Optical: systems and elements – Polarization without modulation – Polarizarion by dichroism
Reexamination Certificate
2000-02-22
2002-01-22
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarizarion by dichroism
C359S490020, C362S019000
Reexamination Certificate
active
06341038
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of art to which this invention relates is an apparatus for polarization conversion. It is more particularly directed to such apparatus which make use of reflective polarizer films and light sources having parabolic mirrors.
2. Discussion of the Prior Art
Intensity in optical projectors is usually limited by source brightness. In addition, projectors that use polarized light, such as many projection displays, suffer further intensity loss if the unused polarization is discarded. Frequently, an effort is made to capture the rejected component and re-inject it into the system after rotating its polarization to match that of the primary component.
FIG. 1
illustrates a known arrangement for accomplishing this.
FIG. 1
shows a lamp
102
providing light having both S and P linear polarizations (where S polarization is denoted by “∘” and P polarization is denoted by “
”). The light enters a polarization beam splitter (PBS) cube
104
which transmits light of one of the polarizations, P polarization in this example, and reflects light of the other polarization, S polarization in this example. The reflected S polarized light is again reflected by a mirror
106
and directed towards a ½ waveplate
108
, where its polarization is converted to P polarization. Thus, the incident light is converted to one polarization and directed to a lens
110
or other optical component.
Two problems with the
FIG. 1
approach are added cost and the need to increase optical etendue (NA times field size) when one polarized source image is, by itself, large enough to fill the lens pupil. Increases in etendue add to cost, and the most cost effective step is usually to use a large enough lamp
1
that the image in a single polarization almost fills the pupil aperture chosen; this reduces the benefit from re-injecting the second polarization. Because of arc inhomogeneities, the
FIG. 1
arrangement can, in practice, still provide some intensity increase, but the benefit is limited.
A known way to partially circumvent the etendue problem is to recycle the converted polarization through the arc, as shown in FIG.
2
. It is thermodynamically impossible to increase the brightness of a black-body source of fixed temperature, but arc discharges are not fully opaque. The
FIG. 2
system increases the effective source emissivity by redirecting rays through the arc. (Emissivity equals absorbance, according to Kirchoff's Law. By tracing rays backwards through the
FIG. 2
system, one can see that the recycling elements also increase arc absorbance).
FIG. 2
shows a lamp
102
having a parabolic mirror
102
a
. The lamp provides both S and P polarized light, both of which pass through a ¼ waveplate
114
and are directed to a PBS
104
. The P polarized light is transmitted
116
and the S polarized light
118
is reflected to a mirror
112
. The S polarized light is then reflected back to the PBS
104
and again reflected back towards the parabolic mirror
102
a
, first passing through the ¼ waveplate where it is converted to circular polarized light having a right handedness
120
. The circularly polarized light having a right handedness
120
is then reflected by the parabolic mirror
102
a
which converts its handedness to left-handedness
122
. This light then reflects of the opposite side of the parabolic mirror
102
a
which converts its handedness back to right handedness
124
. The circular polarized light having a right handedness
124
then passes back through the ¼ waveplate
114
once again, which converts the light back to linear polarized light but having S polarization
126
. The S polarized light
126
is then reflected once again by the PBS
104
towards the mirror
112
, and back again towards the lamp
102
. In this embodiment, an increase in brightness is not obtained unless there is a phase difference between the S and P components of the reflected light at the parabolic mirror
102
a.
The return mirror
112
in the
FIG. 2
system can be slightly tipped so that the two arc images are only partially overlapped; this can improve collected intensity when the system is not fully brightness-limited (due to arc inhomogeneities). However, in practice, the
FIG. 2
arrangement is typically reported to have limited efficiency in converting the returned polarization to the desired output state. Also, in most projectors, the PBS
104
in the
FIG. 2
system must be added as a new component (though in a few systems, a PBS
104
already present in the optics can also perform the recycling function). A PBS
104
is a fairly expensive optical component.
What is needed is a way to improve the efficiency of recycling and, at the same time, lower its cost.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide an apparatus for polarization conversion which overcomes the deficiencies of the prior art.
Accordingly, a first embodiment of an apparatus for polarization conversion is provided. The apparatus of the first embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror has a mirror coating to induce a phase shift of 90° between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. Lastly, a ¼ waveplate is disposed in the optical path between the polarizer means and the parabolic mirror. The ¼ waveplate has quarter wave retardance for converting the reflected linear polarization from the polarizer means to circular polarization before being incident upon the parabolic mirror and for converting the reflected circular polarization from the parabolic mirror to the transmitted polarization which is directed towards, and transmitted by, the polarizer means.
A second embodiment of an apparatus for polarization conversion is also provided. The apparatus of the second embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror having a mirror coating to induce a phase shift of 0° between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. Lastly, a ¼ waveplate is disposed in the optical path between the polarizer means and the parabolic mirror. The ¼ waveplate has opposing segments each having quarter wave retardance but having axes which are antiparallel to each other, for converting the reflected linear polarization from the polarizer means to circular polarization through one of the segments before being incident upon the parabolic mirror and for converting the reflected circular polarization from the parabolic mirror to the transmitted polarization through the other segment which is directed towards, and transmitted by, the polarizer means.
A third embodiment of an apparatus for polarization conversion is also provided. The apparatus of the third embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror has a mirror coating which may induce an arbitrary phase shift between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. Lastly, a ½ waveplate is disposed in the optical path between the polarizer means and the parabolic mirror. The ½ waveplate has a plurality of segment pairs. Wherein each individual segment has half
Budd Russell Alan
Dove Derek Brian
Rosenbluth Alan Edward
Morris, Esq. Daniel P.
Schuberg Darren
Scully Scott Murphy & Presser
LandOfFree
Apparatus for polarization conversion does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus for polarization conversion, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus for polarization conversion will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2866955