Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Patent
1998-10-19
2000-08-15
Parker, Kenneth
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
349 98, G02F 1133
Patent
active
061044563
DESCRIPTION:
BRIEF SUMMARY
This application relates to a liquid crystal element, more particularly to one suitable for use in liquid crystal display applications.
Prior art switches using cholesteric mirrors are known, for example, from the Conference Proceedings of the 13th International Display Research Conference, Strasbourg, Aug. 31-Sep. 3, 1993, pages 317-320 in the article entitled `Applications of cholesteric mirrors in the study of electrooptic effects in liquid crystals` by Kerllenevich and Coche. This article describes fast switches for use in modulating laser (and hence monochromatic) light beams. The switches have either twisted or `homogenous` nematic liquid crystal cells between cholesteric mirrors. The first cholesteric mirror or filter passes only light of one circular polarisation. The liquid-crystal cell is switched between a neutral and a half-wave state, the former having no relevant effect and the latter reversing the handedness of the light. The second cholesteric mirror then blocks (or passes) the circularly polarised light depending on its direction of circular polarisation, i.e. right or left handed.
In the case of homogenous liquid crystals, the curves of intensity against voltage have multiple peaks, and have to be used with both transmission-off and transmission-on voltages away from zero volts. This makes the cells difficult to use in multiplexed liquid crystal display applications in which the voltage cannot be held at precise levels. Other examples relate to twisted or supertwisted nematic cells between cholesteric mirrors, which have much better voltage characteristics.
According to the invention there is provided a display element having a modulator for circularly polarised activation light comprising a liquid crystal cell which reverses the handedness of incident light in one setting and does not affect the handedness in another and a filter for passing only that part of the light from the liquid crystal that has a predetermined handedness; and, preferably, a photoluminescent screen which emits visible light when the activation light passed by the filter is incident upon it.
Advantageously, the filter passing light only of one handedness is a cholesteric mirror. The display element also advantageously comprises another cholesteric mirror to provide the circularly polarised activation light from a randomly polarised source. The display element according to the invention can thus avoid the use of the linear polarisers associated with standard liquid crystal cells and so is particularly beneficial for use with incident ultra-violet light, since ultraviolet polarisers have low transmission coefficients.
The cholesteric mirrors are preferably of opposite handedness. This means that the state in which the polarisation is not changed, normally achieved by applying a voltage, is the dark state This in turn results in a higher contrast than having cholesteric mirrors of the same handedness since in the switched state small errors in the cell thickness or other parameters will not result in the unwanted passage of light, while the cell will still pass most of the light when unswitched.
However, for applications requiring high brightness and for which contrast is less important the cholesteric mirrors may have the same handedness. This version has the advantage that only one type of cholesteric mirror need be produced. The chiral nature of the liquid crystals in the cholesteric mirrors can result in one handedness of cholesteric mirror being, as at present, more difficult to produce, and hence expensive. The increased transmission in the on state, which approaches 100% in this case, can also be highly advantageous.
For most applications, such as when using cholesteric filters, the display element is adapted to work with incident light of a predetermined wavelength. The light may be UV light, in particular UVA light, or it may be visible light.
The photoluminescent screen converts the incident light at the predetermined wavelength into the desired frequency or frequencies of visible light and to this end can in
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Parker Kenneth
The Secretary of State for Defence in Her Britannic Majesty's Go
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