Optics: image projectors – Distortion compensation
Reexamination Certificate
2000-12-22
2003-02-18
Dowling, William (Department: 2851)
Optics: image projectors
Distortion compensation
C353S070000
Reexamination Certificate
active
06520646
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for displaying an optically corrected image using a small throw ratio, off axis projection display system. In a is particular embodiment, the present invention relates to an off-axis integrated front projection system having a throw ratio less or equal to 1.0 that coordinates specialized projection optics and electronics optimized to work together to create a high-quality viewing image and correct lens induced optical distortion, keystone distortion, and anamorphic distortion.
Electronic display systems are devices capable of presenting electronically generated images. Whether for use in home-entertainment, advertising, videoconferencing, computing, data-conferencing or group presentations, the demand exists for an appropriate display device.
Image quality remains a very important factor in choosing a video display device. However, as the need increases for display devices offering a larger picture, factors such as cost and device size and weight become vital considerations. Larger display systems are preferable for group or interactive presentations. The size of the display system cabinet has proven an important factor, particularly for home or office use, where space to place a large housing or cabinet may not be available.
Currently, the most common video display device is the typical CRT monitor, usually recognized as a television screen. CRT devices are relatively inexpensive for applications requiring small to medium size images (e.g., 9″ to 27″, ~23 to 70 cms). (image size traditionally is measured along the diagonal dimension of a rectangular screen). However, as image size increases, the massive proportions and weight of large CRT monitors become cumbersome and severely restrict the use and placement of the monitors. Also, screen curvature issues appear as the screen size increases. Large CRT monitors consume a substantial amount of electrical power and produce significant electromagnetic radiation. Finally, the cost of very large CRT monitors may be prohibitive for many applications.
A new category of presentation systems includes so-called thin plasma displays. Much attention has been given to the ability of plasma displays to provide a relatively thin (about 75-100 mm) cabinet, which may be placed on a wall as a picture display in an integrated compact package. However, at the present time, plasma displays are costly and suffer from the disadvantages of low brightness (approx. 200-400 cd/m
2
range) and difficulty in making repairs. Plasma display panels are heavy (~80-100 lbs., 36-45 kg.), and walls on which they are placed may require structural strengthening.
A traditional type of video presentation device is the projection system, including both rear and front projection. In projection systems, one or more imagers creates an image that is projected using optical lenses. An imager generally is an electronically controlled array of pixels that can be turned on or off to create an image. Imagers, or light valves as they are sometimes called, may be reflective (an “on” pixel reflects incident light to form the image) or transmissive (an “on” pixel transmits incident light). Common imager types include liquid crystal display devices and digital micromirror devices.
Rear projection generally comprises a projection mechanism or engine contained within a large housing for projection to the rear of a transmissive screen. Back-projection screens are designed so that the projection mechanism and the viewer are on opposite sides of the screen. The screen has light transmitting properties to direct the transmitted image to the viewer.
A front-projection system is one that has the projection mechanism and the viewer on the same side of the screen. Front projection systems present many different optical and arrangement challenges not present in rear projection systems, as the image is reflected back to the audience, rather than transmitted. An example of a front projection system is a portable front projector and a front projection screen, for use in meeting room settings or in locations such as an airplane cabin.
Front projection systems have traditionally not been considered attractive for interactive applications because of factors such as blocking of the image by the projector or the presenter and image distortion.
Traditional electronic front projectors typically require a room that affords the projection volume necessary for image expansion without physical obstructions. Although images may be projected upon a large flat surface, such as a wall, better image quality is achieved by the use of a separate screen.
FIGS. 1 and 2
illustrate a traditional front projection system. A projector
10
is placed on a table or other elevated surface to project an image upon a screen or projection surface
20
.
Traditional integrated projectors require optical adjustment, such as focusing every time the projector is repositioned, as well as mechanical adjustment, such as raising of front support feet to position the image on the projection screen. Electronic connections, such as those to a laptop computer, generally are made directly to the projector, thus necessitating that the projector be readily accessible to the presenter or that the presenter runs the necessary wiring in advance.
To achieve a suitable image size, and also due to focus limitations, the projector
10
requires a certain “projection zone” and distance from the screen
20
. Table A lists the published specifications for some common electronic projectors currently in the market.
TABLE A
Smallest
Shortest
Maximum
Projector
Lens Focal
Imager
Screen
Throw
Throw
Keystone
Type
Length
Diagonal
Diagonal
Distance
Ratio
Correction
CTX Opto
*
163 mm
1.0 m
1.1 m
1.1
20° offset/
ExPro 580
Transmissive
optical
LCD
InFocus
*
18 mm
1.3 m
1.5 m
1.2
18° offset
LP425
Reflective
DMD
Chisholm
43-58.5 mm
23 mm
0.55 m
1.2 m
2.2
15° electronic
Dakota
Reflective
X800
LCD
Epson
55-72 mm
33.5 Transmis-
0.58 m
1.1 m
1.9
*
Powerlite
sive LCD
7300
Proxima
45-59 mm
23 mm
0.5 m
1.0 m
2.0
12° offset
Impression
Transmissive
A2
LCD
3M
167 mm
163 mm
1.0 m
1.2 m
1.2
16° offset/
MP8620
Transmissive
optical
LCD
* Not given in published specifications
Throw distance is defined as the distance from the projection lens to the projection screen measured along the optical axis of the projection lens. Throw ratio usually is defined as the ratio of throw distance to screen diagonal. Short throw distance is defined as at most one meter. To achieve a large image, between ~40 to ~60 inches (~1 to ~1.5 meters), a projector having a throw ratio of approximately 1.5 must be positioned at least 3.5 to 7.5 feet (approximately ~1.5 to ~2.25 meters) away from the wall or screen.
The existence of this “projection zone” in front of the screen prevents the viewer from interacting closely with the projected image. If the presenter, for example, wishes to approach the image, the presenter will block the projection and cast a shadow on the screen.
The projection zone may be reduced by moving the projector closer or off-axis from the screen. However, optical distortion effects significantly affect the quality of a projected image at short throw, small throw ratio, and offset angles. There are three distortion effects of particular concern in off-axis projection, especially in front projection systems: keystone geometric distortion, anamorphic geometric distortion, and projection lens imaging distortions, such as third order pincushion distortion. The effects of these distortion components are increased, the closer the projection lens is to the screen.
Those familiar with the use of electronic projectors will appreciate that placing the projector at an angle to the central normal axis of the screen (i.e., off-axis) produces a trapezoidal shape distortion of the image, known as a keystone effect. Keystoning is a geometric image distortion where the projection of a rectangular or square image results in a screen image that resembles a keystone or trapezoid, that is a quadrilateral h
DeLuca Patricia H.
Rodriguez, Jr. Ernesto M.
Saxe Steven G.
3M Innovative Properties Company
Dowling William
Florczak Yen Tong
Ho Nestor F.
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