Abstract:
A head up display (HUD) has multiple light sources of varying intensities for projecting a high quality image under various lighting conditions. The light sources provide illumination through an electronic display (e.g. an AMLCD) in the HUD. A high brightness light source illuminates the electronic display during bright or day light conditions. A low brightness light source illuminates the electronic display during night or low light conditions. Both light sources illuminate the electronic display during intermediate lighting conditions. A light panel conducts and diffuses light from the light sources to illuminate the rear of the electronic display. An optional light mixer reduces uneven lighting of the high brightness light source. In operation, the intensities of the light sources are controlled as a function of ambient lighting conditions to optimise the projected image quality.

Description:
TECHNICAL FIELD 
     The present invention generally relates to head up display (HUD) systems and, more particularly, to backlighting for such systems for improved illumination in both daylight and night operation. 
     BACKGROUND OF THE INVENTION 
     Head up displays (HUD) are known and are currently used in many military and commercial aircraft. HUDs are also finding application in automobiles. Conventionally a HUD unit is mounted in the automobile to project an image in front of the driver. Adapting HUD units for use in automobiles necessitates making the units rugged, reliable, cost effective, and functional in a wide range of environmental conditions. 
     One problem with HUDs in automobiles is maintaining satisfactory image illumination over a wide range of lighting conditions, for example, daylight and nighttime lighting conditions. Daylight conditions generally require a bright or high intensity light source so that the projected image is visible in the bright light environment. Conversely, nighttime or low light conditions require low brightness or dim light to project an appropriately illuminated image that provides a safe and comfortable viewing experience for an operator. 
     Conventional HUD units have attempted to address this problem by using a high brightness light source and simply dimming the light source as needed. However, proper illumination in all light conditions may require dimming ratios of 1000:1 or more. This is difficult to achieve since high brightness light sources typically do not function well over such a wide lighting range. Several problems may occur as a high brightness light source is dimmed to low illumination. Such problems include reducing the lifetime of the light source, color shift (e.g. changes from white to yellow), and uneven illumination or bright spots. 
     Accordingly, it is therefore desirable to provide for a HUD that has improved operation in both daytime and nighttime conditions. It is also desirable to provide a HUD system design that preserves the lifetime of the light source. It is further desirable to improve the light color for a HUD operating in low light conditions. Yet, it is further desirable to reduce the effect of uneven lighting or bright spots generated in low light conditions. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a head up display (HUD) having multiple light sources of varying intensities for improving the projected image under a wide range of ambient lighting conditions. A high brightness light source illuminates a HUD active matrix liquid crystal display (AMLCD) during bright or daylight conditions. A light panel is interposed between the high brightness light source and the AMLCD and diffuses the high brightness light before it illuminates the rear of the AMLCD. A low brightness light source illuminates an edge of the light panel during low light and nighttime conditions. The light panel diffuses the low brightness light and reflects the light out of the front of the light panel to illuminate the rear of the AMLCD. Light from both the high and low brightness light sources are mixed by the light panel and illuminate the AMLCD during intermediate ambient lighting conditions. The light sources are continuously dimmed or brightened responsive to changes in ambient lighting conditions to provide an improved projected image. An optional light mixer, adjacent to the high brightness light source, also mixes the high brightness light thereby further reducing uneven lighting and light spots. 
     The high brightness light source may be any of a variety of bright light sources including, but not limited to, halogen lamps, bright LEDs, arc lamps, and fluorescent lamps. Preferably, the high brightness light source is sufficiently bright to project a good quality image on the windshield or combiner during bright daylight conditions and is capable of being dimmed for use in intermediate lighting conditions. 
     The low brightness light source also illuminates the rear of the AMLCD via the light panel. However, the low brightness light source is preferably located to the side of the light panel and illuminates the edge of the panel. The light panel reflects and diffuses the light from the low brightness light source and illuminates the rear of the AMLCD. The low brightness light source is any of a variety of light sources including, but not limited to, fluorescent lamps and LEDs. The low brightness light source preferably generates good quality light in low light conditions and is capable of being intensified to generate satisfactory light in intermediate lighting conditions. 
     According to more specific aspects of the present invention, the light panel is light transmissive and allows light from the high brightness light source to pass through and illuminate the rear of the AMLCD. Similarly, the panel reflects light from the low brightness source to the rear of the AMLCD. The light panel is preferably made of acrylic material, includes reflective edges, and at least one textured surface. The reflective edges reflect light back into the panel thereby increasing brightness. The textured surface, preferably on the front (nearest the AMLCD), diffuses the light from both sources for even light distribution. 
     The invention also includes a light mixer for mixing the light from the high brightness light source. High brightness light sources often produce uneven light or light spots as they are dimmed to low light levels. The preferred embodiment uses a square light mixer that reduces the effects of the uneven light and/or the light spots. 
     The intensity of the light sources is controlled responsive to ambient light conditions. A light sensor senses ambient light conditions and communicates a light signal representative thereof to a controller. The controller analyzes the light signal and controls the intensity of the light sources for optimum projected image illumination. Brightness of the lights is also adjustable by an operator. The operating ranges of the light sources overlap for improved lighting quality and so that transition between the light sources is smooth and transparent to a viewer. The light sources are controlled as follows. The high brightness light source is active for daylight conditions. As ambient light conditions dim, the high brightness light source is also dimmed. The low brightness light source is activated during intermediate ambient light conditions. Light from the low brightness light source mixes with light from the high brightness light source to reduce color shift (i.e. color temperature light) problems. As ambient light conditions dim further, the high brightness light source is turned off. In low light conditions only the low brightness light source is active, and is dimmed or brightened responsive to changes in ambient light conditions. 
     The design of the present invention advantageously reduces the problems of the prior art to provide an improved daylight and low light HUD. First, mixing light from multiple sources solves the color temperature light problem. Second, light from both sources mix to reduce uneven lighting or light spots. Third, the lifetime of the high brightness light source is preserved since it need not be dimmed to very low brightness levels. 
     It is envisioned that multiple high brightness and low brightness light sources may be used. Further, light sources of various light intensities may be used. For example, a high brightness light source may be combined with an intermediate brightness light source and a low brightness light source. 
     These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a HUD system in an automobile. 
     FIG. 2 is a side view of the backlighting system of the invention. 
     FIG. 3 is a front view of the backlightning system of the invention. 
     FIG. 4 is a perspective view of a light panel/light pipe. 
     FIG. 5 is a block diagram of a HUD system. 
     FIG. 6 is cross sectional view taken through the backlighting system further showing an intermediate brightness light source according to one embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to FIG. 1, an automobile  10  is shown equipped with a head up display system (HUD)  11 . HUD  11  communicates information to the vehicle operator  12  via an image projected in front of the operator and preferably within the operator&#39;s visual line of sight. The HUD  11  allows the operator to receive information without taking his eyes off of the road. 
     HUD  11  includes a video processor  13 , display unit (i.e., an image source)  14 , mirror  15 , and combiner or windshield  16 . Video processor  13  receives data from multiple systems and sensors in automobile  10  and formats the data into a suitable display signal  13 A that is communicated to display unit  14 . For example, data may be received from an infrared sensor for detecting objects in the roadway ahead of the vehicle, vehicle speed sensors, engine sensors, light sensors, and GPS position related data to name a few. Video processor  13  is of conventional design and therefore is not described herein in detail. The preferred embodiment uses a digital microprocessor, memory, and related circuitry to implement the video processor  13 . 
     Processor  13  receives light signal  17 A from light sensor  17 . Light signal  17 A is representative of ambient light conditions. Processor  13  generates multiple signals responsive to light signal  17 A for controlling the brightness of the backlight light sources that are part of display unit  14  and are described below. One signal, the high brightness signal  18 A, controls the high brightness light source. A second signal, the low brightness signal  18 B, controls the low brightness light source. It should be appreciated by those skilled in the art that alternate, yet equivalent, embodiments are possible. For example, a light signal from the light sensor could be communicated directly to the light sources or to a dedicated brightness controller for controlling the brightness of the light sources. 
     Display unit  14  generates and projects the desired image. Display unit  14  may be of conventional design except for the backlight system that is the focus of this specification. Display unit  14  includes an electronic display and a backlight system that is described below. Responsive to display signal  13 A, symbology is formed on the screen of the electronic display. The electronic display is illuminated and an image is projected onto a combiner or windshield  16  via mirror  15 . 
     Referring to FIG. 2, a side view of the backlight system and display of the current invention is illustrated therein. Backlight  20  includes high brightness light sources  21 , low brightness light sources  22 A and  22 B, light panel  23 , and reflectors  24 A and  24 B. 
     High brightness light sources  21  are controlled by processor  13  via high brightness signal  18 A. High brightness light sources  21  preferably include one or more halogen bulbs that produce bright light  21 A. During daylight conditions only the high brightness light sources  21  are used to illuminate electronic display  26 . Processor  13  continuously monitors ambient light conditions via light signal  17 A. When ambient light conditions change, processor  13  controls high brightness light sources  21  to either dim or brighten responsive to the changing conditions. When ambient light conditions reach a predetermined low level, the high brightness light sources  21  are turned off, thus preserving their lifetime. The low brightness light sources  22 A and  22 B are activated to provide illumination when the high brightness light sources are turned off. High brightness light sources  21  preferably operate over the range of about 31,000 footlamberts (fl) to 310,000 fl. 
     Bright light  21 A is transmitted through panel  23  where it is diffused prior to exiting light panel  23  and illuminating the rear of electronic display  26 . When both the high brightness light source  21  and low brightness light sources  22 A and  22 B are active, bright light  21 A is mixed with dim light  22 C in panel  23 . 
     Similarly, low brightness light sources  22 A and  22 B are controlled by processor  13  via low brightness signal  18 B. Low brightness light sources  22 A and  22 B preferably include one or more fluorescent lights suitable for producing low brightness light. During low light or nighttime conditions only the low brightness light sources  22 A and  22 B are turned on. Processor  13  continuously monitors ambient light signal  17 A. When ambient light conditions change, processor  13  controls the low brightness light sources  22 A and  22 B via low brightness signal  18 B to either dim or brighten the light. When ambient light conditions reach a predetermined level, low brightness light sources  22 A and  22 B are turned off since they are no longer needed in bright light conditions. The predetermined light levels for turning light sources on and off are variable depending on a particular implementation. For example, the characteristics of the particular light sources used will affect the selection of the light levels. 
     Dim light  22 C from low brightness light sources  22 A and  22 B illuminate edges  23 A and  23 B of light panel  23  where it is reflected, mixed with bright light  21 A, and diffused before exiting light panel  23  to illuminate the rear of display  26 . Low brightness light sources  22 A an  22 B preferably operate over the range of about 310 fl to 3,100 fl. 
     Light panel  23  conducts light from all light sources to illuminate the electronic display. Also known as a light pipe, light panel  23  is made of a light transmissive material, preferably acrylic, and reflects and mixes light. Bright light  21 A enters the backside  23 D of light panel  23  and exits the front side  23 C to illuminate the rear  26 A of display  26 . The front side  23 C of light panel  23  is textured to diffuse light and provide uniform illumination of display  26 . The backside  23 D of light panel  23  may also be textured, if desired, to further diffuse bright light  21 A. Low brightness light sources  22 A and  22 B illuminate edges  23 A and  23 B of light panel  23 . Edges  23 E that are not illuminated have a reflective coating (not shown here) to reflect light back into panel  23  and thereby reduce light loss. The reflective coating may include any conventional reflective coating such as an aluminised coating, reflective foil, white paint, or the like. 
     Light mixer  27  is of conventional design. Its purpose is to eliminate uneven lighting or bright spots that occur when the high brightness light source is dimmed to low light levels. Mixer  27  is square and has reflective inner surfaces. Alternate shapes may also be used. Mixer  27  is optional since light panel  23  also functions to perform some mixing and diffusing of uneven lighting. Mixer  27  may be added to the backlighting system if uneven lighting or bright spots are a problem. 
     Display  26  may include a conventional backlit electronic display. The preferred embodiment of display  26  uses an active matrix liquid crystal display (AMLCD), however; alternate displays may be used. For example, many LCDs may be used and may include either monochrome or color. 
     FIG. 3 illustrates the front view of the backlighting system. Shown in FIG. 3 are high brightness light sources  21 , low brightness light sources  22 A and  22 B, light panel  23 , and mask  28 . In the embodiment shown, two high brightness light sources  21  illuminate the center portion of light panel  23 . Mask  28  reflects light back into light panel  23 . High brightness light sources  21  illuminate a smaller portion of light panel  23  because a smaller field of view is used for daylight conditions. Low brightness light sources  22 A and  22 B illuminate the entire light panel  23  thereby providing a wider field of view for nighttime operations. 
     Referring to FIG. 4, the light panel/light pipe  23  is shown. Light panel  23  is shown as a rectangular panel of light transmissive material. Dim light  22 C enters from the top edge  23 A and the bottom edge  23 B. Bright light  21 A enters from the backside  23 D of the panel  23 . Light from all sources is mixed in light panel  23  and exits front side  23 C of light panel  23 . Front side  23 C is textured to diffuse light exiting light panel  23 . The texturing preferably comprises pyramid diffusion shapes or the like. Side edges  23 E are coated with a reflective material  40  to reflect light back into light panel  23 . 
     A HUD system that may employ the backlighting system of the present invention is shown in FIG.  5 . Video processor  13  receives inputs from various sensors and systems throughout the vehicle  10 . Light sensor  17  measures ambient light conditions and communicates light signal  17 A to processor  13 . IR sensor  51  detects objects in the road ahead of the vehicle and communicates IR signal  51  A to processor  13 . Speed sensor  52  senses vehicle velocity and communicates speed signal  52 A to processor  13 . Similarly, engine sensors  53  sense and measure various engine related parameters and generates an engine signal  53 A. Position sensors  54  include sensors such as GPS, compass, acceleration, and gyros. Position sensors  54  generate position signal  54 A. 
     Video processor  13  analyzes the various input signals and data and generates a display signal  13 A that commands electronic display  26  to display desired symbology. Video processor  13  also controls the brightness of the high brightness light source  21  via high brightness signal  18 A and the low brightness light source  22 A and  22 B via low brightness signal  18 B. 
     Referring to FIG. 6, an intermediate brightness light source embodiment is illustrated. FIG. 6 is similar to FIG. 4; however, low brightness light source  22 B in FIG. 4 has been replaced with intermediate light source  61 . Light sources of varying brightness help to eliminate color temperature problems, uneven lighting, and extend the lifetime of light sources since they are not dimmed and brightened over wide ranges. 
     In this embodiment the three different light sources are turned on and off and are brightened or dimmed responsive to ambient light conditions. The high brightness light source is used primarily for bright ambient light conditions, the intermediate brightness light source is used for intermediate ambient light conditions, and the low brightness light source is used primarily for low ambient light conditions. Two of the light sources are active simultaneously as the ambient light condition transitions between the predetermined boundaries. This prowess is further described below. 
     Beginning in bright ambient light conditions, the high brightness light source  21  is used exclusively. As ambient light conditions become less bright, the high brightness light source  21  is dimmed. When ambient light conditions reach a predetermined level, intermediate brightness light source  61  is turned on. As ambient light conditions dim further, the high brightness light source  21  is dimmed. When ambient light conditions reach another predetermined level, the high brightness light source is turned off. As ambient light conditions dim to yet another predetermined level, the low brightness light source  22 A is turned on. The intermediate brightness light source  61  is dimmed further responsive to ambient light conditions until the intermediate brightness light source is turned off. In low ambient light conditions the low brightness light source is used exclusively. 
     Also illustrated in FIG. 6 is textured surface  60  on the front of light panel  23 . Textured surface  60  diffuses light from all of the light sources for even lighting of electronic display  26 . 
     The method of the invention follows from the previous description. The steps for backlighting the display include transmitting light from a high brightness light source through a light panel to illuminate the rear of an electronic display, illuminating an edge of the light panel with a low brightness light source, and sensing the ambient lighting conditions. Further steps include controlling the brightness of the light sources responsive to changes in ambient light conditions. The controlling step includes turning the light sources on and off and also dimming and brightening the light sources to smoothly transition through varying ambient lighting conditions. Yet another step is illuminating the light panel with an intermediate brightness light source. 
     The head up display  11  of the present invention achieves dimming ratios exceeding 1000:1 while maintaining excellent light quality and is particularly useful in HUD systems, especially those employed on automobiles. The system achieves improved performance and improved light source lifetimes. 
     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.