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ColorLink
Unveils PolarCorrect Family
07.14.2003
ColorLink (Boulder, CO) (www.colorlink.com)
has introduced a new family of devices designed to correct for polarization
inefficiencies in projection systems. Called PolarCorrect, the first
products are likely to be o-plate compensators that can be used
to improve system performance in LCOS and HTPS-LCD projection systems.
In a paper at SID, author Mike Robinson described how the o-plate
PolarCorrect device is used in LCOS systems. The device is attached
to conventional cube-type PBS assemblies to correct geometrical
depolarizing effects that can degrade contrast.
What's important about the development is that it provides a way
for conventional MacNeil-type PBS assemblies to now provide a very
high level of contrast to effectively compete with alternative high-contrast
designs, such as the Moxtek-based UltreX configuration and Vikuiti
film-based design from 3M. Don't be surprised to see suppliers of
MacNeil-type PBS assemblies begin to offer "Super PBS"
assemblies, which include the o-plate compensator and offer high-contrast
performance over the full visible spectrum with f/2.0 performance.
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When using a MacNeil cube PBS in an LCOS
system, geometrical rotation of polarization axis of skew (off-axis)
rays is a well-known problem that can reduce contrast to as low
as 50:1. Most architectures employ a quarter wave retarder plate
between the LCOS panel and PBS to correct the skew rays. But such
solutions require accurate alignment and good uniformity to provide
a high-contrast correction. And, they can only be used in systems
where the thin-film stacks of the PBS act as both the polarizer
and analyzer.
Alternatively, the PBS can be configured as a single-pass element,
acting as either the polarizer or analyzer. In this case, a second
element preceding or following the PBS provides the complementary
function of polarizer or analyzer. ColorLink has developed PolarCorrect
to provide skew ray compensation for systems of this configuration.
In essence, the device offers a low-cost alternative to conventional
film polarizers, which provide very little compensation.
The figure shows the modeled transmission of the unwanted s-polarized
light at 550nm through an ideal PBS (n=1.65) as a function of ray
angle. The top plot shows the case without the PolarCorrect device
and exhibits leakage of more than 3% of the s-polarized light. By
using the PolarCorrect compensator, shown in the bottom plot, polarization
leakage is significantly reduced.
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ColorLink has modeled the performance of a two-panel
LCOS architecture and |
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determined that without an o-plate compensator,
system contrast will be limited to around 500:1. But with o-plate
PolarCorrect components, as shown in the figure, contrast is now
limited by other elements in the engine - mainly the LCOS panels
themselves. Sequential contrast well in excess of 1000:1 has already
been demonstrated and is likely to reach 2000:1. Even higher contrast
should be possible with the 3-PBS/X-cube architecture.
Use of the PolarCorrect family of compensators is also being evaluated
in HTPS-LCD projection systems, but implementations will likely
be customer specific. We may have more on this later in the year.
Physically, the o-plate compensator is constructed by manufacturing
a splayed birefringent structure. ColorLink says this can be accomplished
by two methods, one of which it is unable to disclose at this
time.
The process that is likely to become a commercial product first
is based upon UV aligning a liquid crystal polymer layer on a
glass substrate. Such layers are thermally and mechanically stable,
and robust enough for use in projection systems. The process is
already in mass production for other applications, so o-plate
compensators can be manufactured at low cost. ColorLink plans
to commercialize the o-plate this summer, manufacturing the devices
in Japan.
ColorLink has been evaluating these new PolarCorrect components
in a series of reliability tests that include cold soak, temperature/humidity
storage, temperature/humidity cycling and illumination testing.
Illumination testing, for example, exposes the device to 18 million
lux of UV filtered light at 70 degrees C for 2500 hours. The PolarCorrect
elements passed this and the other reliability tests.
The o-plate compensator will be the first in a family of products.
Others could include low retardance a-plate compensators, crossed
high retardance a-plates, negative c-plates, or a combination
of these. The approach makes the compensation more robust to uniformity
and alignment issues.
ColorLink, John Korah, 303-545-5843, Korah@Colorlink.com
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