Switchable light filter, and lighting apparatus and screen with such switchable light filter

A switchable light filter comprising:

• a first optical element configured to transmit or at least partially absorb light according to a polarization state and/or an incidence direction thereof, wherein a first preferred direction is selectable for the first optical element and is set at an angle α relative to a central normal of the first optical element, and the angle α being measured in a selectable first plane including the central normal;
• means for selectively generating a first electric field or a second electric field;
• a first linear polarizer having a transmission maximum for light linearly polarized parallel to the selectable first preferred direction;
• optionally, a retardation layer;
• a liquid crystal layer disposed between the first linear polarizer and the first optical element, the first electric field or the second electric field being applied to the liquid crystal layer to influence a polarization state of light passing therethrough,
• so that a transmission property of the switchable light filter varies between a first operation mode B1 in which the first electric field is applied and a second operation mode B2 in which the second electric field is applied,
• wherein the first electric field or the second electric field is generated by applying a voltage U to the means for selectively generating a first electric field or a second electric field, where x1U0≤U≤x2U0 with
• U
• 0
• =
• π
• ⁢
• κ
• ❘
• "\[LeftBracketingBar]"
• ε
• p
• -
• ε
• s
• ❘
• "\[RightBracketingBar]"
• ,
• κ=k11 parallel to a surface or κ=k33 perpendicular to a surface, εp and εs corresponding to dielectric constants parallel and perpendicular to a long molecular axis respectively, and x1=1 and x2=10, so that for the first operation mode B1 or the second operation mode B2, magnitudes of the transmittance values T(α−45°) and T(α+45°) differ by at least a factor of 1.5.