Liquid crystal color displays which cool from isotropic to twist

Details Liquid crystal color displays which cool from isotropic to twist Liquid crystal color displays which cool from isotropic to twist

1990-10-31

1993-02-23

Sikes, William L.

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

359 64, 359 90, G02F 113

Patent

active

051895340

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to liquid crystal colour displays in which information is displayed in one colour on a background of another colour.
2. Discussion of Prior Art
One known form of colour display uses a smectic liquid crystal material addressed by a laser in projection displays and digital storage devices; see for example F. J. Kahn, Applied Physics Letters, 22, p. 111 (1973); A. G. Dewey, Optical Engineering, Vol. 23, No. 3, p. 230 (1984); and H. Birecki et al, SPIE Proceedings Vol. 420 edited by A. E. Bell and A. A. Jamberdino (1983).
A typical projection display with smectic liquid crystal materials uses a scattering and a transmissive state of the smectic material to provide a display. Both the light scattering and transmissive states can be stable and co-exist in different parts of a display at the same time. Information is written as areas of light scattering texture on a background of light transmissive texture i.e. dark features on a light background; or visa versa. The display is monochrome. Multi-colour may be provided by using a plurality of systems, each with an associated filter, and combining the outputs say on a screen. This is quite complicated and expensive. The displays need a small highly localised light source, e.g. an arc lamp, and Schlieren stop optics. These are wasteful of power and make it difficult to obtain adequate display brightness.


SUMMARY OF THE INVENTION

According to this invention these difficulties are overcome by providing a two colour display using a single cell of smectic material arranged as a polarisation switch together with coloured polarisers. The two colours may be black, considered as a colour or absence of light, and white, considered as a colour or the absence of other colours; or two colours such as red and green.
According to this invention a liquid crystal colour display comprises a smectic liquid crystal cell formed of a layer of smectic liquid crystal material located between two cell walls both coated with sheet electrodes for applying an electric field across the smectic material layer, a light source for illuminating the cell, a lens system for directing light from the light source via the cell to a display screen, a laser light source for locally heating the cell, and a scanning system for scanning laser light across the cell, characterised by: smectic and isotropic phase, cell walls, the alignment direction on one wall being at least 45.degree. to that on the other wall, from the isotropic state in the absence of an applied electric field adopts a twisted light rotating configuration, and in cooling from the isotropic state in the presence of an electric field adopts a non-rotating configuration configurations cooperate to display information in one colour on the background of the other colour.
The polariser receiving light from the cell may be a neutral grey polariser, in which case a black and white display is produced; a non-black coloured polariser alone, in which case a non-black colour and white display is produced; or two different non-black coloured polarisers may be used to produce a display of two different colours with information of one colour on the background of the other colour.
The two coloured polariser are preferably arranged with their optic axis orthogonal to one another and orthogonal or parallel to the alignment direction on the nearest cell wall.
A small amount of cholesteric material, e.g. 0.1 to 10% wt., may be dissolved in the smectic material to assist in providing a uniform twist direction.
The alignment direction on the cell walls may be 90.degree.+/-10.degree. preferably +/-5.degree. or less.
A small amount of a dye, absorbing to the laser wavelength, may be dissolved in the liquid crystal material.
The light source is preferrably an incandescent filament bulb such as a halogen bulb, backed by a reflector.


BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings of which:
FIG.

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patent: 4751509 (1988-06-01), Kubota et al.
Raynes, "Cholesteric Texture and Phase Charge Effects", Nonemissive Electrooptic Displays, ed. Kmetz, Plenum Press, N.Y. 1976.
IBM Technical Disclosure Bulletin, vol. 19, No. 5, Oct. 1976, G. J. Sprokel: "Twisted Smectic Storage Display Cells", p. 1888.
Optical Engineering, vol. 23, No. 3, May-Jun. 1984, A. G. Dewey: "Laser-Addressed Liquid Crystal Displays", pp. 230-240.

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