Compound polarization beam splitters

Abstract

A compound polarization beam splitter ( 33 ) for use with a reflective, polarization-modulating, imaging device ( 10 ), e.g., a LCoS device, is provided. The compound PBS has: (a) an input prism ( 20 ); (b) an output prism ( 30 ), and (c) a polarizer ( 13 ), which is located between the two prisms ( 20,30 ) and which may be a wire grid polarizer ( 13 a ) or a multi-layer reflective polarizer ( 13 b ). Polarized illumination light ( 11 ) enters the input prism ( 20 ) through a first surface ( 21 ) and undergoes total internal reflection at a second surface ( 22 ) before being reflected from the polarizer ( 13 ) and polarization-modulated at the imaging device ( 10 ). The polarizer's tilt angle (β) is less than 45°, which reduces astigmatism and the required back working distance of the system's projection lens ( 74 ).

1 . A projection system, comprising: 
 an imaging device capable of modulating an illumination light beam to produce modulated light; and    a prism unit proximate the imaging device, the prism unit comprising a first prism, a second prism and a reflective polarizer disposed between the first and second prisms, the first prism having a first face facing the imaging device and a second face facing the second prism, the prism unit arranged and configured so that the modulated light from the imaging device passes through the first and second faces of the first prism to the reflective polarizer, modulated light incident at the reflective polarizer in a first polarization state being transmitted through the reflective polarizer as image light, the image light passing successively through first and second faces of the second prism, and modulated light incident at the reflective polarizer in a second polarization state orthogonal to the first polarization state from the imaging device being reflected back into the first prism by the reflective polarizer and being totally internally reflected by at least one side of the first prism.    
   
   
       2 . A system as recited in  claim 1 , wherein the light reflected by the reflective polarizer layer is totally internally reflected at the first face of the first prism.  
   
   
       3 . A system as recited in  claim 2 , wherein, after being totally internally reflected at the first face, the totally internally reflected light is totally internally reflected at a third face of the first prism and exits out of the first face of the first prism.  
   
   
       4 . A system as recited in  claim 2 , wherein, after being totally internally reflected at the first side, the totally internally reflected light exits the first prism through a third face.  
   
   
       5 . A system as recited in  claim 1 , wherein the reflective polarizer is a multi-layer reflective polarizer.  
   
   
       6 . A system as recited in  claim 1 , wherein the reflective polarizer is a wire grid polarizer.  
   
   
       7 . A system as recited in  claim 1 , wherein the first face of the first prism and the second face of the second prism are substantially parallel.  
   
   
       8 . A system as recited in  claim 1 , further comprising a projection lens arranged to project the image light transmitted through the reflective polarizer and the second prism.  
   
   
       9 . A system as recited in  claim 1 , wherein an internal angle, β, between the first and second faces of the first prism is not less than β min , where β min =0.5.(γ+α), where γ=sin −1 (½.n 1 .F no ) and α=sin −1 (1/n 1 ), F no  being the f-number of the illumination light and n 1  being the refractive index of the first prism.  
   
   
       10 . A system as recited in  claim 8 , wherein the value of β is not more than β max , where β max =α−γ.  
   
   
       11 . A method of separating a modulated light beam into two beams, comprising: 
 modulating a light beam;    passing the modulated light beam through a first prism;    transmitting a first portion of the modulated light beam, in a first polarization state, after passing through the first prism;    reflecting a second portion of the modulated light beam, in a second polarization state orthogonal to the first polarization state, after passing through the first prism; and    totally internally reflecting the second portion of the modulated light beam within the first prism.    
   
   
       12 . A method as recited in  claim 11 , wherein passing the modulated light beam through the first prism comprises passing the modulated light beam successively through first and second faces of the first prism, and totally internally reflecting the second portion of the modulated light beam comprises totally internally reflecting the second portion of the modulated light beam at the first face of the first prism.  
   
   
       13 . A method as recited in  claim 12 , further comprising totally internally reflecting the second portion of the modulated light beam at a third face of the first prism after totally internally reflecting the second portion of the modulated light beam at the first face of the first prism, and then extracting the second portion of the modulated light beam through the first face of the first prism.  
   
   
       14 . A method as recited in  claim 12 , further comprising extracting the second portion of the modulated light beam through a third face of the first prism after totally internally reflecting the second portion of the modulated light beam at the first face if the first prism.  
   
   
       15 . A method as recited in  claim 11 , wherein reflecting the second portion of the modulated light beam comprises reflecting the second portion of the modulated light beam with a multi-layer reflective polarizer.  
   
   
       16 . A method as recited in  claim 11 , wherein reflecting the second portion of the modulated light beam comprises reflecting the second portion of the modulated light beam with a wire grid polarizer.  
   
   
       17 . A method as recited in  claim 11 , further comprising transmitting the first portion of the modulated light beam through a second prism having an output face parallel to an input face of the first prism.  
   
   
       18 . A method as recited in  claim 11 , further comprising projecting the first portion of the modulated light beam to a screen.  
   
   
       19 . An optical system, comprising: 
 an imaging device capable of polarization modulating an illumination light beam to produce a modulated light beam;    a reflective polarizer; and    a first prism disposed between the imaging device and the reflective polarizer, the first prism having a first side and a second side non-parallel to the first side, the first side facing the imaging device and the second side facing the reflective polarizer;    wherein the modulated light beam passes from the imaging device, through the first and second sides of the first prism to the reflective polarizer, a first portion of the modulated light beam in a first polarization state being transmitted through the reflective polarizer and a second portion of the modulated light beam, in a second polarization state orthogonal to the first polarization state, is reflected by the reflective polarizer through the first prism to the first side.    
   
   
       20 . A system as recited in  claim 19 , wherein the light in the second polarization state reflected to the first side of the first prism is totally internally reflected at the first side of the first prism towards a third side of the first prism.  
   
   
       21 . A system as recited in  claim 20 , wherein the light totally internally reflected at the first side of the first prism exits the first prism through the third side of the first prism.  
   
   
       22 . A system as recited in  claim 20 , wherein the light totally internally reflected at the first side of the first prism totally internally reflects at the third side of the first prism and exits the first prism through the first side.  
   
   
       23 . A system as recited in  claim 19 , wherein the reflective polarizer is a multi-layer reflective polarizer.  
   
   
       24 . A system as recited in  claim 19 , wherein the reflective polarizer is a wire grid polarizer.  
   
   
       25 . A system as recited in  claim 19 , further comprising a second prism, the reflective polarizer positioned between the first and second prisms, the second prism having a first side facing the reflective polarizer, the first side of the second polarizer being substantially parallel to the second side of the first prism.  
   
   
       26 . A system as recited in  claim 19 , further comprising a projection lens arranged to project the light transmitted through the reflective polarizer.  
   
   
       27 . A system as recited in  claim 19 , wherein an internal angle, β, between the first and second sides of the first prism is not less than β min , where β min =0.5.(γ+α), where γ=sin −1 (½.n 1 .F no ) and α=sin −1 (1/n 1 ), F no  being the f-number of the illumination light beam and n 1  being the refractive index of the first prism.  
   
   
       28 . A system as recited in  claim 27 , wherein the value of β is not more than β max , where β max =α−γ.