A LIQUID CRYSTAL SHUTTER AND A LIGHT SHIELDING DEVICE INCLUDING SUCH A SHUTTER

Abstract: A liquid crystal shutter construction, suitable for glass shields or automatically darkening welding glass filters, which is switchable between a first state with high transmission of light and a second state with low transmission of light, and vice versa, in response to an electric control signal. The construction has a nematic type liquid crystal cell disposed between transparent plates having electrodes for providing an electric field in response to the control signal. The plates have mutually facing surfaces, each of which is provided with alignment means for defining a respective molecule alignment direction for molecules in the proximity of said alignment means in the absence of said electric field. The liquid crystal cell is mounted between polarisers and comprises a band pass filter. The liquid crystal cell and the band pass filter are matched so that the wavelength dependency of the transmission characteristics of the band pass filter in a certain wavelength range is substantially complementary to the wavelength dependency of the transmission characteristics of the liquid crystal cell in the absence of the electric field.

Description: A LIQUID CRYSTAL SHUTTER AND A LIGHT SHIELDING DEVICE INCLUDING SUCH A SHUTTER Technical Field of the Invention The present invention relates to liquid crystal shutters and electro-optical eye-protection devices with variable transmission density, and then more specifically to constructions according to the preamble of claim 1. 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 transparent, or light low light-absorbing state and a dark high light-absorbing state. By combining polarisation filters and layers or cells of liquid crystal molecules that are alignable by means of an electric influence, the light transmittance of a liquid crystal shutter construction is made variable in response to a change in the electric influence. A state ofthe art liquid crystal cell in this context comprises liquid crystal molecules sandwiched between two glass plates. The plates have mutually facing surfaces, each of which is provided with alignment means for defining a respective molecule alignment direction for molecules in the proximity of said alignment means in the absence of an electric field. The alignment means may include a plate surface portion having a physical surface shape which co¬ operates with the shape of a liquid crystal molecule in the absence of the electric field. This may be accomplished by treating the glass plate surfaces in a uniform direction so that the liquid crystal molecules in the proximity of such a surface tend to align parallel with the treatment direction. The surface may for example be grooved. By twisting the glass plates so that the treatment directions are not parallel, a helical structure of liquid crystal molecules is formed between the glass plates. For example, the standard 90° twisted nematic (TN) cell includes two mutually facing surfaces which are formed with a twist angle of 90° between the molecule alignment directions ofthe glass plates. Smaller twist angles have been described in the copending but not yet published patent applications SE 9401423-0 and corresponding PCT/SE95/00455. The molecules of the liquid crystal types being used have an inherent dielectric anisotropy and can therefore be predominantly aligned upon application of an electric field with a voltage higher than a cell specific threshold value. The helical structure in the cell is then dissolved 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. With such a liquid crystal cell being placed between crossed polarisers, the cell construction has a high transmittance, in the absence of any stimulating voltage and is said to have a normally white mode. In contrast, positioning ofthe cell between parallel polarisers results in a cell construction having a low transmittance, in the absence of a stimulating voltage and is said to have a normally black mode. A device having a high transmittance means that light may pass through the device with only a small reduction of light intensity A high transmittance may also be referred to as a low optical density or a low shade number. Conversely a device having a low transmittance may be described as having a high optical density and a high shade number. The optical density is conventionally defined as D = 1 + 7/3 x 10 log (1/T), where T is the transmission coefficient. In particular, when the above described kind of state ofthe art shutter is applied as a light filter in for example an eye-protection device, such as an automatically darkening welding glass shield in which the welding glass is activated and darkens in response to detected welding light, it is for safety reasons important to ensure that the fastest response time possible from the light state to the dark state is achieved. Basically, there are two switching times involved in the operation of a liquid crystal cell. The first switching time involves switching the cell from the inactivated state to an activated state upon application of a driving voltage and it takes typically less than a millisecond for the crystal to react. The second switching time occurs in connection with the reverse process where crystal relaxation takes place upon removal ofthe driving voltage and takes about twenty times longer time. Therefore, for shutters requiring very fast switching times from the light state to the dark state it is common to employ liquid crystal cells in the normally white mode. However, should the driving electronics malfunction and for some reason fail to deliver an activating voltage, a state ofthe art normally white mode shutter has the drawback that it is left in a potentially hazardous light state. According to prior art, this problem can be alleviated by using a cell in the normally black mode, but this is then at the expense ofthe switching speed from the light to the dark, protective state. Therefore this is not a satisfactory solution. In case the welding glass shield has a long switching time from the light state to the dark state, the person who is using the welding shield literally is exerted to light with a high intensity for a long time at commencement of a welding operation. The European standard EN 169: 1992 "Personal Eye Protection-Filters for Welding and Related Techniques - Transmittance Requirements and Recommended Utilisation" stipulates that the maximum permissible difference in shade number steps, between the inactivated state and the dark welding state should be no more than 9 shade numbers. For example, a welding filter that attains a shade number 13 in the dark state, therefore can be no lighter than shade number 4 in the inactivated state. In order to provide sufficient light intensity to the user prior to the commencement of a welding operation, there has been a development in known techniques for automatically darkening welding filters that provide a shade number in the range of 3 in the light state. Consequently there is instead a problem to achieve a satisfactorily shaded dark state. Summary It is an object ofthe present invention to solve the problems described above. The present invention also relates to the problem of achieving a liquid crystal shutter which provides a satisfactorily light state in the absence of intense light, and which still provides a satisfactorily dark state in the presence of intense light. A further problem to be solved by the present invention, and thus an object ofthe invention, is to achieve a liquid crystal shutter with an increased optical density value (or shade number) in the inactivated state. Another object ofthe present invention is to provide a glare shielding device and a welding glass construction with an increased transmission density value (or shade number) in the inactivated state for the purpose of achieving an improved level of safety. Another object is to provide a liquid crystal shutter which by its level of transmittance indicates whether it is in a functional state or not. Another object is to provide a liquid crystal shutter that employs at least one liquid crystal element placed between mutually crossed polarisers in order to maintain a fast switching time from the light state to the dark protective state. A further object is to achieve a shutter ofthe mentioned kind with a highly symmetric shade geometry in its dark state and with a broad contrast range in the activated dark state. According to the invention, the objects are achieved by providing a voltage controllable liquid crystal cell, placed between angularly twisted polarisers and having a transmission characteristic that varies in relation to the wavelength of incident light, with a filter having, in the visible wavelength range, a transmission characteristic which is complementary to the transmission characteristic ofthe liquid crystal cell in the absence of a control voltage applied on said cell. Thus, in accordance with one aspect ofthe invention, a normally white mode liquid crystal cell, that in the inactivated state has a high transmittance for specific wavelength ranges, is combined with a band-pass filter that reduces the transmittance in said specific wavelength ranges, so that a darkened state is achieved in said inactivated state. Furthermore, the band pass filter is devised to have a high transmittance for wavelengths that are transmitted with a controllable transmittance by the liquid crystal cell in its activated state. Consequently, the inventive liquid crystal shutter construction is provided with low transmittance in the inactivated state, high transmittance in a first activated, light, state and low transmittance in a second activated, dark, state, whilst the fast response time from the light to the dark state is maintained. In other words, the liquid crystal cell and the band pass filter should be devised and matched such that the wavelength dependency ofthe transmission characteristics ofthe band pass filter in a certain wavelength range is substantially complementary to the wavelength dependency ofthe transmission characteristics ofthe liquid crystal cell in the absence of an electric field. Brief Description of the Drawings The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which: Fig 1 is an exploded schematic view of a first embodiment of a device according to the invention. Fig 2 is an exploded schematic view of a second embodiment of device according to the invention. Fig 3 A shows, in full lines, the wavelength dependency ofthe transmission characteristics of a 90° twisted nematic liquid crystal cell in the absence of a control signal and, in dashed lines, fig 3 A also shows the wavelength dependency ofthe transmission characteristics of a band pass filter. Fig 3B shows, in full lines, the wavelength dependency ofthe transmission characteristics of the 90° twisted nematic cell in a first activated state under the influence of a first drive voltage. Fig 3C shows the transmission characteristics for the cell according to Figs 3 A and B in a second activated state under the influence of a second drive voltage. Fig 4 A shows, in full lines, the wavelength dependency ofthe transmission characteristics of a 0° twisted nematic cell in the absence of a control signal, and, in dashed lines, Fig 4A also shows the wavelength dependency ofthe transmission characteristics of a band pass filter. Fig 4B shows the wavelength dependency ofthe cell according to Fig 4 A under the influence of a first control voltage. Fig 4 c shows the wavelength dependency ofthe transmission characteristics ofthe cell under the influence ofthe second control voltage. Fig 5 illustrates the light transmittance properties for a device in accordance with one embodiment ofthe invention as a function of the control voltage applied to the device. Fig 6 illustrates a third embodiment of a device according to the invention including a retardation film. Fig 7 shows the transmission characteristics of a device according to the invention, including a retardation film and the transmission characteristics ofthe device without any compensating retardation film. Figures 8A and 8B illustrate the shade geometry for different devices according to the invention. Fig 9 shows a shade geometry for a device according to one embodiment ofthe invention. Fig 10 illustrates the light transmittance properties as a function ofthe control voltage applied to a prior art device. Detailed description of embodiments Fig 1 shows a schematic view ofthe components of an embodiment of a liquid crystal shutter construction 1 according to the invention. A nematic type liquid crystal cell 2 comprising transparent plates having electrodes which are connectable to a voltage source and provided with means for defining the direction of molecule alignment near the plate surfaces, therewith causing the liquid crystal molecular directions to be aligned in the absence of an electric field between the plates. The angular difference in molecule alignment (also called the twist angle) between the molecule alignment directions can, according to the invention, be for example 0 degrees. Alternatively the angular difference may be substantially 90°, or a value between 90 degrees and zero degrees. The liquid crystal cell 2 is disposed between two polarisers 3 and 4 having mutually orthogonal polarising directions. The construction 1 is also provided with a band pass filter 5, which is positioned in the path of a light beam LB which passes through the construction 1. Optionally, the shutter construction may comprise an interference filter 6 with a function of eliminating UV-light and ER-light and which also limits the wavelength range. According to another aspect of the invention the band pass filter 5 also blocks out UV-light and IR-light. Fig 2 shows a further embodiment ofthe shutter construction comprising a first polariser 3, a first liquid crystal cell 2, a second polariser 4 whose polarisation direction is at right angles to the polarisation direction of the first polariser 3, a second liquid crystal cell 6, a third polariser 7 which has the same direction of polarisation as the first polariser 3, and a band pass filter 5. Fig 3 A, B and C show in full lines the spectral response of a 90° twisted nematic cell with the product of anisotropy Δn and the thickness d ofthe cell Δn*d = 0.80 micrometers. Fig 3 A, B and C also shows, in dashed lines, the spectral response of a band pass filter. The diagram in Figs 3 a, b and c illustrate the transmission rate plotted as a function ofthe wavelength substantially in the visible wavelength range. In Fig 3 a, the fully drawn curve shows the spectral response of a 90° twisted nematic cell in the electrically inactivated phase with a Δn*d value of 0.78 micrometers placed between crossed polarisers aligned at 45° and 135°, respectively, relative to the entrance molecular direction vector. In this inactivated phase the cell has a transmission maximum in the range of 400 nm and a transmission minimum in the range of 550 nm. The dashed curve shows the spectral response ofthe band pass filter, which has a high optical transmittance ofthe central part of the visible spectrum, i.e. in the wavelength range of 500 to 600 nm. The low transmittance ofthe liquid crystal cell over the 500 to 600 nm range, together with the low transmittance ofthe band pass filter in the ranges 350 to 500 nm and 620 nm and upwards, results in an overall dark state being attained when combining such a liquid crystal cell and a band pass filter with this characteristic. Fig 3B shows the spectral response ofthe same liquid crystal cell and the same band pass filter as in Fig 3 A, in a first activated state when a small driving voltage in the range of 2.5 volts is applied to the liquid crystal cell. It is clear from the diagram that the transmission maximum for the liquid crystal cell and the band pass filter coincide in the visible wavelength range of 500 to 600 nm. Thus, the combined system at this first activated state is in a transparent, high transmittance mode. Fig 3C shows again the spectral responses ofthe system components of Fig 3 a and Fig 3 b, now in a second electrically activated state with an applied voltage in the range of 5 volts. With this higher voltage supplied, the transmittance ofthe liquid crystal cell is again reduced in the central part ofthe visible wavelength range. In fact the transmittance is controllable in the visible wavelength range by varying the voltage from about 2 volts and upwards until the liquid crystal cell reaches a transmittance minimum. The liquid crystal cell component of such a combined construction operates in a way similar to a normally white mode and therefore the switching time for the device according to the invention is advantageously short. This advantage is achieved in combination with the advantage of having a device which reverts back to a dark, resting state in absence of an electric field between the plates in the cell. This is described in further detail in connection to Fig 5. Fig 4 a shows the spectral response of a 0° twisted nematic cell and a band pass filter. The fully drawn curve thus shows the optical response of a 0° birefringent cell in the inactivated phase with a Δn*d value of 0.55 micrometers placed between crossed polarisers oriented at 45° and 135° respectively, relative to the entrance molecule director vector. The dashed curve shows the spectral response of a band pass filter selected to have a high optical transmittance over the central part ofthe visible spectrum, i.e. in the range of 500 to 600 nm. It can be seen from Fig 4 a that the transmission minimum of the liquid crystal cell, in the central part 500 to 600 nm of the visible wavelength area, coincides with the transmission maximum ofthe band pass filter. Thus, when combining this liquid crystal cell and the band pass filter, in accordance with the invention, a low transmission state is achieved. When the voltage over the cell is zero, that is when there is an absence of an electric field between the plates in the liquid crystal cell. Fig 4 b shows the spectral response ofthe same liquid crystal cell and the band pass filter with an applied voltage of between 2 and 3 volts and, as has been explained above, a transparent state is then achieved. Fig 4 c shows the second activated state with a voltage of 5 volts supplied. The second activated state is a dark state which is attained as has been explained above. Calculations indicate that there are mainly two cell types that display this phenomenon. The first is a 90° twisted nematic cell with a Δn*d value in the range of 0.80 micrometers placed between crossed polarisers aligned at 45° and 135°, respectively, relative to the entrance molecular alignment director. The other cell type is a 0° (untwisted) birefπngent cell with a Δn*d value in the range of 0.55 micrometers, again with a polarisers oriented at 45° and 135° respectively relative to the entrance molecular alignment direction. A glare shielding device according to the invention includes a sensor for providing a sensor signal in response to the intensity of a detected light. The sensor signal is provided to a controller including a signal generator. The signal generator is set up to generate a control signal in response to the sensor signal. A liquid crystal construction according to the invention includes a liquid crystal cell having two surfaces provided with electrodes for providing an electric field between these surfaces. The electric field is created by applying the control signal to the electrodes. When the control signal is applied to the electrodes, a certain control signal voltage will create a corresponding electric field in the liquid crystal cell between the electrodes. Fig 5 shows a diagram on the electro-optic properties of a liquid crystal cell combination with the transmission density or shade number plotted against applied voltage. The curve in Fig 5 shows the electro-optic properties of an 8 micrometers 90° twisted nematic cell with, for the purpose of this example, the Merck mic 6096 liquid crystal giving a Δn*d value of about 0.78 micrometers. A cell is placed between crossed polarisers aligned at 45° and 135° respectively relative to the entrance molecular director vector and together with the band pass filter that has a high optical transmittance over the central part ofthe visible spectrum in the range of 500 and 600 nm. In the inactivated phase, in the absence of a control signal voltage, the optical density value is just above 5.5. When applying a voltage of about 2 volt the optical density value decreases to about 3.3 in a first activated state which renders it more transparent than in the inactivated state. With an applied voltage above 2 volts up to the range of 10 volts the optical density value is variable in the range between the minimum value attained in the first activated state and up to a value of about 1 1. Despite the fact that a 90° twisted nematic cell with Δn*d in the range of 7.81 micrometers has good electro-optic properties in a direction lying parallel to the surface normal, the optical angular property of this cell combination may be somewhat insufficient for applications where a wide viewing field is required. Positioning of two such cells together, such that the face to face molecule alignment directions are substantially peφendicular, provides further advantages according to the invention. Such positioning generates a certain degree of cell compensation and an adequate viewing field can be obtained. Such an embodiment ofthe invention is described in Fig 2 and for example has its application in an automatically darkening welding glass filter. Thanks to both a low twist angle and a reduced Δn*d parameter, advantageous optical angular properties ofthe 0° birefringent cell with Δn*d in the range of 0.5 micrometers are found to be highly favourable and clearly suitable for both a single cell and a double cell liquid crystal shutter construction. Such advantageous properties allow a wide and symmetrical viewing field. However, due to the large remnant optical retardation present in the 0° cell when driven at voltages even in the region of 10 volts, the available cell contrast from such a device is small in comparison to that for a 90° twisted nematic liquid crystal cell. In accordance with an embodiment of the invention, the cell contrast is improved by means of an addition of a compensating retardation film. In the 0° birefringent cell embodiment a small retardation value of about 20 to 50 nm is appropriate. In order to maximize the compensation effect, the retardation film should preferably be aligned such that the fast axis direction is perpendicular to the entrance and exit molecular direction vectors. The compensating retardation layer for the 0° birefringent cell can for example be in the form of a single, uniaxially stretched retardation film, with a value of between 25 to 30 nm. In another embodiment, the compensating retardation layer may be implemented by means of retardation films that are aligned such that the net over all retardation generated by these two films is given by the difference between the two values ofthe film sheets. With, for example, a 27 nm compensating retardation film applied in the shutter construction, the optimum Δn*d value ofthe cell combination is increased from 0.55 micrometers to 0.77 micrometers. Fig 6 shows in principle a prepared position of a retardation film 10 in accordance with the invention. In Fig 6 the retardation film 10 is positioned on one side ofthe liquid crystal cell 2 between the polarisation filters 3 and 4. Alternatively the retardation film may be comprised within the liquid crystal cell 2 between the molecular alignment directing plates. Fig 7 shows the electro-optic properties of a liquid crystal construction including a 4 micrometer 0° birefringent cell. The birefringent cell, in this instance comprising the Merck ZLI-4246 liquid crystal giving a Δn*d value of about 0.52 micrometers, is placed between crossed polarisers aligned at 45° and 135° relative to the entrance molecular director vector. The cell is, in correspondence with the above embodiments, placed together with a band pass filter that has a high optical transmittance over the central part ofthe visible spectrum, that is in the range of 500 to 600 nm. Curve 20 shows the optical response ofthe cell combination without any compensating retardation film, whereas curve 22 shows a cell combination including a 26 nm compensating retardation film oriented such that the fast axis is peφendicular to the entrance molecular director. The improvement in cell contrast with the compensation retardation film is clearly seen in this figure. Fig 8a, 8b and Fig 9 show in principle the shade geometry in a central viewing cone for the above described cell combinations. More specifically, Fig 8a shows the shade geometry A of a 90° twisted nematic single cell, and it is clear that the shade geometry a is asymmetric to a fairly large extent although the cell contrast is still high. Fig 8 b shows the shading geometry A and B of a combination of two 90° twisted nematic cells arranged with antisymmetrical polarisers. Shading geometry from each cell is still asymmetric but in combination a resulting symmetric shade geometry with high contrast is obtained. In contrast, a to a large extent symmetric shade geometry, as shown in Fig 9, is obtained by means of a 0° low twist single cell and in combination with a retardation film a substantially symmetric shade geometry together with a high contrast is obtained.

Claims: Claims 1. A liquid crystal shutter construction, suitable for glass shields or automatically darkening welding glass filters, the construction being switchable between a first state with high transmission of light and a second state with low transmission of light, and vice versa, in response to an electric control signal; the construction having a nematic type liquid crystal cell disposed between transparent plates having electrodes for providing an electric field in response to the control signal; said plates having mutually facing surfaces, each of which is provided with alignment means for defining a respective molecule alignment direction for molecules in the proximity of said alignment means in the absence of said electric field; and said liquid crystal cell being mounted between polarisers, characterized in that said liquid crystal shutter construction comprises a band pass filter, said liquid crystal cell and said band pass filter being devised such that the wavelength dependency ofthe transmission characteristics of the band pass filter in a certain wavelength range is substantially complementary to the wavelength dependency ofthe transmission characteristics ofthe liquid crystal cell in the absence of an electric field. 2. A shutter construction according to claim 1, characterized in that said band pass filter has a transmission maximum in a first wavelength range; and in that the thickness ofthe liquid crystal cell is selected such that it has a transmission minimum in a second wavelength range in the absence of an electric field, said second wavelength range substantially corresponding to said first wavelength range. 3. A shutter construction according to claim 1 or 2, characterized in that two mutually facing surfaces provide an angular difference between the molecule alignment directions, said angular difference being ninety degrees; and in that the product between the optical anisotropy and the thickness ofthe cell is substantially 0.8 micrometers. 4. A shutter construction according to claim 1 or 2, characterized in that two mutually facing surfaces provide parallel molecule alignment directions, and in that the product between the optical anisotropy and the thickness ofthe cell is substantially 0.55 micrometers. 5. A shutter construction according to claim 4, characterized in that a retardation film is disposed between the polarisers. 6. A shutter construction according to claim 5, characterized in that the retardation film is disposed between the transparent plates ofthe liquid crystal cell. 7. A shutter construction according to any ofthe preceding claims, characterized in said polarisers have substantially orthogonal polarisation directions. 8. A shutter construction according to any ofthe preceding claims, characterized in that the construction comprises another nematic type liquid crystal cell which is disposed between one of said polarisers and a further polariser, said one polariser and said further polariser having substantially orthogonal polarisation directions. 9. A shutter construction according to any ofthe preceding claims, characterized in that said certain wavelength range is the central part ofthe visible wavelength range, i.e. essentially between 500 nm and 600 nm. 10. A light shielding device including a shutter construction according to any one ofthe preceding claims. 11. A light shielding device according to claim 10 characterized in that it includes sensor means for providing a sensor signal in response to the intensity of light; and a signal generator for generating said electric control signal in response to said sensor signal.

Inventors: HOERNELL AAKE, PALMER STEPHEN, HOERNELL, AAKE, PALMER, STEPHEN
Applicants: HOERNELL AAKE, HOERNELL INTERNATIONAL AB, PALMER STEPHEN, HOERNELL, AAKE, PALMER, STEPHEN
Application year: 1996
Main classification: A61F9
IPC codes: A61F9, G02F1