Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2001-10-11
2003-09-16
Niebling, John F. (Department: 2812)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C359S831000
Reexamination Certificate
active
06621533
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarisation separation element that separates incident unpolarised or partially polarised light into two angularly separated output beams having different polarisation states. The invention also relates to a polarisation conversion system that converts light that is unpolarised or partially polarized to light that is substantially completely polarised. The invention also relates to an optical element that comprises two lens arrays disposed on opposite faces of a substrate. The invention also relates to a projection display system incorporating such a polarisation-conversion system and possibly such an optical element.
2. Description of the Related Art
Many optical systems require that they are illuminated by light; that is substantially completely polarised. Where such a device is operated with light that is unpolarised, or that is partially polarised, it is necessary for the light to be completely polarised—that is, converted to a single polarisation state—before it is incident upon the optical system.
One way of converting unpolarised light to completely polarised light is the well-known linear polariser. An idealised linear polariser transmits light that is linearly polarised in one direction without loss and completely absorbs light that is linearly polarised in an orthogonal direction, so that unpolarised light incident on the polariser is converted into light that is completely linearly polarised. While such a linear polariser is a straightforward means for producing linearly polarised light, it has the disadvantage of having a low efficiency. An ideal linear polariser, in which there is no loss of the polarisation component that is intended to be transmitted owing to absorption within the polariser and/or reflection at the surfaces of the polariser, has an efficiency of only 50%, and the efficiency of a practical linear polariser is generally within the range 40-45%.
Another known means for converting unpolarised light to polarised light is a polarisation conversion system. In a polarisation conversion system, incident light that is already polarised in a desired polarisation state is transmitted unchanged. Light that is polarised in a polarisation state orthogonal to the desired polarisation state is converted to light of the desired polarisation state, rather than being blocked as happens if a conventional linear polarise is used.
A polarisation conversion system consists essentially of a polarisation splitting element (PSE) that splits incident unpolarised or partially polarised light, so that light of one polarisation state is emitted from the PSE spatially or angularly separated from light having an orthogonal polarisation state. A polarisation conversion system also comprises a polarisation conversion element for changing the polarisation state of one of the components emitted by the PSE into the orthogonal polarisation state.
Many polarisation separation elements are known. As one example,
FIG. 19
shows an embodiment of the well-known Wollaston prism in which two birefringent wedges W
1
,W
2
are joined to form a composite block, with the hypotenuse faces of the two prisms adjacent to one another. In this embodiment of the Wollaston prism, described in EP-A-0 993 323, the two wedges W
1
,W
2
are embodied as liquid crystal layers having varying thickness. The direction of the optic axis in each wedge rotates through 90° across the thickness of the wedge, with the optic axis of the two wedges being perpendicular to one another at the interface between the two wedges.
U.S. Pat. No. 5,978,136 discloses a conventional PCOS, which is illustrated in FIG.
21
(
a
) of the accompanying drawings. This polarisation conversion system comprises two microlens arrays
5
and
6
. The elements of the first microlens array
5
image to corresponding elements of the second microlens array
6
. A set of polarising beam splitter cubes
2
that contains polarising separation films
2
a
then spatially separates P and S components of the light so that only the P or only the S component is incident upon a set of retarder stripes
3
. The retarder stripes are configured to be substantially half wave plates, such that light incident on a retarder stripe is converted to its substantially orthogonal state. Light leaving the polarising conversion system is now substantially polarised. The polarising beam splitter cubes further contain reflecting films
2
b
that reflect the other polarisation component so that it leaves the PCOS in a direction that is substantially parallel to the direction in which light leaves the retarder stripes
3
. An opaque mask
9
is disposed between the second microlens array
6
and the polarising beam splitter array
2
to reduce cross-talk. The minimum volume of the PCOS is constrained by the tolerances of the half-wave plates.
The set of polarising beam splitter cubes
2
of the PCOS of FIG.
21
(
a
) may be obtained by obliquely cutting a stack of PBS plates
49
, as shown in FIG.
21
(
b
). A suitable cutting cross-section is indicated by reference
50
in FIG.
21
(
b
).
Ogiwara et al describe in “PS Polarisation Converting Device for LC Projector Using Holographic Polymer-Dispersed LC Films”, SID 1999, a further conventional PCOS. This device it shown in FIG.
20
and comprises two microlens arrays
5
,
6
, two polymer dispersed liquid crystal (PDLC) gratings
2
,
4
and a set of half wave retarder elements
3
. Polarisation splitting is achieved by the PDLC grating
2
that diffracts substantially only light of one linear polarisation (P) and transmits light having the orthogonal linear polarisation (S) without significant diffraction.
The half wave retardation plates
3
are mounted on the second grating
4
and are arranged in the path of p-polarised light emitted by the grating
2
. When p-linearly polarised light passes through one of the half wave plates
3
, it will be converted to s-linearly polarised light.
The half wave retardation plates
3
are arranged so that the s-linearly polarised light emitted by the grating
2
does not pass through the half wave retardation plates
3
. The s-polarised light emitted by the grating
2
is therefore not affected by the half wave plates
3
. After passing through the array of half wave plates, the light is therefore completely s-polarised.
In use, the polarisation conversion system is illuminated by collimated light produced by a lamp
7
and a parabolic mirror
8
, and incident light is focused by the first microlens array
5
. The second microlens array
6
has a similar focal length and pitch to the first microlens array
6
. The first and second microlens arrays are separated by approximately their focal length.
This PCOS again has the disadvantage that the minimum volume is constrained by the tolerances of the half-wave retarder elements. A further disadvantage is that this system uses two polarisation splitting elements to reduce dispersion, and this increases the cost and complexity of the PCOS.
The dimensions of the polarisation conversion system shown in FIG.
21
(
a
) are typically of the order 50 mm×50 mm×70 mm. When the polarisation conversion optical system (PCOS) is used with a projector, it significantly increases the overall volume of the projector. The volume of the PCOS of FIG.
21
(
a
) can only be reduced if the focal length of the microlens arrays is reduced, and this requires a corresponding reduction in the pitch of the microlenses, the half way plates and the polarisation splitting cubes. The microlens arrays used in a conventional PCOS of the type shown in FIG.
21
(
a
) would typically have a pitch p of 6 mm, and it would be desirable to reduce this to under 1 mm with a corresponding reduction in optical system throw. It is, however, difficult to do this in the case of a PCOS that incorporates conventional half-wave retarder elements and conventional polarisation splitting cubes, since it becomes difficult to align the elements with one another with the required tolerance. Fa
Bourhill Grant
Hara Masaharu
Inoko Kazuhiro
Khazova Marina Vladimirovna
Mitani Keisuke
Lattin Christopher
Niebling John F.
Renner Otto Boisselle & Sklar
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