Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2001-10-25
2004-06-08
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C349S005000, C349S008000, C353S034000
Reexamination Certificate
active
06747709
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal projector, and more particularly to an optical system of a liquid crystal projector that is capable of minimizing the size and the number of optical elements of an optical system using three reflective liquid crystal displays.
2. Description of the Related Art
Nowadays, there has been highlighted a flat panel display that is capable of replacing a cathode ray tube display having a limit in the size of screen and a large system size and realizing both a thin thickness and a large-scale screen. The flat panel display includes a projector that projects a small-field picture onto a large screen on an expanded scale.
The projector employs a cathode ray tube or a liquid crystal display (LCD) as a display device for implementing a small-field picture. Recently, there has been mainly used the LCD to keep up with a trend toward a thin thickness of the projector. Generally, a liquid crystal projector adopts a transmissive or reflective LCD. The liquid crystal projector has been developed with intent to obtain small bulk, low weight and high brightness, whereas an LCD panel has been developed with intent to obtain high aperture ratio and high resolution. Thus, there has been used the reflective LCD panel so as to keep up with a recent trend toward a liquid crystal projector realizing high resolution, small bulk and low cost.
The liquid crystal projector takes advantage of a light emitting from a light source to implement a picture on the LCD panel. The liquid crystal projector images a picture of the LCD panel onto a screen using a projective optical system to observe a picture imaged on the screen. When the projector is implemented by directly projecting a picture of the LCD panel onto a rear screen, a projection distance should be assured between the screen and the projective optical system. Accordingly, since the rear side of the screen requires a relatively large space to enlarge a thickness of the projector, it is difficult to obtain a thin thickness of the projector.
In order to solve this problem, a reflecting mirror is introduced between the screen and the projective optical system to change a light path, thereby reducing a thickness of the projector. However, since an arrangement angle of the reflecting mirror has to be more than a critical angle thereof so as to project a picture onto the rear screen without any distortion, there is a limit in reducing a thickness of the system. Also, there is a limit in reducing a thickness of the system due to an inherent full length of an optical system consisting of an illumination system, the LCD and the projective lens system of the projector.
Referring to
FIG. 1
, there is shown an optical system of a conventional liquid crystal projector. The liquid crystal projector includes first and second fly eye lenses (FEL's)
6
and
8
, a polarizing beam splitter (PBS) array
10
and a first condensing lens
12
arranged between a light source
4
and a full-reflecting mirror
14
, and a second condensing lens
16
arranged between the full-reflecting mirror
14
and a first Dichroic mirror
18
.
A white color light emitting from a lamp of the light source
4
is reflected by a parabolic mirror to be progressed toward the first FEL
6
. The first FEL
6
divides an incident light into the cell units and allows them to be focused onto each lens cell of the second FEL
8
. The second FEL
8
converts an incident light into a parallel light for a specific portion and transmits the same to the PBS array
10
. The PBS array
10
separates an incident light into linear polarized lights having any one light axis, that is, a P-polarized light and a S-polarized light. Then, a half wavelength plate (not shown) partially attached to the rear side of the PBS array
10
converts the transmitted P-polarized light into a S-polarized light. Thus, all incident lights are converted into S-polarized lights by means of the PBS array
10
, thereby permitting most lights emitting from the light source
4
to be incident to picture implementing elements
26
R,
26
G and
26
B of the LCD panel.
The first condensing lens
12
focuses an incident light from the PBS array
10
onto the full-reflecting mirror
14
. The full-reflecting mirror
14
makes a full reflection of an incident light from the first condensing lens
12
and allows the same to be progressed toward the second condensing lens
16
. The second condensing lens
16
focuses an incident light from the full-reflecting mirror
14
onto the first Dichroic mirror
18
. The first Dichroic mirror
18
transmits a light at a blue color area in the incident lights while reflecting lights at a green color area and at a red color area having a larger wavelength than a blue light.
Further, the optical system of the liquid crystal projector shown in
FIG. 1
includes a second Dichroic mirror
20
, a first polarizing film
22
R and a first polarizing beam splitter prism (PBSP)
24
R arranged between the first Dichroic mirror
18
and the red LCD panel
26
R, a second polarizing film
22
G and a second PBSP
24
G arranged between the second Dichroic mirror
20
and the green LCD panel
26
G, a first relay lens
27
, a second full-reflecting mirror
28
, a second relay lens
29
, a third polarizing film
22
B and a third PBSP
24
B arranged between the first Dichroic mirror
18
and the blue LCD panel
26
B, a Dichroic prism
30
arranged among the first to third PBSP's
24
R,
24
G and
24
B, and a projection lens
32
installed in opposition to an light output surface of the Dichroic prism
30
.
The second Dichroic mirror
20
reflects a light at the blue area in lights reflected from the first Dichroic mirror
18
and being incident thereto to progress it toward the second polarizing film
22
G while transmitting a light a the red area to progress it toward the first polarizing film
22
R. The second full-reflecting mirror
28
reflects a light at the blue area transmitted from the first Dichroic mirror
18
and being incident thereto to progress it toward the third polarizing film
22
B.
The first and second relay lenses
27
and
29
are field lenses, which relay an imaging point of a light at the blue area to re-image the blue light onto the blue LCD panel
26
B. Each of the first to third polarizing films
22
R,
22
G and
22
B transmits only a S-polarized light parallel to its optical axis in the incident lights and allows it to be progressed toward the first to third PBSP's
24
R,
24
G and
24
B, respectively.
The first to third PBSP's
24
R,
24
G and
24
B reflects red, green and blue S-polarized lights transmitted from the first to third polarizing films
22
R,
22
G and
22
B and being incident thereto and allows them to be progressed into the red, green and blue LCD panels
26
R,
26
G and
26
B, respectively. Further, the first to third PBSP's
24
R,
24
G and
24
B obtain picture information from the red, green and blue LCD panel
26
R,
26
G and
26
B to transmit red, green and blue lights converted into P-polarized lights, respectively and allows them to be progressed toward the Dichroic prism
30
.
Each of the red, green and blue LCD panels
26
R,
26
G and
26
B is a reflective LCD panel, which converts a S-polarized light reflected from each of the first to third PBSP
24
R,
24
G and
24
B and being incident thereto into a P-polarized light to thereby implement a picture.
The Dichroic prism
30
obtains picture information from the red, green and blue LCD panels
26
R,
26
G and
26
B to combine the incident red, green and blue lights and output the combined light to the projection lens
32
. First and second polarization converting films (not shown) for converting P-polarized lights from the first and third PBSP's
24
R and
24
B into S-polarized lights are arranged between the first and third PBSP's
24
R and
24
B and the Dichroic prism
30
, respectively. Accordingly, the Dichroic prism
30
reflects red and blue lights having a S-polarization component being inpu
Kwon Soon Hyung
Na Man Ho
Fleshner & Kim LLP
Kim Robert H.
LG Electronics Inc.
Schechter Andrew
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