistinguishably included in heat transfer substrate  200 .  
       FIG. 2  shows a top view of an example of an assembly including implementations  110  of light source  100  (straight fluorescent tubes) and a suitably dimensioned implementation  202  of heat transfer substrate  200 . In the examples described herein, light sources  100  produce white light, but in other implementations light sources of two or more different colors may be used.  
       FIG. 3  shows a top view of an example of an assembly including implementations  120  of light source  100 , which are U-shaped fluorescent tubes. A U-shaped fluorescent lamp is typically more efficient than a straight one. In one example, each lamp  120  is about 435 millimeters long, with a distance of 16.4 millimeters between the axes of the lamp legs, and a tube diameter of 4.6 millimeters. In one assembly, a planar arrangement of ten tubes is used, with the adjacent legs of each pair of tubes being the same distance apart as the legs of each tube. In other embodiments, light sources having other shapes (such as spiral, serpentine, or circular shape) may be used, and an assembly may include light sources having more than one shape and/or light sources of more than one technology (such as fluorescent tubes and LEDs).  
       FIG. 4  shows another planar arrangement including U-shaped lamps  120 , in which each tube is oriented in the same direction. It may be desirable to control the phase at which the lamps  120  are driven such that adjacent lamps are driven out-of-phase (for example, according to the polarities shown in  FIG. 4 ) to minimize losses from high-voltage differences between the lamps.  
       FIG. 5A  shows another planar arrangement including an implementation  208  of heat transfer substrate  200  in which the curves of the lamps  120  extend beyond the edge of the substrate. Such an arrangement may provide better illumination uniformity over the area of heat transfer substrate  208  (and thus better illumination uniformity over the area of a matching display panel). Elongated light sources such as lamps  120  may be arrayed along a short dimension of a front surface of heat transfer substrate  200 , as shown in  FIGS. 2-5A , or along a long dimension of a front surface of heat transfer substrate  200 , as shown in  FIG. 5B .  
      Heat transfer substrate  200  may have undesirable electrical properties. For example, the dielectric constant of PMMA (ε is about 4 at 60 Hz) is about four times higher than that of air (ε=1). At the high voltages used to drive fluorescent lamps, this property may lead to increased electrical losses due to a reduced impedance to the high-frequency signal that powers the light sources. This parasitic capacitance may cause losses and lower lamp efficiency.  
       FIG. 6  shows an arrangement in which an implementation  210  of heat transfer substrate  200  includes slots  250  between the legs of each lamp  120 . Slots  250  may help to reduce losses between lamp legs by reducing the dielectric constant in regions of high voltage.  FIG. 7  shows another arrangement in which a similar implementation  212  of heat transfer substrate  200  includes slots between the legs of adjacent lamps  120 . Such slots may not be needed if adjacent lamps may be driven out-of-phase as shown in  FIG. 4 , but they may be included nevertheless in case of phase drift between the driving currents of adjacent lamps. It is expressly noted that slots as shown in FIGS.  6  and/or  7  may also be used in any of the configurations shown in  FIGS. 2-5B .  
      As shown in the cross-section of  FIG. 8A , a slot  250  may be implemented as a depression  252  between light sources (or between legs of a light source). Alternatively, as shown in the cross-section of  FIG. 8B , a slot  250  may be implemented as a hole or gap  254  in heat transfer substrate  200 . In a further alternative as shown in the cross-section of  FIG. 8C , heat transfer substrate  200  may be implemented as strips, such that a slot  250  is formed by a space  256  between adjacent strips. In this example, legs of adjacent lamps  120  are supported by a strip of the substrate, while legs  1 , 2  of the same lamp  120 - b  are separated by slot  256 . Further implementations of heat transfer substrate  200  may include any combination of these three alternatives. For example, a slot  250  may be implemented as one or more depressions and/or holes of any desired shape, having sharp and/or rounded edges and corners. A particular shape of slot  250  may be selected based on factors such as cost of fabrication and manufacture, desired degree of electrical isolation, desired degree of optical continuity, and desired degree of structural rigidity of heat transfer substrate  200 .  
      As described above, slots  250  may be implemented as air gaps. Alternatively, one or more of slots  250  may be filled with another substantially transparent material having a low dielectric constant. For example, polyethylene may be used (ε of about 2), or a silicone having suitable electrical and optical properties. Optically clear silicones having a dielectric constant less than three are currently available.  
       FIG. 9  shows several views of an implementation  214  of heat transfer substrate  200 . Specifically,  FIG. 9  includes a top view in the center, two cross-sections on the left, and edge views on the top and right side of the figure. In this example, heat transfer substrate  214  is a generally planar sheet measuring about 320 by 380 millimeters.  
       FIG. 10A  shows a cross-section of an assembly including a backplate  400 . Backplate  400  includes a reflecting surface disposed to reflect light back into heat transfer substrate  200 . In examples as described herein, backplate  400  also functions as a heat sink. The reflecting surface of backplate  400  may be made of aluminum, silver, or any other metal or alloy that forms a highly reflective surface. The reflecting surface may be implemented as a foil, sheet, layer, or film and may have a specular finish.  
      In some cases, the reflecting surface is a layer or film that is deposited on the back surface of heat transfer substrate  200 . In other implementations, the reflecting surface may be a high-reflectance diffusing or scattering surface, such as white powder, plastic, or paint, which in some cases may also be deposited on the back surface of heat transfer substrate  200 . As shown in  FIG. 10B , the reflecting surface may be optically coupled to heat transfer substrate  200  by an optical coupling layer  440 . In an example as described herein, layer  440  is implemented as a transparent and thermally conductive film or sheet (such as silicone). Such coupling may reduce internal reflections at the surface of heat transfer substrate  200 .  
       FIG. 10C  shows a cross-section of an implementation  402  of backplate  400 . Backplate  402  includes a reflector  410  (for example, a foil, sheet, layer, or film), having the reflecting surface as described above, and a collector  420 . Reflector  410  may be directly mounted to collector  420  via fasteners (for example, screws or clips securing backplate  402  to heat transfer substrate  200 ). Alternatively, as shown in  FIG. 10D , reflector  410  may be joined to collector  420  via a heat coupling layer  460  such as an adhesive (e.g. an epoxy) or thermally conductive paste. Examples of such a paste include suspensions of zinc oxide, aluminum oxide, aluminum nitride, and/or precipitated silver.  
      Collector  420  is made of a material of high thermal conductivity. Examples include a metal such as aluminum, copper, magnesium, titanium, silver, or stainless steel; an alloy including one or more such metals; or a polymer composite material. It may be desirable for collector  420  to have an appropriate thickness and/or mass to provide sufficient heat sinking capacity. A back side of collector  420  may have fins, or an otherwise increased surface area, for increased transfer of heat to the air. For example, collector  420  may include a substantially planar sheet that is thermally coupled to a finned heat sink. It may also be desirable for collector  420  to be cooled with forced air (e.g. by one or more fans). Collector  420  may also be cooled using one or more Peltier devices. In other implementations, collector  420  is cooled by a passive or forced liquid or gas, such as water, benzene or other cooling fluid or gas.  
       FIG. 11  shows a perspective view of an assembly including heat transfer substrate  214 , ten light sources  120 , and an implementation of reflector  410 .  FIG. 12  shows a perspective view of such an assembly including an implementation of collector  420 .  
      It may be desired for backplate  420  to be electrically connected to a ground potential of the lighting assembly. In such case, it may also be desirable to drive the lamps between symmetrical voltages around the ground potential of backplate  420  (e.g. between −500 and +500 volts), instead of between the ground potential and a maximum potential (e.g. between 0 and +1000 volts). Such symmetrical driving may help to minimize leakage to ground (e.g. via a parasitic capacitance across heat transfer substrate  200 ).  
      It may be desired to include additional elements having higher thermal conductivity, which may also be opaque, in a region at the back side of light source  100 .  FIG. 13A  shows a cross-section of one such arrangement that includes one or more heat conductors  510  between a light source  100  and backplate  400 .  FIG. 13B  shows a cross-section of an arrangement in which one or more heat conductors  520  take the place of a portion of the interface  300 .  FIG. 13C  shows a cross-section of an arrangement in which one or more heat conductors  530  take the place of a portion of optical coupling layer  440 . Heat conductors  510 - 530  may be implemented variously as, for example, spots or beads of thermally conductive paste or epoxy; or metal pieces, strips, or plugs. In the case of metal pieces, strips, or plugs, the heat conductors may be integrated with or fastened to backplate  400 , passing through gaps in heat transfer substrate  200 . The temperature distribution may not be homogeneous along the lamp, as typically the local lamp temperature decreases as distance from the electrodes increases. Therefore, it may be desirable to locate or concentrate such heat conductors  510 - 530  near the electrodes.  
      In a further embodiment, a microstructure or other texture is applied to the top surface of the heat transfer substrate.  FIGS. 14A and 14B  show cross-sections of two assemblies having such a texture on a top surface of heat transfer substrate  200 . The microstructure may be applied or deposited on the surface. Alternatively, the microstructure may be created chemically (such as by etching the surface of substrate  200 ) and/or mechanically (such as by abrading and/or scoring the surface of substrate  200 ). The texture may have a regular pattern (such as a set of grooves in one or more directions) or may be irregular or random. Such a microstructure or texture may serve to diffuse light shining out of heat transfer substrate  200 , to improve light transmission from substrate  200 , and/or to reduce internal reflection within substrate  200 .  
      High operating temperatures adversely affect the luminous efficiency and operating lifetimes of fluorescent lamps. The same is also true of low operating temperatures, and for a constant driving current there exists a particular operating temperature or temperature range at which the lamp reaches an optimal efficiency and operating lifetime, typically between about 30 and 75 degrees Celsius. More specifically, an optimal operating temperature for a CCFL is typically about 40 degrees Celsius.  FIG. 15  shows one example of a relation between luminance and lamp temperature for a constant driving current.  
      Thermal coupling of the light source to the backplate may also provide opportunities for improved temperature control of the light sources, and further embodiments include systems and methods of temperature control. Some methods include a characterization operation, which uses optical and temperature sensors to identify an optimal operating temperature (in other words, a temperature at which luminance output is maximum for a constant driving current). These sensors may be placed in any of various locations, and the temperature or luminance output may be taken as an average of the outputs of the individual sensors. For example, sensors  700  (temperature sensors  710  and/or light sensors  720 ) may be embedded or inserted into heat transfer substrate  200 , between the light sources  100  (as shown in the cross-section of  FIG. 16A ) and/or behind the light sources  100  (as shown in the cross-section of  FIG. 16B ). As shown in the cross-section of  FIG. 17 , temperature sensors  710  may be located on the outer surface of backplate  400 . Without limitation, temperature sensors  710  may be implemented using silicon devices or thermistors.  
      Light or temperature sensors may also be mounted, fixed, or otherwise positioned in other locations near to the light sources  100 . For example,  FIG. 18  shows a top view of an arrangement in which luminance sensors  720  are located to indicate the light output of each light source  120 . Without limitation, luminance sensors  720  may be implemented using photovoltaic or photoresistive elements.  
      The optimal operating temperature as identified during the characterization operation (or, equivalently, a temperature sensor reading corresponding to that temperature) may be entered into a storage element of the assembly such as a nonvolatile memory or a DIP switch. Alternatively, the characterization operation may be omitted and a desired temperature may be selected according to other information, such as a known operating profile of the light source or a characterization of a similar assembly. During operation of the lighting assembly, a cooling device (such as one or more fans and/or Peltier devices) and/or a heating device (such as a resistive heater) is controlled to cool or heat backplate  420  to maintain the desired temperature.  
       FIGS. 19-21  show examples of several different temperature control schemes.  FIG. 19  shows an example of a scheme in which a cooling unit is activated when a temperature T 2  is reached and deactivated when a lower temperature T 1  is reached. The temperature points T 1  and T 2  may be selected to be slightly lower and higher, respectively, than the desired target temperature.  FIG. 20  shows an example of a scheme in which a fan is off until a temperature T 1  is reached. Temperature T 1  may be selected to be near to the desired target temperature. Between temperatures T 1  and T 2 , the speed of the fan is increased linearly according to the sensed temperature. At temperature T 2 , the fan speed is maximum. In this example, a thermal cutoff shuts down power to the assembly if a critical temperature T 3  is reached.  FIG. 21  shows an example of a scheme similar to that of  FIG. 19  in which both heating and cooling are controlled. In this case, the desired target temperature may lie between temperature points T 2  and T 3 .  
      In other methods, temperature control is performed according to sensed luminance output.  FIG. 22  shows a flowchart of one example M 100  of such a method. Task T 110  determines whether luminance is currently decreasing. In one example, task T 110  makes this decision based on the two most recent luminance measurements. If luminance is not decreasing, then task T 120  resets a counter and task T 100  repeats after some measurement interval. If task T 110  determines that luminance is decreasing, task T 130  tests the current value of the counter. If the counter value has not reached a threshold, task T 140  increments the counter value, and task T 110  repeats after some measurement interval. In this example, the threshold value is four. If task T 130  determines that the counter value has reached the threshold, then task T 150  changes the state of the cooling unit between activation and deactivation. The counter value may be selected based on an interval between luminance measurements (or an interval between repetition of task T 110 ) and according to a desired hysteresis delay.  
      Sensing of the luminance output of each light source (for example, as in the configuration shown in  FIG. 18 ) may also be used to obtain increased uniformity of illumination. In further systems and methods, the driving circuitry of the light sources is configured to control the individual light sources according to their luminance outputs (for example, by adjusting their individual driving currents) such that the light sources have equal luminance outputs. Such a method may be used in conjunction with a method of temperature control by luminance monitoring, such as the method shown in  FIG. 22 .  
      Methods of temperature and/or luminance control as described herein may be performed by a control unit including one or more heating devices and/or cooling devices as described herein. A control unit may also include one or more arrays of logic elements (for example, a microprocessor or embedded processor) executing one or more routines in firmware and/or software. Embodiments also include data storage media (for example, semiconductor memory, optical disks, or magnetic disks) having one or more sets of machine-executable instructions for performing operations of a method as disclosed herein.  
      A display panel may include a lighting assembly, according to one or more of the implementations disclosed herein, being used as a backlight for an LCD or other imaging panel. Such a panel may have a resolution of 1280×1024, or 1600×1200 pixels, or more (for example, 2560×1600, 2560×1920, or 3480×2400 pixels). The LCD panel may be transmissive or transreflective, and may be a monochrome or color LCD. Suitable technologies include active matrix (AM), thin-film transistor (TFT), and super twist nematic (STN). A lighting assembly according to one or more of the implementations disclosed herein may also be used as a backlight to an imaging panel according to another LCD technology or some other light valve, transmissive, or transreflective technology.  
      Principles as disclosed herein may be applied to any configuration in which it is desired to increase a degree of thermal coupling of one or more light sources to a heat sink. For example,  FIG. 23  shows a cross-section of an edge-lit backlight according to an embodiment.  
      A generally planar light guide  800  receives light from the light source  100  along an edge. Light guide  800  may be patterned, printed, etched, molded, tapered, and/or faceted to provide a desired distribution of the illumination across the back surface of an imaging panel. For example, such a pattern may be on the order of 10 to 100 microns. Light guide  800  may be made from a material such as glass or PMMA or another suitable resin. In the example of  FIG. 23 , light source  100  is implemented as a CCFL disposed in a channel along an edge of light guide  800 . In other embodiments, light source  100  is a two-legged CCFL, disposed in dual channels along the edge of light guide  800 , or is made of another technology, such as LEDs arrayed along an edge of the light guide. The cross-section may include more than one light source  100 , arranged side by side and/or one above another.  
      In the example of  FIG. 23 , heat transfer substrate  200  is implemented in a semi-cylindrical shape  220 . In other embodiments, the cross-section of the substrate has another shape, such as parabolic, and/or the heat transfer substrate extends to embed light source  100  more completely or even entirely.  
      Reflector  400 , implemented as a foil, sheet, layer, or film as described herein, is arranged here to follow the contour of heat transfer substrate  220 . In this example, implementation  412  of reflector  410  also extends in cross-section to enclose an end of light guide  800 , although other embodiments according to  FIG. 23  are contemplated in which the reflector does not extend beyond the top surface of heat transfer substrate  220  on at least one side of light guide  800 . It may be desirable for reflector  412  to extend along substantially all of the edge of light guide  800  or at least along a light-generating portion of light source  100 .  
      Collector  420 , likewise implemented as described herein, is also arranged here to follow the contour of heat transfer substrate  220 . It may be desirable for collector  422  to extend along substantially all of the edge of light guide  800  or at least along a heat-generating portion of light source  100 . A heat coupling layer  460  as described herein may also be used.  
      Heat sink  480  is thermally coupled to collector  422  and may be integrated with collector  422 . Heat sink  480  may also be thermally coupled to a back cover of the lighting assembly, which may be generally planar and substantially parallel to light guide  800  and/or the imaging panel. In another example, heat sink  480  is absent and collector  422  is thermally coupled to or integrated with the back cover. In other embodiments, heat sink  480  may be finned, cooled, and/or have any other shape suitable to the particular application.  
      Further embodiments include assemblies in which an arrangement as shown in  FIG. 23  is disposed along more than one edge of a light guide (along opposite edges, for example, or along all four edges).  
      The foregoing presentation of the described embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments are possible, and the generic principles presented herein may be applied to other embodiments as well. In one example, light sources  100  and heat transfer substrate  200  are enclosed in a low-pressure or vacuum chamber, which may reduce heat transfer to an element in front of light sources  100  such as a display panel. In other cases, a chamber enclosing light sources  100  and heat transfer substrate  200  may be cooled by a circulating fluid or gas.  
      A lighting assembly as described herein may be applied to large panels (such as announcement panels for use in airports, train stations, or other public venues); flat-panel televisions and wall displays; and desktop computer monitors. Such an assembly may also be used in smaller embedded display panels in such applications as vehicle satellite navigation systems, avionic instrumentation display units, automatic teller machines, and consumer dispenser machines (such as fuel pumps and beverage dispensers).  
      Circuitry of an LCD panel may include an interface card or other circuit configured to convert an incoming video signal in analog or digital (e.g. DVI) format into an LVDS format for processing by the panel driving circuitry. Such circuitry may also include one or more inverters to generate a high-voltage current to drive lamps of the lighting assembly. In some applications, the display panel may also include a CPU. Such integration, which may reduce total system weight and/or size, may be desired in an application such as a vehicular display application. It may also be desired to configure the display CPU as a thin client and possibly to include other functionality such as a USB interface for enhanced connectivity and/or a GPU for enhanced graphics capability. It may be desired to mount such circuitry on the back of the backplate, with electrical insulation, thermal insulation, and/or cooling being provided as appropriate.  
      A lighting assembly as described herein may also be used in other applications in which a uniform illumination field (especially, a high-intensity field) across a planar or substantially planar surface is desired. Such applications may include automated inspection, identification, and/or monitoring applications, for example, or photographic and photolithographic exposure applications. Thus, the present invention is not intended to be limited to the embodiments shown above but rather is to be accorded the widest scope consistent with the principles and novel features disclosed in any fashion herein.