Abstract:
A system corrects differential focus arising from keystone errors in projection displays. An electrically variable optical element may correct the differential focus of the projected image. The focus of the image may be corrected so that opposed portions of the image, otherwise subject to keystone errors, are both correctly focused.

Description:
BACKGROUND 
     This invention relates generally to managing differential focusing of projection displays including displays that may display computer information. 
     Projection displays may project an enlarged image on a display screen to be more easily viewable, for example, by a number of users. Projection display screens may also be mounted in housings with an image projected on one side of a screen, being viewed on the other side of the screen. In some cases, projection displays may be utilized as the displays for computer systems. 
     Projection displays project onto a display surface by angling the projection display with respect to a horizontal plane. Correct optical projection of the image assumes that the display surface is perpendicular to the axis of projection. Typically, the projection axis is at an angle to the projection surface. As a result, the image that appears on the projection surface no longer has a rectangular aspect ratio but instead is keystone shaped. This is a form of differential focus because the image is not focused everywhere. The keystone shaped image and its differential focus are different manifestations of the same problem. The keystone shaped image is a geometrical projection of a rectangle onto a plane at a angle. 
     Differential focus arises from the fact that the image plane distance varies because of the angle of projection. Due to the larger f numbers used by projectors, the angle of projection causes a shallow depth of focus. 
     If one attempts to correct the focus on the bottom portion of the keystone, the focus at the top of the keystone becomes worse and vice versa. There is no simple way to manually adjust the focus to correct the keystone effect. 
     Thus, there is a continuing need for ways of correct differential focus and keystone errors in projection displays. 
     SUMMARY 
     In accordance with one aspect of the present invention, a method of projecting an image on a display surface includes projecting the image through an electrically variable optical element. The focal length of the optical element is adjusted to correct for differential focus. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of one embodiment of the present invention; 
     FIG. 2 is a schematic depiction of a pendulum style tilt indicator; 
     FIG. 3 is a schematic cross-sectional view of an electrically variable optical element; 
     FIG. 4 is a flow chart showing software for implementing one embodiment of the present invention; 
     FIG. 5 is a flow chart showing software for implementing another embodiment of the present invention; 
     FIG. 6 is a simplified depiction of a circular zone plate; 
     FIG. 7 is a block diagram showing one embodiment for implementing a computer which drives a projection display; 
     FIG. 8 is a block diagram of one embodiment of hardware for controlling a projection display; and 
     FIG. 9 is a layout showing the effect of the electrically variable optical element in correcting differential focus. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a projection display  10 , in one embodiment of the invention, may include a projection device  12  which projects a light beam A onto a screen  20 . As examples, the projection device  12  may use transmissive or reflective liquid crystal techniques or the projection device may be a Schlieren optical projector. The beam A may be reflected by the screen  20 , at least in part, back towards the projection device  12 . In one embodiment of the invention, the display  10  detects the returning, reflected light beam B using a sensor  22 . A system  24  uses information about the returning light to adjust the focus of the display or the system  25  controlling the display. The sensor  22  may include one or more digital video cameras in one embodiment. 
     In one embodiment of the present invention, the device  12  may include a light valve  14  which generates the output light of the projector. The light valve may include a liquid crystal display (LCD) such as a reflective or a transmissive LCD. That output light may pass unaffected through a beam splitter  16  to be projected onto the screen  20  using a projection lens  18 . Returning, reflected light, indicated as B in FIG. 1, passes back through the lens  18  and is reflected by the beam splitter  16  to be captured by the sensor  22 . The beam B may convey information about the focus of the image displayed on the screen  20 . 
     The light valve  14  may receive a video signal from a processor-based system  25  while the sensor  22  may be coupled to a processor-based system  24 . For example, the systems  24  and  25  may be coupled by a bus  41  which may be a Video Electronics Standards Association (VESA) interface. The VESA standard is further described in the Computer Display Timing Specification v.1, rev. 0.8, (available from VESA, San Jose, Calif. 95131 or on the Internet at www.vesa.org/standards.html). Alternatively, a digital interface may be used as described in the Digital Visual Interface (DVI) Specification, Revision 1.0, Apr. 2, 1999, prepared by the Digital Display Working Group (DDWG). 
     In one embodiment of the present invention, a tilt indicator  23  may be coupled to the sensor  22  and the system  24 . One embodiment of the tilt indicator  23 , shown in more detail in FIG. 2, may include a pendulum  94  with a small weight  92  on one end and an angular potentiometer  96  on the other end. When the projection device  12  is tilted, for example to project an image further up a display surface  20 , the tilt indicator  23  indicates the angle of tilt. In particular, the weight  92  keeps the pendulum  94  relatively aligned with gravitational force resulting in a pivoting action of the upper end of the pendulum  94  relative to the angular potentiometer  96 . The tilt indicator  23  produces a electrical signal indicative of the degree or angle of tilt of the projection device  12 . In this way, the system  24  may receive information about the orientation of the projection device  12  and thereby, by geometric transformation, the extent of keystoning which may result due to the angle between the axis of projection of the device  12 , indicated by the arrow A in FIG. 1, and the display surface  20 . 
     An electrically variable optical element  21  may be positioned along the projection axis such that the image projected along the axis A and the returning image along the axis B, pass through the element  21 . The element  21  electrically manages the differential focus arising from keystoning. That is, the element  21  is designed to produce the desired optical effect to correct the keystoning distortion. 
     Referring to FIG. 3, the element  21  may include a birefringent lens  98  which may be flat lens such as a Fresnel, a flat diffractive optic or a holographic plate. In any case, the lens  98  provides the maximum differential focus correction. Its effect may be moderated by varying the voltage on the electro-optic material  102 . 
     The electro-optic material  102  is confined between electrodes  100  which may be formed, for example, of indium tin oxide (ITO) or any other transparent conductive material. The optical properties of the electro-optic material  102  may be varied by a varying the potential applied from a potential source  104  to the electrodes  100 . A light transmissive top plate  106  completes the assembly. 
     The effect of the combination of the birefringent lens  98  and the electro-optic material  102  is to produce an aspheric optic which counteracts the keystoning effect to an electrically programmable degree. The greater the keystoning detected by the tilt indicator or from the returning light B, the greater the keystoning correction provided by the optical element  21 . 
     Electro-optic materials such as liquid crystal materials have an optical anisotropy that cause their index of refraction to vary according to the angle of their molecules. The angle of their molecules is a function of any alignment layers and the torque supplied by an externally applied voltage. This variable angle varies the index of refraction between the so-called ordinary and extraordinary indices, rotating the angle of the polarized light. The variation of this angle is in the plane to the display. 
     As the angle is varied, the effect is to smoothly vary the index of refraction between the maximum extremes of the ordinary and extraordinary indices. The birefringent lens produces a different focal length for each polarization of light incident upon it. The electro-optic material  102  may have the property that it may select one of the polarization states of light incident on it according to the applied electrical field. Consequently, a liquid crystal material may be used to select from among the focal lengths of the birefringent lens  98 . An electrically variable optical element is described for example in U.S. Pat. 5,712,721, assigned to Technology Partnership, PLC. The element  21  is “electrically variable” because its focal properties may be changed in response to an electric or magnetic field. 
     Referring next to FIG. 4, the software  30  for providing an autofocus feature in the embodiment shown in FIG. 1 may begin by receiving a first image to be displayed by the light valve  14  as indicated at block  32 . The first image is then projected onto a screen  20  using the projection device  12 , as indicated in block  34 . Information about the first image is received, in the form of a second image, by the sensor  22 , as indicated in block  36 . 
     The image received for projection in the light valve  14  from the system  25  and the image received by the sensor  22  and the system  24  are then compared as indicated in diamond  38 . If they are sufficiently similar in focus, as explained hereinafter, the flow ends. Otherwise, the focus is adjusted as indicated in block  40 . Conventional and differential focus corrections may be made automatically. 
     Referring next to FIG. 5, software  42  for implementing a more detailed embodiment of an autofocus feature for embodiments of the type shown in FIG. 1 begins by displaying a circular zone plate as indicated at block  44 . A zone plate includes a plurality of circles of decreasing intra-circle spacing as shown in a simplified depiction in FIG.  6 . 
     The image of the zone plate displayed by the projection system is then analyzed to locate a first line and a point along the line (block  45 ) of 50 percent scale gray value without edges, in one embodiment of the invention. This corresponds to the point where the adjacent circles merge together to form a 50 percent gray scale image without showing circle edges, as indicated in block  46 . 
     The focus is then adjusted in either of two directions, either towards or away from the projection screen, until the focus sharpens. In other words, the focus is adjusted in a first direction to determine if the focus improves and then in a second direction if the focus did not improve by the adjustments in the first direction, as indicated in block  48 . 
     Next, the projected image is compared with the image of what was projected as indicated in block  50 . If the point of the 50 percent gray scale is at the same radial distance, or substantially the same radial distance (the distance R 1  for example, on the zone plate shown in FIG. 6) in both images, the display is substantially in focus along the line R 1 . Otherwise, the focus is adjusted until the 50 percent gray scale points substantially match as indicated in block  51 . 
     After the focus has been completed along a first line R 1  for example, a second line R 2 , shown in FIG. 6, and a point along the second line may be chosen and the procedure may be repeated for the second line as indicated in FIG.  5 . The focal corrections along each line R 1  and R 2  are determined. Thus, if the lines R 1  and R 2  are on different sides of a central horizontal axis C shown in FIG. 6, the differential focus that they indicate is a measure of the differential focus arising from the keystone error due to tilting the projector up or down. 
     Once the two focal adjustments have been determined as indicated in diamond  52  in FIG. 5, the differential focus may be eliminated as indicated in block  53  using the electrically variable optical element  21 . Namely, a potential is applied to aspherically alter the light passing through the element to correct the keystoning effect. 
     Referring to FIG. 9, an image  20   a  projected on the projection surface  20  may be subject to keystone distortion as indicated on the left in FIG.  9 . As a result, there is differential focus about the axis C. Therefore, by obtaining a focal correction along the line R 1  and the line R 2 , the differential focus can be quantified. As a result of the correction applied through the electrical variable optical element  21 , a rectangular image  20   b  may be projected on the surface  20  as indicated on the right in FIG.  9 . 
     Referring to FIG. 7, in one embodiment, the system  24  may include a processor  54  coupled to an accelerated graphics port (AGP) chipset  56  for implementing an accelerated graphics port embodiment (see the Accelerated Graphics Port Interface Specification, version 1.0, published on Jul. 31, 1996, by Intel Corporation, Santa Clara, Calif.). The chipset  56  may communicate with the AGP port  58  and the graphics accelerator  60 . The system  25  may be coupled by the bus  41  to receive the output signal from the graphics accelerator  60 . The system  25  may use a display controller which provides appropriate corrections (such as color or gamma correction) for the given type of display. The display controller may also include an appropriate frame buffer and timing generator. The chipset  56  may also be coupled to the system memory  64  and to a bus  66 . 
     The bus  66  may be coupled to a bridge  68  which in turn is coupled to a hard disk drive  70 . The hard disk drive may store the software  30  or  42  in one embodiment of the present invention for execution by the processor  54 . 
     The bridge  68  may also couple another bus  72  which is in turn coupled to a serial input/output (SIO) device  74 . The device  74  may allow serial communication with the sensor  22 , the tilt indicator  23  and the electrically variable optical element  21 . Also coupled to the bus  72  is a basic input/output system (BIOS)  80 . 
     Turning next to FIG. 8, in one embodiment, the system  25  for controlling the light valve  14  may be coupled to a bus  41 , such as a VESA local bus. The bus  41  in turn is coupled to an analog to digital converter  84 . The converter  84  may receive information from a frame buffer  86  controlled by a timing generator  88 . A microcontroller  90  may store information for controlling the light valve  14 . For example, the microcontroller  90  may store information which provides gamma correction and color conversions for a particular display or light valve. 
     The system  24  may implement the differential focus corrections automatically in response to tilt information from the tilt indicator  23  or using the feedback technique illustrated in FIGS. 1 and 4. Alternatively, the power of the optical element  21  may be fine tuned by hand by the operator. As still another alternative, the focus may be adjusted by simply measuring the angle of tilt and manually inputting the appropriate correction to the system  24  and element  21 . 
     Normally, projectors are designed to project upwardly or downwardly. For example, table mounted projectors usually project upwardly to the display surface. Conversely, ceiling mounted projectors are designed to project downwardly to the display screen. Thus, the optical element  21  may correct for the keystoning effect arising from either upward or downward projection. That is, the optical element  21  need only correct for either the keystone with a wider bottom edge or wider top edge. This simplifies the design of the optical element. Of course, more than one optical element may be provided to enable the system to work on either upwardly or downwardly projected images. For example, a shutter mechanism may be provided to move one or the other of two electrically variable optical elements into the projection axis depending on whether the projector projects upwardly or downwardly. In addition, a pair of such elements may be provided in series along the projection axis. One of the elements may be electrically selected while the other is electrically deselected. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. For example, the angular deflection of the pendulum  94  may be detected by other angular detectors instead of the angular potentiometer  96 . As an additional example, an optical encoder wheel with periodic angular slots may be used. The passage of the slots may be optically counted to measure angular displacement. As another example, a mirror may be mounted on the pendulum. As the pendulum rotates, the amount of light produced by a fixed light source and detected by a fixed photodetector, reflected from the mirror varies with angular displacement. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the present invention.