Projection optics for reflective light valves

Abstract

Reflective x-Si backplanes allow projection displays to evolve toward higher pixel count and greater miniaturization, extending the range of competitive application. As light valve area A is reduced, projector output into solid angle S≡πNA2 can in many cases be considered to decrease roughly as approx. (A*S)0.5, with the 0.5 exponent representing typical microdisplay operation in a regime that is neither purely brightness-limited nor purely power-limited. Polarization modulation entails a modified scaling approx. (A*S/2)0.5; color sequential operation, approx. (1/3)*(A*S)0.5; spatially divided single-light-valve RGB projection, approx. (A*S/3)0.5. Projection lenses for three-light-valve systems must provide an increased working distance to accommodate a color recombiner (as well as polarizing beam splitter [PBS], or a total reflection prism). Zoom lenses are often required in front projectors, and rear projection usually entails a short lens-to-screen distance. It has become cost-effective to use plastic aspherical elements to meet these requirements. Periodic strip-PBS arrays have been widely adopted for polarization recycling, but aperiodic homogenizers are sometimes used to correct the uneven magnification and symmetry limitations of conic reflectors. Bright-state and dark-state beams must occupy distinct etendues in the half space above a reflective light valve, creating a vulnerability to crosstalk. Crosstalk from a polarizing beamsplitter gives rise to a residual background intensity approx. 0.3*NA2, unless a quarterwave corrector is used. Crosstalk can also arise from stress birefringence in prism substrates. Stray light makes an indirect contribution to background, but can sometimes be corrected by filtering.