Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-14
2003-10-07
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S096000, C349S115000
Reexamination Certificate
active
06630974
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to polarizing broad bands of light and particularly to polarizing broad bands of light at large angles of incidence from any azimuth without polarization or color distortion in both transmission and reflection modes.
2. Description of the Related Art
Cholesteric Liquid Crystals (CLC) have the property of being able to reflect or transmit circularly polarized light depending on the handedness of the light, reflecting one circular polarization and transmitting the opposite circular polarization. For example, a right handed cholesteric liquid crystal will reflect right handed circularly polarized light and transmit left handed circularly polarized light. Cholesteric liquid crystals can be made which function over a large bandwidth such that all the colors in the spectrum can be reflected or transmitted. These properties are very useful in many applications and work well when the incident light is normal to the cholesteric liquid crystal films. However, polarization and spectral distortions occur at large angles of incidence and vary at different viewing angles due to the eigen-states of the light being elliptical rather than circular. These distortions reduce the polarizer efficiency and produce color shifts. Many attempts to correct these polarization and color distortions have been made in the past.
For example in U.S. Pat. No. 5,731,886 entitled “Birefringent Compensator for Reflective Polarizers”, Taber et al. issued Mar. 24, 1998 the invention comprises a CLC material layer and a compensator comprising a positively birefringent C-plate layer which is a uniaxial film with an optical axis perpendicular to the surfaces of the film. The compensation film is inserted on the path of the light transmitted through the CLC polarizer, and incorporated into a brightness enhancement system for a liquid crystal display.
The simulation results presented in the patent indicate an improvement in color and luminance behavior of the LCD brightness enhancement system when the C-plate is incorporated into the system. However, no spectra of the CLC polarizers used in these simulations are presented, either with or without an included compensator. The only information about the CLC polarizers is their end-point values of the pitch (0.26 and 0.402 microns) which suggest a reflection band from about 400 nm to 600-650 nm. Improvement in chromaticity behavior of the brightness enhancement system does not necessarily indicate that the CLC polarizer angular behavior has been completely compensated. In addition, the chromaticity of the system is presented for only one azimuthal orientation, and need not be the same at all azimuthal orientations. This compensation technique improves the polar angular behavior of the chromaticity at one azimuthal orientation only. European Patent Application EP 0860717 A2 published Aug. 26, 1998 entitled “Broadband Cholesteric Optical Device, Polarizer, Filter, Liquid Crystal Device, and Polarizing Beam-Splitter”, proposes to improve the viewing angle behavior of the broadband CLC-based polarizers by using compensation films, which are uniaxial and have their optical axes perpendicular to the surfaces. The compensation films typically consist of two layers. The layer closest to the CLC polarizer has a positive birefringence, and the second layer has a negative birefringence. These two-layer compensation films may be inserted either in front of the CLC polarizer, or at the back, or both. This compensation technique requires dispersion of the refractive indices of the compensation films. The patent application gives formulas indicating the necessary dispersion relationships.
The CLC polarizer used in the simulations described in the patent has a bandwidth from about 380 nm to 700 nm, and thus would not cover the entire visible range for all incident angles. As the patent indicates, the proposed technique is capable of improving the angular performance of this CLC polarizer up to incident angles of 49 degrees. In order to achieve the proposed compensation, dispersion of the refractive indices of both compensation films is a prerequisite. The authors have derived formulas to specify the required dispersion behavior. These requirements may be quite difficult to satisfy in practice.
Impractical material requirements, however, are not the major drawback of this compensation method. The CLC polarizer spectra presented in the patent indicate that even at normal incidence the polarization state that has to be reflected from the polarizer is actually 20-30% transmitted. When unpolarized light is incident on such a polarizer, saturated reflection will not be achieved, that is, more than 50% of the incident light will be transmitted, and very importantly, the transmitted light will be only partially polarized. This feature will severely degrade the contrast ratio of any system utilizing this CLC reflective polarizer.
A very important aspect of this compensation technique is that it actually would be ineffective if the polarizers were designed so that a saturated reflection did occur for the entire visible range at all incident angles (that is, if the polarizers were thicker, with higher birefringence, and larger bandwidth).
European Patent Application EP 0860716 A2 published Sep. 26, 1998 entitled “Broadband Cholesteric Optical Device, Polarizer, Filter, Liquid Crystal Device, and Polarizing Beam-Splitter” discloses a multi-layer system to serve as a broadband reflective polarizer with improved viewing angle performance. The system consists of alternating narrow-band CLC and homeotropic films with positive birefringence. Broadband reflection is achieved by requiring each CLC layer to have a different peak reflection wavelength. The multi-layer design is to achieve a reflective circular polarizer with superior angular performance. The patent application presents simulation results for a 15-layer system, which is capable of reflecting in the range from 380 nm to 630 nm at normal incidence. The angular behavior of such a polarizer is improved for incident angles of up to 40 degrees.
To produce a reflective circular polarizer with a larger bandwidth and superior angular performance would require the addition of more CLC and homeotropic layers, all having precisely controlled parameters. These requirements make the fabrication of such a system difficult, prone to defects, and expensive.
U.S. Pat. No. 5,808,794 entitled “Reflective Polarizers Having Extended Red Band Edge for Controlled Off-Axis Color”, issued Sep. 15, 1998 attempts to solve the angular behavior problem of the broadband polarizers, with a broadband reflective linear polarizer consisting of alternating birefringent and isotropic layers with variable thickness. This linear polarizer does not have polarization distortions at large viewing angles. However, in order to achieve a reflection band covering the entire visible range at all incident angles, it must have at least 800 layers with precisely controlled layer thickness. This complicated structure makes the manufacturing procedure prone to defects and expensive. In addition, the layers have been found to delaminate at extreme environmental conditions, which makes the polarizer inappropriate for military and avionics applications.
SUMMARY OF THE INVENTION
A broadband polarizer and analyzer with two compensation films to compensate the polarization distortions of the light at large angles caused in the polarizer and analyzer layer. The compensation layers bring the light back to circularly polarized light without distortion of chromaticity for all incident angles at all azimuthal orientations.
The broadband polarizer and analyzer is typically a broadband CLC film having a variable pitch helix. Light incident normal to the CLC layer is reflected or transmitted as circularly polarized light with no distortion. Light incident at small angles to normal will have very small distortions. However, light incident at larger angles has more distortions and needs to be corrected for. The first layer of compens
Galabova Hristina
Li Le
Crispino Ralph J.
Nguyen Dung
Reveo Inc.
Sampson Richard L.
Sikes William L.
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