Abstract:
A projection display has a display screen for displaying images formed by image engines illuminated by a high-reliability illumination source. The illumination source contains lamps for illumination of the image engines. Optical components conduct the light from the lamps to the image engines such that when a lamp fails illumination is maintained on the display screen by varying the power to the remaining lamps. A lamp power controller senses failure of a lamp and varies the power to the remaining lamps to compensate for the lamp failure.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to aircraft cockpit projection displays, projection liquids crystal displays (LCD), and more specifically to illumination of LCD cockpit projections displays. 
     While cathode ray tube (CRT) and color active matrix liquid crystal display (AMLCD) technologies remain a viable display media for some cockpit applications, projection displays offer several unique advantages that render them highly attractive for aircraft applications. 
     Projection display technology provides an important advantage over other competing technologies. A single compact optical engine design may be utilized for a display image source for varied display sizes, achieving a scaleable display design. An optical engine comprises an image engine that generates the image and projection optics that project the image on a screen. A type of image engine that may be used is a reflective micro LCD device with a size of less than one inch diagonal that is manufactured on silicon. One projection optical engine combined with slightly modified folded optics may be used for displays ranging in size from 5″ square to 32″ diagonal. This flexibility allows leveraging a current design into new products, which results in lower development costs and faster timing to market. Projection display technology will also enable customers to benefit from continuing advances in commercial projection components without having to undergo major system changes to current products. For example, as the commercial market drives higher resolution and more efficient reflective LCDs and lamps, projection display products may incorporate these improvements by replacing individual components with minimal impact to the system. Projection displays offer performance such as luminance, contrast ratio, and chromaticity equal or superior to an AMLCD. The versatility of projection displays makes them a viable candidate technology for both military and commercial cockpit designs for new aircraft as well as cockpit redesigns for existing aircraft. 
     A projection LCD has several essential subsystems. Among them, the illumination subsystem is very critical. The failure of the illumination subsystem will immediately cause the projection display to stop displaying information. In order to use projection displays as aircraft cockpit displays, the illumination subsystem must be very reliable. When an aircraft is airborne under normal operating and environmental conditions, the illumination subsystem of a cockpit projection display should never fail. However, none of the current single lamp illumination systems has such a high reliability. What is needed is a projection display illumination configuration that provides illumination when a lamp fails. 
     SUMMARY OF THE INVENTION 
     A projection display with a high reliability illumination system is disclosed. Image signal sources provide image signals to be displayed on the projection display. Controllers convert the image signals from the image signal sources into signal formats for displaying and providing image drive signals. Image engines are connected to the controllers and form the display images for display. The illumination system provides light to illuminate the image engines to project the display images. The illumination system has variable power lamps to provide the light. Optical components conduct the light from the lamps to the image engines. The optical components are disposed to conduct the light to the image engines such that when a lamp fails illumination is maintained by varying the power to the remaining lamps. Projection optics channel the display image to a screen for displaying the image. 
     The illumination system further comprises a first channel with a first lamp providing light to a linear polarizer for polarizing the light from the first lamp into s-polarized light. A second lamp provides light to a first polarizing beam splitter for receiving the light from the second lamp and splitting the light from the second lamp into s-polarized light and p-polarized light. The first polarizing beam splitter receives the s-polarized light from the linear polarizer and combines the s-polarized light from the linear polarizer with the split p-polarized light. A second polarizing beam splitter receives the combined s- and p-polarized light from the first polarizing beam splitter and splits the combined s- and p-polarized light into s-polarized light and p-polarized light. A first mirror reflects the s-polarized light from the second polarizing beam splitter. A first half-wave retarder receives the s-polarized light reflected from the first mirror and shifts the s-polarized light to p-polarized light. A first converging lens combines the p-polarized light from the second polarizing beam splitter and the p-polarized light from the first half-wave retarder. A first light pipe channels the combined light from the converging lens to a first image engine. 
     The illumination system may further comprise a second channel with a third lamp for providing light. A third polarizing beam splitter receives the light from the third lamp and splits the light from the third lamp into s-polarized light and p-polarized light and receives the s-polarized light from the first polarizing beam splitter and combines the s-polarized light from the first polarizing beam splitter with the split p-polarized light. A fourth polarizing beam splitter receives the combined s- and p-polarized light from the third polarizing beam splitter and splits the combined s- and p-polarized light into s-polarized light and p-polarized light. A second mirror reflects the s-polarized light from the fourth polarizing beam splitter. A second half-wave retarder receives the s-polarized light reflected from the mirror and shifts it to p-polarized light. A second converging lens combines the p-polarized light from the fourth polarizing beam splitter and the p-polarized light from the second half-wave retarder. A second light pipe channels the combined light from the converging lens to a second image engine. 
     A lamp controller powers the lamps and a lamp sensor detects a failure of one of the lamps and causes the lamp controller to adjust power to the operating lamps to maintain illumination of the screen. 
     It is an object of the present invention to provide an illumination system that provides light for a projection display when a lamp fails. 
     It is an object of the present invention to provide a multi-channel LCD projection display with an illumination system with high reliability. 
     It is an advantage of the present invention to maintain illumination of a projection display automatically when a lamp fails. 
     It is an advantage of the present invention to easily expand to more channels as required. 
     It is a feature of the present invention to use commercially available components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following, on of the preferred embodiments of the invention in conjunction with the drawings wherein: 
     FIG. 1 is a block diagram of a single-channel liquid crystal projection display; 
     FIG. 2 is a block of a multi-channel liquid crystal projection display; 
     FIG. 3 is a diagram of an illumination system of the present invention for the multi-channel projection display of FIG. 2; and 
     FIG. 4 is a diagram of the illumination system of the present invention implemented with three channels. 
    
    
     DETAILED DESCRIPTION 
     A block diagram of a basic single-channel projection display  100  is shown in FIG.  1 . There are two major types of projection display  100 . One is a front projection display where the projector is behind a viewer and both the viewer and projector face a screen  135  as in a movie theater. The other type of projection display is a rear projection display where the projector is on one side of the screen  135  and the viewer is on the other side looking at the screen as in a projection television set. The rear projection display is the type most suitable for aircraft cockpit display applications. 
     The basic single-channel projection display  100  comprises several subsystems as shown FIG.  1 . An image signal source  105  provides image signals to be displayed on the screen  135 . The image signals  105  may come from a wide variety of sources and may be analog or digital signals. In an aircraft cockpit display, the image signals  105  may come from such data sources as flight control systems, altimeters, engine indicating systems, or any other avionics data source for display of data to a pilot in an aircraft. 
     The image signals  105  are passed to a controller  110  that converts the image signals  105  to a format compatible with an image engine  115  and to drive the image engine  115  to generate an image for projection on the screen  135 . The image engine  115  may be one of a wide variety of image engines known in the art. Two major categories of image engine  115  include transmissive panels that either block light or transmit light to create an image and reflective panels that reflect light from the panel surface. The image engine  115  itself may be a very small LCD panel such as less than an inch diagonally. Among these is a reflective micro LCD device that is manufactured on silicon. 
     An illumination system  120  comprising a light source  121  and optics  125  in FIG. 1 supply light for the projection display  100 . The light source  121  may comprise a reflector and a lamp of some type. Types of lamps include metal halide lamps and ultra high pressure arc lamps. The optics  125  in the illumination system  120  are used to channel the light from the light source  121  to the image engine  115 . 
     Projection optics  130  are used in front of the image engine  115  to project the image on the screen  135 . The type of projection optics  130  depends on the type of image engine  115  and the type of projection display system, front or rear. 
     A multi-channel display  200  shown in FIG. 2 may be assembled by duplicating various portions of the projection display  100  of FIG.  1 . The multi-channel system  200  of FIG. 2 shows two channels while more than two may be implemented. The purpose of multi-channel display  200  is to accommodate additional image signal sources  105  on board an aircraft to be displayed on a single screen  135 . Avionics systems typically have many image signal sources  105  that require displaying on a common screen  135 . With the additional image signal sources  105 , additional controllers  110  and image engines  115  are also required. Each additional image engine  115  also has an additional illumination system  120 . Additional projection optics  130  after each image engine  115  are used to combine the images for display on the common screen  135 . 
     The illumination system  120  comprises the light source or lamp  121  and the optics  125 . Generally, optical components such as lenses, mirrors, retarders, and filters never fail. The main source of failure in the illumination system  120  is the lamp  121 . 
     An illumination system  300  of the present invention is shown in FIG.  3 . Shown in FIG. 3 is a two-channel illumination system  300  comprising a first channel  302  and a second channel  311  that may be incorporated in the illumination systems  120  for the two-channel cockpit projection display  200  of FIG.  2 . The first channel  302  may be incorporated in the illumination system  120  in the single-channel cockpit projection display  100  of FIG.  1 . Additional channels may be added as will be discussed below. The illumination system  300  of the present invention addresses the problem of a lamp failure in a projection display through the use of multiple commercially available lamps and optical components to compensate for a lamp failure. 
     As shown in FIG. 3, three lamp-reflector combinations are used comprising lamp  1   305 , lamp  2   307 , and lamp  3   309  in the two-channel implementation. The first channel  302  has two lamps lamp  1   305  and lamp  2   307 . Light from lamp  1   305  is passed through a linear polarizer  306  where it is shifted to s-polarized light. The s-polarized light from the linear polarizer  306  is passed to a first polarized beam splitter (PBS)  308 . Polarizing beam splitters are known in the art and are commercially available. A polarizing beam splitter is a cube formed from two optical quality glass right angle prisms cemented together at the hypotenuse that splits incoming unpolarized light in to two orthogonal polarized light beams, s-polarized and p-polarized. The light from lamp  2   307  is split in PBS  308  into s- and p-polarized light with the s-polarized light reflected to the right of the first channel  302  in FIG.  3 . The s-polarized light from the linear polarizer  306  and lamp  1   305  is reflected down from the PBS  308  beam splitting surface and combined with the p-polarized light from lamp  2   307  split by PBS  308 . This combined light from PBS  308  enters a second PBS  310 . PBS  310  splits the combined light from PBS  308  into s-polarized light again reflected to the right and p-polarized light that is passed down through PBS  310 . The s-polarized light coming from PBS  310  on the right in FIG. 3 is reflected by a first mirror  315  down to a first half-wave retarder plate  317  where it is shifted to p-polarized light. The p-polarized light from PBS  310  and from half-wave retarder plate  317  are combined in a first converging lens  320 . The light from the converging lens  320  is passed to a first light pipe  325  where it is conducted to the image engine  115  to illuminate the LCD display. 
     The s-polarized light that is reflected to the right of PBS  308  from the first channel  302  is passed to the second channel  311 . The second channel  311  is very similar to the first channel  302  except that the second channel  311  has only one lamp, lamp  3   309 . The s-polarized light from the first channel  302  takes the place of light from lamp  1   305  and linear polarizer  306 . The remainder of the second channel  311  operates in the same fashion as the first channel  302 . The s-polarized light from the first channel  302  is passed to a third PBS  326 . The light from lamp  3   309  is split in PBS  326  into s- and p-polarized light with the s-polarized light reflected to the right of the second channel  311 . The s-polarized light from the first channel  302  is reflected down from the PBS  326  beam splitting surface and combined with the p-polarized light from lamp  3   309  split by PBS  326 . This combined light from PBS  326  enters a fourth PBS  327 . PBS  327  splits the combined light from PBS  326  into s-polarized light again reflected to the right and p-polarized light that is passed down through PBS  327 . The s-polarized light coming from PBS  327  to the right is reflected by a second mirror  328  down to a second half-wave retarder plate  329  where it is shifted to p-polarized light. The p-polarized light from PBS  327  and from half-wave retarder plate  329  are combined in a second converging lens  330 . The light from the converging lens  330  is passed to a second light pipe  335  where it is conducted to another image engine  115  to illuminate the LCD display. 
     Additional channels may be added to the illumination system  300  of FIG. 3 by adding channels similar to the second channel  311 . The next channel added to the system of FIG. 3 would use the s-polarized light coming from PBS  326  in the same fashion as the second channel  311 . A diagram of a three-channel illumination system  400  is shown in FIG.  4 . In the third channel  405 , a lamp  4   409  provides the light to a fifth polarizing beam splitter  410  that splits the beam and passes light to a sixth PBS  415  as in the first channel  302  and the second channel  311 . The light is reflected by a third mirror  420 , shifted in polarity by third retarder  425 , and combined in third converging lens  430  as before. 
     In FIG. 4, a lamp power controller  401  provides and controls power to the lamps. The lamps in the three-channel illumination system  400  of FIG. 4 are initially operating at some power level less than full power. If any one of the four lamps in FIG. 4 fails, the failure is sensed and the power to the three remaining operable lamps is adjusted to compensate for the failure. A light sensor (not shown) located near the screen  135  may be used to sense light at the screen  135  and provide a fail signal to the lamp power controller  401 . To sense a lamp failure the lamp light output or current draw of the lamps may be sensed with techniques known in the art. Light sensors (not shown) may be located near each lamp to sense a lamp failure and provide a fail signal to the lamp power controller  401 . The lamp power controller  401  may include current sense circuitry to sense a lamp failure. 
     As an example, in FIG. 4 assume that lamp  1   305  fails. The light entering PBS  308  from lamp  1   305  is lost as and the amount of s- and p-polarized light leaving PBS  308  down to PBS  310  is reduced. To compensate for the loss of lamp  1   305  as detected by a lamp sensor (not shown), the power to lamp  2  is increased by the lamp power controller  401  to maintain the light entering PBS  310  and ultimately converging lens  320  and image engine  115 . The increase in s-polarized light from lamp  2   307  reflected to the second channel  311  from PBS  308  also increases. To compensate for the increased light entering the second channel  311  and PBS  326  from PBS  308  in the first channel  302 , the lamp power controller  401  reduces power to lamp  3   309  to maintain the light entering PBS  327 , converging lens  330  and image engine  115 . This decrease in power from lamp  3   309  results in a decrease in power to the third channel  405 . This decrease in power from the second channel  311  must be compensated for by an increase in power to lamp  4   409 . As can be seen from this example, failure of a single lamp does not result in a catastrophic failure of the illumination system  400  and the projection display. The display screen  135  remains illuminated with a lamp failure with the present invention. 
     The table below shows operational states of the four lamps for the three-channel LCD projection display shown in FIG.  4 . The various possible conditions are shown down the left side of the table as case  1  through case  5 . Each lamp is listed across the top of the table. The lamp power controller  401  adjusts power to the lamps in accordance to this table when sensing a failure. A x in the table indicates lamp failure, a + indicates increasing lamp power, a − indicates decreasing lamp power, and a blank indicates no change. Case  1  with a blank for each lamp is the normal case with no failures. Case  2  shows a x for lamp  1  failure and is the case described above in detail. Lamp  1  fails, lamp  2  increases in power, lamp  3  decreases in power, and lamp  4  increases in power in case  2 . Cases  3 ,  4 , and  5  show failures of lamps  2 ,  3 , and  4  respectively and the operating conditions of the other lamps. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Lamp 1 
                 Lamp 2 
                 Lamp 3 
                 Lamp 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Case 1 
                   
                   
                   
                   
               
               
                   
                 Case 2 
                 x 
                 + 
                 − 
                 + 
               
               
                   
                 Case 3 
                 + 
                 x 
                 + 
                 − 
               
               
                   
                 Case 4 
                 − 
                 + 
                 x 
                 + 
               
               
                   
                 Case 5 
                 + 
                 − 
                 + 
                 x 
               
               
                   
                   
               
             
          
         
       
     
     While the present invention is generally described in terms of a display for displaying avionics data in an aircraft cockpit application, the present invention is not limited to such applications. The present invention may be used in a single-channel or any multi-channel projection display application. 
     It is believed that the illumination configuration for multi-channel cockpit LCD projections displays of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages, the form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.