Liquid crystal cells – elements and systems – Liquid crystal optical element
Reexamination Certificate
1999-08-03
2003-12-09
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal optical element
C349S001000, C349S009000, C349S017000, C359S001000
Reexamination Certificate
active
06661495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an image forming apparatus, and more particularly to a pancake window display system employing one or more switchable holographic elements.
2. Description of the Relevant Art
Pancake window display systems (often times referred to as in-line infinity display systems) are capable of forming images at or closer than infinity of an object or a plurality of optically superimposed objects. Pancake window display systems find application in aircraft simulators, spacecraft simulators, or in head mounted displays for use in, for example, virtual imaging. Pancake window display systems, so called because of their minimal depth, represent a major achievement in terms of simultaneously maximizing field of view, eye relief, and image quality in a compact and lightweight configuration.
FIG. 18
shows one embodiment of a prior art pancake window display system
310
that includes a first linear polarizer
312
, a curved, spherical beam splitting mirror
314
, a first quarter wave plate
316
, a beam splitting mirror
318
, a second quarter wave plate
320
, and a second linear polarizer
322
. In operation, first linear polarizer
312
imposes linear polarization on light from a source (not shown in
FIG. 18
) passing therethrough. The direction of the polarization of first linear polarizer
312
is represented by vertical arrow
324
. Linearly polarized light encounters first quarter wave plate
316
after being transmitted through partially transmitting, spherical beam splitting mirror
314
. The first quarter wave plate imparts a circular polarization to light passing therethrough. Right curved arrow
326
shows that light emerging from first quarter wave plate
316
is right circularly polarized. This right circularly polarized light next encounters partially transmitting, partially reflecting beam splitting mirror
318
, a fraction of which passes therethough to encounter second quarter wave plate
320
. Second quarter wave plate
320
acts to change the write circularly polarized light transmitted through mirror
318
back to linearly polarized light having a direction of polarization oriented at 90° to the direction of polarization
324
of the first linear polarizer
312
. This is indicated in
FIG. 18
by horizontal arrow
330
. The linear polarized light emitted from second quarter wave plate
320
is blocked at second linear polarizer
322
which has a direction of polarization parallel to that of first linear polarizer
312
.
The fraction of right circularly polarized light from first quarter wave plate
316
which is reflected at beam splitting mirror
318
is converted by such reflection into circularly polarized light of the opposite rotation, i.e., into left circularly polarized light in the case assumed. This is indicated in
FIG. 18
by left curved arrow
332
. In its reflective passage back towards first linear polarizer
312
, this left circularly polarized light again encounters first quarter wave plate
316
which transforms the left circularly polarized light into linearly polarized light with a direction of polarization perpendicular with respect to the direction of polarization provided by first linear polarizer
312
as represented by horizontal arrow
334
. A portion of this linearly polarized light is reflected and collimated by spherical beam splitting mirror
314
without change in the orientation of its polarization direction. The light so reflected and collimated becomes left circularly polarized after passing through first quarter wave plate
316
as indicated by left curve arrow
336
. A fraction of this light is then transmitted through beam splitting mirror
318
and converted by second quarter wave plate
320
into linearly polarized light having a polarization direction parallel to the polarization direction of the first linear polarizer as indicated by arrow
340
. This light, accordingly, is permitted to pass through second linear polarizer
322
and constitutes only a fraction of the unpolarized light from the image source (not shown in
FIG. 18
) that is visible to an observer
342
.
The arrangement of the pancake window display system
310
shown in
FIG. 18
obviates the use of an oblique beam splitting mirror across the axis of the spherical mirror
314
so that the optical elements can be assembled into a compact package. With the exception of the curved mirror
314
, all the elements of pancake window display system
310
are in the form of flat sheets, thereby imparting a relatively thin cross section. However, the curved mirror by its nature cannot be reduced to flat sheet form.
FIG. 19
shows a second prior art pancake window display system
350
which includes all of the elements
312
-
322
of pancake window display system
310
shown in
FIG. 18
except for curved, spherical beam splitting mirror
314
. Pancake window display system
350
of
FIG. 19
employs a static, reflective type holographic analog
352
of a curved mirror in place of the conventional curved mirror
314
of FIG.
18
. Typically, such analog
352
is formed by superimposing a coherent monochromatic “reference” beam of light and a coherent “object” beam of light upon a transparent photo sensitive layer to form an interference pattern within the photosensitive layer. The layer is then photographically developed to produce the holographic analog.
As shown in
FIG. 19
, pancake window display system
350
operates in a manner substantially similar to the pancake window display system
310
shown in FIG.
18
. Advantageously, all elements of pancake window display
350
are reduced to a flat sheet form that reduces a longitudinal thickness and weight substantially. However, while pancake window display system
310
shown in
FIG. 18
provides a broad band, or colored image to viewer
342
, pancake window display system
350
shown in
FIG. 19
can provide only a narrow band or monochromatic image to viewer
342
as a result of employing the static, reflective type holographic analog
352
.
SUMMARY OF THE INVENTION
The present invention provides one or more dynamic, switchable holographic elements which can be used in a pancake window display system. Each of the one or more switchable holographic elements is configured to operate between active and inactive modes. In the active mode, the switchable holographic element substantially alters a substantial portion of light incident thereon. In one embodiment, the switchable holographic element operating in the active state, reflects and collimates a substantial portion of light incident thereon. In this embodiment, the switchable holographic element in the active state defines a holographic analog of a concave, spherical mirror. In the inactive state, the switchable holographic element transmits substantially all light incident thereon without substantial alteration. In one embodiment, the holographic analog of the concave, spherical mirror of the switchable holographic element is erased.
In another embodiment, the present invention provides three, dynamic, switchable holographic elements, each one of which operates between the active and inactive states in accordance with signals provided by a logic control circuit. Each of the three switchable holographic elements operating in the active state defines a holographic analog of a concave, spherical mirror which reflects and collimates a select bandwidth of light incident thereon. In the inactive state, each of the three switchable holographic elements erases its holographic analog of the concave spherical mirror so that substantially all light incident thereon is transmitted thereto substantially unaltered. In one embodiment when active, the first of the three holographic optical elements is configured to reflect and collimate narrow band red light, the second holographic optical element is configured to reflect and collimate narrow band blue light, and the third holographic optical elements is configured to reflect and collimate narrow band green light. In this embodiment, the cont
Campbell Stephenson Ascolese LLP
Chowdhury Tarifur R.
DigiLens Inc.
Nguyen Dung
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