Patent Publication Number: US-2007115397-A1

Title: Projection display with internal calibration bezel for video

Description:
FIELD OF THE INVENTIONS  
      The inventions described below relate to the field of video projection and more specifically to automated calibration of video projection systems.  
     BACKGROUND OF THE INVENTION  
      It is advantageous in large screen rear projection monitors to provide a camera that acts as a sensor in a feedback control loop. This camera can watch the images displayed by the image projection system in order to notice defects in the projected images. These defects may include but are not limited to distorted images, images that are not correctly centered (linearly or rotationally) on the display screen, chromatic aberrations, or the like.  
      Shipping and handling a large screen rear projection monitor may cause optical components such as projection lenses and fold mirrors to move out of alignment, resulting in the previously mentioned problems. Any of these problems is undesirable to the consumer watching the large screen rear projection monitor.  
      The angle of reflection of the projected image from the back of the projector screen does not provide the best surface for gauging the quality of the projected image. The shallowness of the light rays&#39; angles leads to image related problems. The electronics that monitor the images taken by the feedback control camera have difficulty recognizing portions of pictures at the outside edge of the camera&#39;s field of view. This is due to the way the camera&#39;s lens collects light and projects it onto a camera sensor, such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Light rays from features at the center of the display screen to the camera have very steep angles (with respect to the display screen), and the angle between light rays pointing from the boundaries of such features (for example, a one inch square) will be comparatively large. If, however, these same features are located at the very edge of the display screen, then the light rays from the features to the camera will be very shallow (again, with respect to the screen), and the angle between light rays emanating from the boundaries of a feature (for example, a one inch square) will be extremely small. The magnitude of the angle between the boundary light rays is proportional to the number of camera sensor pixels that the feature is projected onto. The more pixels devoted to a feature in an image, the easier it is for electronics or software to recognize that feature. Thus, due to the geometry of the camera/lens/display screen setup, features at the center of the screen take up comparatively more pixels versus features of the same linear size at the very edge of the screen. This makes it very difficult for electronics to see the edge of the screen and consequently images projected on the screen.  
      It is important during both an initial camera calibration stage and in subsequent use to locate the edge of the screen within the images taken by the feedback control camera.  
      What is needed is a technique for automatically detecting and correcting misalignments, aberrations or other imperfections in the projected image from a surface that provides better reflectivity.  
     SUMMARY  
      A video projection monitoring and calibration technique discussed below includes a camera or other image sensor capable of watching images displayed on the internal side of the image screen of a rear projection video monitor. The image projection system can monitor how projected images look and can adjust the way the images are projected in order to correct detected problems.  
      For aesthetic and calibration related reasons the camera should be located within the large screen rear projection monitor, looking at the surface of the display screen where images are projected. This surface is opposite the surface that viewers watch. Unfortunately, locating the camera within the large screen rear projection monitor cabinet forces the camera to sit very close to the display screen, especially in the case of thin cabinet rear projection monitors. This means that the light rays reaching the camera from the furthest edges of the display screen are close to parallel to the screen. The camera therefore needs an extremely wide-angle lens, or a fish-eye lens, to see the entire area of the display screen. For example, suppose a feedback control camera is located six inches directly behind the center of a fifty inch (diagonal), 16:9 aspect ratio display screen. A light ray pointing from a corner of the screen to the camera will have a roughly 13.5 degree angle with respect to the screen.  
      During the initial camera calibration stage a mapping function is created that maps the location of pixels within the camera&#39;s images to the physical locations on the display screen. Finding the edge of the screen quickly reveals the geometry of the camera relative to the display screen. This mapping function is then stored in the electronics within the large screen rear projection monitor. When used, the electronics can use images collected by the feedback control camera and, combined with the mapping function, determine the location of a projected image on the display screen and thereby establish if it has shifted or warped out of position. Again, being able to quickly locate the edge of the display screen facilitates this diagnostic process.  
      A video projection system may include a transmissive display screen having a front side and a rear side, a projector having one or more optical elements forming an image path to the rear side of the display screen, a calibration bezel in the image path, means for collecting calibration data having a view of a portion of the display screen or a portion of the calibration bezel, or both, and an image processing means using collected calibration data to adjust image data for projection by the projector.  
      The calibration bezel forms a visual fiducial or reference at the edge of a display screen in a large screen rear projection monitor. The calibration bezel may include one or more elements to form a complete or partial border around the display screen and the bezel elements may be oriented at an angle relative to the display screen. Thus, a comparatively larger angle exists between light rays reflected from the bezel to the camera than in a conventional setup that has a surface planar with respect to the display screen. This makes it easier for electronics or software to locate the edge of the display in images captured by the camera, facilitating a more efficient camera setup process and an improved picture alignment process. The calibration bezel further allows electronics to quickly locate and see images at the perimeter that defines the edge of a large screen rear projection monitor screen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side view block diagram of a rear projection video display including a calibration system.  
       FIG. 2  is a cross section view of the calibration system of  FIG. 1 .  
       FIG. 3  is a front view of a video projection screen and bezels.  
       FIG. 4  is a cross section view of the calibration system of  FIG. 1  illustrating the geometry of a calibration bezel.  
       FIG. 5  is a cross section view of the calibration system of  FIG. 1  illustrating the geometry of relative to the display screen.  
       FIG. 6  is a rear view of a video projection screen and alternate configuration of a calibration bezel.  
       FIG. 7  illustrates a portion of an image captured by a camera in the back of a rear projection monitor.  
       FIG. 8  illustrates a portion of an image captured by a camera in the back of a rear projection monitor.  
    
    
     DETAILED DESCRIPTION OF THE INVENTIONS  
      Referring now to  FIG. 1 , rear projection display device  10  includes projector  12 , projection optics  14 , control system  16 , screen  18 , cabinet  20 , camera  22 , and front bezel  30  and a calibration bezel  31 . Rear projection display device  10  may be of a thin type, meaning that the depth of cabinet  20 , represented by distance  11 , may be less than fourteen inches. Rear projection display device  10  may be used, for example, as a television, a home cinema, or for any other suitable application. Projector  12  is mounted below horizontal centerline  41  of screen  18  and projects upwards, off-axis, into viewing area  37  of screen  18 . It is set up to electronically receive signals  16 ′ from control system  16 . Projector  12  may be a single microdisplay projector for use in a rear projection imaging system and thus may use a transmissive liquid crystal display (LCD) imager, a digital micromirror devices (DMD) imager, or a liquid crystal on silicon (LCOS) imager, or any other suitable technology.  
      The microdisplay imager in projector  12  may be an HD microdisplay, meaning that the display contains electronically controlled pixels arrayed in 1280 columns by 720 rows. The operation of an HD microdisplay projector is familiar to those of skill in the art. Projector  12  may be a multiple microdisplay projector with resolution greater or less than HD without departing from the spirit of the invention. Note that the distances and relative size of objects in  FIG. 1-6  are not to scale.  
      A projected image radiates from projector  12  through projection optics  14 . Although projection optics  14  are shown schematically in  FIG. 1  as a single lens, projection optics  14  may include multiple lenses, mirrors, and other optical devices. Projection optics  14  projects an image from projector  12  on to rear surface  19  of screen  18 . Screen  18  is transmissive so that the image projected on to surface  19  may be clearly be seen by viewers looking at front surface  21 . Screen  18  may be, for example, a fresnel lens and diffuser or any other suitable diffusive screen material.  
      Control system  16  is responsible for receiving input video images  17  from any suitable video input device  15 , re-sampling the images to convert them to a pixel based images, and turning the corresponding microdisplay pixels on and off in order to display the images. Control system  16  are also responsible for performing the picture alignment process aided by camera  22 . Control system  16  may include non-volatile memory, a microprocessor, integrated circuits, and the like. Similarly, control system  16  may be implemented in hardware, software, firmware or any other suitable combination.  
      Camera  22  is configured to electronically share information with control system  16 . Camera  22  is a low resolution digital camera, such as those manufactured by Micron. Those of skill in the art will recognize that it is possible to replace camera  22  with an image sensor or any other suitable device without departing from the spirit of the invention. Camera  22  is located inside cabinet  20  and is oriented to view surface  19  of screen  18 . Although camera  22  is located directly behind the center of screen  18  as illustrated in  FIG. 1  and is oriented substantially perpendicular to the screen, any other suitable positions and angles of camera  22  may be used. Camera  22  includes lens  23 . Lens  23  may be a fisheye lens, a wide angle lens, or the like, that enables camera  22  to see the entirety of viewing area  37  of screen  18  as well as calibration bezel  31 . Camera  22  may be a VGA digital camera and lens  23  may be a fish-eye lens.  
      For example, if screen  18  is a fifty inch display screen (measured along the diagonal) that measures 43.8 inches along the screen&#39;s horizontal and 24.7 inches along the screen&#39;s vertical, camera  22  may be located a distance (distance  13 ) from the screen; this distance may be, for example, 6.4 inches in a cabinet of 14 inches total depth. These measurements are provided merely for illustrative purposes.  
      Referring now to  FIG. 2 , calibration bezel  31  is attached or otherwise secured to screen  18  for the purpose of aiding camera calibration and picture alignment with respect to screen  18 . Calibration bezel  31  is located inside cabinet  20 , on the same side of screen  18  as surface  19  and may be secured on the inside of screen  18  outside of viewing area  37  and is positioned at an angle β to screen  18 . Angle β may be any suitable angle. In a currently preferred configuration, angle β is between 30 and 90 degrees from surface  19  of screen  18 . Surface  31 ′ of calibration bezel  31  is viewable by camera  22 . The vertical and horizontal members of the calibration bezel  31  may be disposed at a substantial angle from the plane of the screen  18 , and may be substantially perpendicular to screen  18 . The bezel may be constructed from thin rectangular strips of material, such as aluminum plate, and may protrude from screen  18  by approximately an inch. Surface  31 ′ is preferably non-reflective light gray or dark in color. Calibration bezel  31  may form a continuous perimeter around viewing area  37  as illustrated or it may form a discontinuous border around the viewing area. A calibration bezel also need not occupy each edge of viewing screen  37 .  
      The calibration bezel  31  need not be rigidly attached to screen  18  but may instead be attached to a frame securing screen  18  in cabinet  20 . In this particular configuration, calibration bezel  31  need not physically touch screen  18 .  
      As illustrated schematically in  FIGS. 2 and 4 , surface  31 ′ is not flat, but rather is curved. Surface  31 ′ may be generally concave and is designed to diffusively reflect each light ray  40  to lens  23  at the point where the light ray contacts surface  31 . This improves the reflectivity of surface  31 ′, making it more noticeable in images captured by camera  22 . Surface  31 ′ on each element of a calibration bezel may therefore occupy a portion of a spheroid, and the resulting calibration bezel may form a distorted barrel shape.  
      Referring now to  FIG. 6 , surface  31 ′ may contain fiducials  39  or any other suitable reference marks. The reference marks serve to provide identifiable, known positions that can be easily located within an image captured by camera  22 . Video calibration references such as fiducials  39  may be located in predetermined locations; for example, the fiducials may be one inch from corner  35  and one half inch from surface  19  of screen  18 . Fiducials  39  may be painted on surface  31 ′ or, more preferably, molded into the surface of calibration bezel  31 . When molded in to surface  31 ′, a dark spot may be created by making a shadow with the three-dimensionally profiled fiducial. Although two fiducials are shown in  FIG. 6  at corner  35 , more or fewer fiducials may be used and need not be located at the corners of a calibration bezel such as calibration bezel  31 . They may be located at any designated location on surface  31 ′. Any suitable shape or configuration of calibration references such as fiducial  39  may be used.  
      A calibration bezel such as bezel  31  may operate as a visual fiducial in images captured by camera  22 . This aids the control system, whether it is system  16  or other suitable outside electronics connected to the camera, in locating viewing area  37 . Calibration bezel  31  essentially “frames” viewing area  37  in images captured by camera  22  without being visible from front  21 . Knowing the location of viewing area  37  within an image captured by camera  22  allows many tasks to be performed, including but not limited to (1) locating where a projected image falls on screen  18 , and thereby determining if a projected image is centered on screen  18 ; (2) calibrating camera  22  by mapping captured image pixels to specific locations on screen  18  or calibration bezel  31 , and (3) establishing if a portion of an image projected on screen  18  is distorted, discolored or otherwise in need of correction.  
      For a feature in an image captured by camera  22  to be identifiable, there must be a substantial difference in the angles subtended by the light rays extending from the feature&#39;s borders to lens  23 . This large angle corresponds to the feature taking up more pixels on image sensor  22 ′ in camera  22 . The light rays emanating from calibration bezel  31  have a large angle between them, and thus the calibration bezel  31  provides a noticeable boundary around viewing area  37  in images captured by camera  22 . This contrasts with a barely visible boundary around the viewing area in the case where only a planar surface extends beyond the screen.  
      Referring now to  FIG. 4 , in a detailed view of the geometry of  FIG. 2 , a reference surface of calibration bezel  31  may be oriented at an angle β relative to screen  18 .  FIG. 5  is a detailed view of the geometry of  FIG. 2  but with calibration bezel  31  removed. Point  63  is located on the vertical edge of viewing area  37 , and point  64  is located a distance (distance  65 ) along horizontal centerline  41  from point  63  on surface  19 . Point  60  is located in the center of lens  23 . Distance  67  represents half the horizontal length of viewing area  37 . Light ray  62  extends from point  63  to point  60 , and light ray  66  extends from point  64  to point  60 . Angle αa is the angle between light ray  62  and screen  18 , and angle αb is the angle between light ray  66  and screen  18 . The difference between angle αa and angle αb can be found from the following formula: 
 
 αa−αb =tan−1(distance  13 /distance  67 )−tan−1(distance  13 /(distance  67 +distance  65 ))
 
      If, for example, in  FIG. 5  distance  13  equals 6.4 inches, distance  67  equals 21.9 inches, and distance  65  equals 1 inch, then αa and angle αb equal 16.3° and 15.6°, respectively, and the difference between the two angles is only 0.7°.  
      Referring now to  FIG. 5 , the relative positions of points  60 ,  61 , and  63 , as well as distances  13  and  67  are illustrated with reference to display screen  18 . Point  68  is located on the end of calibration bezel  31 . Calibration bezel  31  extends a distance (distance  69 ) out from screen  18 . Light ray  70  originates at point  68  and ends at point  60 , and light ray  62 , as before, extends from point  63  to point  60 . Angle αa is the angle between light ray  62  and screen  18 , and angle αc is the angle between light ray  70  and screen  18 . The difference between angle αa and angle αc can be found from the following formula: 
 
 αa−αc =tan−1(distance  13 /distance  67 )−tan−1(distance  13 −distance  69 )/(distance  67 )
 
      If, for example, in  FIG. 4  distance  69  equals one inch and all the other distances are the same as before, then αa and angle αc equal 16.3° and 13.9°, respectively, and the difference between the two angles is 2.4°. The angular difference between light rays  62  and  70  versus light rays  62  and  66  is over three times bigger; thus, a one inch wide calibration bezel such as bezel  31  will be much more visible in an image captured by camera  22  if it is oriented at some angle such as perpendicular to screen  18  versus parallel to screen  18 .  
      The geometry of the calibration bezel also reflects light rays towards camera  22 , greatly increasing how noticeable the calibration bezel is in images. With reference to  FIGS. 1, 4 , and  5 , it is easy to appreciate how a light ray from projector  12  that reflects off point  63  is more likely to reflect to lens  23  if calibration bezel  31  is in place. Otherwise, the light ray will reflect off the point, away from lens  23 .  
      Referring now to  FIGS. 7 and 8 , the effects a calibration bezel has on captured image quality are illustrated. Both image  100  in  FIG. 7  and image  102  in  FIG. 8  are portions of images captured by camera  22 . The upper left hand corner of screen  18 , with respect to the camera&#39;s viewpoint, is visible in each figure. Calibration bezel  31  was present on screen  18  when image  100  was captured and was not present on screen  18  when image  102  was captured. The calibration bezel in image  100  is painted a flat or non-reflective gray color and is perpendicular to screen  18 . For the portion of the picture visible in  FIG. 8  the calibration bezel forms a continuous boundary around screen  18 .  FIGS. 7 and 8  demonstrate the utility of providing a camera calibration bezel in a rear projection monitor equipped with an alignment camera. The edge of screen  18  is clearly visible in image  100 ; there is a distinct border in image  100  where the screen visibly ends. This is not the case in image  102  where it is difficult to tell where the border of screen  18  is. The fuzzy, indistinct border in image  102  would make it difficult for electronics to find the defining edges of screen  18 .  
      Thus, while the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.