Stress birefringence compensation in polarizing beamsplitters and systems using same

An optical system, comprising:

• a first polarizing beamsplitter (PBS) having a reflective polarizing layer disposed between first and second covers, the first cover being formed of a material having a stress optic coefficient greater than 0.1;
• a first reflecting image-forming device facing a first surface of the first cover so that light passes through the first cover from the reflective polarizing layer of the first PBS to the first reflecting image-forming device and is reflected from the reflecting image-forming device through the first cover to the reflective polarizing layer of the first PBS, the light being subject to stress birefringence in the first cover and to residual birefringence in the reflecting image-forming device;
• and
• a first quarter-wave retarding element disposed to retard the light passing between the first surface of the first cover and the reflecting image-forming device.

An optical system, comprising:

• an image-forming device;
• a reflective polarizing layer disposed to reflect light in a selected polarization state to or from the image forming device;
• at least a first birefringent element disposed on an optical path between the image-forming device and the polarizing layer, the first birefringent element having a face substantially perpendicular to the optical path, the birefringence of the birefringent element being non-uniform so that values of birefringent retardation for light passing through different portions of the face are different;
• and
• a quarter-wave retarding element disposed between the first birefringent element and the image-forming device, the quarter-wave retarding element oriented to at least partially compensate retardation of light on a round trip between the reflective polarizing layer and the image-forming element.

An optical system, comprising:

• a color combiner unit having at least two entrance faces associated with different color bands;
• at least two polarizing beamsplitters (PBSs) operatively disposed relative to the at least two entrance faces respectively, at least a first PBS of the at least two PBSs comprising a first reflective polarizing layer disposed between respective first surfaces of first and second covers, the second cover positioned between the reflective polarizing layer and the color combiner unit, the first cover being formed of a transparent material having a stress-optic coefficient (SOC) greater than 0.1×10−6 mm2 N−1, the first PBS defining a first plane of reflection;
• and
• a first quarter-wave retarding element, the first cover of the first PBS between the quarter-wave retarding element and the reflective polarizing layer, the quarter-wave retarding element having a fast axis oriented non-parallel and non-perpendicular to the first plane of reflection.

A method of compensating for birefringence in an image projection system, comprising:

• polarizing light at a polarizing layer substantially into a selected polarization state;
• directing the polarized light to an image-forming device through a birefringent element;
• reflecting the polarized light at the image-forming device;
• analyzing the reflected polarized light at the polarizing layer;
• and
• passing the polarized light and the reflected polarized light through a quarter-wave retarding element disposed between the birefringent element and the image-forming device, the quarter-wave retarding element oriented to substantially optimize compensation of retardance in the birefringent element.