Color separating/synthesizing apparatus

Details Color separating/synthesizing apparatus Color separating/synthesizing apparatus

2001-05-15

2004-01-13

Miller, John (Department: 2614)

Television

Video display

Projection device

C348S756000, C348S757000, C349S009000, C353S020000, C359S490020

Reexamination Certificate

active

06678015

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection system, and more particularly to a color separating/synthesizing apparatus which uses three polarized beam splitters and one dichroic filter, as compared to a conventional system using four polarized beam splitters, thereby achieving a lightness in weight and a reduction in costs while achieving an improvement in performance.
2. Description of the Prior Art
In pace with the development of large-scale displays, the development of data projectors, projection TVs, and projection monitors, which use projection techniques, have been accelerated. Recently, research has been made in association with reflective liquid crystal panels including a reflective electrode arranged at each pixel to achieve an improvement of the aspect ratio of the pixel. Also, application of such reflective liquid crystal panels to projection type liquid crystal projectors have been made. Reflective liquid crystal panels can make it possible to realize miniature projectors having a high efficiency because they provide an improved aspect ratio, as compared to conventional transmission type liquid crystal panels.
The above mentioned projection system mainly includes an illumination unit, a color separating/synthesizing unit, and a projection unit. Where such a projection system uses a three-plate type reflective liquid crystal display (LCD), its color separating/synthesizing unit may be the most important element for an improvement in the contrast of the screen.
The color separating/synthesizing unit may include a Color Coner™, a Philips prism, a Color Quad TM, or an X-prism. These configurations for the color separating/synthesizing unit are illustrated in
FIGS. 1
to
4
, respectively.
FIG. 1
illustrates a conventional projection system using a Color Coner™ as its color synthesizing/synthesizing unit. Now, the operation of this projection system will be described in conjunction with FIG.
1
. Non-polarized white light emitted from a lamp
11
is splitted into P and S waves by a polarization means. The polarized light is then incident to a first color selecting retarder CS
1
. The first color selecting retarder CS
1
serves to polarize G (green)-color light beams into P waves while transmitting S-polarized light beams. A polarized beam splitter, which is denoted by the reference character PBS, is arranged downstream from the first color selecting retarder CS
1
. The polarized beam splitter PBS allows P-polarized G-color light components of the light incident thereto after being transmitted through the first color selecting polarization plate CS
1
to be transmitted therethrough while reflecting the remaining components of the incident light, thereby changing the travel direction of the remaining light components.
A dichroic filter DIC is arranged downstream from the polarized beam splitter PBS to separate B (blue) and R (red)-color components from the light reflected by the polarized beam splitter PBS. The P-polarized G-color light beams transmitted through the polarized beam splitter PBS, and the B and R-color light beams separated by the dichroic filter DIC are projected onto reflective LCDs
15
G,
15
B, and
15
R, respectively, and then reflected by those reflective LCDs
15
G,
15
B, and
15
R while containing images respectively corresponding thereto. Finally, the image-containing R, G, and B-color light beams are incident to a projection unit
16
.
Where a Color Coner™ is used as the color separating/synthesizing unit of the projection system, as mentioned above, there may be a reflection difference between the P and S waves due to the performance of the dichroic filter DIC, thereby resulting in a loss of light. Furthermore, a reduction in contrast occurs because one polarized beam splitter PBS and color selecting retarders or retarder stacks CS
1
and CS
2
, as polarization elements, are used.
FIG. 2
briefly illustrates a conventional projection system using a Philips prism as its color synthesizing/synthesizing unit. Now, the operation of this projection system will be described in conjunction with FIG.
2
. Non-polarized white light emitted from a lamp
11
is incident to a polarization means (not shown) which, in turn, extracts only S waves from the incident light. The S-polarized light is then reflected by a polarized beam splitter
13
, and then sequentially splitted into a plurality of desired color light components such as red, green, and blue-color light components. A prism assembly not denoted by any reference numeral is also provided to allow the three color light components to be incident to three reflective LCDs
15
R,
15
G, and
15
B, respectively.
The prism assembly includes three prisms spaced from one another by a desired angle while providing two color separation surfaces. A dichroic coating is formed on each color separation surface in order to achieve a desired color separation.
Where such a Philips prism is used as the color separating/synthesizing unit of the projection system, each element of the system should have a very sophisticated structure. As a result, there is a difficulty in manufacturing the projection system. This may result in an increase in the manufacturing cost. In similar to the case using the Color Coner™, there may be a reflection difference between the P and S waves at each color separation surface formed with a dichroic coating, thereby resulting in a loss of light. Furthermore, there is a considerable deviation depending on the performance of the dichroic coating. For this reason, it is difficult to apply this system to a projector using an LCD which is a polarization element.
FIG. 3
briefly illustrates a conventional projection system using an X-prism as its color synthesizing/synthesizing unit. Non-polarized white light emitted from a lamp
11
is directed to a polarization means (not shown) which, in turn, extracts polarized S waves from the light, thereby generating polarized illumination light beams. The polarized S waves are then incident to a polarized beam splitter
13
. The polarized beam splitter
13
completely reflects the incident polarized S waves in accordance with its characteristics. The reflected light is then directed to an X-prism
14
.
The light incident to the X-prism
14
is splitted into R, G, and B-color components which are, in turn, reflected by respective reflective liquid crystal panels
15
R,
15
G, and
15
B. Thereafter, the R, G, and B-color light beams are incident again to the polarized beam splitter
13
along the same optical path.
When a liquid crystal is at its ON state, the light beam modulated into an image by the region of the reflective liquid crystal panel
15
R,
15
G, or
15
B associated with the liquid crystal is emitted in a state in which its polarization direction is rotated by 90°. In other words, an incident S-polarized light beam is converted into a P-polarized light beam while being reflected. As a result, the light beam corresponding to the ON-state region is transmitted through the polarized beam splitter
13
, and then projected onto a screen through a projection lens
16
, thereby forming an image.
Although the color separating/synthesizing unit having the X-prism configuration illustrated in
FIG. 3
is likely to have a simple configuration capable of being advantageous to a miniature, as compared to other configurations as mentioned above, it cannot practically realize a desired performance of projection systems, using known techniques. Practically, desired functions of projection systems are obtained in so far as three polarized beam splitters and three dichroic filters are used, in addition to an X-prism having the configuration shown in FIG.
4
.
Thus, where such an X-prism is used as the color synthesizing/synthesizing unit of the projection system illustrated in
FIG. 4
, it is necessary to appropriately combine and arrange a plurality of constituting elements, thereby resulting in a complicated structure and an increased volume. As a result, there is a degradation in competitiveness.
FIG. 5

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