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1. (WO1997015255) A LIQUID CRYSTAL SHUTTER AND A LIGHT SHIELDING DEVICE INCLUDING SUCH A SHUTTER
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Claims

1. A liquid crystal shutter construction, suitable for glass shields and automatically darkening welding glass filters, the shutter 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 shutter 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, said liquid crystal cell being mounted between polarisers, characterized in a retardation film disposed between the polarisers and being devised to compensate for remaining retardation of the liquid crystal cell when in an electrically activated state.

2. A shutter construction according to claim 1, characterized in that the retardation means is arranged such that a fast axis of the retardation means differs from a fast axis ofthe inherent retardation ofthe liquid crystal cell.

3. A shutter construction according to claim 1 or 2, characterized in that the angle between the fast axis ofthe retardation means and the fast axis of the inherent retardation of the liquid crystal cell is in the range between 45° and 90°.

4. A shutter construction according to claim 2 or 3, characterized in that the fast axis of the retardation means is essentially perpendicular to the fast axis ofthe inherent retardation ofthe liquid crystal cell.

5. A shutter construction according to any of the preceding claims 2-4, characterized in that the fast axis ofthe retardation means differs from the direction ofthe bisectrix ofthe angle between the molecular alignment directions.

6. A shutter construction according to any ofthe preceding claims 2-5, characterized in that the fast axis ofthe retardation means is disposed peφendicular to the bisectrix ofthe angle between the molecular alignment directions.

7. A shutter construction according to any ofthe preceding claims, characterized in that the angular displacement between the molecular alignment directions is in the range between 0° to 85°.

8. A shutter construction according to any ofthe preceding claims, characterized in that the alignment directions are parallel, i.e. the angular displacement between the alignment directions is 0°.

9. A shutter construction according to any ofthe preceding claims, characterized in that it further comprises a second cell and a second polarisation filter.

10. A shutter construction according to any ofthe preceding claims, characterized in a band-pass filter with a transmission characteristics having a transmittance maximum in the central part ofthe visible wavelength range, essentially between 500 and 600 nm, and in that the liquid crystal cell is selected such that it has transmission characteristics essentially complementary to the transmission characteristics ofthe band-pass filter.

11. A shutter construction according to any of the preceding claims, characterized in that the retardation means is a retardation film constituting a compensating layer.

12. A light shielding device including a shutter construction according to any one ofthe preceding claims.

13. A light shielding device according to claim 12, characterized in that it includes a 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.

AMENDED CLAIMS
[received by the International Bureau on 25 March 1997 (25.03.97) ;
original claims 1-13 replaced by amended claims 1-12 (_. pagesjj

1. A liquid crystal shutter construction, suitable for glass shields and automatically darkening welding glass filters, the shutter 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 shutter 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, said liquid crystal cell being mounted between polarisers, characterized in a retardation means disposed between the polarisers and being devised to compensate for remaining retardation ofthe liquid crystal cell when in an electrically activated state, said retardation means being arranged such that a fast axis ofthe retardation means differs from a fast axis ofthe inherent retardation ofthe liquid crystal cell.

2. A shutter construction according to claim 1, characterized in that the angle between the fast axis ofthe retardation means and the fast axis ofthe inherent retardation ofthe liquid crystal cell is in the range between 45° and 90°.

3. A shutter construction according to claim 1 or 2, characterized in that the fast axis of the retardation means is essentially peφendicular to the fast axis ofthe inherent retardation ofthe liquid crystal cell.

4. A shutter construction according to any ofthe preceding claims 1-3, characterized in that the fast axis ofthe retardation means differs from the direction ofthe bisectrix ofthe angle between the molecular alignment directions.

5. A shutter construction according to any ofthe preceding claims 1-4, characterized in that the fast axis ofthe retardation means is disposed peφendicular to the bisectrix ofthe angle between the molecular alignment directions.

6. A shutter construction according to any ofthe preceding claims, characterized in that the angular displacement between the molecular alignment directions is in the range between 0° to 85°.

7. A shutter construction according to any ofthe preceding claims, characterized in that the alignment directions are parallel, i.e. the angular displacement between the alignment directions is 0°.

8. A shutter construction according to any ofthe preceding claims, characterized in that it further comprises a second cell and a second polarisation filter.

9. A shutter construction according to any ofthe preceding claims, characterized in a band-pass filter with a transmission characteristics having a transmittance maximum in the central part ofthe visible wavelength range, essentially between 500 and 600 nm, and in that the liquid crystal cell is selected such that it has transmission characteristics essentially complementary to the transmission characteristics ofthe band-pass filter.

10. A shutter construction according to any ofthe preceding claims, characterized in that the retardation means is a retardation film constituting a compensating layer.

1 1. A light shielding device including a shutter construction according to any one ofthe preceding claims.

12. A light shielding device according to claim 1 1, characterized in that it includes a 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.