GB2215898A

GB2215898A - LC Displays

Info

Publication number: GB2215898A
Application number: GB8901509A
Authority: GB
Inventors: Michael Keil; Dorith Schmidt; Ruediger Salomon
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1988-02-08
Filing date: 1989-01-24
Publication date: 1989-09-27
Also published as: GB8901509D0; FR2627000A1; DD268785A1; DE3900871A1; SE8900394D0; JPH024222A

Description

DESCRIPTION AN ACTIVE DISPLAY ELEMENT BASED ON THE LIQUID-CRYSTAL EFFECT The invention relates to an active display element which can be used to display information in a large variety of fields.

Display systems based on liquid-crystalline effects are well known in the pricr art. In order to recognise the information shown, external illuminating devices are used to achieve an improvement in contrast, in particular when there is low ambient light. Such illuminating devices usually provide a uniform brightening of the backgrcund with respect to area and therefore do not alter the structure or the method of operation of the liquid-crystal cell. In DE-OS 29 33 312, an ultraviolet lamp Waving a layer of luminescent material is disposed on te side of the liquid-crystal display element remote frcm the observer.

A similar solution is described in DE-OS 30 37 224, in which an electroluminescent layer is disposed on the rear side of the liquid-crystal cell. Ultraviolet radiation can, however, shorten the operating life, since it leads to photochfical reactions in the liquidcrystal layer. A further solution for increasing the contrast of the informatizn shown by way of uniform background radiation is described in DD 213 257.

Illumination is effected by light-emitting semiconductor chips, which are disopsed between the liquid-crystal cell and a wafer disposed therebeyond, which is lightscattering at the front and reflecting at the back.

When utilising electro-optical effects on the basis of interference phenomena, it is disadvantageous that exact cell optimisation, fluctuations in temperature and inhomogeneities in the layer thickness of the liquid crystal may lead to errors in colour or contrast in the information display.

DD 250 191 describes an arrangement in which light introduced laterally is reflected in dependence upon the orientation condition of the liquid-crystal molecules at the interface with the liquid crystal layer or is refracted into the layer. The disadvantages of this solution result from the use of the DSM effect. The associated high operating voltages, relatively long switching times, short operating life and limited resolution capability prevent a more wide-spread application, such as, for example, for large-area display elements. A further disadvantage in conventional electro-optical effects is that, as a result of the necessary polarizers, a considerable amount of light is lost and limited visual angle ranges for the information displayed occur.

It is an object of the present invention to provide a liquid-crystal display element for displaying information in a clearly visible manner, which element can be produced inexpensively as a result of its technologically simple structure, and hence ensures a high degree of reliability and image quality.

In accordance with the invention, an active display element based on a liquid-crystal effect, comprises a liquid-crystal cell having the following construction: glass plate - transparent electrodes, in the form of a segment, full or matrix display, - orientation layer liquid crystal - orientation layer - transparent electrodes - glass plate, and wherein the display element includes an illuminating device for laterally introducing linearly polarized light into a rear glass plate remote from an observer side of the element, the liquid crystal is a ferroelectric-smectic liquid crystal and a light-scattering layer is disposed on the side of the liquid-crystal cell facing the observer.

In order to ensure a defined state of total reflection, the liquid crystal is preferably orientated in such a way that, in the optically closed state, the longitudinal axes of the molecules are perpendicular to the polarization direction of the light radiated in.

The present invention therefore has the advantage of providing an active liquid-crystal display element, using which information can be displayed, independently of the ambient light, in high contrast and independently of the visual angle, and which is characterised by a high resolution capability, low temperature dependency and short switching times.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which shows the schematic structure of one embodiment of a display element in accordance with the present invention.

An active display element based on the liquidcrystal effect has the following known layer structure: a glass plate la - transparent electrodes 2a orientation layer 3 - liquid crystal 4 - orientation layer - transparent electrodes 2b - glass plate ib. In order to form a matrix, the transparnt electrodes 2a and 2b are strip-shaped and disposed perpendicularly to one another. -The display element also comprises an illuminating device 5 for laterally radiating linearly polarized light 8 into the glass plate la and is further characterised in that the liquid crystal is a ferroelectric-smectic liquid crystal 4 and a lightscattering layer 6 is disposed on the side of the liquid-crystal cell facing the observer.The ferroelectric-smectic liquid crystal 4 is orientated in such a way that, in the optically closed state, the longitudinal axes of the molecules are perpendicular to the polarization direction of the light 8 introduced.

The function of the display element is based on the principle of controllable total reflection utilising an electrically induced alteration in the refractive index.

The illuminating device 5 is set so that the polarised light falls on one side into the glass plate la and irradiates the entire interface between the orientaticn layer 3 and the liquid crystal 4. The orientation layers 3, whch are made of SiO, lead to a defined initial orientation, in which the longitudinal axes of the molecules of the liquid crystal 4 are perpendicular to the polarisation direction of the introduced light 8 (corresponds to refractive index nO). Excitation using an electrical field reorientates the liquid-crystal molecules, which results in an alteration of the refractive index to the value ne.The light introduction angle Z is selected such that, in the initial conditiion, that is given nO, it is slightly above the limiting angle of the total reflection A at the orientation layer 3/liquid crystal 4 interface, ad the light beam is reflected in total in accordance with the beam course 1. Reorientation produces ne of the liquid crystal 4. Since ne > no, the limiting angle of total reflection B at the above-mentioned interface increases, so that the light introduction angle JL is then below this limiting angle and the light beam is refracted into the liquid crystal 4 in accordance with the beam course II, and is subsequently scattered through the light-scattering layer 6 in all directions of the half-space in front of the glass plate Ib.The glass plate 1b may itself be light-scattering by virtue of increased surface roughness or other methods, as a result of which the light-scattering layer 6 may be omitted.

The thickness of the liquid-crystal layer 4 is 2 sm and is adjusted by spacers 7. In the case of large-area display elements, it is preferable to dispose the spacers 7 not only on the periphery of the component, but aso in the middle. In order to prevent these spacers 7 leading to interfering light points on the display if their dimensions are too large, lowrefracting material is used in order for total reflection to take place at these points.

For the operation mode,it would be ideal to use a ferroelectric-smectic liquid crystal 4 having a theoretical tilt angle of 450. The ferroelectricsmectic liquid crystal 4 used, which has a tilt angle of less than 450, means that, in the excited state, only part of the polarized light 8 introduced is refracted into the liquid crystal 4 and imaged on the display. The other part of the introduced polarized light 8 is reflected, in the optically closed state, at the orientation layer 3/liquid crystal 4 interface. This results only in a reduction in intensity of the information imaged; operation is not impaired. In the embodiment described, the materials listed in Table 1 were used for the various elements.

Table 1 Element Material Refractive Index Glass plates la, b BK 7 1.52 Transparent ITO 2.00 electrodes 2a, b Orientation layer 3 SiO 1.90 Liquid crystal 4 Ferroelectric- 1.455/1.625 smectic phase Spacers 7 SiO2 1.44 The display element described can be used in a large variety of fields to display information by realising segment, full or matrix displays. The specific advantages of the invention are to be found above all in its active character, as a result of which the display is not dependent upon ambient light, and in its improved image quality, which is characterised by a high resolution capability, as well as in the very short switching times and in the high multiplex rate. As a result, large-area display elements are made possible.

The active ferroelectric-smectic display element is further characterised by its technologically simple construction and inexpensive production. A further substantial advantage of the display element in accordance with the invention is that no fluctuations in contrast in the information displayed occur as a result of existing inhomogeneities in layer thickness.

Claims

1. An active display element based on the liquidcrystal effect, comprising a liquid-crystal cell having the following construction: glass plate - transparent electrodes, in the form of a segment, full or matrix display - orientation layer - liquid crystal orientation layer - transparent electrodes - glass plate, and wherein the display element includes an illuminating device for laterally introducing linearly polarized light into a rear glass plate remote from an observer side of the element the liquid crystal is a ferroelectric-smectic liquid crystal and a lightscattering layer is disposed on the side of the liquidcrystal cell facing the observer.

2. A display element as claimed in claim 1, wherein the ferroelectric-smectic liquid crystal is orientated in such a way that, in its optically closed state, the longitudinal axes of the molecules are perpendicular to the polarization direction of the fight introduced.

3. An active display element, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.