Polarization Conversion Systems are for converting all of the light from the lamp assembly from unpolarized to linearly polarized light since several of the current spatial light modulators like the LCD and LCoS modulators must have linearly polarized light to work properly.

Many proj
ection display systems and direct-view flat-panel TVs use unpol arized light sources (e.g., lamps, light-emitting-diodes (LEDs), am
bient light, etc.). However, many devices that employ liquid crystal (LC) materials, including LC on Silicon (LCOS) microdisplays or LC displays (LCD), may require this light to be polarized. While conventional polarizing elements can produce polarized light from unpolarized light (including sheet polarizers or various birefringent prisms) by permitting light of the desired polarization to pass therethrough, they may be inherently inefficient, since they typically operate by either absorbing the unwanted light or redirecting it in an unwanted direction. This can lead to greater than 50% loss of optical efficiency, even before the light enters the opto-electronic component. Such large losses are typically undesirable, especially in portable or battery-powered display systems where battery life is limited.

Polarization Gratings (PGs) employed here is a member of the class of birefringent diffractive optical elements comprised of a spatially periodic birefringence. It functions as a thin-film beamsplitter, whe
re incident light is angularly separated into two, forward-propagating, orthogonal, circular polarizations. As photographed below, its output is governed by the classic grating equation and input polarization state.
Light diffracted from broadband PGs is directed almost entirely into their first diffraction orders for most of the visible wavelength range. These first-orders naturally have orthogonal circular polarizations, but a quarter-wave can be added to convert them to horizontal and vertical polarization. The diffraction transmittance remains high (> 90%) within modestly oblique angles of incidence (±20˚). Note that an entirely different use of polymer PGs in projectors is also being studied, that essentially replaces all polarizing elements with PGs and directs both polarizations into the LC microdisplay simultaneously. While this prior approach presents several advantages, it requires significant re-design of the optical light engine. It should be clear that the PG-based PCS described here is broadly applicable to practically any HTPS or LCoS projector with little modification.

The new PG-based PCS is one of the best options for projectors that require polarized light, because it achieves high polarization conversion. It also manifests improved brightness (ansi-lumens) and color uniformity (duv), as compared to a polarizing beam splitter array based PCS, etc. This concept enabled us to demonstrate a compelling and practical energy efficient portable projector.

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[4] Komanduri, Kim et al. Proc. SPIE 8279, 8279 (2012)
[5] Kim et al. Appl. Opt. 51,
4852-4857 (2012)