High throughput contrast enhancement

Abstract: A display (10) has a linearly polarized image source (11), a quarter-wave retarder (13) for transforming the linearly polarized illumination to circularly polarized illumination, imaging elements (17-19) responsive to the circularly polarized illumination for producing viewable circularly polarized imaging illumination, and a circular polarizer (21) for transforming the viewable circularly polarized imaging illumination to viewable linearly polarized imaging illumination. The circular polarizer (21) includes a quarter-wave retarder (21a) for transforming the circular light to linearly polarized light, and a linear polarizer (21b) configured to transmit the linearly polarized imaging illumination. Contrast enhancement is achieved by the circular polarizer (21) which functions to block ambient light reflected from the front element of the imaging elements as a result of transmission through the circular polarizer (21).

Description: The disclosed invention is directed to a display for a vehicle having a windshield which comprises a display source emitting imaging illumination, an imaging lens comprising a front element and being responsive to the imaging illumination for producing viewable imaging illumination, and an optical combiner associated with the windshield for reflecting the imaging illumination to produce a virtual image viewable by the driver. Such a display which is also generally referred to as a head-up display is known from EP-A-0 321 149. According to the prior art device the imaging illumination emitted from a display source is transmitted by an imaging lens to an optical combiner associated with the windshield for reflecting the imaging illumination to produce a virtual image viewable by the driver. The imaging lens collimates the imaging illumination to appear before the driver at infinity and also compensates for the distortions introduced by the curvature of the windshield. However, such a display and also polarized displays, such as liquid crystal device (LCD) displays, suffer from reduced contrast that results from the reflection of ambient light from the display surface. Absorptive filters of varies types have been commonly utilized to enhance display contrast, but typically at the expense of brightness. In other words, although such filtering increases contrast, the light from the display is attenuated. From FR-A-2 212 559 a polarized display has become known, which uses a circularly polarized light to enhance contrast and to reduce reflections from ambient light. Light emerging from a liquid crystal display source passes a first polarizer and through a quarter wave plate and is thus circularly polarized. The circularly polarized light is then directed onto a diffusor followed by a second quarter wave plate and a second linear polarizer constituting a second circular polarizer. Since the direction of ambient light passing through the second circular polarizer is reversed during reflection off the diffusor, ambient light passing through the second circular polarizer is reversed, when reflected from the diffusor, ambient light is substantially blocked out on the second circular polarizer, while light eminating from the display source and passing through the first circular polarizer is transmitted substantially unattenuated also through the second circular polarizer. This prior art design gives an effective means for reducing reflections from ambient light and for enhancing contrast of a display source. This design, however, does not give an indication for the application of such a device in an instrument panel display for vehicles. A similar device known from WO-A-8 403 778 comprises a liquid crystal cell and an inclined cover plate spaced from the liquid crystal cell and carrying a circular polarizer. On the liquid crystal cell there is arranged a quarter wave delay sheet cooperating with the circular polarizer so as to substantially suppress all the reflections of light coming from the outside and traversing the cover plate. Also, this known device is not directed to an instrument panel display for vehicles. It is an object of the invention to provide contrast enhancement for a polarized display for a vehicle with a windshield which does not substantially attenuate the light output of the display and which is particularly suitable for use as instrument panel display for vehicles. The foregoing and other advantages are provided by a display according to claim 1. The display according to the invention includes a display source for producing linearly polarized imaging illumination, a quarter-wave retarder for transforming the linearly polarized imaging illumination to circularly polarized imaging illumination having a first circular polarization, and an imaging lens responsive to the circularly polarized imaging illumination for producing a viewable image. A circular polarizer transforms the polarization of the image output of the imaging lens to linearly polarized illumination. The circular polarizer particularly includes a quarter-wave retarder configured to transform the circularly polarized imaging illumination to linearly polarized illumination and a linear polarizer that is aligned to transmit such linearly polarized illumination. In any case, the imaging lens includes some element near the front of the display which is subject to ambient light. Ambient light incident on the circular polarizer is absorptively filtered so that the light that passes through to the front element of the imaging lens is substantially of the first circular polarization. Pursuant to reflection from the front element of the imaging lens, the circular polarization of such ambient light is reversed to be that of a second circular orientation, which is substantially blocked by the quarter-wave retarder of the circular polarizer. Thus, the imaging illumination transmitted by the front element of the imaging means passes through the circular polarizer substantially unattenuated while ambient light reflected by the front element of the imaging lens is substantially blocked. The advantages and features of the invention will be readily appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein: Fig. 1 is a schematic diagram depicting some of the basic elements of a polarized display in accordance with the invention. Fig. 2 is a schematic side view of the display of Fig. 1. Fig. 3 is a schematic side view of a display in accordance with the invention. In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. Referring now to Figs. 1 and 2, shown therein, is a polarized display 10, which is useful in automobile instrument displays according to Fig. 3. The polarized display includes an image or display source 11 for providing linearly polarized imaging illumination. The polarized imaging illumination, which by way of illustrative example is indicated as being vertically polarized, passes through a quarter-wave retarder 13 which is configured to transform the linearly polarized imaging illumination to circularly polarized imaging illumination. For ease or reference, circularly polarized illumination will be referred to as circular illumination or circular light. The quarter-wave retarder 13 is particularly oriented so that the incident polarization is at a 45 degree inclination relative to the fast and slow axes of the retarder that are at 90 degrees relative to each other. By way of illustrative example, the fast and slow axes of the retarder 13 can be oriented as shown in FIG. 1. Pursuant to the illustrative example of the orientation of the fast and slow axes of the quarter-wave retarder 13, the circularly polarized imaging illumination from the retarder is left circularly polarized as would be viewed a hypothetical observer toward whom the circular light is moving. The circular imaging illumination provided by the quarter-wave retarder 13 is imaged by a projection lens 15 and a field lens 17 on to a screen 19, for example a diffusion screen, which does not affect polarization and provides diffuse circular imaging illumination that is viewable by an observer. While the projection lens 15 is shown as a simple concave lens for ease of illustration, it would typically be implemented as a multiple element lens for providing diverging illumination. The field lens 17 typically would be some form of convex or converging lens. Illumination from the screen 19 passes to a circular polarizer 21 which includes a quarter-wave retarder 21a configured to transform the diffuse circular imaging illumination to have a linear polarization. The circular polarizer 21 further includes a linear polarizer 21b configured to transmit the linearly polarized illumination from the quarter-wave retarder 21a to the observer. Preferably, for relatively wideband image sources, the linear polarization output of the circular polarizer 21 should be of the same orientation as the output of the image source 11, which in the illustrative example is vertical polarization. This is achieved by configuring the quarter-wave retarder 21a to have its fast and slow axes oriented at 90 degrees relative to the fast and slow axes of the quarter-wave retarder 13, and by configuring the linear polarizer to have a vertical transmission axis. With such particular quarter-wave retarder 21a and linear polarizer 21b, the circular imaging illumination from the screen 19 is transformed by the quarter-wave retarder 21a to vertically polarized imaging illumination that is transmitted by the linear polarizer 21b. The circular polarizer 21 functions as follows relative to ambient illumination, which would be typically randomly polarized. The linearly vertically polarized component of the ambient illumination passes through the linear polarizer 21b substantially unaffected, while the remaining components of the ambient illumination are substantially blocked. The transmitted linearly, vertically polarized ambient light is transformed by the quarter-wave retarder 21a to be left circularly polarized (as described above, the quarter-wave retarder 21a changes the polarization of the left circularly polarized imaging illumination transmitted by the diffusion screen 19). When left circular ambient light transmitted by the quarter-wave retarder 21a is reflected by the diffusion screen 19, the circular polarization of the reflected light is reversed, being reflected right circularly polarized ambient light. The right circular light reflected by the diffusion screen toward the observer will be transformed to horizontally linearly polarized light by the quarter-wave retarder 21a. Such horizontally linearly polarized light is substantially blocked by the linear polarizer 21b which, as described above, is configured for the particular illustrative example to pass linearly vertically polarized light. Thus, display illumination from the diffusion screen 19 is transmitted substantially unattenuated through the circular polarizer 21, while ambient light transmitted by the circular polarizer to the diffusion screen and reflected thereby is substantially blocked by the circular polarizer. Contrast enhancement is accordingly achieved with minimal attenuation of display illumination. In the above illustrative example of the polarized display system 10, the quarter-wave retarders can be essentially identical elements that are rotated at 90 degrees relative to each other about the normal or propagation axis. In other words, the quarter-wave retarders subtract and therefore the polarization axis of the imaging illumination provided by the quarter-wave retarder 21a is parallel to the polarization axis of the imaging illumination provided by the image source 11. For wideband image sources, such subtracting quarter-wave retarders are preferred so as to avoid chromatic aberration that otherwise would occur if the quarter-wave retarders were oriented with their fast axes in parallel and their slow axes in parallel. However, for narrow band image sources, the quarter-wave retarders and the linear polarizer could be configured so that imaging illumination observed by the observer is of a different linear polarization than the illumination provided by the polarized image source 11. The important consideration is to configure the circular polarizer to circularly polarize the incoming ambient light so that the circularly polarized light reflected by the diffusion screen 19 is ultimately absorptively blocked by the linear polarizer 21b. The quarter-wave retarders should be zero order retarders for wideband sources, but can be multi-order for narrow band image sources. The center of the quarter-wave retardance should be centered at about the peak of the output of the image source. In the above discussed illustrative example, the display has been configured to provide vertically linearly polarized illumination to the observer so as to prevent blockage of the display output by polarized sunglasses which typically are adapted to block horizontally polarized illumination. A polarized display in accordance with the invention that utilizes optical elements for producing a virtual image is schematically illustrated in Fig. 3. The display includes a polarized image source 111, a quarter-wave retarder 113, an imaging lens 117, a circular polarizer 121, and an optical combiner 123 at the vehicle windshield for receiving the imaging illumination provided by the circular polarizer. The display of Fig. 3 differs from the display of Figs. 1 and 2 in the use of an optical combiner and an imaging lens. In particular, according to the invention, the combiner reflects imaging illumination to produce a virtual image that is viewable by the driver, and the imaging lens functions to locate the virtual image at a location ahead of the windshield. By way of illustrative examples, the combiner 123 can comprise a reflection hologram supported between the layers of a standard safety windshield, or the inside surface of the windshield. In the display of Fig. 3, the circular polarizer 121 is perpendicular to the windshield, and the quarter-wave retarder 113 is parallel to the circular polarizer 121 to prevent reflections caused by such elements from reaching the eyes of the driver. Relative to ambient light first surface reflections off the imaging lens, the display of Fig. 3 functions in substantially the same manner as explained with respect to Fig. 1 and 2 for substantially blocking the reflection from the imaging lens of ambient light that passes through the circular polarizer to the imaging lens. While the foregoing structure has been discussed in the context of polarized image sources such as LCDs, it should be appreciated that the invention will provide some benefit with image sources that provide partially polarized imaging illumination. The foregoing has been a disclosure of a display which includes a polarized or partially polarized image source and provides for contrast enhancement while providing for minimal attenuation of the display illumination. 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.

Claims: A display for a vehicle having a windshield comprising: a display source (111) emitting imaging illumination; an imaging lens (117) comprising a front element and being responsive to said imaging illumination for producing viewable imaging illumination; and an optical combiner (123) associated with said windshield for reflecting said imaging illumination to produce a virtual image viewable by the driver; characterized in that said display source (111) emits linearly polarized imaging illumination; a first quarter wave retarder (113) is provided between said display source (111) and said imaging lens (117), said first quarter wave retarder (113) having a fast axis for transforming said linearly polarized imaging illumination to circularly polarized imaging illumination; a circular polarizer (121) is provided between said imaging lens and said combiner (123), said circular polarizer (121) comprising a second quarter wave retarder having a fast axis which is oriented 90 degrees relative to the fast axis of the first quarter wave retarder (113) and further comprising a linear polarizer diposed to transmit said linearly polarized imaging illumination; wherein ambient light passing through said circular polarizer (121) and being reflected back to said circular polarizer (121) is substantially blocked by said second quarter wave retarder and said linear polarizer. The display of claim 1, characterized in that the circularly polarized imaging illumination provided by said first quarter wave retarder (113) is of the same orientation as the circularly polarized diffuse illumination provided by said second quarter wave retarder. The display of any of claims 1 or 2, characterized in that said linearly polarized imaging illumination provided by said second quarter wave retarder is of the same polarization orientation as the linearly polarized imaging illumination provided by said display source (11;111). The display of any of claims 1 through 3, characterized in that said display source (111) comprises a liquid crystal device display source. The display of any of claims 1 through 4, characterized in that said optical combiner (123) comprises a reflection hologram.

Inventors: MCDONALD MARK, MCDONALD, MARK
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1990
Main classification: G02F1
IPC codes: G02F1, B60K35, G02B27