Abstract:
A head-up display including a hybrid illumination system is provided. A light mixing unit provides a substantially homogenous light source to a reflective display unit. A concentrating optics unit collects ambient light and directs it towards the light mixing unit. At the same time, an electrically-powered light source emits light which is directed towards the light mixing unit. One or more optical elements direct the ambient light and the light source light into the light mixing unit for homogenization. A condensing unit receives the homogenized light mixture and outputs the condensed light to a polarizing beam splitter. A reflective display modulates the light from the polarizing beam splitter with information from a source of electrical information signals back towards the polarizing beam splitter. A projection unit projects the modulated light to create an image on a windshield.

Description:
FIELD OF INVENTION 
     The present invention relates to head-up displays in general and, more particularly, to illumination systems for head-up displays. 
     BACKGROUND 
     Head-up displays (HUD) are systems which project images onto a viewing surface at a position which allows the viewer to maintain a posture in which the gaze is directed forward rather than downward to a display or instrument panel. Head-up displays are used in various environments such as motor vehicles, aircraft, helmets and other situations in which it is important that the viewer not divert his gaze. 
     Although head-up displays are useful in such environments, the images can sometimes be difficult to discern in bright ambient lighting conditions. Therefore, there is a need in the art for displays which can be easily viewed in bright light. Previously, attempts have been made to harvest ambient light for head-up displays. An example of such a head-up display is found in WO 95/13557 which uses an ambient light source in a display which projects images onto a windshield. Attempts have also been made to combine ambient light with another light source as in U.S. Pat. No. 4,997,263 and U.S. Pat. No. 7,430,349. Although ambient light and another light source are used, the former patent has low ambient light collection efficiency, while the latter involves complex collection fiber optics. Further, neither patent combines the two light sources in such a manner as to provide a uniform light source for the display. 
     Thus there is a need in the art for improved light sources for head-up displays which can provide both adequate brightness of the display and a uniform light source for the display in order to permit clear viewing under high ambient light conditions. 
     SUMMARY OF THE INVENTION 
     The present invention is a head-up display (HUD) unit that utilizes both ambient light from the environment and the visible light from an electrical light source for illumination of the projected image. It comprises a concentrating optics unit, a first beam-shaping optics unit, a second beam-shaping optics unit, a light-mixing unit, a condensing unit, a polarizing beamsplitter (PBS), a reflective display unit, a projection unit, a diffusing film and a light source. Reflective mirrors are optionally used to change the optical paths of the light depending on the locations of the various components. Moreover, it may further comprise a brightness enhancer and a pre-polarizer. 
     The concentrating optics unit collects ambient light from the environment and redirects it towards the axis of the concentrating optics unit so that it falls within a defined cone angle. In a first embodiment of the invention, wherein the axes of the concentrating optics unit and the first beam-shaping optics are parallel and coincident, the light emerging from the concentrating optics unit is fed directly into the first beam-shaping optics unit. In a second embodiment of the invention, wherein the axes of the concentrating optics unit and the first beam-shaping optics are not parallel, the light emerging from the concentrating optics unit is fed into the beam-shaping optics unit via a reflective element. The function of the first beam-shaping optics unit is to collimate the concentrated ambient light, and direct it towards the light-mixing unit via a reflective mirror. 
     The visible light from the light source is passed through the beam-shaping optics unit for collimation. The collimated light from the beam-shaping optics is subsequently directed towards the light-mixing unit via a reflective mirror. 
     The light-mixing unit combines and homogenizes all the light within the light-mixing unit and it controls the spreading angle of the mixed light that will be projected onto the reflective display unit so that the reflective display unit receives a uniform illumination. The mixed light coming out of the light-mixing unit is then fed into the condensing unit, which then directs the light towards the PBS. 
     If a pre-polarizer is present, it is preferably installed between the condensing unit and the PBS. The pre-polarizer is oriented such that only light in the block polarization state of the PBS is transmitted. Therefore, the transmitted light from the pre-polarizer will be reflected at the PBS towards the reflective display unit. At the PBS, light having one specific polarization is completely reflected while light having the orthogonal polarization is transmitted. The light that is reflected off the PBS then reaches the reflective display unit. The reflective display unit, which has a video signal as its input during operative use, spatially modulates the incident light by polarization rotation. The reflected light contains light in both the block and the pass polarizations for the PBS. Only the light in the pass polarization of the PBS, i.e. the image light, is transmitted through the PBS towards the projection unit. Subsequently, the projection unit projects an image from the video signal onto a diffusing film, which then forms a real image that is reflected onto the windshield into the observer&#39;s view of sight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a head-up display incorporating ambient light according to an embodiment of the present invention. 
         FIG. 2  is an external view of a head-up display incorporating ambient light according to one embodiment of the present invention. 
         FIG. 3  is a schematic depiction of the optics used in an embodiment of the head-up display of the present invention. 
         FIG. 4  depicts the interior configuration of the head-up display of  FIG. 2 . 
         FIG. 5  is an external view of a head-up display incorporating ambient light according to another embodiment of the present invention. 
         FIG. 6  depicts the interior configuration of the head-up display of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning to the drawings in which like numerals indicate the same or similar features in each of the several views,  FIG. 1  schematically depicts a head-up display  10  according to an embodiment of the present invention. As seen in  FIG. 1 , the present invention collects ambient light to be used to enhance the illumination optics used in the head-up display. This ambient light, combined with visible light from an electrically-powered light source, such as a light-emitting diode (LED) or a laser diode, is used to form an image with an enhanced level of brightness. 
     When the present invention is being used with a vehicle as depicted in  FIG. 1 , an image  20  which is formed by the head-up display (HUD) unit  10  will be projected onto a diffusing film  220  and reflected to the windshield  30 , which can then be seen by the driver. Optionally, the HUD unit may be positioned on top of the dashboard, as this minimizes the obstruction to the driver&#39;s view ahead. Nonetheless, the HUD unit can also be placed in other appropriate locations within the vehicle such as partially or fully in the dashboard, the vehicle roof, etc. 
     A schematic view of the exterior of HUD unit  10  is depicted in  FIG. 2 . Ambient light enters unit  10  through window  40  and the exit for the projection optics is shown at projection port  50 . Due to the compact design of HUD unit  10 , after-market installation in automobiles is easily accomplished. In this manner, HUD  10  may form part of a navigation/GPS system that may be conveniently installed in any motor vehicle. 
     An overview of the optics used in an embodiment of the present invention is depicted schematically in  FIG. 3 . As seen in  FIG. 3 , ambient light and light from a light source such as one or more LEDs/lasers are combined to form a single, substantially uniform light source for use in the HUD. Ambient light  100  enters concentrating optics unit  110 , where it is optionally reflected by mirror  120  and shaped by a first beam-shaping optics unit  130 . The concentrating optics  110  unit collects light from the ambient environment and redirects it so that it falls within a narrower cone angle. Concentrating optics unit  110  may be selected from a variety of optical devices such as light guides, aspheric lenses, and Fresnel lenses. However this list is not limited; any optical device which can sufficiently collect and deliver ambient light to the HUD is contemplated for use in the present invention. 
     The beam-shaping optics unit  130  may comprise a set of conjugated lenses or TIR lenses. The purpose of the beam-shaping optics unit  130  is to collimate the concentrated ambient light before the light is directed towards the light mixing device. 
     An additional light source  140  is used to emit light to combine with the ambient light collected by the concentrating optics  110 . Light source  140  may be a single-color LED, multiple-color LEDs, lasers, an incandescent light bulb, a halogen lamp, an arc lamp, or any other light emitters sufficient to illuminate the liquid crystal element, or any combination of the above light emitters. 
     The light from light source  140  is then passed into a second beam-shaping optics unit  150 . It is possible to adjust the intensity of light from the light source according to the intensity of ambient light being collected when using an optional feedback control system. The purpose of the second beam-shaping optics is to collimate the light from the light source before it is directed towards the light mixing unit  170 . 
     Reflective mirrors  160  are located and oriented such that light from both light source  140  and the collected ambient light are directed towards the light-mixing unit  170 . Reflective mirrors  160  may comprise flat mirrors, curved mirrors right-angle prisms or a plurality and/or combination of the above. Reflective mirrors  160  may be formed as a single unit or as two or more separate units. Reflective mirrors can also be replaced by a right-angle prism or other optics, which could alter the direction of the light. 
     Because it is important for the liquid crystal element to have a uniform incident light supply, light mixing unit  170  is selected from a variety of devices which can mix the light from the light source  140  and the ambient light and output a substantially uniform light beam. Such devices include, but are not limited to an integrating rod or a macrofocal concentrator. Examples of the macrofocal concentrator include a Compound Parabolic Concentrator (CPC), a Compound Ellipsoidal Concentrator (CEC) and a Compound Hyperbolic Concentrator (CHC). However, any optical element which can uniformly mix the two light sources and provide a uniform output is contemplated as the light mixing unit  170  of the present invention. To further enhance homogenization of the ambient light and light from light source  140 , the sidewalls of the light mixing unit may diffusively reflect light with the mixer. In the light-mixing unit, the concentrated and collimated ambient light is mixed with the collimated light from light source  140 . The function of the light-mixing unit is to combine and homogenize all the light within the light-mixing unit, and to control the spreading angle of the mixed light that will be projected onto the reflective display unit. Consequently, the light that is transmitted out of the light-mixing unit is substantially uniform. 
     The mixed light that emerges from light-mixing unit  170  is then fed into the condensing unit  180 . The condensing unit  180  may comprise a set of conjugated lenses or a set of freeform lenses. Optionally, if a pre-polarizer is present, the condensing unit directs the mixed light towards the pre-polarizer, otherwise, upon exiting condensing unit  180  the mixed light is directed towards the polarizing beamsplitter (PBS)  190 . 
     The pre-polarizer is oriented such that only light in the block polarization state of the PBS is transmitted. Therefore, the transmitted light from the pre-polarizer will be reflected at the PBS towards the reflective display unit  200 . 
     At the PBS  190 , light having one specific polarization is completely reflected while light having the orthogonal polarization is transmitted. The polarization may be linear, circular, or cholesteric. Examples of a linear polarizer include a polymeric multiple layer polarizing film or a wire grid polarizer. It will be appreciated that use of a cholesteric polarizer may also necessitate the introduction of a quarter wave retarder in order to convert light between linear and circular polarization. For example, where the reflective display unit operates on linearly polarized light and the light from the light source is also linearly polarized, then the cholesteric polarizer may be provided with a quarter wave retarder layer on its front surface so as to circularize the polarization of the light prior to incidence on the surface of the cholesteric polarizer. Furthermore, the quarter wave retarder linearizes the polarization of the reflected light before propagating to the reflective image display unit. Where the light from the light source is circularly polarized, the reflective image display unit may be provided with a quarter wave retarder at its input so as to linearize the polarization of the light reflected from the cholesteric polarizer. Furthermore, PBS  190  may be flat, or curved in one or two directions. 
     The light with the block polarization of the PBS will be reflected at PBS  190 , and will travel towards the reflective display unit  200 . The reflective display unit may be a liquid crystal display (LCD) unit, for example a liquid crystal on silicon (LCoS) display, a digital light processing (DLP) display, or any other suitable display units. When the present invention is in operation, a video signal, which may originate from a GPS unit, a navigation unit, one or more dashboard devices (e.g., speedometer, tachometer, fuel gauge), the onboard computer, or any device that can generate a suitable video signal for the reflective display unit, is fed to reflective display unit  200 . The reflective display unit  200  spatially modulates the incident light by polarization rotation. The reflected light contains light in both the block and the pass polarizations for the PBS. Only the light in the pass polarization of the PBS  190 , i.e. the image light, is transmitted through the PBS towards the projection unit  210 . 
     As seen in  FIG. 1 , the projection unit  210  then projects the image light onto diffusing film  220 . The diffusing film may be coupled to the projection unit. A real mirror image will be formed by the diffusing film, and the real mirror image will be reflected onto the windshield into the observer&#39;s view of sight. It is understood that the use of diffusing film  220  is optional; the reflective display  200  can be configured along with the projecting optics to display a suitable image directly from projection unit  210  onto the windshield. 
       FIG. 4  depicts the interior arrangement of the optical components of the head-up display  10  of  FIG. 2 . The elements are substantially similar to those depicted in  FIG. 3  as indicated by the corresponding element numbers. 
       FIG. 5  depicts the exterior of a low-profile embodiment  10  of the head-up display of the present invention. As in the previous embodiment, ambient light enters window  40  and image light exits from projection port  50 .  FIG. 6  depicts the optical component arrangement in the interior of head-up display  10 . The elements are substantially similar to those depicted in  FIG. 3 , as indicated by the corresponding element numbers. As seen in  FIG. 6 , the arrangement of the concentrating optics  110  for the ambient light permit the height of the HUD to be lower than that of the HUD of  FIG. 2 . This embodiment is particularly useful for after-market installation on a vehicle dashboard as the lower profile provides enhanced visibility. 
     It should be understood that the exact positions and orientations of the above-mentioned components of the present invention may be adjusted to alter the size, brightness, and sharpness of the image. 
     Although the present invention has been described in the context of a head-up display (HUD) unit designed primarily for use on a motor vehicle for displaying relevant driving information such as vehicle speed, engine speed (rpm) and global positioning system (GPS) data, it is understood that the present invention also applies to head-up displays in other vehicle contexts, such as aircraft and boats. Further, the present invention can also be applied to other situations that require enhanced illumination, for example as a helmet-mounted display unit or as a pico-projector. 
     While particular embodiments of the present invention have been illustrated and described, it is understood that the invention is not limited to the precise construction depicted herein and that various modifications, changes, and variations are apparent from the foregoing description. Such modifications, changes, and variations are considered to be a part of the scope of the invention as set forth in the following claims.