Optics: image projectors – Reflector
Reexamination Certificate
2001-11-30
2003-11-25
Dowling, William (Department: 2851)
Optics: image projectors
Reflector
C348S771000
Reexamination Certificate
active
06652105
ABSTRACT:
RELATED APPLICATIONS
Not Applicable
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
TECHNICAL FIELD
This invention relates to video and multimedia projectors and more particularly to a patterned-silvered mirror for propagating illumination toward a micro-electromechanical display device (“MDD”) and reflecting image bearing light rays emanating from the MDD toward a projection lens.
BACKGROUND OF THE INVENTION
Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. Such multimedia projection systems typically receive from a personal computer (“PC”) analog video signals representing still, partial-, or full-motion display images that are converted into digital video signals for controlling a digitally driven imageforming device, such as an MDD, a common type of which is a digital micromirror device. MDD-based projectors are popular because the MDD is a very efficient, albeit expensive, display device. Accordingly, MDDs are typically employed in single light path, frame sequential color projector configurations. An example of such a projector is the model LP130 manufactured by In Focus Corporation, Wilsonville, Oreg., the assignee of this application.
Significant effort has been invested into developing light-weight, portable multimedia projectors that produce bright, high-quality, color images. However, the weight, size, and optical performance of such projectors is often less than satisfactory. For example, suitable projected images having adequate brightness are difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
FIG. 1
, shows a typical prior art multimedia projector
30
in which a light source
32
propagates polychromatic light along an optical path
34
including a condenser lens
40
, a color wheel
42
, a light integrating tunnel
44
, a planar fold mirror
46
, a relay lens
48
, an MDD
50
, and a projection lens
52
. One or two field lenses (not shown) typically follow light integrating tunnel
44
. A display controller
56
receives color image data from a PC
58
and processes the image data into frame sequential red, green, and blue image data, sequential frames of which are conveyed to MDD
50
in proper synchronism with the rotating angular position of color wheel
42
. Display controller
56
controls MDD
50
such that light propagating from relay lens
48
is selectively reflected by MDD pixel mirrors either toward projection lens
52
or toward a light-absorbing surface
66
.
To achieve adequate projected image brightness and uniformity, light integrating tunnel
44
collects light exiting color wheel
42
and homogenizes the light during propagation through tunnel
44
to an output aperture
72
. The uniformly bright rectangular light bundle exiting output aperture
72
propagates through the field lenses, reflects off fold mirror
46
, and is imaged by relay lens
48
onto MDD
50
.
Other workers have tried making simpler single path MDD-based projectors. For example, U.S. Pat. No. 6,129,437 for IMAGE DISPLAY APPARATUS describes a similar MDD-based projector in which a concave mirror combines the functions of planar fold mirror
46
and relay lens
48
to simplify the optical path of the projector. While this is an improvement over other optical path configurations, employing mirrors in an MDD light path is not without its problems.
FIG. 2
reveals a source of such problems. An MDD
76
includes an array of micromirrors that each pivot about a hinge axis
78
that, in this embodiment, is parallel to an edge margin of MDD
76
. MDD
76
receives an incident light bundle
80
and reflects a reflected light bundle
82
, the centers of which are separated by an angle
84
corresponding to the mirror tilt angle range of MDD
76
. To achieve the maximum possible projected brightness, incident light bundle
80
and reflected light bundle
82
should each have a low f/#, which results in bundles
80
and
82
almost touching at their closest points. This means that the optical components separating incident light bundle
80
from reflected light bundle
82
must have a sharp cutoff to prevent unwanted spillover of incident light into the reflected light bundle. Such light spillover causes a reduction in contrast ratio of the projected display.
There are several reasons why mirrors are disadvantageous for separating incident light bundles from reflected light bundles in an MDD-based projector. The mirror edge margins are often carefully shaped and positioned to reflect one light bundle while not blocking the other light bundle. The edge shaping is often a curved contour shaped to accommodate the light bundle shapes, projection lens barrel, and folded light paths typically found in compact projectors. Planar mirrors are typically shaped by a “scribe and break” process, which is unreliable for curved breaks. Concave mirrors may also be aspherical, and are shaped by expensive grinding and polishing processes. Both planar and concave mirrors typically have an extra edge margin to accommodate the larger tolerance of the manufacturing processes. Clearly, these manufacturing and adjustment processes work against providing a sharp cutoff between the incident and reflected light bundles.
What is still needed, therefore, is a means of simplifying the optical path of a light-weight, portable projector without reducing the projected image contrast ratio.
SUMMARY OF THE INVENTION
An object of this invention is, therefore, to provide an apparatus and a method for improving the compactness, brightness, and contrast ratio of a MDD-based multimedia projector.
Another object of this invention is to provide improved mirrors for use in MDD-based multimedia projectors.
A first preferred embodiment of this invention provides a multimedia projector including a light source for propagating intense illumination through a color modulator. Light exiting the color modulator enters an input aperture of a light integrating tunnel. The light propagates by multiple internal reflection through the light integrating tunnel and exits through an output aperture. Field lenses image light from the output aperture through a transparent portion of a patterned-silvered mirror, through an optional field lens, and onto the micromirror array in an MDD. Micromirrors in the MDD that are tilted to an image-forming angle, reflect the image-forming light back toward a reflective portion of the patterned-silvered mirror, which reflects the image-forming light through a projection lens. In this embodiment, a linear boundary separates the transparent and reflective portions of the patterned-silvered mirror. The reflective portion is formed by depositing a metallic or dielectric coating, which is accurately positioned by masking and formed by conventional processes. This eliminates the need for either an edge-grinding process or a scribe-and-break process and results in a less costly mirror having a very sharp, well controlled boundary. The sharp cutoff prevents or reduces unwanted spillover of an incident light bundle into a reflected light bundle.
A second preferred embodiment of this provides a multimedia projector including a light source for propagating intense illumination through a color modulator and light integrating tunnel as before, but is configured for off-axis illumination of an MDD having an array of diagonally hinged micromirrors. Field lenses image the light from the light integrating tunnel by reflection off a reflective portion another patterned-silvered mirror, through the optional field lens, and onto the MDD. Micromirrors in the MDD that are tilted to an image-forming angle, reflect the image-forming light back through the optional field lens toward a transparent portion of the patterned-silvered mirror, which propagates the image-forming light through the projection lens. In this embodiment, a non
Gohman Jeffrey A.
Peterson Mark
Dowling William
InFocus Corporation
Schwabe Williamson & Wyatt, PC
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