Abstract:
An LED backlight system for dual mode operation with an NVIS system which utilizes non-edge lit night mode LED and a thermal conduction path from the day mode array of LEDS to a rearward disposed heat sink which cross the night mode array of LEDS. The system includes first and second LED arrays, the second array being located behind the first array. Light from the second LED array pass through a plurality of apertures located on the circuit card supporting the first array.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to liquid crystal displays (LCDs), and more particularly relates to dual mode backlit LCDs, and even more particularly relates to methods and apparatus for backlighting LCD&#39;s with dual modes without the use of an edge lighted or waveguide lighted backlighting scheme. 
     BACKGROUND OF THE INVENTION 
     In the past, LCDs have been used in a wide variety of environments, including displays within the cockpit of an aircraft. In some aircraft it is desirable to have LCDs with dual mode backlighting. These displays have a mode for use during daylight operation and a mode for nighttime operation when a pilot is using a night vision imaging system, hereafter NVIS. It has been well known to use filtered light during the NVIS mode. Filtering the light enhances the performance of the display when NVIS equipment is used. Without other changes, the filtering which helps NVIS performance degrades the optical performance (mainly color and brightness) of the same LCD in daylight. 
     One example of a prior art system is shown in FIG. 1, which includes a heat sink with a plurality of light pipes therethrough disposed about a night mode array of light emitting diodes (LEDs). 
     While these LCD backlighting systems may have many advantages in particular uses, they also have created challenges. One common challenge created by these designs is the relatively high temperatures experienced by the day and night mode LEDs and the concomitant lower luminance, shorter life and/or increased power consumption. 
     Consequently, there exists a need for improved methods and apparatuses for backlighting compact dual mode LCDs. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an LCD which is highly compact. 
     It is another object of the present invention to provide an LCD with improved luminance and luminance uniformity. 
     It is yet another object of the present invention to provide an LCD with extended MTBF. 
     It is yet another object of the present invention to provide an LED backlight for an LCD which consumes less power. 
     It is a feature of the present invention to utilize a thermal path across a night mode LED array with a heat sink disposed behind the night mode LED array. 
     It is a feature of the present invention to package day and night mode LED arrays with reduced detriment to the main cooling structure (heat sink). 
     The present invention is an apparatus and method for backlighting an LCD which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the already articulated advantages. The present invention is carried out in a “wasted space-less” and “wasted power-less” manner in a sense that the size of a fully functional dual mode luminare and the power consumed has been reduced. 
     Accordingly, the present invention is an LCD system including a predetermined sequence and orientation of backlighting LED elements which includes a day mode LED array and a separate night mode LED array where a thermal conduction path from said day mode LED array extends across the night mode LED array to a heat sink disposed behind both LED arrays. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the foregoing description of the preferred embodiments of the invention, in conjunction with the appended drawing wherein: 
     FIG. 1 is an exploded view of a prior art LED backlight. 
     FIG. 2 is a cross-sectional view of a prior art LED backlight of FIG. 1, in a non-exploded view. 
     FIG. 3 is an exploded view of an LED backlight of the present invention. 
     FIG. 4 is a cross-sectional view of an LED backlight of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings wherein like numerals refer to like matter throughout, there is shown an LED backlight system of the prior art, generally designated  100 . LED backlight system  100  includes a day mode circuit board  102  having a day mode array  103  of LEDs disposed thereon. Day mode circuit board  102  is a flex circuit. Day mode circuit board  102  is mounted on light piped heat sink  104 , which is made of AL 6061. A night mode circuit board  106  with a night mode array of LEDs  107  disposed thereon is coupled to the rear side of light piped heat sink  104 . A plurality of light pipe spacers  108  is disposed in the light pipes in light piped heat sink  104 . A backlight driver  110  is disposed rearward of all of the active components. A reflector  112  can be disposed forward of the active components. The LED backlight system shown in FIG. 1 is similar to the prior art LED backlight manufactured by Rockwell Collins Inc. of Cedar Rapids, Iowa, USA, and known as the Back Light Assembly, Night Vision-5ATI, having model number 987-2737-100. 
     A better understanding of the prior art LED backlight as mentioned above and as shown in FIG. 1 can be obtained by now referring to FIG. 2, which shows a cross-sectional view. It is clearly seen that the light piped heat sink  104  is not located behind the night mode circuit board  106 . 
     Now referring to FIG. 3, there is shown an exploded view of the LED backlight of the present invention, generally designated  300 , which includes a day mode LED flex circuit board  302 , which is preferably a multi-layered flex circuit with a thin layer of thermally-conductive pressure sensitive adhesive laminated to the back side; day mode LED flex circuit board  302  has attached thereto a day mode array of LEDs  303  which can be any suitable LED, which is preferably a surface mount device that emits white light. Day mode LED flex circuit board  302  is shown with a plurality of NVIS enabling apertures  301  therein. All of the LEDs of day mode array of LEDs  303  are not shown in FIG.  3 . To aid in the views of NVIS enabling apertures  301 , only a few LEDs in the corners of day mode array of LEDs  303  are shown. Day mode LED flex circuit board  302  is coupled to heat spreader  308 , which is made of AL 6061, and a plurality of NVIS filters  310  is disposed between day mode LED flex circuit board  302  and heat spreader  308  in NVIS enabling passages  309  in heat spreader  308  which are in registration with the NVIS enabling apertures  301  in day mode LED flex circuit board  302 . NVIS filters  310  are well known in the art, and are able to effectively attenuate the near infrared wavelengths of light required for NVIS operation. 
     Night mode LED thin flex circuit board  306  has coupled thereto a night mode array of LEDs  307 . Night mode LED flex circuit board  306  is shown with a plurality of fastener receiving holes  311  therein. All of the LEDs of night mode array of LEDs  307  are not shown in FIG.  3 . To aid in the views of fastener receiving holes  311 , only a few LEDs in the corners of night mode array of LEDs  307  are shown. Preferably night mode LED thin flex circuit board  306  is a thin, single layer flex circuit with a maximum thickness of 0.009″, and a thin layer of thermally-conductive pressure sensitive adhesive laminated to the back side, which is not shown in the Figures because it is preferably a very thin layer; other alternate circuit boards could be employed. It is preferred that the thermal insulating properties of the night mode LED thin flex circuit board  306  be known as tailored, depending upon various design choices being made. 
     Disposed between night mode LED thin flex circuit board  306  and heat spreader  308  is an electrically insulating thermally conductive pad  316  which has a plurality of night mode LED receiving holes and fastener receiving holes therein. Electrically insulating thermally conductive pad  316  is preferably made of a polyimide film coated with an impregnated silicone elastomer, and has a thickness, such as 0.007″ maximum. 
     Rearward disposed heat sink  304  is coupled to night mode LED thin flex circuit board  306 . Preferably, rearward disposed heat sink  304  is made of AL 6061 or other suitable material, and preferably, rearward disposed heat sink  304  is configured with cooling fins, etc. 
     An array of fasteners  318  are used to couple rearward disposed heat sink  304  to heat spreader  308 , and create a suitable compression of electrically insulating thermally conductive pad  316  and night mode LED thin flex circuit board  306  between heat sink  304  and heat spreader  308  to improve heat conduction away from day mode LED flex circuit board  302 . The various other layered components of the present invention may be held by adhesives, such as pressure-sensitive adhesives or others. 
     In operation, and now referring to FIGS. 3 and 4, the apparatus and method of the present invention could function as follows: 
     During daytime operation, the day mode array of LEDs  303  is used to illuminate the LCD, while the night mode array of LEDs  307  may be either on or off. With the present invention, the density of day mode array of LEDs  303  can be relatively high. The heat generated by day mode array of LEDs  303  is conducted through heat spreader  308  across electrically insulating thermally conductive pad  316  across night mode LED thin flex circuit board  306  and into rearward disposed heat sink  304 , where it can be dissipated through cooling fins or other means. 
     During nighttime operation when an NVIS is being employed, the day mode array of LEDs  303  is disabled, leaving only the night mode array of LEDs  307 . The light from night mode array of LEDs  307  passes though holes in electrically insulating thermally conductive pad  316  through NVIS enabling passages  309  disposed in heat spreader  308 , through NVIS filters  310  which provide the requisite light characteristic to be used in conjunction with an NVIS. The light then further passes through NVIS enabling apertures  301  in day mode LED flex circuit board  302  and is incident upon holographic thin film diffuser strips  312 , which spread out the then filtered light to enhance luminance uniformity across the face of the LCD. Finally, the light passes through holes in light reflector  314 , which is used to capture and redirect later reflected light into more useful directions. The heat generated by night mode array of LEDs  307  is conducted across night mode LED thin flex circuit board  306  and into rearward disposed heat sink  304 , where it can be dissipated through cooling fins or other means. 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps and arrangement of the parts and steps thereof without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.