Electro-optical polarization interference filters

Color in a typical liquid crystal display is generated by the use of color filters patterned over individual display pixels. These filters consist of dyed gelatin, dyed polyimide, or colored ink and produce color by absorption. In general, only three filters, red, green, and blue are required to generate the full spectrum. Assuming that optical energy is evenly distributed in the red, green, and blue parts of the spectra then it is clear that only a third of the energy makes it out to the observer. This causes the display to appear very dark and requires the use of high brightness, power hungry backlights. These filters also reduce the display resolution since each pixel has to be subpixelated into three, over which the RGB color filters are applied. There has been a lot of work done recently to produce color in displays without having to use color filters. Such work involves use of stacked cholesteric displays, guest host displays, and birefringence color based displays. All of these solutions however suffer from low brightness, insufficient chromaticity, or the inability to produce full color. In this work we show one promising and unique solution, the use of polarization interference filters for the generation of color. Polarization interference filters (PIF) have the ability to provide high light throughput while retaining excellent chromatic characteristics. PIFs work by introducing a phase shift between two orthogonally polarized field components. This can be achieved by using either a static or dynamic optical element such as a uniaxial retarder or electrically driven liquid crystal cell. Color is generated by the interference of these two components with an analyzer. Since there are no additional losses other than the ones associated with polarizer absorption, the PIF can generate color with a higher luminance than absorptive color filters. PIF's have been the focus of attention for a number of years in areas of study ranging from astronomy to lasers. Recently, the use of such a filter was suggested as a means of generating color in LCD's and also for use as low loss color switches. The designs however are not multiplexable, and no clear route toward a low cost high brightness color display was shown. In this work using computer simulation, the design, use, and optimization of a PIF is shown for both active and passive matrix displays. Computer modeling of the displays incorporating such filters provides a fast and economical means of design and optimization. Using the Extended Jones and Berreman 4x4 optical simulation techniques, we developed a tool that allowed us to perform a detailed characterization of PIF based electrically controlled birefringence (ECB) active matrix and super twisted nematic (STN) passive matrix displays. For the ECB case we investigate the effects of the number of retarders that the PIF comprises of, has on the chromaticity of the display. In the STN case we show for the first time the integration of a PIF filter with a passive matrix optical element. The performance of this multiplexable display is fully characterized in terms of its light throughput, viewing angle characteristics and chromaticity. The final fully optimized device shows excellent color characteristics and brightness with the viewing angle being on par or better than double layer STN (DSTN) based devices.