Two polarization-recycling techniques have been proposed to increase the efficiency of illumination of liquid-crystal display (LCD) panels. The motivation for this proposal lies in the inherent inefficiency of an LCD panel: For proper operation, illumination with polarized light is necessary, but a typical lamp generates unpolarized light. If one simply passes the lamp light through a polarizer on the way to the LCD panel, then one wastes the half of the light that is in the undesired polarization. To increase the efficiency of illumination, one would have to recycle the otherwise wasted light, converting the undesired polarization to the desired one; this is what is meant by "polarization recycling."
Unlike a related older polarization-recycling technique, the two proposed polarization-recycling techniques would not enlarge the cross section of the illuminating beam. (Such enlargement is a disadvantage in a typical application in which one seeks to illuminate a small panel.)
Figure 1 schematically depicts the optical configuration for the first proposed technique. The unpolarized light from a lamp would be concentrated by a reflector and directed to a polarizing beam splitter. The p-polarized light would pass through the beam splitter, while the s-polarized light would be reflected perpendicularly toward an LCD panel. The p-polarized light would strike a flat mirror, then would travel back to the lamp reflector, where it would be reflected twice. After emerging from the lamp reflector, the p-polarized light would pass through a half-wave retarder that would occupy half of the beam cross section. The half-wave retarder would cause the reflected p-polarized light to become s-polarized. In the beam splitter, this newly s-polarized light would be reflected perpendicularly toward the LCD panel, along with the originally s-polarized light.
For the portion of unpolarized lamp light that would hit the half-wave retarder first, the end result would be the same, though the sequence of events would differ: After passing through the half-wave retarder, the initially unpolarized light would remain unpolarized until it reached the polarizing beam splitter. The p-polarized subpart of this part of the light would pass through the beam splitter, while the s-polarized subpart would be reflected perpendicularly toward the LCD panel. The p-polarized light would strike the flat mirror and would go back through the half-wave retarder, which would convert it to s-polarized light. Continuing along its path, this portion of s-polarized light would be reflected twice by the lamp reflector, and would finally be reflected perpendicularly, by the beam splitter, toward the LCD panel.
Figure 2 schematically depicts the configuration for the second proposed technique. Light from a lamp would reach a reflective polarizer; the p-polarized light would pass through to the LCD panel, while the s-polarized light would be reflected. The reflected s-polarized light would pass through a quarter-wave retarder, becoming circularly polarized. The circularly polarized light would be reflected by a flat mirror and would go back through the quarter-wave retarder, which would cause this light to become p-polarized, as needed to join the originally p-polarized light in illuminating the LCD panel.
This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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