Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
1997-11-18
2001-05-08
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S834000
Reexamination Certificate
active
06229651
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns an optical element that comprises a base body and an optically effective layer system of which at least one layer system surface contacts the base body.
The invention also comprises a related manufacturing process, the utilization of such a component or process, as well as an optical projection arrangement.
INTRODUCTION
For example, in DE-40 33 842 a cuboid optical element composed of dichroitic layers is referred to as a “dichroitic prism”.
In the present application the term
X-cube
is used.
The present invention starts with the problems that exist with known X-cubes, as described, for example, in DE-40 33 842, or that occur in its manufacture. The present invention which was developed in order to find a solution to the problems with such elements, can be applied to a number of other optical elements.
For this reason this description begins with the specific problems to be solved on X-cubes and based thereon explains the application of the invention in more general terms.
DESCRIPTION
Based on
FIG. 1
the basic functional principle of an X-cube is explained. Optical elements of this type are principally used in projectors in order to recombine the red/green and blue channels in the spectral range of the visible light. As shown in
FIG. 1
such an X-cube
1
comprises four individual prisms
2
a
to
2
d
which can, for example, be made of BK7 glass. In their cross-section they form right-angled isoceles triangles with an angle of 90°, usually with a tolerance of over ±60 angular seconds, and hypotenuse angles of 45° with tolerances of a few angular minutes. The length of the hypotenuse is typically between 5 mm and 50 mm, preferably 40 mm. Embedded between the two prism pairs
2
a
and
2
b
on the one side,
2
d
and
2
c
on the other side, there is an optically effective layer
5
that largely reflects visible light in the blue range but largely transmits visible light in the green or red range.
FIG. 1
shows a part of the blue reflector layer system as a color splitting system, labeled
5
′, the other
5
″.
Embedded between the two prism pairs
2
a
and
2
d
on the one side,
2
b
and
2
c
on the other side there is an additional, optically effective layer system
7
, that largely reflects light in the red range but largely transmits light in the green range and the blue range. In
FIG. 1
also the two legs of the red reflector layer system are shown as a color splitter system, labeled
7
′ and
7
″.
On the X-cube there are three input channels for red, green and blue light from corresponding sources, for example, LCD controlled, and an output channel with the recombined input signals. On the reflector systems, between each of said prism pairs, the correspondingly colored light, and particularly S-polarized light with an incidence angle of less than 45° is reflected. In addition the hypotenuse surfaces of the prisms
2
can be and usually are coated with an antireflection layer system.
Because the pixels of the red-blue-green input channels should converge as accurately as possible, the angle tolerances on prism
2
and in the assembled X-cube must be very narrow.
Large tolerances result in poor imaging quality because the pictures do not accurately converge: blurring or color fringes occur.
Location
9
, shown with dashes in
FIG. 1
where the four individual prisms
2
meet, is also located within the imaging optical path. Optical interferences created in this location manifest themselves, as mentioned for example, as a blurred picture in the output channel OUT. It is a requirement of such elements and their manufacturing process to minimize the interferences, particularly in this location
9
.
From DE-40 33 842, for example, it is known that X-cubes can be manufactured from four prisms
2
according to FIG.
1
. The four individual prisms are first manufactured in their exact dimensions through milling, grinding and polishing. Subsequently they are coated with the appropriate layer system along their leg and possibly on their hypotenuse surfaces with an antireflection coating. Finally the coated individual prisms
2
are cemented together.
Much earlier than DE-OS 40 33 842, that is, from U.S. Pat. No. 2,737,076 it was known that when different optical layer systems are created on the leg surfaces of prisms, problems occur at the vertex in area
9
as shown in
FIG. 1
with respect to the intersection of the layer systems. In order to solve this problem this application proposes to apply a masking layer to one leg surface each of two 90° prisms and to join the two prisms with the masked surfaces in such a way that the two other leg surfaces are in alignment. These are coated with sulfide and fluoride layers. Subsequently the prisms are broken apart again, one previously coated surface and a leg surface of an uncoated prism are masked, joined, and after removal of the previously applied mask the uncoated, also aligned leg surfaces, are coated with zinc sulfide and lead fluoride coatings—with the latter being a cumulatively acting catalyst poison. The prisms are again separated. Now four of the coated individual prisms are combined into an X-cube after the removal of mask material by means of solvents.
In an advanced development GB-754 590 assumes that it is very difficult to accurately combines the coated individual prisms. For this reason the following solution is adopted:
Two 90° prisms are positioned vertex against vertex with aligned leg surface in such a way that a gap is created between the two other, mutually opposite leg surfaces.
This gap is sealed in the vertex area by means of a soluble mask.
The aligned leg surfaces are coated jointly, the prisms are again separated and the mask removed. The aim is that the coating breaks parallel to the vertex.
A specially coated leg surface and a leg surface of an uncoated prism, with the other leg surfaces aligned on an optically flat block, are cemented together where cement exudation on the vertex is prevented by covers that are removably cemented to the aligned surfaces.
The aligned surface of the prism pair is coated after the cover and the exuded cement have been removed.
A prism pair coated in this manner and a prism pair not coated on the aligned surfaces are cemented together while their relative setting and position are accurately monitored.
Disadvantages of the Known Processes and Known X-Cubes
The effort required for manufacturing the X-cubes as described, for example, in GB-754-590 or DE-40 33 842, is very high: First, each of the three lateral sides of each individual prism
2
must be mounted or fixed by plastering, blocking or wringing as shown in
FIG. 1
before the glass can be worked. Subsequently the surfaces must be cleaned for coating the individual prisms
2
, and then mounted and dismounted for the coating process. On an average two sides per individual prism need to be coated. This laborious handling considerably raises the production costs for such X-cubes.
From
FIG. 1
, particularly location
9
, it is evident that the coating of the red and blue reflecting layer systems must be executed in such a way that the coating does not wrap around the 90° edges of the individual prisms. This requires sophisticated coating fixtures or masking of the legs on which no coating may be deposited. In this respect we refer to the masks according to GB-754 590 and U.S. Pat. No. 2,737,076 (Rock et al.).
During the coating and the handling of the individual prisms
2
, the 90° prism edges are exposed without protection, that is, especially those edges which according to the foregoing explanation must be very accurate. This exposure inevitably leads to chipping unless laborious precautions to protect these edges are taken, which again increases the costs.
If, for example, anything goes wrong during the coating of the individual prisms
2
, such an individual piece must be remounted, ground, and repolished, otherwise it will have to be discarded. Correction processes are at best very difficult to implement.
Cementing of the i
Balzers Aktienresellschaft
Notaro & Michalos P.C.
Spyrou Cassandra
Treas Jared
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