Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
2002-02-05
2003-08-05
Adams, Russell (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S020000, C353S033000, C353S034000, C353S037000, C353S081000, C353S082000, C353S122000, C349S005000, C349S007000, C349S008000, C349S009000
Reexamination Certificate
active
06601957
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection-type image displaying device to be used in a liquid crystal projector.
2. Related Background Art
Conventionally, a cross dichroic prism XDP has been generally used as a color combining optical element to be used in a projection-type image displaying device such as a liquid crystal projector. As shown in
FIG. 15
, the cross dichroic prism XDP is composed of four rectangular prisms
67
,
68
,
69
and
70
and dichroic layers DM
1
and DM
2
that have two kinds of reflection wave ranges different from each other and cross each other in the inside of the cross dichroic prism XDP.
Moreover, in case of the projection-type image displaying device using the cross dichroic prism XDP shown in
FIG. 15
, the incidence side of its projection lens is designed to be telecentric in order to prevent the generation of color shading owing to the cross dichroic prism XDP.
Furthermore, polarizing plates
11
,
18
and
28
and polarizing plates
15
,
22
and
32
are disposed on the incidence sides and the exit sides of liquid crystal panels (or liquid crystal display elements)
13
,
20
and
30
, respectively. And positive lenses
60
,
61
and
66
are disposed on the light source sides of the liquid crystal panels
13
,
20
and
30
, respectively, and then the illumination light is made to be telecentric to the liquid crystal panels
13
,
20
and
30
, in order to prevent the generation of contrast unevenness of the liquid crystal panels
13
,
20
and
30
caused by the changes of the incidence angles of illumination light to the liquid crystal panels
13
,
20
and
30
.
Moreover, for the improvement of contrast in such a projection-type image displaying device, Japanese Patent Application Laid-Open No. 5-323311, No. 6-148628 and No. 7-5421 propose configurations in which phase plates are disposed between polarizing plates disposed on both of the incident sides and the exit sides of liquid crystal display elements.
Moreover, Japanese Patent Application Laid-Open No. 7-13150 and No. 7-13152 propose configurations for improving the view angle characteristic of contrast in direct vision type image displaying devices using liquid crystal panels.
However, in the conventional projection-type image displaying device shown in
FIG. 15
, the dichroic layers DM
1
and DM
2
are bent at apical angles of the rectangular prisms
67
to
70
when the angles of the four rectangular prisms
67
to
70
are not correctly polished. Consequently, the conventional projection-type image displaying device has a problem such that an image projected on a screen (not shown) becomes a double image, which makes a feeling of resolution thereof remarkably worse. Moreover, when the four rectangular prisms
67
to
70
are joined, the joining requires scrupulous care lest differences in level should be occurred in order to keep the resolving power thereof on the screen good.
Furthermore, defects such as kinks, chips and the like are not allowed on the ridges (edge lines) of the rectangular prisms
67
to
70
. When the width of the ridge portion is wide, a problem is also produced in which crossed portions between crossing prisms are projected on a screen as vertical stripes.
As described above, the conventional cross dichroic prism XDP is extremely hard to work a prism and to perform joining of prisms as a prism, and therefore it is difficult to reduce the manufacturing cost thereof.
On the other hand, for resolving such problems of the conventional cross dichroic prism XDP, Japanese Patent Application Laid-Open No. 10-104763 proposes a color separation prism applied to a liquid crystal projector. The color separation prism shown in
FIG. 1
of the reference is composed of three prisms that have been used in a video camera or the like.
However, the shape of the color separation prism is not optimized to make the optical path length of the prism minimum. The color separation prism consequently requires an optical path length nearly twice as long as that of the cross dichroic prism.
That is, the manufacturing of the proposed color separation prism itself becomes easier in comparison with the manufacturing of the cross dichroic prism XDP because the dichroic films of the color separation prism do not cross each other in the inside of the prism. However, because the optical path length of the color separation prism is long, it is needed to set the back focal distance of the projection lens in the case of the use of the color separation prism considerably longer than that in the case of the use of the cross dichroic prism.
Consequently, the size of the projection lens in the case of the use of the color separation prism becomes large, which increases the manufacturing cost of the projection lens. Moreover, as for the performance of the projection lens especially, there is also a problem such that the chromatic aberration of magnification of the projection lens increases as the increase of the back focal distance.
Furthermore, Japanese Patent Application Laid-Open No. 5-323311 proposes a configuration in which phase difference correcting plates for correcting optical anisotropies of liquid crystal panels are disposed between incidence side polarizing plates and the liquid crystal panels for improving the contrast of the liquid crystal display elements.
In a normally white mode liquid crystal display element composed of a liquid crystal cell displaying white when the maximum voltage is applied thereto and polarizing plates disposed on the incidence side and the exit side of the liquid crystal cell, the incidence angle of illumination light at which the maximum contrast can be obtained is inclined by about three degrees from an angle perpendicular to the surface of the liquid crystal cell. Then, the main object of the configuration proposed in Japanese Patent Application Laid-Open No. 5-323311 is to compensate the phase shift produced in the liquid crystal cell by changing the phase differences of phase plates in accordance with positions.
The proposed configuration can set the liquid crystal cell to have the maximum contrast with respect to the principal ray of the illumination light that illuminates the liquid crystal cell. However, since a color image is obtained by superimposing projection images of three colors by the use of three liquid crystal panels and three projection lenses for combining three colors, there is raised a problem that the entire apparatus becomes large. Moreover, because there is only one point on the optical axis of the projection lens where the color images are completely superimposed on one another the configuration can be applied only to a rear projector or a fixed type front projector.
For resolving the constructive problems of the aforesaid configuration proposed in Japanese Patent Application Laid-Open No. 5-323311, Japanese Patent Application Laid-Open No. 6-148628 proposes to realize the improvement of contrast and the miniaturization of its size in a projection-type image displaying device which performs color combining by means of two dichroic mirrors, and in which a phase difference plate is disposed between the liquid crystal cell of a liquid crystal display element and the polarizing plate on the exit side of the liquid crystal display element so as to improve the contrast and to realize the miniaturization.
In the foregoing projection-type image displaying device, however, since the color combining is effected by the two dichroic mirrors, the back focal distance of the projection lens is needed to be twice or more as long as the length of the diagonal line of the effective display surface of the liquid crystal display element, which makes difficult to design the projection lens. Furthermore, the projection-type image displaying device has the problem of the difficulty of the complete superimposition of the projection images of three colors owing to the astigmatism caused by the dichroic mirrors and irregular deflections of their surfaces.
Moreover, the projection-type image displayin
Adams Russell
Koval Melissa J.
Morgan & Finnegan , LLP
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