Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1999-01-22
2001-01-09
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S097000, C349S106000, C349S120000, C349S119000, C348S742000, C348S744000, C359S490020, C359S490020
Reexamination Certificate
active
06172722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to color filters having nematic liquid crystal cells, and to compensating for the chromaticity of the nematic cells and utilizing fast transitions of the nematic cells in switching the color filters.
2. Background of the Invention
Switched-polarizer-filters (SPFs) have one or more stages, each stage consisting of a color polarizer and a two state polarization switch. The color polarizer provides different transmission spectra along orthogonal polarization axes, and the polarization switch is a switchable analyzer which selects which of the two polarizations is transmitted. The SPF 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 are generally dye-type pleochroic color polarizing filters (for example in U.S. Pat. No. 4,582,396 to Bos, U.S. Pat. No. 4,416,514 to Plummer, U.S. Pat. No. 4,758,818 to Vatne and U.S. Pat. No. 5,347,378 to Handschy). Dye-type 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.
Recently, polarizer-retarder-stack (PRS) color polarizers have been described (U.S. patent application Ser. No. 08/447,522, filed May 23, 1995, which is incorporated by reference herein in its entirety). The PRS comprises a linear polarizer in combination with a stack of retarders. The number of retarders in the stack and the retardances and orientations of the retarders are selected such that an additive primary spectrum is transmitted along a first polarization axis and the complementary subtractive primary spectrum is transmitted along the orthogonal polarization axis.
The polarization switch in a SPF can be a liquid crystal (LC) cell in combination with a static polarization analyzer. The switch optimally provides neutral polarization switching. Unfortunately, the performance of these filters is significantly degraded by the chromatic nature of the liquid crystal active element. In general, liquid crystal cells accurately switch between orthogonal polarizations in only one spectral region. Outside of this region the filters are subject to leakage of the blocked bands and reduced throughput of the pass-band.
Smectic liquid crystal polarization switches generally comprise a linear polarizer and a liquid crystal half-wave retarder with fixed retardance and rotatable orientation. Rotating the optic axis of the liquid crystal cell between 0° and 45° with respect to the polarizer switches the polarization between two orthogonal axes. Achromatic smectic liquid crystal polarization switches have recently been described (U.S. patent applications Ser. No. 08/419,593, filed Apr. 7, 1995, and Ser. No. 08/549,963, filed Oct. 30, 1995), and can be used to provide high quality color filters.
Nematic liquid crystal polarizations switches generally comprise a linear polarizer and a nematic liquid crystal cell with fixed orientation and variable retardance. The achromatic polarization switches developed for smectic liquid crystal cells cannot be employed with nematics, leaving nematic liquid crystal color filters beset with leakage and low throughput.
A second drawback to nematic liquid crystal filters is their slow switching speed. Nematic liquid crystal cell typically take a few hundred microseconds to realign in response to an applied electric field and much longer, several milliseconds to reset to the unenergized state. Because of the slow response time, the rate at which the filter can sequence through the three primary colors is limited, which in turn limits the rate at which subframes can be acquired or displayed for camera or display applications.
BRIEF SUMMARY OF THE INVENTION
This invention provides chromaticity compensation for color filters having nematic liquid crystal tuning elements, as well as fast-transition tuning units for use in combination with chromaticity compensation. The filters of this invention have two stages formed by three linear polarizers in series, as least two of which are color polarizers. Within each stage is a tuning unit comprising a nematic liquid crystal cell switchable between a first state wherein the polarization of light is transformed to the orthogonal state (orthogonally transformed) and a second state, the “normal” state, wherein the polarization of light is unchanged. In the first state the liquid crystal cell chromaticity introduces significant distortion to the filter stage, but in the second state it does not. Chromaticity compensation is achieved in this invention by optimizing the use of the undistorted normal states and by judicious choice of the design wavelengths of the liquid crystal cells.
The chromaticity compensating filter of this invention has first and second stages which normally transmit first and second subtractive primary colors, respectively, and normally block first and second additive primary colors. Since two of the primary colors can be simultaneously blocked by both stages (redundant blocking), one of the stages being the normal, undistorted state, the filter does not suffer leakage of these colors. The third primary color does not benefit from redundant blocking and furthermore is only blocked when the liquid crystal cell is in the chromatic state. It is therefore the most compromised by the chromaticity of the liquid crystal cells and is most at risk of leaking. To optimize blocking of the third primary color, the design wavelength of the liquid crystal cells preferably falls within the region of the third primary color.
This invention further provides compound tuning units, for use in the chromaticity compensating filter, that allow the use of fast transitions in switching between three primary colors. Since nematic liquid crystal cells typically have faster rise times than fall times, fast transitions are achieved by using only the unenergized to energized transitions to sequence through the three states. The sequence can be represented digitally as 00, 01 (or 10), and 11, where 0 is an unenergized state and 1 is an energized state. The key is to make these three states correspond to high quality red, green and blue transmission spectra. In the chromaticity compensating filter described above, the switching states corresponding to the three primary colors are 01, 10 and 11, requiring a slow transition in the sequence.
A first embodiment of the compound tuning unit of this invention uses a bias retarder in combination with the nematic liquid crystal cell to pre- or post-condition the polarization of light through the liquid crystal cell. The bias retarder is a half-wave retarder oriented at ±45°, and it orthogonally transforms the polarization of light within the stage. The nematic cell reverses the transformation. The chromaticity caused by the first transformation is reversed by the second transformation. For the compound tuning unit, the unenergized nematic corresponds to the normal state of the tuning unit, wherein the polarization of light is unchanged, while the energized nematic corresponds to the orthogonally transforming state. For the chromaticity compensating filter having this compound tuning unit in one stage, the switching states 00, 01 and 11 therefore provide the three primary colors, and the sequence uses only fast transitions.
Another embodiment
ColorLink, Inc.
Fleshner & Kim LLP
Ngo Julie
Sikes William L.
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