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 discharging of the cell result in a large power consumption in the electrically activated state.
In the U.S. Pat. No. 5,315,099 to Gunz et al it is suggested to reduce the power consumption of a liquid crystal cell, which comprises a corrosion resistant layer and a corrosion-neutral liquid or corrosion inhibiting additives, by applying a relatively low frequency voltage in the range of 0.1 Hertz. For the purpose of reducing the cell's sensitivity to changes of the optical density due to temperature fluctuations and instabilities of the voltage source, the cell is operated at a driving voltage much higher than the threshold voltage, which also has a moderating effect on the optical transmission asymmetry for the two voltage polarities. However, the liquid crystal cell as described in U.S. Pat. No. 5,315,099 is driven at a voltage considerably higher than is required to reach the range of the asymptotic peak value of the transmission density, and consequently this cell is basically a two-state shutter.
Other drawbacks of powering a liquid crystal cell with low frequencies or even a direct current are for example decreased useful life, electrochemical changes and charging of layers within the cell structures. In particular, when using driving voltages close to the threshold voltage, the charging effects result in an optical asymmetry and a time dependent transmission. These drawbacks are also confirmed by technical specifications from manufacturers of liquid crystal cells, where it is stated that only a very low direct current component can be accepted.
In this context, a typical cell construction consists of a twisted nematic (TN) type liquid crystal cell inserted between two mutually crossed polarisation filters, where the defining walls are treated with a plastic layer which has been brushed or rubbed in specific directions, the so-called alignment directions, so that the structure in the liquid crystal defining surfaces will force the nematic molecules to each take specific angular positions and so that the molecules will be twisted mutually through 90.degree.20 between said defining surfaces. Other surface treatment methods which have corresponding effects are also known to the art. In an electrically non-activated state, the polarisation plane will be rotated through 90.degree. as light passes through the filter and the cell becomes transparent. This rotation of the nematic molecules can be stopped to a greater or lesser extent, by applying an electric field and therewith obtain a filter effect that can also be controlled. However, a cell of this kind has a relatively strong angular variation of transmittance in its dark, electrically activated state, with varying absorption of light that is incident at angles other than a right angle, this asymmetry being further amplified by the fact that the nematic molecules nearest the surface, bound by the surface effect, still give rise to a residual optical activity. Thus, when the angles of incident light increase in relation to the normal (i.e. the perpendicular), the filter in the two bisectrix directions between the alignment directions will be more transparent and relatively constant in relation to the directions of the crossed polarisers along the direction of one bisectrix while darkening along the direction of the other bisectrix.
It is known to compensate for the transmittance variation effect by combining two TN cells which twist through 90.degree., such that the "weak" bisectrix of one TN cell will coincide with the bisectrix of the other "strong" bisectrix, and vice versa. However, despite this compensation, the field of vision is still uneven, which is troublesome to the user.
An improvement in respect of the angular dependent transmission asymmetry is provided by the technology described in the copending but not yet published patent applications SE 9401423-0 and corresponding PCT/SE95/00455. These documents show a liquid crystal cell construction comprising two nematic-type liquid crystal cells, each cell being provided with molecule orientating plates defining the molecule alignment directions with mutual angular displacement in an inactivated state and electrically actuatable molecule alignment means for controllable molecule alignment in an activated state. The liquid crystal cells are each mounted between mutually extinguishing polarisation filters and the molecule alignment directions of the cells are so turned as to obtain a compensating effect between the respective asymmetrical light absorptions of the cells when a voltage is applied. The problem of angular dependent transmission is according to these patent applications reduced by at least one of the cells between the molecule alignment directors that is smaller than the previously known twist angle of 90.degree.. In order to be able to apply the same voltage to two different liquid crystal cells, and therewith simplify the electronics required, it is in the SE 9401423-0 and PCT/SE95/00455 technology advantageous to use two mutually identical cells.
The described also provides an improvement with regard to low absorption in the transparent state of the shutter construction. Furthermore, this shutter construction applied for example in a protective welding glass will have variable darkness in its darkened state, so as to enable the same protective glass to be used with very strong welding light and with much weaker welding light, so that all types of welding work can be carried out with one and the same protective glass shield to the best possible extent. It was previously known to the art that the optical activity can be varied by applying different voltages, although the unevenness in the angle-dependent transmission density tends to become more troublesome when the voltage across the cells is increased in the earlier known techniques.
One of the problems encountered when using cells having a smaller twist angle than 90.degree., referred to conveniently as "low-twist cells", resides in achieving high light transmission in the transparent state while, at the same time, obtaining a sufficiently low light transmission in the dark state. Consequently, in accordance with one aspect of the SE 9401423-0 and PCT/SE95/00455 technology, a "symmetric" polarisation filter placement is preferred. When the polarisation filters are disposed at mutually intersecting angles of 90.degree., it is suitable to mount a low-twist cell such that the bisectrix between the surface treatment directions will coincide essentially with a bisectrix between the polarisation directions of the filters. The greatest transmission of light will then be obtained in the electrically non-activated state of the device, i.e. its transparent state.
In accordance with one embodiment of the SE 9401423-0 and PCT/SE95/00455 technology, it is convenient also to reduce the thickness of the liquid crystal cells. This results particularly in a reduced switching time, because the switching time is inversely proportional to the square of cell thickness. Thus, the switching time can be reduced in the order of magnitude of 50%, by reducing the thickness of the liquid crystal cells from 4 mm to 3 mm under otherwise equal conditions. This reduction in cell thickness is also required when low-twist cells, by virtue of a dependency that has been found to exist between the value thickness multiplied by optical anisotropy, the twist angle and the light transmission in the light or transparent state. This dependency can be utilised to construct a protective welding glass which has good optical angular properties, high light transmission in the transparent state, and rapid state-switching properties. This is only possible by using low-twist cells with the polarisation filters placed in the aforedescribed symmetric manner.
The fundamental cause of this thickness problem is that a cell which does not have appreciable thickness will not function to cleanly rotate optically incident polarised light, and elliptically polarised light will be emitted instead. When this cell is placed between two mutually crossing polarisation filters, transmission will vary periodically with the thickness of the cell.
In accordance with another embodiment of the SE 9401423-0 and PCT/SE95/00455 technology, a low-twist cell can be placed anti-symmetrically, meaning that the direction of the bisectrix of the acute angle between the treated molecule alignment directions of the cell (rubbing directions) is placed so as to coincide with the direction of polarisation of one of the mutually crossed polarisers. In a non-activated state, such a construction will exhibit relatively low light transmission, but a more transparent state is obtained when a moderate voltage is applied, this more transparent state returning to a generally darker state when the voltage is again increased. One advantage with this construction is that a loss in voltage will not result in a loss of light absorption and that a given protective effect will remain. This enables the existing standard for protective welding glass which requires the difference between an adjusted state and a state which occurs upon the loss of current supply to be at most nine darkness degrees to be maintained more easily, even at high degrees of darkness. This enables two asymmetric low-twist cells with polarisers placed anti-symmetrically to be used, or one cell with symmetrically placed polarisers and one low-twist cell with anti-symmetrical polarisers.
The low twist technology taught in SE 9401423-0 and PCT/SE95/00455 remedies the angular dependency problem of a liquid crystal cell construction and also provides a construction with voltage dependent, variable transmission density, but with state of the art electric driving methods the power consumption is still relatively high.
The problem to be solved by the present invention, and thus an object of it, is to achieve a liquid crystal shutter with variable transmission density and improved angular properties, i.e. reduced angular dependency of transmission density, together with low power consumption.
Further objects of the present invention is to provide a glare shielding device and a welding glass construction with variable transmission density and improved angular properties, together with low power consumption.
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