Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2000-04-05
2002-11-19
Mahoney, Christopher (Department: 2851)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S453000, C359S460000
Reexamination Certificate
active
06483612
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to projection systems and projection screens and, more particularly, to a projection screen apparatus that provides improved image illumination uniformity.
2. Description of Related Art
Light projection is used to display images on large surfaces, such as large computer displays or television screens. In front projection systems, an image beam is projected from an image source onto the front side of a reflection-type, angle transforming screen, which then reflects the light toward a viewer positioned in front of the screen. In a rear projection system, the image beam is projected onto the rear side of a transmission-type, angle transforming screen and transmitted toward a viewer located in front of the screen.
Referring to
FIG. 1
, wide angle projection systems that include a screen apparatus
10
are known to optimally use a conventional Fresnel lens
11
in combination with some diffusing element, such as a substrate covered with glass beads (e.g., a type of diffuser or diffusive screen)
12
. The combination forms an imaging screen that produces an image. The Fresnel lens
11
and the diffuser
12
are held in relatively rigid or semi-rigid spaced apart relation to assure proper operation of the combination. Such screens, known generally in the art as “black matrix bead” or “BMB” screens, are commercially available from Minnesota Mining & Manufacturing Company and others. Fresnel lenses are sold by Fresnel Optics and are manufactured by Minnesota Mining & Manufacturing Company, for example, as used in devices such as overhead projectors. The Fresnel lens
11
element is constructed to provide the optical properties of a much thicker lens. however, with smaller size and weight. Concentric steps or discontinuities
11
A allow these optical and physical properties to be realized. Each of the steps has a curved profile, in cross-section, that exhibits optical power to redirect incident light
13
. The cut-out sections that define the steps reduce the overall size and weight.
In
FIG. 1
, the Fresnel lens
11
receives the incoming light
13
from a projection image engine or image projector
14
(e.g., a liquid crystal display imager, a light source, and a projection lens that produce image light in response to input video or other signals). The break in the light path of the light
13
shown in
FIG. 1
is included to recognize that the light
13
may be processed or filtered, for example, projected by the projection or other lens (not shown), and is generally indicated by numeral
13
A. The screen apparatus
10
and the image engine
14
are arranged such that a light beam exiting the Fresnel lens
11
is collimated, as shown by parallel rays of light
15
. The collimated rays
15
pass across an air gap
16
to a matrix of glass beads
17
-
21
in the diffuser
12
. The glass beads
17
-
21
are mounted upon an adhesive black mask layer
22
that is on a front surface
23
of a substrate
24
of the diffuser
12
. As the collimated light rays
15
strike any of the glass beads
17
-
21
, the rays are focused as light
25
in FIG.
1
. The substrate
24
is light transparent so that a viewer
27
can see an image from the light
25
that passes through a surface
26
(e.g., an acrylic, polystyrene, other polymer or like surface) of the screen apparatus
10
. The screen apparatus
10
can be an “intelligent” television screen, having a large diagonal dimension, for example, substantially 60 inches, or a computer monitor screen.
For wide angle projection, there are currently no satisfactory methods of collimating light at a display screen. The conventional Fresnel lens
11
may create objectionable shadows and ghosts (i.e., ghost images from light scattered in undesired directions) that degrade the display image. The discontinuities in the Fresnel lens
11
lead to shadows and ghosts being introduced on illumination of the screen apparatus
10
. The formation of a ghost image from discontinuous surfaces of a Fresnel lens is schematically illustrated in FIG.
2
and its appearance on the viewing side of a Fresnel lens/diffuser combination screen is schematically illustrated in FIG.
3
. For discussion on the disadvantages of such stepped lenses, see
Antenna Theory
by Constantine A. Balanis, Harper and Row, New York, 1982, p. 650 and
Antenna Engineering Handbook
, H. Jasik (ed.) (Chapter 14 by S. B. Cohn), McGraw-Hill, New York, 1961, pp. 14-1 to 14-43. Moreover, the Fresnel lens
11
is also disadvantageous because it may be relatively expensive, easily damaged, have visible rings, and cannot be laminated (e.g., index matched) on both sides. Removing the Fresnel lens
11
and relying only on the diffusive screen
12
itself to achieve uniformity may result in a “hot spot” in the center of the screen and wasted light diffused out of the field of view of the viewer
27
. The air gap
16
between the Fresnel lens
11
and the glass beads
17
-
21
also prevents the screen apparatus
10
from being as compact or as mechanically stable as might otherwise be possible.
The present invention is directed to overcoming or substantially limiting some or all of the above shortcomings of the Fresnel lens/diffuser combination screens, and the occurrence of the hot spot when no Fresnel lens is used.
SUMMARY OF THE INVENTION
In one aspect, the invention features a screen apparatus. The screen apparatus includes a holographic optical element adapted to receive image light and to redirect the image light. The screen apparatus also includes a diffuser adapted to receive the redirected image light from the holographic optical element and to scatter the redirected image light.
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Duke University
Fleshner & Kim LLP
Mahoney Christopher
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