Liquid crystal cells – elements and systems – Liquid crystal system – Liquid crystal eyewear
Patent
1997-06-25
2000-08-01
Parker, Kenneth
Liquid crystal cells, elements and systems
Liquid crystal system
Liquid crystal eyewear
349 33, 349 77, G02F 11337
Patent
active
060974516
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to liquid crystal shutters suitable for electro-optical eye-protection devices, glare shields and automatically darkening welding glass filters.
BACKGROUND
Liquid crystal shutters are useful in various applications concerning the transmittance of light through an aperture in which it should be possible to switch the shutter between a low light-absorbing state with a low transmission density value and a high light-absorbing state with a high transmission density value. By combining polarisation filters and cells of liquid crystals that are alignable by means of an electric influence, the transmittance of state of the art liquid crystal shutters is made variable in response to a change in the electric influence.
This kind of shutters are favourably applied as light filters in for example eye-protection devices, such as automatically darkening welding glass shields. However, prior art liquid crystal shutters suffer from the problem that the transmission density is asymmetric and dependent on the angle of incidence of transmitted light. This angular dependence is due to an incomplete molecular alignment with the applied electric field when driven in an intermediate voltage range typically between 2 and 10 volts and is especially disadvantageous in applications that require a large field of view. Another problem is the high power consumption of such prior art shutters, which leads to rapidly consumed battery cells in mobile equipment and has well known economic as well as safety and environmental aspects.
A state of the art liquid crystal cell in this context consists of a liquid mixture of elongated molecules sandwiched between two delimiting glass plates. The liquid mixture facing surface layers of the glass plates are treated so that alignment directors, for example grooves, are formed having a uniform direction and the liquid crystal molecules close to such a surface layer tend to align parallel with the directors. By twisting the glass plates so that the directors are not-parallel, a helical structure of liquid crystal molecules is formed between the glass plates. For example, the standard 90.degree. twisted nematic (TN) cell is formed with a twist angle between the molecule alignment directions of the glass plates of 90.degree.. The molecules of liquid crystal normally used in this context, have an inherent positive dielectric anisotropy and can therefore be predominantly aligned upon the application of an electric field with a voltage higher than a cell specific threshold value. The helical molecule structure in the cell is dissolved under the electric influence and the crystal molecules are instead oriented according to the electrical field. When placed between polarisers, the transmission density of such a cell assembly can be controlled by varying the applied electrical field above the threshold voltage, whereby the transmission characteristic is typically asymptotic. The mentioned optical angular asynmmetry, however, appears in this electrically activated state.
Impurities in a liquid crystal cell tend to interfere with the liquid crystal structure, and in particular the presence of seemingly unavoidable alkaline earth metal ions causes, in the electrically activated state, a leakage current flow across the cell. If the switching of a cell is driven with a DC or low frequency voltage, such impurity ions can migrate towards the alignment layers and become embedded at the inner cell surfaces. Upon removal of the driving voltage, an electrical field across the crystal may still exist due to captured ions and may affect cell switching, therefore, liquid crystal cells are normally driven with an alternating voltage, for example a square wave voltage, where the polarity is rapidly switched in order to prevent impurity ion migration and ensuing cell degradation. Under these conditions the cell approximates to a parallel plate capacitor and must continually be charged and discharged upon polarity reversal. The leakage current flow together with the continual charging and disc
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Hornell .ANG.ke
Palmer Stephen
Hornell International AB
Parker Kenneth
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