Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-01-22
2001-01-16
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S096000, C349S098000, C349S120000
Reexamination Certificate
active
06175400
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a broadband cholesteric optical device, for instance for use in polarisers, filters, liquid crystal devices and polarising beam-splitters. Such devices may be used in displays such as liquid crystal displays and as colour or notch filters. Such devices may also be used in head mounted displays, optical and electronic measuring and sensing systems, compensators and for high flux applications.
DISCUSSION OF THE RELATED ART
As is well known in the art, a cholesteric liquid crystal is one in which the director rotates through the material, forming a helical structure. The term “cholesteric” is synonymous with “chiral nematic”.
EP 0 720 041 discloses patterned cholesteric colour filters and polarisers which comprise several layers which are active in different defined spectral bands.
EP 0 634 674 discloses a wide spectral and angular bandwidth rear. polariser for direct view displays. The broadband polariser is made using high birefringence cholesteric materials or by using stacks of lower birefringence cholesteric films.
EP 0 606 940 discloses a broadband cholesteric polariser which is made using a combination of ultraviolet (UV) intensity profile and diffusion to expand the polariser bandwidth. The intensity profile results from using a polymerising wavelength in a range where the maximum of the sum of the absorptions of the cholesteric material and the photoinitiator exists. Alternatively, an appropriate UV absorbing dye is added to the cholesteric mixture. The polariser comprises a graded pitch structure which varies monotonically from one surface of the polariser to the other.
Although not shown, similar off-axis birefringence effects occur if a thick cholesteric layer is provided after each reflecting layer.
“Optics of cholesteric liquid crystals”, V. A. Belyakov et al, Sov. Phys. Usp. 22(2), pp 63-88, Febrary 1979 and “Optical properties of the interface between a twisted liquid crystal and an isotropic transparent medium” G. Joly et al, J Optics, vol 25 pp 177-186 (1994) disclose that, for a single pitch cholesteric film, the polarisation state of reflected and transmitted light has a complex dependence on wavelength and angle of illumination. For graded pitch cholesteric films providing wider reflection bandwidths, the angular dependence is more complex but has not been studied. “Theory of light reflection by cholesteric liquid crystals possessing a pitch gradient” L. E. Hajdo et al, J.Opt. Soc. Am. vol 69, No.7, July 1979 considers only normal incidence.
WO96/02016 discloses a backlight illumination system for a liquid crystal device (LCD) comprising a broadband cholesteric polariser. This patent discloses that improved off-axis performance may be achieved by orienting the cholesteric liquid crystal polymer (CLCP) polariser such that the largest pitch is closest to the illumination source. Also, a negative birefringence quarter wavefilm may be used to provide a further improvement to the off-axis performance as well as to convert light to a linearly polarised state.
It is known to use compensators in LCDs in order to reduce or eliminate the unwanted effects of birefringence. Various types of compensators for dealing with specified birefringence problems have been disclosed. For instance, a negative birefringence film whose optic axis is normal to the film plane is disclosed in Japan Display '92 247-250 for improving the viewing angle of a normally white mode twisted nematic LCD. Also, angular compensation for a normally white mode twisted nematic or super twisted nematic LCD using a short pitch cholesteric liquid crystal polymer film such that the intra-plane refractive index is substantially averaged and larger than the refractive index in the thickness direction is disclosed in EP 0 531 120. The compensation film essentially has a negative uniaxial structure whose optic axis is normal to the plane of the film. Multilayer films and holographically formed grating structures have also been used as negative birefringence compensators for normally white mode twisted nematic LCDs. SID '95, P47, 555-558, S. T. Wu discloses the use of biaxial compensators to improve the contrast ratio both on-axis and off-axis.
SID '95, P50 Nishimura “Colour compensation” discloses the use of a liquid crystal polymer film with a super twisted nematic structure and controllable retardation, twist angle and dispersion for improving the contrast ratio of super twisted nematic LCDs over the visible spectrum at normal incidence.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a broadband cholesteric optical device comprising a broadband cholesteric layer, characterised by a first compensator for providing a desired off-axis device performance, the first compensator comprising a first layer having positive birefringence and an optic axis substantially perpendicular to the first layer.
The first compensator may comprise a second layer having negative birefringence and an optic axis substantially perpendicular to the second layer.
The desired off-axis device performance may be reduced angular dependence.
The cholesteric layer may have a graded pitch which increases monotonically from a first surface to a second surface thereof.
The cholesteric layer may have a graded refractive index which increases monotonically from a first surface to a second surface thereof.
The sum of the off-axis birefringence of the first and second layers may be substantially equal to zero for a wavelength corresponding to the shortest pitch of the cholesteric layer and substantially equal to but opposite that of the cholesteric layer for a wavelength corresponding to the longest pitch of the cholesteric layer, and the first compensator may be disposed adjacent the first surface of the cholesteric layer.
The refractive index dispersions of the first and second layers may be such that:
|&Dgr;
n
1
(400)/&Dgr;
n
1
(700)−&Dgr;
n
2
(400)/&Dgr;
n
2
(700)|>0
where &Dgr;n
1
(400) and &Dgr;n
1
(700) are the birefringence
5
of the first layer at wavelengths of 400 and 700 nanometres respectively, and &Dgr;n
2
(400) and &Dgr;n
2
(700) are the birefringences of the second layer at wavelengths of 400 and 700 nanometres, respectively; i.e. &Dgr;n is the magnitude of the difference between the refractive indices in the plane and perpendicular to the plane.
The sum of the off-axis birefringence of the first and second layers may be substantially equal to zero for a wavelength corresponding to the longest pitch of the cholesteric layer and substantially equal to but opposite that of the cholesteric layer for a wavelength corresponding to the shortest pitch of the cholesteric layer, and the first compensator may be disposed adjacent the second surface of the cholesteric layer.
The refractive index dispersions of the first and second layers may be such that:
|&Dgr;
n
2
(400)/&Dgr;
n
2
(700)−&Dgr;
n
1
(400)/&Dgr;
n
1
(700)|>0
where &Dgr;n
1
(400) and &Dgr;n
1
(700) are the birefringences of the first layer at wavelengths of 400 and 700 nanometres, respectively, and &Dgr;n
2
(400) and &Dgr;n
2
(700) are the birefringences of the second layer at wavelengths of 400 and 700 nanometres, respectively.
The first layer may comprise a reactive mesogenic material. The first layer may comprise a homeotropically aligned reactive mesogenic material.
The first layer may comprise part of the cholesteric layer having a predetermined alignment.
The first layer may comprise at least one stretched polymer film.
The first layer may comprise a plurality of uniaxial films, each of which has negative birefringence and an optic axis substantially in the plane thereof, the optic axes of the or each adjacent pair of the uniaxial films being angularly spaced by a non-zero angle. The first layer may comprise two uniaxial films whose optic axes are substantially perpendicular to each other.
The first layer may comprise a plurality of biaxial films, each of which has a refractive index in a direction perpendi
Brown Robert George
Davis Gillian Margaret
Duncan Anderson James
Walsh Kathryn
Chowdhury Tarifur R.
Renner, Otto, Boiselle & Sklar LLP
Sharp Kabushiki Kaisha
Sikes William L.
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