Abstract:
The invention generally relates to an integrated head up displaying (HUD) device including a housing that houses an active-matrix image projecting system and its accompanying electronics, and an optical combiner (for providing a see-through image thereon) connected to the housing via a retractable arm attached at one end to the main body and holds the HUD optical panel at its other end. In particular, the telescopically retractable arm allows the assembly to extend or retract for a desirable combiner height. Further, the LED backlighting array is placed within a reflective light chamber which is lined with a safe, light-weight, low cost, specular, reflective sheet for high reflectivity. The non-conductive, reflective sheet also covers the LED printed circuit board.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention generally relates to an integrated head up displaying (HUD) device including a housing that houses an active-matrix image projecting system and its accompanying electronics, and an optical combiner (for providing a see-through image thereon) connected to the housing via a retractable arm attached at one end to the main body and holds the HUD optical panel at its other end. In particular, the telescopically retractable arm allows the assembly to extend or retract for a desirable combiner height. Further, the LED backlighting array is placed within a reflective light chamber which is lined with a safe, light-weight, low cost, specular, reflective sheet for high reflectivity. The non-conductive, reflective sheet also covers the LED printed circuit board.  
         [0003]     2. Description of the Related Art  
         [0004]     Head-up display (HUD) devices are widely used in vehicles for both business and home purposes. The use of and developments in HUD technology continue to explode. Current HUD systems differ from one another in terms of projection technology, resolution, positioning-adjustability, viewing flexibility, size, shape, and installation convenience. For example, U.S. Pat. Nos. 3,503,146 and 5,841,408 teach an automatic apparatus for moving a combiner assembly away from the pilot&#39;s head only in the event of a crash. U.S. Pat. No. 3,945,716 rotates the HUD about an axis in line with the pilot&#39;s head. It is slaved to a Forward-Looking-Infra-Red (FLIR) or TV tracker and the entire unit (including the combiner) turns with the pilot&#39;s head. The system uses a CRT for the display and an expensive holographic lens. It has only one pivot point to move the combiner.  
         [0005]     U.S. Pat. No. 5,381,267 uses a four bar linkage to support a HUD combiner to swing between an upper storage position and a lower operating position solely to stow the combiner assembly. It is not possible to adjust the display (arranged at an identical desired viewing angle) to different vertical positions.  
         [0006]     U.S. Pat. No. 4,188,090 provides a complex HUD combiner and optical projector, combined as a solid single unit for one eye, i.e., a monocular system. The unit is hinged and rotates to a stowed position. However, the monocular system is designed so the viewer&#39;s eyes are always approximate to the focal point of the objective lens thereby maintaining an image subtended size and a viewing angle, rather than providing viewing flexibility. In addition, the flat combiner does not support any focal change.  
         [0007]     U.S. Pat. No. 4,937,665 shows a small mirror adhered to the inside of a car windshield and above the driver&#39;s line of sight, for reflecting back a video, movie, TV program, etc. to one eye of the driver. A display is separately mounted overhead and is focused to infinity.  
         [0008]     U.S. Pat. No. 6,078,428 mounts a head-up collimator (including a projector and a semi-transparent mirror that is used by being placed before the user&#39;s eye) in a support. By adjusting bolts and hinges of the support with the feature of potting a tapered pin in resin, exact alignment of the support with respect to the structure of the aircraft in axes X, Y and Z during reinstallation after maintenance becomes possible. However, the semi-transparent mirror is only allowed to rotate about the x-axis, but cannot be vertically lifted with respect to the projector without rotation.  
         [0009]     U.S. Pat. No. 6,445,506 mounts a combiner on an axis so as to flip to a stowed position on top of the projection with a motor analogous to flipping a page of a book. The rotational axis, the display surface of the combiner, and the projector are arranged at fixed angles.  
         [0010]     U.S. Pat. No. 4,775,218 uses a LED alignment detector to determine whether a holographic combiner based with an infinity focus is arranged with a separately mounted cathode-ray tube within a prearranged operational tolerance. The combiner only stows rearward, and the sensitive holographic optics requires unique pilot calibration such that the combiner cannot be easily adjusted to a pilot&#39;s preference.  
         [0011]     U.S. Pat. No. 5,517,337 shows a retractable holographic combiner with a movable holographic mirror placed before the pilot&#39;s eyes, and designed for presenting light images collimated at infinity, superimposed on the external scene, and coming from a projector positioned up-line with respect to the combiner. The holographic mirror is connected to a fixed part of the aircraft by a hinged support constituted by a single lateral connecting arm to the fixed part of the aircraft. However, the projector and the arm are separately fixed to the aircraft rather than integrated into one unit to be fixed to an aircraft. In addition, the arm only stows rearward. Further, the system uses holographic components which are sensitive to alignment.  
         [0012]     U.S. Pat. No. 5,867,287 shows a HUD with a lens de-centered to the optical axis of the bright display backlighting illumination which requires a high intensity halogen bulb, IR filters, Fresnel lens and cooling fans. In addition, its display emits a yellow orange color due to the low dimming levels of the incandescent bulb such that a green display used in aircraft would turn brown at night rather than stay the same color. Furthermore, the halogen bulb reflectors result in a “Flashlight beam” effect that must be de-centered and diffused to provide even illumination. Holographic optical elements, including lenses, mirrors, gratings, prisms and beam splitters made by holographic methods, operate efficiently only over a narrow band of wavelengths.  
         [0013]     U.S. Pat. No. 6,072,444 provides a HUD device mounting assembly that enables optical alignment between the HUD projector and the image combiner, although the combiner assembly and the optical projector are separately mounted to the cockpit of an aircraft. The components are not integrated into a single unit to reduce the likelihood of tolerance issues.  
         [0014]     U.S. Pat. No. 6,343,863 uses a bulky frame to support and align an optical unit and a head-up display in a cockpit of an aircraft, and a sensor coupled to the frame for sensing the vertical position of the frame and providing a signal to the computer to adjust at least part of the image displayed on the combiner to correspond to the position of the frame. The frame only moves in a generally vertical direction relative to the cockpit, rather than any other directions, and it requires a motor to move the bulky frame.  
         [0015]     U.S. Pat. No. 4,725,125 stows a projector and a combiner flatly into the dashboard of a vehicle using kinematic chains. Dash mounting (rather than head-mounting) causes significant sun intrusion issues.  
         [0016]     U.S. Pat. No. 6,301,053 shows a combiner assembly  20  having an optical combiner  21  mounted to a housing  22 , which is further arranged for mounting to a roof structure of a flight deck of an aircraft. Via the combiner, a user views an outside scene overlaid with a projected image. However, the combiner and stowage arm assembly is separately mounted to the aircraft from a display forming section (i.e., a projector; not shown). In addition, the combiner and stowage arm assembly is complex since it requires kinematic linkages with four pivot points.  
         [0017]     In view of the state of the prior art as discussed above, there is a demand for a self-contained, universal compatible, low-cost heads-up display devices to “fit” into existing vehicles, while delivering acceptable optical and image brightness performance.  
       SUMMARY OF THE INVENTION  
       [0018]     The engineering design philosophy of the HUD system of the present invention creates a unique configuration that encompasses all aspects of safety, low cost, functionality, human factors and good optical principles, especially for aircraft. Years of aircraft HUD history provide an abundance of resources, hardware configurations and optical layouts that contribute to finding a solution that will meet a variety of difficult requirements. This vast knowledge has led to the present invention embodying a unique and novel design that is highly functional and meets the economic requirements of, among other industries, today&#39;s general aviation marketplace.  
         [0019]     Initial studies of the HUD system concept were configured with the HUD installed in the top of the aircraft instrument panel. The instrument panel installation results in a HUD package position approximately 25 inches away from the pilot for most general aviation installations. Field of view requirements, paralleled with the far combiner distance, results in designs that have overly large combiners and optical packages. It was found that installation and retrofit of these designs, into existing aircraft panels, would cause substantial cutting of the existing structure and repositioning of impacted instruments. Besides compromising the original aircraft panel and related components, this option would result in tremendous financial and FAA certification issues for the aircraft owner.  
         [0020]     The mounting of the HUD system of the present invention in an overhead position provided several advantages: 
        No compromising of the existing instrument panel structure.     Allows the combiner to be placed closer to the pilot&#39;s eyes, reducing size.     No intrusion of direct sunlight into the optical system.     Allows easy access to adjustments and controls.     Allows for a straight refractive optical system providing simplicity and quality.        
 
         [0026]     Adjustment of pilot seated height, in relation to positioning and calibration of the HUD virtual image to the horizon, is key to the HUD system&#39;s configuration. This critical adjustment is made through the use of an expandable telescopic arm that is integrated to the main housing. The end of the arm holds the combiner and allows for fine calibration of the virtual image vertical position. The combiner moves up and down to keep it centered between the pilots seated eye position and the horizon, allowing for maximum eyebox and conformal information overlay. The direction of telescopic travel is down/forward and up/rearward to correspond with the correlation of leg and torso length variance.  
         [0027]     Selection of a combiner type (spherical, aspheric or flat) is also a principle characteristic of the HUD system of the present invention. The selection of an optically flat combiner allows for the adjustment of distances between the main objective lens and the combiner itself, which is necessary due to changes in pilot height adjustment. Since light exiting the housing of the HUD system exhibits all the necessary magnification and is nearly collimated, the selection for use of a flat combiner becomes clear. The combiner can be moved up and down via the telescopic arm without changes in system magnification, apparent image size or distance. Changes in magnification would have the negative result of varying image size and distance in relation to the pilot seated height.  
         [0028]     The reflection of an image off a flat combiner surface retains it&#39;s optical characteristics through changes in reflection angle. This is not so with the use of curved combiners (spherical or aspheric.) The reflection angle off a curved combiner (approximately 54 degrees) would create a slight “smile” to the image, which would wash from side to side with head movement. A flat combiner would not exhibit these drawbacks.  
         [0029]     The key issues with the combiner will be second surface reflections and the material used. Several solutions are available for these issues. Since the combiner is close to the eye, the quality of optically ground glass is not necessary. Glass presents safety concerns in the cockpit. The recommendation is for commercially available clear cast 3/16 acrylic plastic.  
         [0030]     One approach for elimination of second surface reflections would be the application of an anti-reflective coating on the back side of the plastic combiner, further enhancement would be accomplished by the application of a slight (&lt;30%) aluminum reflective coat on the face of the combiner.  
         [0031]     A unique and novel approach to eliminate second surface reflections and thereby eliminate the need for expensive coatings involves optically grinding and polishing a slight wedge angle into the acrylic sheet. This “wedge” shape would superimpose the primary image from the face of the combiner with the reflection caused by the second surface. If this process is not commercially economical, the parts could be cast with the proper shape from CR39 optical lens material in higher volumes.  
         [0032]     Contrary to known concerns about the use of refractive optics, the positioning of the HUD system of the present invention into an overhead position prevents the unwanted intrusion of direct sunlight. This eliminates issues of visible sun reflections and magnified sunlight focusing on internal components. It now becomes possible to utilize low cost refractive spherical or aspheric lenses that provide good optical performance, such as those listed below, at higher magnification levels: 
        Vertical and horizontal disparity: &lt;0.50 milliradians     Optical back focus length: 275 mm     Objective lens: 290 mm focal length×150 mm diameter     Field of View (FOV): 5 degrees vertical×10 degrees horizontal     Virtual image brightness: &gt;1,000 Ft. Lamberts to the eye     Virtual Image Distance 6.0 meters        
 
         [0039]     In view of all the above, it is a purpose of this invention to provide a self-contained, universally-compatible, low-cost HUD device that can be fitted into any one of a variety of vehicles, including but not limited to general aviation aircraft (both airplanes and helicopters), commercial or passenger aircraft, private powered boats, commercial vessels, passenger ships, trucks, and special-purpose land vehicles.  
         [0040]     It is another purpose of this invention to provide a HUD system that will allow the elimination of or minimizing the need for a PC-based or conventional display-based navigation system (i.e., displaying on a conventional display monitor).  
         [0041]     It is still another purpose of this invention to provide a HUD system in which the combiner can be operationally positioned and re-positioned repetitively and consistently, while only causing a minimum degree of interference with a user&#39;s field of view.  
         [0042]     It is still another purpose of this invention to provide a HUD system that ensures the frontward retraction of the combiner in the event of shock or an emergency when it is in an operational position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]     The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings in which like reference numerals designate like elements and wherein:  
         [0044]      FIG. 1  shows a side view of a cockpit comprising a HUD system according to the invention;  
         [0045]      FIG. 2A  is a overall perspective view of a HUD system according to the invention;  
         [0046]      FIG. 2B  is an overall side view of the HUD system according to the present invention;  
         [0047]      FIG. 2C  is an overall rear view of the HUD system according to the present invention;  
         [0048]      FIG. 2D  is an overall bottom view of the HUD system according to the present invention;  
         [0049]      FIG. 2E  is an overall side view of the HUD system illustrating the adjustable positioning of the combiner according to the present invention;  
         [0050]      FIG. 3A  is an overall exterior perspective diagram showing the main components of the present invention in one implementation of the present invention;  
         [0051]      FIG. 3B  is an interior perspective diagram showing the main components of the present invention as mounted in the one implementation of the present invention;  
         [0052]      FIG. 3C  is a first interior perspective view highlighting the interior components of the one implementation of the present invention;  
         [0053]      FIG. 3D  is a second interior perspective view highlighting the interior components of the one implementation of the present invention;  
         [0054]      FIG. 4A  is a side view of the HUD system illustrating an optical ray trace pattern representative of the system in operation;  
         [0055]      FIG. 4B  is a bottom view of the HUD system illustrating an optical ray trace pattern representative of the system in operation;  
         [0056]      FIG. 5  is a view of the combiner of the HUD system showing the video or display imagery projected onto the combiner according to the present invention; and  
         [0057]      FIG. 6  illustrates how the field of view is calculated for the HUD system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]     With reference to the figures, like reference characters will be used to indicate like elements throughout the several embodiments and views thereof.  FIG. 1  shows a schematic view of a pilot&#39;s cabin or cockpit, wherein the ceiling A of the cabin and the pilot&#39;s station B set limits of visibility, i.e., the field of view for the pilot. A HUD  10  is mounted above the pilot&#39;s head on the ceiling via fixing means, such as screws or other conventional mounting devices, as will be discussed further hereinbelow. Resin/pin alignment techniques are also applicable for fixing the HUD  10  to the ceiling A.  
         [0059]     As shown in  FIGS. 2A-2D , the HUD  10  has a housing  12 , mounting elements  14 , a retractable arm  16 , a high resolution display screen/optical combiner  18 , control panel  20 , and a window  22  visible from outside. Inside the housing  12 , (see  FIGS. 3C and 3D ), there is a controller  34  (in this embodiment, composed of power controller board  34 A and digital video board  34 B) that provides the electronic circuitry controlling the HUD  10 , as well as a LCD panel  24 , a first fold mirror  26 , a main objective lens  28 , a second fold mirror  30  and a main mirror  32  optically assembled into an optical system to generate the images to be displayed externally on the optical combiner  18  for the pilot to view. In at least one embodiment, the housing  12  is preferably 3.4-3.55″ thick, 10.7″ wide, and 12-13.5″ long.  
         [0060]     The controller  34  may be composed of a microprocessor circuit or other similar programmable data processing circuit having the necessary circuitry and processing capacity to generate a video or visual image display that will be updated at least periodically or preferably in real-time (i.e., as new data for display is processed, as circumstances change). As such, the controller  34  should be able to provide at least the following functions: (1) power to all internal circuitry and electrical components (i.e., power controller board  34 A); and (2) processing of inputted data and generating of video data to be inputted into the LCD patent  24  (i.e., digital video board  34 B). Data signals providing navigational information may be inputted into the controller  34  via conventional data connection and transmission devices (i.e., RS-232 cabling, USB connection, serial connection, ribbon cabling, etc.) available in the aircraft or other vehicles.  
         [0061]     As shown in  FIG. 2A , the mounting elements  14  may consist of screws, mounting brackets or other fixed mounting devices known in the art. One variation as shown in  FIG. 3A  would use a mounting track  14 A wherein the housing  12  would be allowed to slide along the track  14 A in order to achieve another degree of adjustability. However, such a mounting track would require a locking mechanism (not shown) that would lock the housing  12  in the selected position. The implementation of such a locking mechanism in conjunction with the mounting track  14 A would be known to one of skill in the art.  
         [0062]     As shown in  FIGS. 3B-3D , the first fold mirror  26  is used to effect a horizontal side fold of the image emitted from the LCD panel  24  through the main objective lens  28 , and then onto the second fold mirror  30 . The second fold mirror  30  effects a vertical fold of the image reflected from the first fold mirror  26 , onto the main mirror  32  and then out to the combiner  18 . In one embodiment, these mirrors will be manufactured out acrylic plastic for cockpit safety. The optical folds effected by the mirrors are used to reduce package size and properly direct the optical rays. This results in the images generated on the LCD panel  24  being provided to the combiner  18  to be flipped vertically, so when put through the optical system, the images are right side up.  
         [0063]     Referring specifically to  FIG. 3B , the LED light from the LCD panel  24  is refracted at 90 degrees by the first fold mirror  26  towards the main lens  28 , and then refracted by the second fold mirror  30  passing the window  22  onto the optical combiner  18 .  
         [0064]      FIG. 5  illustrates one arrangement of display images that would be projected onto the combiner  18 . For example, green computer-generated flight instrument symbology generated by the controller  34  (i.e., video control board  34 B) based in inputted data from outside data sources (i.e., flight instrumentation data, GPS signals, navigation telemetry) may be presented on the combiner  18 . Such symbology may include compass direction, air speed, distance to destination, current location, wind direction, fuel level, travel path and various warning symbols. The symbology may also be varied depending on the type of vehicle using the HUD system of the present invention. Specifically, the symbology used in an aircraft may be different from the symbology that would be used on a boat or a special-purpose ground vehicle.  
         [0065]     In the embodiment shown in  FIG. 2A , the control panel  20  has at least an on/off switch  21 , a control input  23 , and a brightness adjust knob  25 . In other embodiments, the control panel  20  may include an on/off switch  21 , a control input  23 , a brightness adjust knob  25 , a video signal input (e.g., supporting NTSC Standard Single RCA connection), a vertical FLIP control, a horizontal FLIP control, and a DC power input. The DC power to be inputted may be defined as 9-15 Volts (12V nominal) with 800 mA minimum current and 1.2 amps nominal (at full brightness), and the input is a 5.5 mm-diameter OD male (ground) with a 2.3 mm-diameter power pin.  
         [0066]     As shown in  FIG. 2E , the HUD system  10  stows the combiner  18  overhead with a rotatable/telescopic composite arm  16 . When rotated in the a direction, the arm  16  is stowed and typically rests close to the ceiling of the cabin adjacent the housing  12 . When in use, the arm  16  is pivoted at the first pivot point  36  so to move the optical combiner  18  towards the pilot&#39;s head. The arm  16  may be stowed forward in the direction with detented forward motion availability to avoid the pilot&#39;s head coming in contact the arm  16 . The arm  16  allows the assembly to fold forward for temporary in-flight stowage and rearward for more permanent stowage, system removal and portability. In addition, the optical combiner  18  can be angled towards or away from the pilot at the second pivot point  38 . Further, the single composite telescopic arm  16  allows vertical adjustment for different pilot seat heights. Once locked into position and the combiner  18  height is adjusted to pilot preference, fine vertical image adjustments can be accomplished, such as by adjustment of the combiner  18  about the horizontal top edge axis. A thin horizontal axis bar  18 A (see  FIG. 3A ) serves as both the primary fastening location of the combiner  18 , but also provides the pilot with a true aircraft/vehicle horizontal reference in direct view for comparison with a displayed horizontal situation indicator. In this embodiment, the combiner  18  is formed as a flat element, whereby no focal change is necessary when the position of the combiner  18  is adjusted.  
         [0067]     A locking mechanism  40  including a groove and a screw slidably moveable in the groove  42  is provided to lock the arm  16  at a desired vertical position. Image subtended size (viewing angle) can be maintained in that the HUD system is designed so that the pilot&#39;s eyes are positioned to be always at approximately the same distance to the focal point of the main objective lens  28 .  
         [0068]     The rotational and telescopic movement of the composite arm  16  provides flexible and unique positioning without the need for a complex Kinematic linkage system. The structure of the arm  16  takes into consideration pilot head contact through its arm angle and detented forward rotation capabilities. The optical exit angle positions the combiner  18  in space such that the connection of the arm  16 , back to the housing  12 , creates a safe angle for incident head contact. Such adjustment of the display screen or combiner  18  closer or further from the housing  12  of the HUD system  10  would be impossible in the prior art systems, since it would result in optical misalignment, de-focusing and magnification issues.  
         [0069]     As shown in  FIG. 3D , the optical system within the housing  12  has a LED backlighting array  42  for diffusing broad illumination. The LED backlighting array projects evenly distributed LED light into a reflective light chamber  44  that utilizes, for example, a Dupont® 99% reflective polymer film (not shown) on the internal walls. This design provides the brightness and high internal reflectivity of a solid glass prism (or acrylic light-pipe)  48  without the mass and cost, and eliminates the need for hot high brightness halogen bulbs, IR filters, fresnel lenses, cooling fans, optical de-centering, etc.  
         [0070]     In one embodiment of the present invention, the LED backlighting array  42  is selected using such units that are currently the brightest projection sources available for non-military head-up displays. Such LED backlighting arrays can generate 100,000 foot-Lamberts of light energy output versus 20,000 foot-Lamberts from lighting units used in conventional HUD devices. Preferably, the LED lighting unit  42  applies high brightness 532 nm green LEDs in a rectangular array, in conjunction with a polished “reflection tube” having properties of a large lightpipe. The inventor has determined that this wavelength of LEDs in conjunction with a TFT-LCD display panel presenting graphics in a Cyan (Light blue/green) allows maximum transmission of LED light through the LCD panel  24 .  
         [0071]     Referring to  FIGS. 3A, 4A  and  4 B, an ambient light sensor  32  may be installed at the end of the arm  16  to sense any light refracted by the second fold mirror  30  to ensure the refracted light (i.e., the virtual image) is presented at approximately 6 meters (20 feet) from the pilot&#39;s eyes and superimposed on the distant, background scenery. The refracted light is not focused at infinity like other prior art HUD systems. The inventor has determined that the focal distance of 6 meters is the optimal distance for a pilot&#39;s ease of viewing and readability.  
         [0072]     In particular, using a theoretical optical system with a fixed constant of 10× magnification, the following Table 1 shows the ratio of system optical path length and display dimensions (for a 5°×10° FOV) as virtual image distance increases. In effect, the result is that the larger the package size, the greater the weight and the cost.  
         [0073]     Further, Table 1 below illustrates, using a 2.5 meter image distance, that the HUD box needs to package a 160.5 mm (6¼ inch) optical system. Thus, for optical infinity, the same 10× geometry system would need to package an optical system almost 3 feet long.  
                                                                               TABLE 1                                       Virtual Image Distance (meters)                    2.50   3.00   4.00   5.00   Optical ∞                        Optical path   160.5   208.6   304.6   400.6   &gt;880       length (mm)       Display size   44 × 22   53 × 26   70 × 35   88 × 44   &gt;176 × 87       (mm) H × V                  
 
         [0074]     Another significant contributor to package size is the virtual image field of view (FOV.) If, conceptually, the image forming combiner is placed just beyond instrument panel distance, for example 560 mm (22 in.), FOV and eye box size have a dramatic effect on the combiner size. Using a fixed eye box size of 130(h)×30(v) mm and an image distance of 4.0 meters, the following table illustrates combiner size in relation to field of view, where the calculation show below is illustrated in  FIG. 6 :  
                                                                                 TABLE 2                           Combiner size = (Y + (L * TAN A) − ((Y * L)/X)) * 2                Image Size (FOV) (degrees)                    2° × 4°   4° × 8°   5° × 10°   7.5° × 15°   10° × 20°                        Horizontal   150.9   190.1   209.8   259.2   309.3       Combiner Size       (mm)       Vertical   45.3   64.9   74.7   99.2   123.8       Combiner Size       (mm)                  
 
         [0075]     Table 2 above shows with a 10°×20° FOV the combiner would need to be over 12 inches wide and almost 5 inches tall. The accompanying package, to properly stow and provide clear optics, would be much too large for general aviation aircraft. This justifies the need to keep the FOV as small as possible.  
         [0076]     Taking all the above factors into consideration, the inventor as noted above that 6 meters as an optimum focal distance. However, if other factors are taken into consideration based on the above-discussed calculations, different focal distances may be used to accomplish the same purpose intended by the present invention.  
         [0077]     The LCD panel  24  may be a thin-film transistor (TFT) LCD device with a resolution 480(w)×234(H), a color peak of 532 nm green, and a contrast of 150:1. When positioned with a LED light source discussed above, the TFT-LCD device has a minimum brightness (before off) of 2 foot-Lambert and a maximum brightness of 1120 foot-Lambert.  
         [0078]     The HUD  10  of the present invention is optically designed to obtain high reflectivity without the use of expensive glass laminated holography, as is done in the prior art. Holography is sometimes necessary to create off-axis reflections in a wavelength peak in some traditional display units. Without using sensitive holographic optics, the HUD  10  can be easily positioned and adjusted to each individual pilot&#39;s preference. Without using sensitive holographic optics, The HUD  10  does not require conformal information for individual pilot calibration. For example, U.S. Pat. No. 5,867,287 emits a yellow-orange color due to the low dimming levels of the incandescent light that is used. A green display, as used in aircraft, would turn brown at night using the system described in the &#39;287 patent. In contrast, the LED lighting unit  42  maintains its color spectrum at all illumination levels. In addition, using halogen light reflectors creates a “flashlight beam” effect that must be de-centered and diffused to provide even illumination.  
         [0079]     The size of the combiner  18  and the size of the window  22  are paired such that the pilot does not see the mechanical components related to the housing assembly. In one embodiment of the present invention, the HUD  10  supports a field of view of 10 degrees horizontally×7.5 degrees vertically, and an aspect ratio of 4:3, an image distance of 6 meters (20 ft), an eyebox size of 72 mm horizontal (cyclopean) and 30 mm vertical.  
         [0080]     The overhead mounting and system architecture is designed such that objectionable sunlight cannot enter the optical system. Off-axis reflection is not necessary and broad spectrum reflectivity can be obtained. A safe, light-weight, low cost, cast acrylic sheet is laminated on the glass combiner  18  which was not practical or functional in the prior art. Light-weight materials reduce the likelihood of objectionable vibration, as would be prevalent with a heavier laminated glass combiner.  
         [0081]     The HUD  10  integrates the arm  16  and the optical display housing  18 , allowing the entire system to be installed and removed as one unit thereby reducing the likelihood of tolerance issues. This unique HUD  10  provides contrast, brightness, resolution and optical clarity comparable with systems of substantially greater cost. The HUD  12  is simple, robust and measured-up to the performance expected by today&#39;s general aviation pilots.  
         [0082]     The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not limited to the particular embodiments disclosed. The embodiments described herein are illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.