Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1994-01-18
1995-12-26
Sikes, William L.
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 78, 359100, G02F 11335
Patent
active
054792819
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to display devices, and particularly to ferroelectric liquid crystal display devices.
The majority of liquid crystal displays (LCDs) rely on the use of two polarisers to render visible the difference between the switched and unswitched regions. These polarisers are disposed one each side of the LC cell. The appearance of the display depends critically on both the performance and the alignment of these polarisers.
For example, in a twisted nematic LCD, one polariser is aligned so that its absorption axis is either parallel or perpendicular to the alignment direction of the liquid crystal molecules (the director) on the neighbouring wall of the cell. The other polariser is arranged either parallel (for a dark OFF state) or perpendicular (for a bright OFF state) to the first polariser, and is placed on the second wall of the cell. Such arrangements are well known in the art.
In ferroelectric LCDs, which operate by switching the LC director in the plane of the LC layer, the polarisers are generally set to be crossed, with one polariser axis parallel to the LC director in one of the display states. Such polariser alignments provide displays with high contrast which are suitable for viewing in transmission and are well known to those skilled in the art.
Reflective LCDs are constructed in the same way, but with an additional reflective layer (generally non-specular) disposed behind the rear polariser. Therefore light passes through the polariser/LC cell/polariser combination twice in opposite directions. It is found that displays constructed using conventional methods described above are very dull and difficult to read. They maintain high contrast, but, because of the double passage of the light through the display, the nominally "bright" states are, in fact, also rather dark, making them difficult to use.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved reflective ferroelectric liquid crystal display.
According to the invention there is provided a reflective liquid crystal display device, comprising a ferroelectric liquid crystal cell; a first polariser through which light from a light source passes before passing through the cell; a second polariser through which the light passes after passing through the cell; and means to reflect the light back through the second polariser, the cell and the first polariser in that order for viewing by an observer; wherein the first and second polarisers are orientated with their directions of polarisation at angles of .alpha. and .beta., respectively, relative to an alignment direction, the value of .alpha.+.beta. being determined from the formula ##EQU2## where the optic axes in the two switching states of the liquid crystal material are at angles of +.theta..sub.c and -.theta..sub.c, respectively, to the alignment direction.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which
FIG. 1 is a schematic sectional view of a reflective ferroelectric liquid crystal display device, and
FIG. 2 illustrates, schematically, angles relevant to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a reflective liquid crystal display device 1 comprises substrates 3 and 5 which contain there between a layer 7 of a ferroelectric liquid crystal material. The inner surfaces 9,11, respectively, of the substrates 3 and 5 are rubbed in a predetermined alignment direction, the direction being the same for both substrates. A first polariser 13 is located at the upper surface of the substrate 3 and a second polariser (analyser) 15 is located at the lower surface of the substrate 5, as viewed in FIG. 1. A reflective layer 17 is disposed beneath the polariser 15. In use of the device 1, light, as indicated by arrows 19, passes through the polariser 13, the substrate 3, the liquid crystal layer 7, the substrate 5 and the polariser 15, and is reflected by the layer 17 so
REFERENCES:
patent: 4709994 (1987-12-01), Kanbe
patent: 4878740 (1989-11-01), Inaba et al.
patent: 5082352 (1992-01-01), Kawagishi
Optical Engineering, vol. 26, No. 5, May, 1987, pp. 373-384, "Properties and Applications of Ferroelectric Liquid Crystals", J. S. Patel et al.
Molecular Crystals and Liquid Crystals, vol. 143, Feb., 1987, pp. 101-112, "Electrooptics of a Thin Ferroelectric Smectic C* Liquid Crystal Layer", M. I. Barnik et al.
Charsley Mark F. W.
Wiltshire Michael C. K.
Yeoh Colin T. H.
GEC--Marconi Limited
Nguyen Tiep H.
Sikes William L.
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