Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2001-04-06
2003-12-30
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C348S336000, C348S338000, C353S031000, C353S081000
Reexamination Certificate
active
06671101
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color combining optical element and projection type image display apparatus using it and is suitably applicable, for example, to color liquid crystal projectors for projecting an enlarged image from projection original images based on color liquid crystal panels, onto a screen surface.
Related Background Art
A variety of proposals have been made heretofore for projection apparatus (liquid crystal projectors) constructed to project an enlarged image from projection original images based on liquid crystal light valves, onto the screen surface.
In the projection apparatus of this type, image display elements (liquid crystal panels) based on images of three colors of R, G, and B are illuminated by respective color light beams obtained by color separation of light from a light source. The respective color beams transmitted by the image display elements are guided through a color combining means to be projected through one projection lens onto the screen surface or the like.
Conventionally, a cross dichroic prism is known as one of color combining optical elements used in the liquid crystal projectors.
FIG. 5
is a schematic view to show the major part of a projection type image display apparatus using the conventional cross dichroic prism. The cross dichroic prism XDP used in the projection image display apparatus illustrated in
FIG. 5
is composed of four rectangular prisms
31
,
32
,
33
, and
34
, and dichroic layers DM
1
, DM
2
having two types of reflection wavelength regions cross inside the prism.
In the projection image display apparatus of
FIG. 5
, a parabolic mirror
2
converts white light emitted from light source
1
, into a nearly parallel beam, and a first fly-eye lens
3
consisting of an array of rectangular lenses forms light source images in almost center regions of respective lenses in a second fly-eye lens
4
consisting of an array of rectangular lenses. Then a polarization converting element
5
emits beams of only one polarization component and the beams of one polarization component are laid over image modulating means
16
,
18
,
20
by a first positive lens
6
. Blue light reflected by a blue reflecting dichroic mirror
8
is guided via a light reflecting mirror
9
and a second positive lens
15
to be converged on a display portion of image modulating means
16
for blue. The green component out of green and red components transmitted by the blue reflecting dichroic mirror
8
is reflected by a dichroic mirror
10
for reflecting green, and is guided through a third positive lens
17
to be converged on a display portion of image modulating means
18
for green. The light of the red component transmitted by the dichroic mirror
10
is guided via a fourth positive lens
27
, a high reflecting mirror
28
, a fifth positive lens
29
, a high reflecting mirror
30
, and a sixth positive lens
26
to be converged on a display portion of image modulating means
16
for red.
Light beams modulated by the respective color image modulating means
16
,
18
,
20
undergo color composition in the cross dichroic prism XDP composed of the first prism
31
, second prism
32
, third prism
33
, and fourth prism
34
, to be projected as a color image onto the screen not shown, by a projection lens
35
.
Unless the cross dichroic prism as a color combining prism illustrated in
FIG. 5
is made by accurately forming the angles of the four rectangular prisms and polishing each of the surfaces thereof, the dichroic layers DM
1
, DM
2
will be bent at the apexes of the rectangular prisms.
This caused the problem that the projected image appeared double on the unrepresented screen and thus resolution sensation became considerably poor. In order to keep good resolution on the screen, the four rectangular prisms
31
,
32
,
33
,
34
had to be joined without a level difference at their joint surfaces, and thus close attention had to be paid for the joining work. The right-angled ridge portions of the rectangular prisms had to be made without any such defects as kinks, chips, or the like, and if the width of the ridge portions was too wide there arose the problem that the cross part of the cross prism was projected as a vertical stripe on the screen. As described, the conventional cross dichroic prism was made by the extremely difficult prism processing and prism joining works and thus required considerable time and expenditure for manufacturing.
On the other hand, in order to avoid the above-stated problems of the cross dichroic prism, Japanese Patent Application Laid-Open No. 10-104763 describes the proposal of a liquid crystal projector using a color separation prism consisting of three prisms, which has been used as a color separation means in video cameras and the like. However, since the shape of the color separation prism in this application was not optimized so as to minimize the optical path length of the prism, it necessitated the prism path length close to double that of the cross dichroic prism of
FIG. 7A
, as illustrated in FIG.
8
A.
Since the specification of the above application describes nothing about the material and refractive index of the prism, it is not clear whether the air-reduced path length is reduced by increasing the refractive index of the prism.
In the proposal of the above application, as described above, the manufacturing of the prism itself was easier than that of the cross-dichroic prism, but, because of the large size of the prism and the long path length of the prism, the back focus of the projection lens had to be set considerably longer than that in the case of use of the cross dichroic prism. For this reason, the projection lens became large in size and, as to the optical performance of the projection lens, there arose the problem of increase in chromatic aberration of magnification in particular.
SUMMARY OF THE INVENTION
An object of the present invention is to propose a compact color combining optical element and a compact color separation optical element shorter in the optical path length and easier to make up than the prism of the above application and provide projection type image display apparatus using the color combining optical element and color separation optical element.
A color combining optical element according to one aspect of the present invention is a color combining optical element comprising three or more prisms, which has a prism comprising a common surface as a surface serving as a total reflection surface and as a transmissive surface on the extreme exit side and which has two dichroic mirror layers for reflecting light of mutually different wavelength regions, wherein the two dichroic mirror layers are located so as not to cross each other, an outline of a cross section of a prism placed between the two dichroic mirror layers, cut by a plane along a color composition direction, is comprised of four or more segments, and at least one of flection points in the cross section of surfaces without the dichroic mirror layers is present at an inner position of the color separation optical element with respect to a segment connecting two ends of segments in the cross section of surfaces with the two dichroic mirror layers.
A color combining optical element according to a further aspect of the invention is a color combining optical element comprising three or more prisms, which has a prism comprising a common surface as a surface serving as a total reflection surface and as a transmissive surface on the extreme exit side and which has two dichroic mirror layers for reflecting light of mutually different wavelength regions, wherein the two dichroic mirror layers are located so as not to cross each other, an outline of a cross section of a prism placed between the two dichroic mirror layers, cut by a plane along a color composition direction, is comprised of four or more segments, and at least one of interior angles in the outline of the cross section is an angle exceeding 180°.
A color combining optical element according to a stil
Canon Kabushiki Kaisha
Dang Hung Xuan
Martinez Joseph
Morgan & Finnegan L.L.P.
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