1. Field of the Invention
This invention relates to the field of polarization recycling or recovery systems, and in particular, to the field of polarization recycling or recovery systems adapted for use in illumination system based LCD or LCOS imagers.
2. Description of Related Art
LCOS or LCD imagers need polarized light to function properly. In a conventional illumination system for LCD or LCOS imagers, the light is polarized by a sheet polarizer that is absorbs one polarization component. There are two undesired effects. The polarizer is overheated and eventually damaged and more that half of the light available for illumination is lost.
Light is polarized if the electrical field associated with a ray is vibrating in one plane, perpendicular to the direction of propagation of the ray. Generally a light ray is randomly polarized, which means that the electric field can vibrate in any direction perpendicular to the direction of propagation. When a randomly polarized light ray hits a reflecting polarizer, for example, the polarizer transmits light that has the electric field vibrating in a first plane perpendicular to the direction of the propagation. The orientation of the plane is determined by the orientation of the polarizer. The polarizer also reflects light that has an electric field that vibrates in a second plane perpendicular to the direction of propagation. The transmitted electric field and the reflected electric field vibrate in planes that are perpendicular one to another. Hence, the reflecting polarizer passes or transmits light having one polarization and reflects light having a perpendicular, or second, polarization. The plane of polarization can be changed, for example by passing light through a quarter wave plate whose fast axis is at 45° from the orientation of the light ray's polarization. Passing light through two quarter wave plates, or passing light twice through the same quarter wave plate, for example, rotates the polarization plane by 90°.
The state of the art for recycling the polarization involves an array of polarization beam splitters (PBS) either associated with an integrating rod (e.g., Japanese Patent 10232430) or a fly eye lens system. While both means have excellent light throughput and illumination uniformity, there are some significant disadvantages. The price of a PBS array is quite high. Additional optics needed to implement operation a PBS array. PBS arrays require a lot of space, whereas space is often in short supply.
There is an urgent need for a new polarization recovery system that is much simpler and less expensive to implement, that can be implemented without additional lenses or space and that can be substituted into an existing light engine with little, if any, architectural change. Moreover, there is a continuing need for a new polarization recovery or recycling system that avoids thermally damaging polarizers, and at the same time, provides more polarized light for illuminating LCD or LCOS imagers more fully and efficiently.
The inventive arrangements taught herein satisfy the urgent and long-felt needs for a new polarization recovery or recycling system that is much simpler and less expensive to implement, that can be implemented without additional lenses or space, that can be substituted into an existing light engine with little, if any, architectural change, that avoids thermally damaging polarizers and that provides more polarized light for illuminating LCD or LCOS imagers more fully and efficiently.
In accordance with the inventive arrangements, polarization transforming means, for example a quarter wave plate, can be provided in a light integrator to recycle light originally reflected by the reflecting polarizer. The polarization transforming means can be positioned to enable the reflected light to pass through twice, for example during after back and forth reflections in the light path of the light integrator, thus transforming the reflected light to the correct polarization needed to be passed through the reflective polarizer. Accordingly, the light that is normally lost by being back reflected is recycled, or recovered, by transforming its polarization. The system advantageously exhibits very little losses, and therefore supplies significantly more polarized light more efficiently than prior art systems.
A polarization recovery system in accordance with the inventive arrangements comprises: a light channel; the light channel having a light injection aperture for receiving light from a source of illumination radiating light with first and second polarizations; the light channel having a reflecting polarizer that transmits the injected light having the first polarization and that reflects the injected light having the second polarization; at least one reflective surface for further reflecting the reflected light back toward the reflecting polarizer; and, means positioned in the light channel for transforming the light reflected by the reflecting polarizer into light having the first polarization, the transformed light also being transmitted by the reflecting polarizer.
A method for recovering polarization in an illumination system for a liquid crystal display, in accordance with the inventive arrangements, comprises the steps of: guiding light having first and second polarizations in a forward direction along a light path; passing the guided light having the first polarization out of the path; reflecting the light having the second polarization backwardly and forwardly along the light path; transforming the reflected light during the reflecting step to have the first polarization; and, passing the light transformed to the first polarization out of the path, enabling more of the light having the first polarization to be passed out of the path.
An illumination system for a liquid crystal imager, comprising: a source of randomly polarized light; a light integrator having an aperture at one end for receiving the randomly polarized light and having reflective surfaces defining an internal light path; a reflecting polarizer disposed at an opposite end of the light path that transmits light having a first polarization and that reflects light having a second polarization, the light having the second polarization being reflected back and forth along the light path; and, means positioned in the light channel for transforming the back and forth reflected light into light having the first polarization, the transformed light also being transmitted by the reflecting polarizer. A liquid crystal imager can be more efficiently illuminated by both components of the light transmitted by the reflecting polarizer and having the first polarization.
FIGS. 2(a) and 2(b) are useful for explaining how to choose the size of the integrating light pipe, or rod.
FIGS. 3(a) and 3(b) are useful for explaining injection of the source into the integrating light pipe, or rod.
FIGS. 4(a), 4(b), 4(c) and 4(d) illustrate four embodiments for recovering polarization in accordance with the inventive arrangements.
An illumination system 10 in accordance with the inventive arrangements and adapted for use with a liquid crystal imager, for example an LCD or LCOS (liquid crystal on silicon) imager, is shown in
The principle of the integrating rod for the illumination system is well established and widely used. It's functionality is twofold: beam shaping from round to rectangular and illumination uniformity. The light coming out of a source is focused at the input side of an integrating rod. The light bounces into the rod by either reflecting from its mirrored sides or by total internal reflection if the rod is made out of glass. At the end of the rod, the illumination is uniform. The rod having a cross section of the same ratio than the imager, it's output side can be imaged onto a LCOS or LCD imager with a lens system. Hence the illumination is uniform and very efficient because of the format conversion between the collecting means (reflector) and imager. If polarized light is needed for the imager, more than half of the available light will be lost by polarizing the illumination unless polarization is achieved by a means that recycles one of the components.
An integrating rod forming the light channel 20 is shown in
FIGS. 4(a)-4(d) show four embodiments as to how the inventive arrangements can be implemented. In the embodiment shown in
The quarter wave plate can also be located near the input port or aperture 18 of the light integrator, as shown in
FIGS. 4(c) and 4(d) show alternative systems for reflecting the polarization. The mirrored surfaces 26 of the light integrator can be a dielectric coating that does not rotate the polarization, for example a Silflex brand coating from the Unaxis company. The integrator can also be a rod made out of glass. Then, the output reflecting polarizer is effectively glued or otherwise adhered to the output port. In FIGS. 4(c) and 4(d) the reflecting polarizer is an assembly, as opposed to the single optical device in FIGS. 4(a) and 4(b). In
The light injection aperture on the input side of the light path can be made by a reflecting coating having a hole. A retardation film 42 is disposed in the light path 27, either between the output surface of the rod and the reflecting polarizer or at its input. The film is represented by a dashed line, disposed near the input end of the light path in FIGS. 4(c) and disposed near the output of the light path in
A system in accordance with the inventive arrangements has been simulated with ASAP according to the following conditions. The lamp is a Radiant imaging 16 bit model of a widely used high pressure discharge lamp. The reflector is elliptical and focuses light into an integrator that has an input size of 11.08×6.23 mm. The sides of the integrator reflect 98% of the light, regardless of the angle of incidence, the polarization and the wavelength (Silflex mirror). The reflecting polarizer is a Proflux brand from Moxtek with a transmission of 85% of the p polarization. It reflects the s polarization, which is recycled by a quarter wave plate located at the input side of the integrator with an efficiency of 85%. The size of the injection aperture has been varied and the efficiency of the system plotted against the radius of the aperture. The graph in
Significant advantages of this system compared to those based on a linear array of PBS's are the extreme compactness and the fact that the system can easily fit into already existing designs without much change in the geometry, since the system does not require extra optical means aside from a relay lens system to image the output of the integrator onto the imager. Moreover, such a relay system is already implemented in a system using an integrating light pipe. Better gains can be achieved with light sources that have a smaller burner than the one used in this simulation case, which had a gap of 1.3 mm. The system also has cost advantages, since there are only a few new parts added, and no major tooling is needed, as would be the case for fly's eye lens and PBS arrays.
In
Number | Date | Country | Kind |
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01503044.9 | Nov 2001 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/US02/37166 | 11/19/2002 | WO |