SE516183C2 - liquid crystal - Google Patents

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

U: 10 15 25 5.16 183 if; '§ï * .g "§ ° _f =." I = zïï. in filed but not yet published patent applications SE 9401423-0 and the corresponding PCT / SE95 / 00455. The molecules in the types of liquid crystal used have their own built-in dielectric anisotropy and can therefore be predominantly aligned by applying an electric field with a voltage which exceeds a threshold value specific to the cell. The screw structure in the cell is then dissolved, and the crystal molecules are instead oriented along the electric field. When placed between polarizers, the transmission density of such a cell array can be controlled by varying the applied electric field. When such a liquid crystal cell is placed between crossed polarizers, the cell structure has a high transmittance, in the presence of each stimulating voltage, and is then said to have a normal white mode. In contrast, a placement of the cell between parallel polarizers results in a cell construct that has low transmittance in the absence of stimulating voltage and is said to have normal black mode. A device with high transmittance means that light can pass through the device with only a small reduction in the light intensity. High transmittance can also be described as low optical density or low dark number. A device with low transmittance, on the other hand, can be described as having a high optical density and a high dark number.

The optical density is conventionally denoted as D = 1 + 7/3 x * ° 10g (1 / 'r), where T is the transmission coefficient.

When, more specifically, the known shutter described above is used as a light filter in, for example, an eye protection device, such as an automatically darkening welding glass screen, where the welding glass is activated and darkens corresponding to detected welding light, it is important for safety reasons to ensure the fastest possible response time. achieved from light state to dark state. Essentially, there are two significant switching times when operating a liquid crystal cell.

The first switching time involves switching the cell from inactivated state to activated state by applying a drive voltage, and it typically takes less than a millisecond for the crystal to react. The second switching time occurs in the opposite process, where crystal relaxation occurs when the drive voltage is removed, and takes about 20 times longer.

For shutters that require very fast switching time from light state to dark state, it is therefore common to use liquid crystal cells in normal white mode. However, if the drive electronics do not emit and for some reason do not supply activation voltage, a shutter of the normal white mode type of known type has the disadvantage that it remains in a potentially dangerously bright state. In the prior art, this problem can be alleviated by using a cell in northern black mode, but this is done at the cost of the switching speed from light to dark, protective state. Consequently, this is not a satisfactory solution. If the welding glass cover has a long switching time from light state to dark state, then the user is literally exposed to high intensity light for a long time at the beginning of a welding operation.

The European standard EN 169: 1992 “Personal Eye Protection Alerts for Welding and Related Techniques - Transrnittance Requirements and Recommended Utilization” stipulates that the maximum permitted difference in dark number steps between inactivated state and dark welding state must not exceed 9 dark numbers. A weld filter, for example, which achieves a dark number 13 in the dark state can therefore not be brighter than dark number 4 in the inactivated state. In order to provide sufficient light intensity for the user before starting a welding operation, a development has taken place in known technology for automatically darkened welding filters, which gives a dark number in the vicinity of 3 in the light state. Consequently, there is instead a problem in achieving a satisfactory dark condition.

It is an object of the present invention to solve the problems described above. The present invention also relates to the problem of obtaining a liquid crystal shutter which has a satisfactory light state in the absence of intense light, and which nevertheless gives a satisfactory dark state in the presence of intense light. An additional problem, which should be solved with the present invention, and thus an object of the invention, is to achieve a liquid crystal shutter with increased optical density value (or dark number) in the inactivated state.

Another object of the invention is to provide a mixing protection device and a welding glass structure with improved transmission density value (or dark number) in the inactivated state in order to achieve an improved level of safety. Yet another object is to provide a liquid crystal shutter which, by its transmission level, indicates whether it is in a functional state or not. Another object is to provide a liquid crystal shutter which utilizes at least one liquid crystal element placed between mutually crossed polarizers to achieve fast switching time from light state to dark protective state.

A further object is to provide a shutter of the type mentioned with a very symmetrical darkness geometry in its dark state and a wide contrast range when the dark state is activated. In accordance with the invention, these objects are achieved by arranging a voltage controllable liquid crystal cell, placed between angularly polarized polarizers and having a transmission characteristic varying according to the wavelength of incident light, with a filter having a transmission range in the visible wavelength range. rala characteristics, which are complementary to the transmission characteristics of the liquid crystal cell in the absence of control voltage applied to the cell.

Thus, in accordance with one aspect of the invention, a liquid white cell with normal white mode, which in the inactivated state has high transmittance for specific wavelength ranges, is combined with a bandpass filter which reduces the transmittance in said specific wavelength ranges to achieve a predetermined state. said disabled state. Furthermore, the bandpass filter is arranged to have high transmittance for wavelengths, which is transmitted with controllable transmittance through the liquid crystal cell in its activated state. Accordingly, the liquid crystal shutter consumption has low transmittance in the inactivated state, high transmittance in a first activated light state and low transmittance in a second activated dark state, while maintaining the fast reaction time from light to dark state.

Brief description The invention will now be described in more detail with reference to exemplary embodiments and with reference to the drawings, in which: Fig. 1 shows a schematic exploded view of a first embodiment of a device according to the invention.

Fig. 2 shows a schematic exploded view of a second embodiment of a device according to the invention.

Fig. 3A shows in solid lines the wavelength dependence of the transmission characteristic of a 90 ° twisted nematic liquid crystal cell in the absence of a control signal, and, with dashed lines, fi g 3A also shows the wavelength dependence of a bandpass filter's transmission properties.

Fig. 3B shows the solid wavelength dependence of the transmission characteristic of the 90 ° TN cell in a first activated state under the influence of a first driving voltage.

Fig. 3C shows the transmission characteristics according to the cell for fi g 3A and B in a second activated state under the influence of a second driving voltage.

Fig. 4A shows, in solid lines, the wavelength dependence of the transmission characteristic fi can in a 0 ° slvruvvirilclad nematic cell in the absence of control signal, and, in fi g 4A also shows 10 15 20 25 30 516 18 3 šï? šïïí * ïïê T * - 'IF- I? 6. . . . ... in dashed lines the wavelength dependence of a bandpass filter's transmission characteristics.

Fig. 4B shows the wavelength dependence of the cell according to fi g 4A under the influence of a first control voltage.

Fig. 4C shows the wavelength dependence of the transmission characteristics of the cell under the influence of the second control voltage.

Fig. 5 shows the light transmitting properties of a device in accordance with an exemplary embodiment of the design as a feature of a control cut-off applied to the device.

Fig. 6 shows a third embodiment of a device according to the invention, which comprises a deceleration arm.

Fig. 7 shows the transmission characteristics of a device according to the invention, which comprises a deceleration fi ch and a transmission characteristic ct of the same device without compensating deceleration m ch.

Figs. 8A and B show the dark geometry of different devices according to the invention.

Fig. 9 shows the dark geometry of a device according to an exemplary embodiment of the invention.

Fig. 10 shows the light transmission characteristics as a function of the control voltage applied to a device of a previously known type.

Detailed Description of Embodiment Fig. 1 shows a schematic view of the components of an embodiment of a liquid crystal shutter structure 1 according to the invention. A liquid crystal cell 2 of nema 10 15 20 25 30 516 718 3. _ f; type: having transparent plates with electrodes which can be connected to a voltage source and provided with means for defining the direction of the molecular direction near the plate surfaces, so that the molecular directions of the liquid crystal are aligned in the absence of an electric field between the plates . The angular difference in molecular alignment (also called screw angle) between the molecular alignment rims can according to the invention be, for example, 0 °. Alternatively, the angular difference may be substantially 90 ° or some value between 90 ° and 0 °. The liquid crystal cell 2 is arranged between two polarizers 3 and 4, which have mutually orthogonal polarization directions. The construction 1 also has a bandpass filter 5, which is placed in the path of a light column LB, which passes through the construction 1. Optionally, the shutter construction may comprise an interference filter 6 with the function of eliminating UV light and IR light and which also limits the wavelength range. In accordance with another aspect of the invention, the bandpass filter 5 also blocks out large UV lights and IR lights.

Fig. 2 shows a further embodiment of the shutter construction, comprising a first polarizer 3, a first liquid crystal cell 2, a second polarizer 4, the polarization direction ring of which forms a right angle to the polarization direction of the first polarizer 3, a second liquid crystal cell 6, a third polarizer 7, which has the same polarization direction as the first polarizer 3, and a bandpass filter 5.

Fig. 3 A, B and C show in solid lines the spectral properties of a 90 ° TN cell with the product between the anisotropy An and the thickness d of the cell Anld = 0.80 micrometers. Figures 3A, B and C also show in dashed lines the spectral properties of a bandpass filter. The diagrams in fi g 3A, B and C show the transmission plotted as a function of the wavelength, essentially in the visible wavelength range. In fi g 3A, the solid curve shows the spectral transmission of a 90 ° TN cell in an electrically inactivated phase with a value of An'd of 0.78 micrometers, placed between crossed polarizers, which form the angles 45 ° and 135 ° to the molecular direction vector at the input. At this inactivated phase, the cell has a transmission maximum near 400 nm and a transmission minimum number at approximately 550 nm. The dashed curve shows the spectral ratio of the bandpass filter, which has high optical transmittance at the center portion of the visible spectrum, i.e. in the wavelength range 500-600 nm.

The low transmittance of the liquid crystal cell over the range of 500-600 mn, together with the low transmittance of the bandpass filter in the range of 350-500 mn and from 620 nm and upwards, a substantially dark state is achieved when such a liquid crystal cell is combined with a bandpass filter. characteristics.

Fig. 3B shows the spectral response of the same liquid crystal cell and the same bandpass filter as in Fig. 3A, in a first activated state when a low drive voltage of approximately 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 for the bandpass filter coincides in the visible wavelength range 500-600 nm. The combined system is thus in this first activated state in a transparent, highly transmissive mode.

Fig. 3C again shows the spectral conditions of the system components in fi g 3A and fi g 3B, now in a second electrically activated state with applied voltage of approximately 5 volts. With this higher voltage applied, the transmittance of the liquid crystal cell in the cross section of the visible wavelength range is again reduced. In practice, the transmittance can be controlled 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 steel cell component of such a combined structure functions in a manner similar to normal white mode, and consequently the switching time of the device according to the invention is advantageously short. This advantage is achieved in combination with the advantage of obtaining a device which, in the absence of an electric field between the plates in the cell, falls back to a dark state of dormancy. This is described in more detail in connection with fi g 5.

Fig. 4a shows the spectral ratio at a 0 ° twisted nematic cell and a bandpass filter. The solid curve thus shows the optical characteristics of a 0 ° birefringent cell in the inactivated phase with an Anid value of 0.55 rnjrometer, placed between crossed polarizers, oriented by 45 ° and 135 °, respectively, relative to 10 15 20 25 30 516 1983 šï * '; “- @ I?: LI; - :, I栗: = the initially present molecular director vector. The dashed curve shows the spectral ratio of a bandpass filter, which is selected to have high optical transmittance over the cross section of the visible spectrum, i.e. in the range 500-600 nm while blocking low light. From Fig. 4A it appears that the transmission minimum for the liquid crystal cell, at the center pa 500-600 mn of the visible wavelength zone council coincides with the transmission maximum for the bandpass filter. Thus, when combining this liquid crystal cell and the bandpass filter according to the invention, a low transmission state is achieved when the voltage across the cell is zero, i.e. in the absence of electric field between the plates in the liquid crystal cell.

Fig. 4B shows the spectral ratio of the same liquid crystal cell and bandpass filter with applied voltage of between 2 and 3 volts, and, as explained above, a transparent state is then achieved.

Fig. 4C shows the second activated state with a voltage of 5 volts applied. The second activated state is a dark state, which is achieved as described OVaIl.

Calculations indicate that there are mainly two cell types, which show this phenomenon. The first is a 90 ° TN cell with an An'd = value in the range 0.80 micrometers, placed between crossed polarizers, directed at a 45 ° and 135 ° angle, respectively, towards the incident molecular alignment director. The second cell type is a 0 ° (unscrewed) birefringent cell with a value An'd of about 0.55 micrometers, again with polarizers oriented at 45 ° and 135 °, respectively, relative to the constituent molecular excitation corrugation.

An anti-glare device according to the invention comprises a sensor for emitting a sensor signal corresponding to the intensity of a detected light. The sensor signal is connected to a control unit comprising a signal generator. The signal generator is arranged to generate a control signal corresponding to the sensor signal. A liquid crystal structure according to the invention comprises a liquid crystal cell having two surfaces provided with electrodes to provide an electric field between these surfaces. The electric field When the control signal is applied to the electrodes, a certain control signal voltage will generate a corresponding electric field in the liquid crystal cell between the electrodes.

Fig. 5 shows a diagram of the electro-optical properties of a liquid crystal cell combination with the transmission density or the dark number plotted against applied blocking. The curve in Fig. 5 shows the electro-optical properties of an 8 micron 90 ° TN cell with, in this example, the liquid crystal Merck mlc 6096, giving a value Antd of about 0.78 micrometers. A cell is placed between crossed polarizers, directed at 45 ° and 135 °, respectively, relative to the constituent molecular director vector and together with the bandpass filter, which has high optical transmittance over the center portion of the visible spectrum in the range of 500-600 nm. In the deactivated phase and in the absence of control signal voltage, the optical density value is well above 5.5.

When a voltage of about 2 volts is applied, the optical density value drops to about 3.3 in a first activated state, which makes it more transparent than in the inactivated state. With an applied voltage above 2 volts up to about 10 volts, the optical density value is variable within a range between the minimum value achieved in the first activated state and up to a value of about 11.

Despite the fact that a 90 ° TN cell with An'd at about 7.81 milcrometers has good electro-optical properties in a direction parallel to the surface normal, the optical angular properties of this cell combination may be somewhat insufficient for applications where a large oxygen field is required. By placing two such cells together, so that the molecular pointing devices side by side are substantially perpendicular, further advantages according to the invention are obtained. Such a positioning provides a certain degree of cell compensation, and a sufficiently good field of view can be obtained. Such an embodiment of the invention is shown in Figure 2 and has, for example, its application to an automatically darkening welding glass filter.

Thanks to both low screw angle and reduced pairing angles Anid is found that favorable optical angular properties are obtained with 0 ° birefringent cells with Anïld in the range around 0.5 micrometers and are particularly advantageous and clearly suitable for liquid crystal shutter designs both with single cell and with double cell. Such advantageous properties enable a large and symmetrical field of view. However, due to the large residual optical deceleration present in the 0 ° cell, even at voltages up to 10 volts, the available cell contrast from such a device is low compared to a 90 ° TN liquid crystal cell. In accordance with an embodiment of the invention, the cell contrast is improved by the addition of a compensating deceleration film. In the 0 ° birefringent cell design, a small retardation value of about 20-50 nm is appropriate. In order to maximize the compensation effect, the deceleration fi should preferably be so directed that its fast axis direction is perpendicular to the input and output molecular director vector. The compensating retardation layer for the 0 ° birefringent cell may, for example, be in the form of a single, uniaxially stretched retardation m ch with a value of between 25 and 30 nm. In another embodiment, the compensating deceleration layer can be implemented by means of deceleration layers which are so aligned that the total sum of the deceleration which is observed by these two films is determined by the difference between the two values of the film layers.

With, for example, a 27 nm compensating deceleration film inserted in the shutter structure, the optimum value An'd for the cell combination is increased from 0.55 micrometers to 0.77 micrometers.

Fig. 6 shows in principle a suitable position for a deceleration loop 10 in accordance with the invention. In Fig. 6, the retardation film 10 is located on one side of the liquid crystal cell 2, between the polarizing filters 3 and 4. Alternatively, the retardation film may be arranged within the liquid crystal cell 2, between the molecule-aligning plates. 10 15 20 25 516 183 = If2fï * .ë "§" f = šï. 12. Fig. 7 shows the electro-optical properties of a liquid crystal cell structure including a 4 micrometer 0 ° birefringent cell. The birefringent cell, in this case comprising the liquid crystal Merck ZLI-4246, which gives a value Anld of about 0.52 micrometers, placed between crossed polarizers, arranged at 450 and 135 °, respectively, in relation to the constituent molecular targeting vector. The cell, like the embodiments described above, is placed together with a bandpass filter that has high optical transmittance over the middle part of the visible spectrum, i.e. in the range 500-600 nm. Curve 20 shows the optical properties of the cell combination without compensating deceleration m lm while curve 22 shows a cell combination, comprising a 26 nm compensating deceleration m lm, oriented so that the fast axis is perpendicular to the incoming molecular director.

The improvement in cell contrast with the compensating retardation film is clearly shown in this figure.

Figures 8a, b and fi g 9 show in principle the blackening geometry in a central visual field cone for the cell combinations described above. More specifically, fi g 8 shows the blackening geometry A a at a 90 ° TN single cell, and it is clear that the blackening geometry a is asymmetric to a fairly large extent, although the cell contrast is still high.

Fig. 8 b shows the blackening geometry A and B for a combination of two 90 ° TN cells arranged with antisymmetric polarizers. The blackening geometry of each cell is still asymmetric, but in combination a resulting symmetrical blackening geometry with high contrast is obtained. In contrast, a largely symmetrical blackening geometry, shown in fi g 9, is obtained by a 0 ° low screw single cell, and in combination with a deceleration fi lm a substantially symmetrical blackening geometry together with high contrast.

Claims (9)

10 15 20 25 30 s1e_1s3 13 Patent claims A liquid shutter shutter structure, a mandatory glass protection element and an automatically darkening welding glass filter, which structure is switchable between a first state of high light transmission and a second state of low light transmission and vice versa, corresponding to an electrical control signal which construction has a nematic-type liquid crystal cell arranged between transparent plates provided with electrodes for arranging an electric field corresponding to the control signal, which plates have facing surfaces, provided with aligning means for defining each molecular alignment direction for molecules in the vicinity. of said alignment means in the absence of adjacent electric fields, and which liquid crystal cell is mounted between polarizers, the liquid crystal structure comprising a bandpass filter and the liquid crystal cell and the bandpass filter are so adapted to each other that the transmission passband of the bandpass filter e in a certain wavelength range is substantially complementary to the wavelength dependence of the transmission characteristics of the liquid sample cell in the absence of said electric field, characterized in that the two facing surfaces have parallel molecular alignment directions, that a deceleration fi lm is arranged between the polarizers. and in that the product between the optical anisotropy and the thickness of the cell is substantially 0.55 micrometers. Shutter construction according to Claim 1, characterized in that the deceleration means is arranged between the transparent plates of the liquid crystal cell. Liquid crystal shutter construction, suitable for glass protection and automatic darkening welding glass filters, which design is switchable between a first state with high light transmission and a second state with low light transmission and vice versa, corresponding to an electrical control signal, n .no which construction has a liquid crystal cell of nematic type arranged between transparent plates provided with electrodes for arranging an electric field in opposition to the control signal, which plates have facing surfaces, provided with aligning means because they each have their own molecular alignment direction for molecules in the vicinity of said alignment means in the absence of said electric fields, and which liquid crystal cell is mounted between polarizers, the liquid crystal structure comprising a band pass and the liquid to bond crystals to each other. the wavelength dependence of properties in a certain wavelength range is substantially complementary to the wavelength dependence of the transmission properties of the liquid crystal cell in the absence of said electric field, characterized in that the two facing surfaces have an angular difference between the molecular alignment directions; and in that the product between the optical anisotropy and the thickness of the cell is substantially 0.8 in micrometers. Shutter construction according to one of the preceding claims, characterized in that the bandpass filter has a transmission maximum in a first wavelength range; and in that the thickness of the liquid crystal cell is so selected that it has a transmission minimum in a second wavelength range in the absence of said electric field, which second wavelength range substantially corresponds to said first wavelength range. Shutter construction according to one of the preceding claims, characterized in that the polarizers have substantially orthogonal polarization ridges. Shutter construction according to any one of the preceding claims, characterized in that the construction comprises a further liquid-cell cell of the nematic type, which is arranged between one of said polarizers and a further polarizer, wherein n now go 10 15 '516 183 15 a polarizer and said additional polarizer have substantially orthogonal polarization directions. Shutter construction according to any one of the preceding claims, characterized in that said certain wavelength irradiation is the central portion of the visible wavelength irradiation, i.e. substantially between 500 nm and 600 nm. Light protection device comprising a shutter construction according to any one of the preceding claims. Light protection device according to claim 8, characterized in that it comprises a sensor means for emitting a sensor signal corresponding to the light intensity; and a signal generator for generating said electrical control signal corresponding to the sensor signal.