Projection display

Details Projection display Projection display

2001-03-30

2003-07-22

Dowling, William (Department: 2851)

Optics: image projectors

Miscellaneous

C353S098000, C359S015000, C359S016000

Reexamination Certificate

active

06595648

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a projection display. Such a display may be in the form of a compact convertible projector for displaying enlarged images from conventional direct view spatial light modulators, such as liquid crystal devices, for multiple viewers. Displays of this type have uses in portable office equipment, desktop office equipment, television equipment and display presentations.
BACKGROUND ART
U.S. Pat. No. 5,629,806 discloses a display system for providing private viewing and for displaying a relatively large image from a small direct view image source. The system comprises an image source such as a cathode-ray tube, electro-luminescent display or direct view liquid crystal display (LCD), focusing, conjugating and folding optics. The conjugating optics include a retro-reflector and a beamsplitter.
U.S. Pat. No. 5,418,584 discloses a retro-reflective array projection screen for displaying virtual images. The screen comprises a large collimating element and an array of small retro-reflective elements for reflecting the projected image back to an external exit pupil. The retro-reflective elements are of the diffractive type with a rear mirror for reflecting light back through the diffractive element. This system is also provided for private viewing.
FIG. 1
illustrates a known type of overhead projector of the reflection type for images fixed on transparencies. A projection light source including a condensing optic
1
illuminates a transparency
2
which is placed on a reflective Fresnel lens
3
. The axis of the lens
3
is laterally spaced from the axis of the condensing optic
1
so that the lens
3
images the illuminator at a projection lens
4
, which is laterally spaced from the condensing optic
1
. A folding mirror
5
directs light onto a projection screen (not shown).
U.S. Pat. No. 5,132,823 discloses a multipurpose LCD suitable for use as a reflective display and overhead projection panel. The LCD comprises a liquid crystal layer which is PIXELLATEd with the picture elements (pixels) being switchable between high and low scattering states. The LCD is disposed on top of a removable corner cube retro-reflector. The retro-reflector is used to improve the image contrast.
U.S. Pat. No. 5,353,075 discloses an arrangement which is convertible between direct view operation and overhead projection operation. For direct view operation, an LCD is disposed over a backlight. For projection operation, the LCD is used in place of a transparency in a conventional projection display.
U.S. Pat. No. 5,668,695 discloses a portable computer whose main body and lid are linked together and which may be used with a conventional overhead projector.
U.S. Pat. No. 5,593,221 discloses the use of an LCD as a projection transparency for a conventional type of overhead projector.
Engberg S. J., “Holographic techniques change shape of retro-reflectors”, Euro Photonics, December/January, 1998, p 37-38 and Hardin R. W., “Diffraction brings street signs to life”, Photonics Spectra, December, 1997, p 40 disclose broadband diffractive retro-reflectors in the form of diffractive Fresnel zone plates for creating light sources on the front surface of a retro-reflector with textured back reflective surface to produce semi-Lambertian scatter to improve the acceptance angle.
U.S. Pat. No. 5,515,354 discloses the use of a diffractive retro-reflector in an optical pick up in the form of a blazed diffraction grating with a reflective film on its rear surface.
U.S. Pat. No. 5,801,793 discloses an LCD which has a removable backlight so that the LCD may be used in direct view and projection modes.
Gallagher T., “Standard registration mark-please”, Holography News, vol. 11, no. 5, p. 4, 1997 discloses the use of a holographic retro-reflection registration mark for use in precise positioning of embossed holograms in the printing industry. In particular, an embossed transparent plastic hologram having a rear metal reflecting layer is disclosed.
Ralli P. J. and Wenyon M. M., “Imagix(™) holographic diffusers for reflective liquid crystal displays”, SID
96
disclose the use of a holographic reflector made of a photopolymer with an STN backlit display device to allow such a device to be used with ambient overhead illumination.
FIG. 2
a
illustrates such an arrangement in the direct view mode whereas
FIG. 2
b
illustrates such a system in the reflection mode.
As shown in
FIG. 2
a,
a transmission mode LCD
10
is disposed above a holographic reflector
11
, which is disposed above a backlight
12
. In the backlight mode, the backlight
12
is illuminated and directs light
16
through the holographic reflector
11
, which has no visible effect, and the LCD
10
towards an observer whose eye is shown at
14
.
In the reflection mode illustrated in
FIG. 2
b,
the LCD
10
is illuminated from a suitable light source to provide the overhead illumination
13
within a predetermined acceptable angle of the holographic reflector
11
. Provided the illumination occurs within the acceptance angle; the holographic reflector
11
acts as a reflector and directs diffracted light
18
back through the LCD
10
towards the eye
14
of the observer. The holographic reflector
11
may also work in association with a diffuse rear metallic reflector
17
.
FIG. 3
illustrates a conventional projection display using a transmission mode LCD
10
. An illuminator comprises a light emitter in the form of a lamp
20
and collecting optics shown as a parabolic mirror
21
. The resulting collimated light beam is supplied to an homogeniser comprising a first homogeniser lens array
22
and a second homogeniser lens array
23
. Light from the homogeniser is passed to an array of polarisation beam splitter cubes and half-waveplate strips
24
, a first condensing optic
1
and a second condensing optic
25
.
Light from the light source illuminates the LCD
10
and is modulated by the displayed image. The output light is supplied to a projection lens
4
which projects an enlarged image onto a front-projection or back-projection screen (not shown). The collecting optics
21
illuminate the first homogeniser lens array
22
with collimated light from the lamp
20
. The array
22
produces an image of the light source formed by the lamp
20
and the collecting optics
21
at each of the lenses of the second homogeniser lens array
23
. The lens array
23
and the first condensing optic
1
produce an image of each of the lenses of the array
22
at the plane of the LCD
10
. The polarisation recirculation cubes in conjunction with the array of half-waveplates
24
convert the polarisation so that all of the light supplied to the LCD
10
is of the same linear polarisation. Images of the array
22
at the plane of the LCD
10
are overlaid by means of the first condensing optic
1
. The second condensing optic
25
forms an image of the light source at the entrance pupil of the projection lens
4
, which images the LCD
10
at the screen.
FIG. 4
illustrates a known type of projection display using telecentric imaging to avoid the need for a field lens such as
25
. The display shown in
FIG. 4
uses a reflection-mode LCD
10
provided with a rear metallic plane reflector internal to the liquid crystal layer and has a folded optical path provided by a turning beam splitter
26
which may be a polarising beam splitter.
The illuminator shown in
FIG. 4
is of the same type as shown in FIG.
3
. Light from the condensing optic
1
is reflected by the beam splitter
26
onto the LCD. Light passes through the LCD in accordance with the modulation by the displayed image and is reflected back through the LCD
10
by the rear metallic reflector. However, the output light from the LCD has a greater spread than systems based on field lenses and, after passing through the beam splitter
26
, requires that the projection lens
4
have a greater input aperture size than the size of the LCD.
U.S. Pat. No. 5,663,816 discloses an arrangement which is similar to that shown in
FIGS. 2
a
and
2
b
of the accompanying drawings.
WO 95/

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