Light diffusion control by electrically reconfigurable...

Details Light diffusion control by electrically reconfigurable... Light diffusion control by electrically reconfigurable...

1999-07-09

2001-02-20

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

C359S022000, C359S023000, C359S024000, C359S462000, C349S201000, C349S202000, C353S007000

Reexamination Certificate

active

06191876

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to projection systems and more particularly to a projection screen that utilizes a holographic optical element.
DESCRIPTION OF THE RELATED ART
Projection systems operate to display an image or a sequence of images by projecting the image(s) onto a screen. These systems tend to be designed for multiple viewers and usually require the projection screen to have a large viewing area to achieve large fields of view at comfortable viewing distances. The projected images vary in size, depending on the particular projection system. For example, projection systems that are used in theaters are able to display projected images that are extremely large, while projection systems that are used in offices for presentations are typically limited to much smaller projected images.
A significant disadvantage of the projection systems over other display systems is that the quality of the displayed image on a typical projection screen deteriorates as a viewer deviates further away from an axis normal to the center of the screen. This disadvantage can be illustrated by describing the image displaying operation of a standard projection system
10
, shown in FIG.
1
. The standard projection system includes a projector
12
and a projection screen
14
. The projector
12
contains an image generator
16
and projection optics
18
. The image generator operates to generate and project an input image onto the screen. The projection optics includes lenses (not shown) that magnify and/or focus the input image that is being projected to the screen. The projection screen is designed to display the projected input image by diffusing the input image in order for an observer
20
to view the displayed image. The light intensity of the displayed image viewed by the observer is largely dependent on the position of the observer from the center axis of the screen.
In
FIG. 1
, three light rays
22
,
24
and
26
defining beam directions of the projected input image are shown to propagate from the projection optics
18
to the screen
14
. The path of the light ray
24
is the center axis of the screen. When the light rays
22
-
26
travel through the screen, the light beams are diffused by the screen. The light rays
22
-
26
emanate from the screen at points
28
,
30
and
32
, respectively. The diffusion patterns of the light rays from these points are schematically illustrated by polar diagrams
34
,
36
and
38
. For each polar diagram, the light intensity from the emanating point on the screen depends on the viewing location of the observer
20
. The highest intensity of light is in the original direction of the light rays
22
-
26
, as represented by arrows
40
,
42
and
44
. The intensity of light decreases as the observer moves further away from the direction of the highest light intensity.
From the location of the observer
20
, the light intensities from the points
28
-
32
on the screen
14
vary in accordance with the polar diagrams
34
-
38
. The light intensities from the points
28
-
32
, as viewed by the observer
20
, are illustrated by arrows
46
,
48
and
50
in the polar diagrams. The lengths of these arrows represent the light intensity from the various points on the screen, in which greater lengths equate to higher light intensity. As shown in
FIG. 1
, the light intensity from the point
32
toward the observer is low, as illustrated by the short length of the arrow
50
. Therefore, the displayed image will appear dim at certain regions on the screen, such as the point
32
.
One way to increase the light intensities from different points on the screen
14
, as viewed from the location of the observer
20
, is to modify the diffusing characteristic of the screen by including a holographic optical element (HOE) in the screen. An HOE is a hologram having recorded interference fringes to optically manipulate propagating light. Depending on the recording, the HOE is able to perform optical functions that are associated with traditional optical elements, such as lenses and prisms, as well as more sophisticated optical operations.
Turning to
FIG. 2
, the projection system
10
has been modified by replacing the screen
14
with an HOE
52
that functions as a projection screen. The HOE is holographically configured to cause the light beams
22
,
24
and
26
to be diffused in a different diffusion pattern than the screen
14
of
FIG. 1
, as illustrated by polar diagrams
54
,
56
and
58
. As a result, the light intensities from the points
28
-
32
on the HOE
52
to the observer
20
are much higher that the light intensities from the same points on the screen
14
, as indicated by arrows
60
,
62
and
64
. Therefore, the displayed image on the HOE will appear to be uniformly brighter to the observer
20
than the displayed image on the screen
14
.
A concern with the projection system
10
of
FIG. 2
is that at other viewing locations, the displayed image will again appear dim at certain regions on the HOE
52
. In addition, the optical operation of the HOE
52
would need to function over the full visual bandwidth to display a color image, which would cause chromatic aberrations due to the intrinsically monochromatic nature of the holograms.
There are a number of U.S. patents that disclose holographic projection screens designed to correct chromatic aberrations. U.S. Pat. No. 5,046,793 to Hockley et al. describes a screen that is holographically configured to correct chromatic aberrations within predefined “eyeboxes.” The holographic screen of Hockley et al. is fabricated by recording multiple holographic exposures for each eyebox. The recording creates interference fringes that correspond to the multiple exposures. These interference fringes collectively mix appropriate amounts of different color lights to color balance the resulting displayed image on the screen. A concern with the holographic screen of Hockley et al. is that the multiple exposure procedure results in separate holograms having a reduced diffraction efficiency. U.S. Pat. No. 5,796,499 to Wenyon describes a screen that is also holographically configured to correct chromatic aberrations. The holographic projection screen of Wenyon is designed to produce particular angular intensity distributions to chromatically balance diffused light for light beams having wavelengths of 400 nm to 700 nm and impinging at an incident angle of 0° to ±30° relative to normal incidence.
Although the conventional projection screens operate well for their intended purposes, what is needed is a projection screen having a controllable diffusing characteristic that can produce a color-balanced and intensity-consistent displayed image toward one or more optimal viewing regions.
SUMMARY OF THE INVENTION
A system and a method of displaying projected images on a projection screen of the system utilize one or more reconfigurable holographic optical elements (HOEs) in the screen to optically manipulate the projected images impinging on the screen. The reconfigurable HOEs may be designed to perform simple optical functions that are commonly associated with traditional optical devices, such as those performed by lenses, prisms and mirrors. However, the reconfigurable HOEs are also designed to perform sophisticated optical manipulations, such as varying angular intensity of diffused light toward predefined viewing regions. Depending on the optical characteristics of the reconfigurable HOEs, the projection screen of the system may be designed for “reflective viewing,” i.e., viewing from the side of the projection screen facing a projector, or for “transmissive viewing,” i.e., viewing from the side of the screen opposite to the projector.
Each reconfigurable HOE includes a hologram that is sandwiched between two electrode layers. The hologram is a holographic photopolymeric film that has been combined with liquid crystal. The presence of the liquid crystal allows the hologram to exhibit optical characteristics that are dependent on an applied electrical field. Preferably, the hologram is a Bra

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