Reflective projector

Details Reflective projector Reflective projector

1999-04-08

2002-11-12

Dowling, William (Department: 2851)

Optics: image projectors

Composite projected image

Multicolor picture

C353S020000, C349S008000

Reexamination Certificate

active

06478429

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective projector. More particularly, the present invention relates to a reflective projector which can display an image on a large screen by using a color separation/synthesis part, a reflective liquid crystal panel and the like.
Conventionally, as a projective liquid crystal projector for displaying an image on a large screen, a translucent-type liquid crystal panel having a TN liquid crystal panel is widely used.
In recent years, research on a reflective liquid crystal panel in which the aperture rate of a pixel is increased by providing a reflector electrode in each pixel is being conducted. In addition, the reflective liquid crystal panel is applied to the projective-type liquid crystal projector. The aperture rate of the reflective liquid crystal panel is higher than that of the conventional translucent-type liquid crystal panel. Therefore, a small and efficient projector can be realized.
2. Description of the Related Art
FIG. 1
is a block diagram showing an example of a conventional reflective projector.
As shown in the figure, the reflective projector
10
includes a light source
11
, a polarization separation prism (PBS)
12
, a dichroic prism
14
, reflective liquid crystal panels
16
R,
16
G, and
16
B, a projection lens
17
, and so on. Here, R ,G, and B indicate red, green and blue respectively.
In the configuration, the polarization separation prism
12
extracts linearly polarized light from the light flux which is emitted from the light source
11
. At the same time, the direction of the linearly polarized light is changed by 90° Then, the linearly polarized light enters the dichroic prism
14
.
The incident light beam to the dichroic prism
14
is separated into light beams of red, green, and blue (RGB), and the light beams exit from the dichroic prism
14
. After being reflected by respective reflective liquid crystal panels
16
R,
16
G, and
16
B corresponding to each color, the light beams enter the polarization separation prism
12
again through the same optical path.
At this time, among the light beams which are modulated in respective reflective liquid crystal panels
16
R,
16
G, and
16
B, light beams which are modulated in areas of activated liquid crystal exit after the direction of the polarization of the light beam is turned by 90°. Therefore, exiting light beams modulated in the area pass through the polarization separation prism
12
, and are projected on a screen (not shown in
FIG. 1
) from the projection lens
17
so as to form an image.
Since the polarization separation prism
12
which is an expensive optical device is used for the above-mentioned conventional reflective projector
10
, there is a problem that the cost of the reflective projector
10
will increase. Further, there is also a problem that it is difficult for the polarization separation prism
12
to separate polarization of the light beam from the light source
11
which spreads for example by ±2°.
Further, the conventional reflective projector
10
makes RGB light beams by separating white light from the light source
11
into three light beams. For this reason, there is a problem that color purity falls since the components of cyan (Cy) and yellow (Ye) which exist between G-B and between G-R respectively are mixed among the three light beams and an illumination for emitting a convergence light flux can not be used.
One measure against this problem is to provide a cyan (Cy) cut-off filter and a yellow (Ye) cut-off filter between the light source
11
and the dichroic prism
14
or between the dichroic prism
14
and the projection lens
17
(refer to
FIG. 1
) so as to cut unnecessary wavelengths.
However, the cut rates of the cyan cut-off filter and the yellow cut-off filter which are band-pass filters are not enough for cutting a 10-20 nm band which is the unnecessary wavelength region. Here, the cut rate is a reflection/absorption rate of any wavelength region. Therefore, light loss may occur by cutting a required wavelength region, or, since the unnecessary wavelengths are not fully cut, sufficient color purity cannot be assured. Therefore, assuring color purity without the loss of light has not been realized.
Further, since two optical elements which are the dichroic prism
14
and the polarization separation prism
12
are provided between the reflective liquid crystal panels
16
R,
16
G, and
16
B and the projection lens
17
in the conventional reflective projector
10
, there is a problem that the spacing between the reflective liquid crystal panels
16
R,
16
G, and
16
B and the projection lens
17
becomes long. The result is that the size of the projector becomes large.
Furthermore, since the optical path of the dichroic prism
14
is long, spreading of light occurs. Therefore, the polarization separation prism
12
, the dichroic prism
14
, and the projection lens
17
need to be made large-sized in order to cope with the spreading of light.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a reflective projector which is small, efficient, inexpensive, and has high color purity.
The above object of the present invention is achieved by a reflective projector including:
a light source;
a reflective liquid crystal panel;
a color separation/synthesis unit for separating a light beam from the light source into light beams of predetermined colors and/or for synthesizing the light beams of predetermined colors into a light beam;
a projection lens;
a first polarizer provided between the light source and the reflective liquid crystal panel; and
a second polarizer provided between the reflective liquid crystal panel and the projection lens.
According to the above-mentioned invention, a good contrast image can be obtained at low cost since an expensive polarization beam splitter which is conventionally used becomes unnecessary.


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