Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-07-30
2001-10-30
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S018000, C349S061000
Reexamination Certificate
active
06310673
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to color polarizers consisting of a polarizer in combination with a stack of retardation films, and to their use in liquid crystal switchable color filters and color displays with high saturation and high throughput.
BACKGROUND OF THE INVENTION
Switched Polarizer Filters
There are two basic classes of liquid crystal color switching filters: polarization interference filters (PIFs) and switched-polarizer-filters (SPFs). The basic unit of an SPF is a stage, consisting of a color polarizer and a two-state neutral polarization switch. This class is intrinsically binary tunable, such that each filter stage permits switching between two colors. Stages are cascaded in order to provide additional output colors. Color polarizers used in SPFs include single retardation films on neutral linear polarizers and pleochroic color polarizing filters. The polarization switch can be a liquid crystal (LC) polarization switch preceding a static polarization analyzer. The switch optimally provides neutral polarization switching. The chromatic nature of the active element degrades performance and is ideally suppressed in an SPF.
Shutters based on a color polarizer consisting of a neutral-polarizer followed by a single retarder are well reported in the art (for example in U.S. Pat. Nos. 4,003,081 to Hilsum, 4,091,808 to Scheffer and 4,232,984 to Shanks). While the polarizer/retarder structure can be described as a complementary color polarizer in the sense that it is possible to produce two distinct hues by rotating the polarizer through 90 degrees, using this type of color polarizer in an SPF does not result in saturated colors.
Shutters based on pleochroic color polarizers are also well reported (for example in U.S. Pat. Nos. 4,582,396 to Bos, 4,416,514 to Plummer, 4,758,818 to Vatne and 5,347,378 to Handschy). Pleochroic color polarizers are films that function as linear polarizers in specific wavelength bands. They are formed by doping a polymer with long-chain pleochroic dyes. Incident white light polarized along one axis is fully transmitted, but is selectively absorbed along the orthogonal axis. For instance, a cyan color polarizer functions as a linear polarizer by absorbing the red along one axis. A color polarizer that passes a primary color (either additive or subtractive) along each axis can be formed as a composite consisting of two films with crossed axes. Colors are typically selected using crossed complementary color (eg. red/cyan) polarizer films coupled with a switchable polarizer. A full-color device can comprise five polarizing films (one neutral), and two switching means. The resulting structures provide poor overall peak transmission.
Polarization Interference Filters
The simplest PIFs are essentially two-beam interferometers, where a uniaxial material induces a phase shift between orthogonally polarized field components. Color is generated by interfering these components with an analyzing polarizer. Color switching is accomplished by changing the phase shift between the arms. The most rudimentary color switches comprise a single variable retarding means between neutral polarizers. Single stage devices can also incorporate passive bias retarders with variable birefringence devices. However, these single stage PIFs are incapable of providing saturated color.
PIFs often comprise cascaded filter units in a Lyot structure, each performing a distinct filtering operation to achieve improved selectivity. A polarization analyzer is required between each phase retarder, reducing transmission. Though adequate color saturation is obtained, multiple-stage birefringent filters are by definition incapable of-functioning as color polarizer. This is quite simply because color polarizers must transmit both orthogonal polarizations, which does not permit internal polarizers.
Tuning is accomplished by varying the retardance of active elements in each stage, maintaining specific relationships between retardances, in order to shift the pass-band. PIFs use LC elements as variable retarders in order to shift the transmission spectrum. As such, in contrast to SPF, the chromaticity of the active element retardance is not only acceptable, it is often an integral aspect of the design. In PIF designs, an analyzing polarizer is a static component and tuning is accomplished by changing the retardance of the filter elements. When multiple active stages are used, the retardances are typically changed in unison to shift the pass-band, while maintaining the basic design. Variable birefringence PIFs can be tuned to provide peak transmission at any wavelength. By contrast, SPFs do not provide tunable color.
Solc filters (Solc (1965), J. Opt. Soc. Am. 55:621) provide high finesse spectra using a cascade of identical phase retarders, with complete elimination of internal polarizers. The Solc filter is a specific example of a much broader class of filters. In this generalization, Harris et al. (Harris et al. (1964), J. Opt. Soc. Am. 54:1267) showed that any finite impulse response (FIR) filter transmission function can in principle be generated using a stack of properly oriented identical retardation plates. Numerous researchers have used the network synthesis technique, along with standard signal processing methods, to generate FIR filter designs. These designs have focussed on high resolution as opposed to broad pass-bands. Tunability, when mentioned, requires that all retardances are varied in unison.
The color polarizer of the present invention, though using some of the design principles of polarization interference filter technology, is a component for use in switched-polarizer-filter structures. Prior art PIF devices require fully active retarder stacks to effectively shift the design wavelength of the filter while maintaining the basic design. Conversely, the color polarizers of the present invention use a passive retarder stack to generate a particular fixed spectral profile which is, as in SPF, invertible by using effective rotation of the analyzing polarizer.
SUMMARY OF THE INVENTION
This invention provides a complementary color polarizer using a single neutral polarizing film followed by a stack of two or more retardation films. This technology is termed “polarizer-retarder-stack” (PRS). The use of more than one retarder in the retarder stack increases the span of the filter impulse response and the retarder orientations are selected to control the amplitudes of the impulses. In a preferred embodiment, the color polarizers of this invention produce orthogonally polarized complementary primary colors (red/cyan, green/magenta, or blue/yellow). In combination with a blocking filter, the color polarizer of this invention produces two orthogonally polarized additive primary colors. Similarly, when a pleochroic color polarizer follows a PRS, it can produce two orthogonally polarized additive primaries. In an alternative structure, a pleochroic color polarizer is used instead of the neutral polarizer, and the color polarizer produces two orthogonally polarized additive primary colors in combination with one unpolarized additive primary to give the appearance of two subtractive primary colors.
This invention further provides color filters which utilize the color polarizers of this invention in combination with a polarization separator to separate the orthogonally polarized colors. The polarization separator can be passive, such as a polarizing beam splitter, or active, such as a switchable polarizer. The switchable polarizer can be a rotatable polarizer or a polarization switch in combination with a fixed polarizer. The polarization switch can employ a nematic or a smectic liquid crystal cell. The fixed polarizer can be a pleochroic color polarizer, in which case one additive primary is transmitted unpolarized. Two color filters of this invention can be cascaded to provide a three color (RGB) filter with an off-state. In combination with a monochrome display, the color filters of this invention provide color displays. In combination with a camera or electronic receiv
Fleshner & Kim LLP
Parker Kenneth
University Technology Corporation
LandOfFree
Transmissive optical polarizing filters designed to maximize... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Transmissive optical polarizing filters designed to maximize..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Transmissive optical polarizing filters designed to maximize... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2595417