Automotive head-up display with high brightness in daytime and high contrast in nighttime

Abstract: A head-up display has an image source (111) for producing imaging illumination and an optical combiner for partially reflecting the imaging illumination to produce a virtual image of the image source (111) that is viewable by the operator of the vehicle. The head-up display further comprises a polarizer (112) which can be controllably interposed between the image source (111) and the optical combiner for nighttime use. The polarizer (112) tends to block the illumination that otherwise would produce objectionable ghost images.

Description: The disclosed invention is directed to a head-up display comprising an image source for producing imaging illumination, an optical combiner for partially reflecting the imaging illumination to produce a virtual image of said image source viewable by the operator and a polarizer for interposing between said image source and said combiner. Head-up displays are utilized in vehicles such as automobiles to produce virtual images of vehicle operating parameter indicators that appear to be ahead of the vehicle windshield and are therefore viewable by the vehicle operator without diversion of his or her eyes to an instrument panel inside the vehicle and without refocusing. As a result of the thickness of the vehicle windshield and reflections at a glass/air interface that is not the primary reflecting surface, ghost images are produced. Depending on ambient lighting conditions and intensity relative to primary image intensity, ghost images can be objectionable. From EP 0 286 962 A3 an automotive head-up display has become known using an image source for producing imaging illumination applied to the surface of a vehicle windshield which is partially reflecting. Thereby a virtual image of the image source is produced which can be viewed by the operator of the vehicle. To reduce the intensity of ghost images a polarizer foil may be introduced into the light path, in particular may be inserted between the front layer and the back layer of the windshield made up by two or more glass layers. From EP 0 278 395 A2 a further automotive head-up display has become known comprising a holographic display system. In this prior art system light from a display source illuminates a first hologram diffracting the received light. A second hologram receives the light from the first hologram and diffractively reflects the received light forming a virtual image of the display source in the viewer's field of view. The first and second holograms are non-parallel, thereby moving the ghost image out of the normal driver's field of view. It is an objective of the invention to provide a head-up display for vehicles that provides better visibility for daytime use and for nighttime use. According to the invention this objective is achieved with a head-up display for a vehicle, comprising an image source for producing a beam comprising imaging information, an optical combiner for partially reflecting said beam to produce a virtual image of said image source viewable by the operator of the vehicle, a polarizer interposed between said image source and said combiner, whereby the display further comprises means for selectively interposing said polarizer between said image source and said combiner. According to the invention it was found that requirements for head-up displays include sufficient image brightness that is not too bright, ghost image intensity that is not distractingly objectionable, and acceptable image contrast. The foregoing requirements for head-up displays typically require different specific characteristics for day and night use. Daytime use requires high image brightness, and permits higher ghost image intensity since the ghost images tend to blend with the ambient scene. Nighttime use requires lower primary image brightness, and lower ghost image intensity since ghost images are readily noticed against the generally dark ambient scene. According to the invention primary image to ghost image intensity ratio is greater at night, leading to a high contrast and bright picture with an acceptable ghost image intensity during daytime use and less bright image with a somewhat lower contrast but low ghost image intensity during nighttime use. The optical combiner preferably comprises a beamsplitter coating for partially reflecting the imaging illumination to produce a virtual image of the image source that is viewable by the operator of the vehicle. The polarizer when selectively introduced into the light path tends to block the illumination that otherwise would produce ghost images. The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailled descriptions when read in conjunction with the drawing wherein: FIG. 1 is a schematic illustration of the major components of the disclosed vehicle head-up display system. FIG. 2 is a schematic exploded view of the imaging illumination source of the head-up display system of FIG. 1. DETAILED DESCRIPTION In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. Referring now to FIG. 1, shown therein is a head-up vehicle instrument display system that includes an imaging illumination source 20 and a combiner element 11 that comprises a partially reflecting beamsplitter coating on the inside surface of the vehicle windshield, for example. The imaging illumination source 20 can be on the dashboard or within the dashboard with an appropriate opening for passage of the imaging illumination. The combiner element 11 partially reflects the imaging illumination toward the driver so as to produce a virtual image of a light emitting image source within the imaging illumination source 20 at a location ahead of the vehicle windshield, for example. While the disclosed embodiments are described in the context of a vehicle such as an automobile, it should be appreciated that the term vehicle encompasses vehicles that include a windshield or similar transparent protective device, as well as vehicles that do not have windshields, in which case the combiner would be a partially reflecting coating applied to a transparent panel, for example. By way of illustrative example, the combiner element 11 comprises a multi-layer dielectric coating or a metallic coating that does not reduce light transmission below the pertinent federal standard, presently 70%. The coating would be applied to either the inside surface of the windshield or between the layers of the windshield. As a further alternative, the reflection element 11 can be a reflection hologram. Preferably, the imaging illumination is incident on the windshield inside surface at an incident angle that is close to tho Brewster's angle for the material comprising the Windshield (56 degrees relative to normal for glass in air). At the glass/air Brewster's angle, 100% of the P-polarized component of incident light passes through glass/air interfaces without reflection, while the S-polarized light is partially reflected off each glass/air interface. However, at the glass/air Brewster's angle, a hologram or coating will reflect some of the P-polarized light, the amount depending on the particular coating or hologram. A simple quarter wave stack dielectric coating or a singly-exposed volume reflection hologram is less reflective for P-polarized light than for S-polarized light; nevertheless, a good portion of the P-polarized light can be reflected. Furthermore, there are techniques Well known in the art for designing dielectric coatings as to which P-reflectance at a particular incident angle is greatly improved. See for example Optical Interference Coating Technology , Lecture Notes, Short Course at UCLA (Engineering 823.17), November 28 - December 2, 1983, Philip Baumeister, instructor. Referring now to FIG. 2, shown therein is a detail view of the imaging illumination source 20 which includes an image source 111 comprising a high intensity vacuum fluorescent display (VFD), for example, which is secured in a housing 110. VFD's are known display devices which are commercially available, for example, from Futaba Corporation of America, Plymouth, Michigan, and commonly include segmented elements that are selectively energizable to form light emitting numerical and/or alphabetical symbols, as well as other indicia. Also, the image source 111 can be a segmented or matrix addressable liquid crystal display (LCD), which can be obtained from Seiko of Japan for example. Also, a spectrally narrow source such as light emitting diodes available from Stanley could be utilized, particularly where the combiner 11 is a hologram. Inputs to the image source 111 are provided by appropriate transducing circuitry to display selected vehicle and/or engine operating parameters and conditions. A movable polarizer 112 is configured to be controllably locatable in front of the image source 111, for example by a small electric motor 121, to polarize the imaging illumination when desired. As discussed more fully herein, the plane of polarization for the polarizer 112 depends on the type of combiner 11 utilized. If the image source 111 provides polarized light as the imaging illumination (an LCD, for example), it should be adapted to provide light of the same polarization as the polarizer 112. A planar fold mirror 113 secured in the housing 110 relays the imaging illumination from the image source 111 to an off-axis, aspheric mirror 115, also secured in the housing 110, which in turn relays the imaging illumination through a transparent cover 117 to the combiner element 11. While a relay mirror is included, it should be appreciated that depending on the location of the image source 111 in the imaging illumination source 20 and the location of the imaging illumination source 20 relative to the windshield, the relay mirror 115 might not be necessary. It should also be appreciated that with an odd number of reflecting elements (e.g., three), the image source 111 must adapted to be a mirror image of the intended virtual image. With an even number of reflecting elements (e.g., two), the image source 111 would be adapted to be oriented the same as the intended virtual image. The aspheric mirror 115 is generally concave and magnifies the image relayed to the combiner element 11, and further functions to locate the virtual image relatively far ahead of the observer. The particular curvatures of the aspheric mirror can be defined so as to compensate for the distortion introduced by the off-axis configuration of the aspheric mirror and the distortion introduced by the curvatures of the windshield. While an aspheric mirror 115 is included in the foregoing illustrative example, a flat mirror could be utilized if it is not necessary to locate the virtual image relatively far ahead of the observer. It should also be realized that the mirrors 113 and 115 could be eliminated by directing the image source directly at the combiner 11. The polarizer 112 is designed to reduce the brightness of the ghost image or images relative to that of the main or primary image. For combiners which comprise a metallic or dielectric coating or a hologram, the polarizer 112 should be P-polarized. With such polarization, if the imaging illumination is incident on the combiner at or near the Brewster's angle, virtually no light is reflected off the glass/air interface or interfaces of the windshield, but there will be appreciable light reflected by the coating or hologram. Thus, the resulting image produced by the combiner will include primarily the image off the coating or hologram, and will be relatively free of ghost images. Even if the incidence angle is significantly different from the glass/air Brewster's angle, the invention can still provide appreciable reductions in ghost image intensity relative to primary image intensity. For example, for an incidence angle of 62.5°, only about .6% of P-polarized light is reflected from a glass/air interface, while 20.6% of S-polarized light is reflected from a glass-air interface. The reflection of P-polarized light from a beamsplitter coating or hologram will likely be much greater than .6%. For a combiner comprising only the inside surface of the windshield (i.e., no coating or hologram), no benefit is provided by inserting a P-polarizer in front of the image source 111, which would result in close to zero reflection of P-polarized light off each of the two glass/air interfaces and the relative brightness of the two images reflected from the outside and inside surfaces would be about equal. However, making the polarizer 112 S-polarized provides small but useful reductions in ghost image intensity relative to the primary image intensity in this case where the ghost image is off the outside glass/air interface and the primary image is off the inside glass/air interface. The reflections at the glass/adhesive interfaces for safety windshields are negligible since the indices of refraction are very closely matched. The benefit of S-polarization with a combiner comprising only the inside surface of the windshield can be appreciated by consideration of the example of imaging light incident on a windshield at 70° relative to normal. The P-polarized reflection is 4.1% while the S-polarized reflection is 30.8%. In terms of incident illumination having a normalized intensity of 1, the relative intensity of the ghost image is based on the normalized values for (a) the amount of incident illumination transmitted to the outside glass/air interface, (b) the amount of illumination reflected back by the outside glass/air interface, and (c) the amount of reflected illumination transmitted at the inside glass/air interface. Assuming that normalized transmittance is (1 - reflectance), and for simplicity neglecting any windshield tint, the amount of P-polarized light reflected at the outside glass/air interface and transmitted by the inside glass/air interface is 0.959 x 0.041 x 0.959 = .0377. With the same assumptions, the amount of S-polarized light reflected at the outside glass/air interface and transmitted by the inside glass/air interface is .692 x .308 x .692 = .147. If the light source is randomly polarized and no polarizer is placed in front of it, the ghost/main image ratio is (.0377 + .147)/(.041 + .308) = .529. If a P-polarizer is placed in front of the light source, the ghost/main image ratio is .0377/.041 = .920. If an S-polarizer is placed in front of the light source, the ghost/main image ratio is .147/.308 = .477, which is a small reduction compared to when no polarizer is in place. The effect of the windshield tint is to reduce the ghost/main image ratio by a factor which is independent of polarization. Therefore, when using a bare windshield as a combiner, small reductions in ghost image visibility can be made with an S-polarizer, as well as significant reductions in overall image brightness. Further as to the operation of the head-up display, the polarizer 112 is controlled to be in front of the image source during nighttime or otherwise dark ambient lighting conditions and functions to reduce the intensity of ghost images relative to the primary image intensity. For nighttime use, the intensity of the imaging illumination provided to the combiner 11 is reduced, which is achieved by the polarizer 112 and by appropriately reducing the output of the image source 111. While the polarizer 112 would also function to reduce the ghost image intensity relative to the primary image intensity during daytime use, the resulting primary image might not be sufficiently bright, depending on the image source 111. Ghost images during daytime use will in any event not be as objectionable due to the bright ambient scene. The disclosed system provides the further benefit of increased contrast for an image source such as a liquid crystal display wherein the "off" portions of the display (the background of a digital display, for example) are not completely off. The resulting glow of the off areas is not noticeable during daytime use, but is noticeable during nighttime use and results in reduced image contrast. Since part of the off area leakage illumination is of a different polarization from the on areas that emit illumination of the same polarization as the polarizer 112, the off area leakage will be substantially blocked by the polarizer, resulting in increased image contrast. The foregoing has been a disclosure of a head-up display system that provides for sufficient brightness for daytime use and non-objectionable ghost image intensity for nighttime use. The polarizer not only functions to reduce the relative ghost image intensity, but also reduces the primary image brightness, thereby providing further control over the primary image brightness for nighttime use. For liquid crystal device image sources, the disclosed invention advantageously increases image contrast for nighttime display by blocking the illumination leakage from the off areas of the image source display. Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope of the invention as defined by the following claims. Further details of automotive head-up displays may be found in applicant's co-pending European patent application 90 105 902.2, the disclosure of which is hereby incorporated into this application by way of reference.

Claims: A head-up display for a vehicle, comprising an image source (111) for producing a beam comprising imaging information; an optical combiner (11) for partially reflecting said beam to produce a virtual image of said image source (111) viewable by the operator of the vehicle; and a polarizer (112) for interposing between said image source (111) and said combiner (11), characterized in that the display further comprises means for selectively interposing said polarizer (112) between said image source (111) and said combiner (112). The head-up display of claim 1, characterized in that said optical combiner (11) comprises a coating supported by a windshield. The head-up display of claim 1, characterized in that said optical combiner (11) comprises a reflection hologram supported by a windshield. The head-up display of claim 1, characterized in that said optical combiner (11) comprises a windshield surface. The head-up display of any of claims 1 - 4, characterized in that said polarizer (112) comprises a P-polarizer. The head-up display of any of claims 1 - 4, characterized in that said polarizer (112) comprises a S-polarizer. The head-up display of any of claims 1 - 6, characterized in that said image source (111) produces imaging illumination that is randomly polarized. The head-up display of any of the preceding claims, characterized in that said optical combiner (11) comprises a partially reflecting beamsplitter coating supported by the vehicle windshield. The head-up display of any of claims 1 - 8, characterized in that said image source (111) comprises a vacuum fluorescent display. The head-up display of any of claims 1 - 8, characterized in that said image source (111) comprises a liquid crystal display. The head-up display of any of claims 1 - 10, characterized in that said image source (111) produces imaging illumination that is spectrally narrow.

Inventors: CHERN MAO-JIN, HEGG RONALD G, SMITH RONALD T, CHERN, MAO-JIN, HEGG, RONALD G., SMITH, RONALD T.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1990
Main classification: G02B27
IPC codes: G02F1, G02B5, G02B27, G09F9, B60K35