1.  Field of the Invention
The present invention relates generally to optical systems, and more particularly, to a high efficiency polarization device for converting substantially all incoming light into a single polarization. 
2.  Description of the Related Art
Applications that use liquid crystal cells to render an image usually require linearly polarized input light to function properly.  Conventional light sources are typically unpolarized.  In order to polarize this light, it is passed through a polarizer, which transmits the desired polarization state and absorbs or deflects the unusable, perpendicular polarization state.  Thus, approximately half of the incoming source light has to be discarded, resulting in reduced system efficiency and brightness. 
Solutions to the foregoing efficiency problem have been developed to transform the light from the unusable polarization state into the desired state.  One solution developed by 3M Company is a dual brightness enhancement film which is used in laptop LCD displays to increase screen brightness.  The film transmits the desired polarization state and reflects the perpendicular polarization state back to the light source.  Due to scatter and reflection, part of this light is reflected back in the desired polarization state and passes to the screen.  However, due to high brightness requirements and light absorption in the source, this approach is of limited efficiency for use in LCD projection applications. 
Another solution in the art is the use of a polarizing beam splitter of the type shown in FIG.  1.  Unpolarized light 10 is directed to a polarizing beam splitter 12.  A desired light polarization is transmitted by the polarizing beam splitter 12 and is represented by the light beam 14.  The polarizing beam splitter 12 reflects the perpendicular state of polarization.  This reflected perpendicular state of polarization is then directed by a mirror 16 through a half wave plate 18, which functions to change the polarization state of the reflected light into light 20 with the desired state of polarization.  In theory, the total amount of unpolarized incoming light is transformed into linearly polarized light.  However, the resulting light consists of two distinct optical beams, which are difficult to utilize in an optical system.  Large and more expensive optics would be required in order to facilitate the utilization of both beams, and the optical efficiency of such a complex system would typically not be optimum.  Accordingly, there continues to be an efficiency problem in the conversion of light to a single polarization. 
The invention provides a polarization device that comprises a light path, a micro lens array, a polarization filter and a reflecting polarization converter.  The micro lens array is disposed in the light path for separating light into a plurality of individual beams of light.  The polarization filter comprises a polarizing film and is disposed in the light path to receive a plurality of the beams of light.  The polarization filter transmits the received light of a desired polarization state and reflects the received light of the perpendicular polarization state that is perpendicular to the desired polarization state. 
The reflecting polarization converter is disposed to receive and reflect the reflected light of the perpendicular polarization state.  The reflecting polarization converter converts the perpendicular polarization state to the desired polarization state.  The polarization filter and the reflecting polarization converter are separated by a non-solid filled gap.  The reflecting polarization converter re-reflects at least a portion of the converted light back onto the polarization filter. 
The invention also provides a polarizing device that comprises a light path, a micro lens array, a polarization filter and a reflecting polarization converter.  The micro lens array is disposed in the light path for separating light into a plurality of individual beams of light.  The micro lens array comprises a plurality of micro lenses, wherein each of the micro lenses in the micro lens array has a centerline.  The polarization filter comprises a polarizing film and is disposed in the light path to receive a plurality of the beams of light and to transmit the received light of a desired polarization state and to reflect the received light of a perpendicular polarization state that is perpendicular to the desired polarization state.  The polarization filter is shaped to comprise a plurality of surface features, wherein each of the surface features is shaped symmetrically about a center line to reflect light from a different one of the individual beams of light in at least two directions at an acute angle to the different one of the individual beams of light. 
The reflecting polarization converter is disposed between the micro lens array and the polarization filter, separated from the polarization filter by a non-solid filled gap.  The reflecting polarization converter receives and reflects the reflected light of the perpendicular polarization state and converts the perpendicular polarization state to the desired polarization state.  The reflecting polarization converter includes a plurality of transmissive regions, wherein each of the transmissive regions has a centerline aligned with a different one of the individual beams of light to transmit the beam of light aligned with it.  The plurality of micro lenses each has its centerline aligned with the centerline for one of the transmissive regions and with the centerline for one of the features of the polarization filter.  The reflecting polarization converter re-reflects at least a portion of the converted light back onto the polarization filter. 
The invention also provides a method for polarizing light.  In the method, light in a light path is separated into a plurality of individual beams of light, wherein each of the plurality of individual beams of light is passed through a different associated transmissive region in an element.  The plurality of individual beams of light is also passed through a non-solid filled gap to a polarization filter comprising a polarizing film.  The plurality of individual beams is filtered with the polarization filter into light beams with a desired polarization state and light beams with a perpendicular polarization state substantially perpendicular to the desired polarization state.  The light beams of the desired polarization state are transmitted with a first range of emission angles.  The light beams with the perpendicular polarization state are reflected, not straight back into the transmissive region associated with the light beam, but back through the gap toward the element onto a surface on the element adjacent the associated transmissive region in the element.  The perpendicular polarization state is converted to the desired polarization state.  The reflected light beams of the perpendicular polarization state are re-reflected from the surface back through the gap toward the polarization filter.  The re-reflected light beams have substantially the same range of emission angles as the first range of emission angles of the transmitted light beams of the desired polarization state. 
The invention also provides a polarization device that comprises a light path, a micro lens array, a polarization filter and a reflecting polarization converter.  The micro lens array is disposed in the light path for separating light into a plurality of individual beams of light.  The micro-lens array comprises a plurality of micro-lenses, each of which has a centerline.  The polarization filter comprises a polarizing film and is disposed in the light path to receive a plurality of the beams of light and to transmit the received light of a desired polarization state and to reflect the received light of a perpendicular polarization state that is perpendicular to the desired polarization state.  The polarization filter additionally comprises a plurality of pyramid shaped surface features, wherein each of the pyramid shaped surface features is formed symmetrically about a different respective centerline. 
The reflecting polarization converter is disposed to receive and reflect the reflected light of the perpendicular polarization state and to convert the perpendicular polarization state to the desired polarization state.  The polarization converter comprises a mirror backing, a quarter wave plate coupled to the mirror backing, and a plurality of transmissive regions.  The transmissive regions have a centerline.  The polarization filter and the reflecting polarization converter are separated by a non-solid filled gap. 
The plurality of the micro lenses each has its centerline aligned with the centerline of one of the transmissive regions in the reflecting polarization converter and with the centerline for one of the pyramid shape surface features of the polarization filter.  The reflecting polarization converter re-reflects at least a portion of the converted light back onto the polarization filter. 
Finally, the invention provides a polarization device that comprises a light path, a micro lens array, a polarization filter and a reflecting polarization converter.  The micro lens array is disposed in the light path for separating light into a plurality of individual beams of light.  The micro lens array comprises a plurality of lenses each with a centerline.  The polarization filter comprises a polarizing film and is disposed in the light path to receive a plurality of the beams of light and to transmit the received light of a desired polarization state and to reflect the received light of a perpendicular polarization state that is perpendicular to the desired polarization state.  The polarization filter additionally comprises a plurality of surface features each shaped symmetrically about a centerline.  The reflecting polarization converter is disposed to receive and reflect the reflected light of the perpendicular polarization state and to convert the perpendicular polarization state to the desired polarization state.  The reflecting polarization converter is separated from the polarization filter by a non-solid filled gap.  The reflecting polarization converter re-reflects at least a portion of the converted light back onto the polarization filter.