Optical element suitable for projection display apparatus

Optics: image projectors – Methods

Reexamination Certificate

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C353S020000, C359S487030, C359S485050, C349S009000

Reexamination Certificate

active

06523962

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical element suitable for a projection display apparatus, and a method for fabricating the optical element.
2. Description of the Related Art
Japanese Patent Laid-open Gazette No. 7-294906 discloses an optical element, called polarization converting element, for use in converting light having random polarization directions to light having one polarization direction. Such an optical element is shown in plan view in FIG.
1
(A) and in perspective view in FIG.
1
(B). This optical element comprises a polarization beam splitter array
22
comprising alternately adhered linear polarization beam splitters
30
having polarization splitting films
36
and linear prisms
40
having reflecting films
46
. Portions of the exit surface of the polarization beam splitter array
22
are selectively provided with &lgr;/2 optical phase plates
24
.
The linear polarization beam splitter
30
includes two rectangular prisms
32
,
34
and the polarization splitting film
36
formed at the slant plane constituted by the interface between the rectangular prisms
32
,
34
. During fabrication of the polarization beam splitter
30
, the polarization splitting film
36
is formed on the slant plane of one of the rectangular prisms and the two rectangular prisms
32
,
34
are then bonded with an optical adhesive.
The linear prism
40
includes two rectangular prisms
42
,
44
and the reflecting film
46
formed at the slant plane at the interface between rectangular prisms
42
,
44
. During fabrication of the prism
40
, the reflecting film
46
is formed on the slant plane of one of the rectangular prisms, and the two rectangular prisms
42
,
44
are then bonded with an optical adhesive. The reflecting film
46
is formed of an aluminum or other metal film.
Multiple linear polarization beam splitters
30
and linear prisms
40
prepared in this manner are adhered alternately with an optical adhesive to fabricate the polarization beam splitter array
22
. The &lgr;/2 optical phase plates
24
are then selectively bonded to the exit surface of the linear polarization beam splitter
30
.
Light including an S polarized light component and a P polarized light component enters from the incident surface. The incident light is first separated into S polarized light and P polarized light by the polarization splitting film
36
. The S polarized light is reflected at substantially a right angle by the polarization splitting film
36
, is further reflected at a right angle by the reflecting film
46
, and exits the prism
40
. The P polarized light passes straight through the polarization splitting film
36
, is converted to S polarized light by the &lgr;/2 optical phase plate
24
, and exits therefrom. As a result, a light beam having random polarization directions entering this optical element emerges entirely as an S polarized light beam.
The conventional optical element shown in FIGS.
1
(A) and
1
(B) has four rectangular prisms
32
,
34
,
42
,
44
adhered by optical adhesive. Between entering and exiting the optical element, the S polarized light and P polarized light must therefore pass repeatedly through the optical adhesive layers formed at the prism interfaces. Since the optical adhesive absorbs some of the light, the intensity of the light decreases with each passage through an optical adhesive layer. This results in a considerable decline in light utilization efficiency.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to enhance the light utilization efficiency of the optical element.
Another object of the present invention is to provide the optical element which is easy to fabricate.
In order to attain at least part of the above and other objects, the present invention provides an optical element comprising a plurality of first transparent members and a plurality of second transparent members, which are alternately arranged with and secured to each other. Each of the plurality of first transparent members has a first incident surface and a first exit surface substantially parallel to each other, first and second film forming surfaces substantially parallel to each other and making a prescribed angle with the first incident surface and the first exit surface. A polarization splitting film is formed on the first film forming surface, and a reflecting film is formed on the second film forming surface. Each of the plurality of second transparent members has a second incident surface and a second exit surface parallel to each other. The plurality of second transparent members are alternately arranged with and secured to the plurality of first transparent members at the first and second film forming surfaces across the polarization-splitting film and the reflecting film respectively so that the second incident surfaces are aligned with the first incident surfaces to form an incident plane and that the second exit surfaces are aligned with the first exit surfaces to form an exit plane.
In the above optical element, after the light enters through the incident surface of the first transparent member, the polarized light component thereof reflected by the polarizing-splitting film is reflected by the reflecting film without passing through a layer of optical adhesive and then exits from the optical element. The light utilization efficiency is improved because the number of times this polarized light component passes through layers of the optical adhesive can therefore be reduced.
In a preferred embodiment, the reflecting film has a dielectric multi-layer film. A reflecting film formed of a multi-layer dielectric films enables the reflectance for a specific linearly polarized light component to be increased over that in the case of a reflecting film formed of an aluminum or other metal film. A further increase in the light utilization efficiency can therefore be attained.
In the embodiment, the optical element further comprises polarization direction converting means associated with either of the first exit surface and the second exit surface. Linearly polarized light components of different polarization direction exit from the exit surface portion of the first transparent member and the exit surface portion of the second transparent member. Thus, by providing a polarization direction converting means on one of the exit surface portions, the light beam exiting from the optical element can be entirely converted to one linearly polarized light component.
The optical element may further comprise light shielding means associated with the second incident surface. If light enters from the second incident surface of the second transparent member, this light will, after reflection by the reflecting film, pass through optical adhesive layers repeatedly before being converted into S polarized light and P polarized light by the polarization splitting film. If this kind of light is shut out by providing light shielding means with respect to the second incident surface of the second transparent member, repeated passage of the light entering the optical element through optical adhesive layers can be prevented.
The optical element further comprises adhesive layers between the first and second transparent members, and at least one of a thickness of the adhesive layers and thicknesses of the first and second transparent members are adjusted to make intervals between the polarization splitting films and the reflecting films substantially constant throughout the optical element. Since this makes the intervals between the polarization-splitting films and the reflecting films equal, the positional accuracy of the films in the optical element can be improved to increase the light utilization efficiency.
Preferably, the thickness of the second transparent members is set smaller than the thickness of the first transparent member. More preferably, the thickness of the second transparent member is in the range of 80% to 90% of the thickness of the first transparent member. For example, the thickness of the f

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