Digitally Reconfigurable Neutral Density Filter

A zero-Rth achromatic polarization rotator switch, comprising:

• a first linear polarizer with absorption axis orientation 0° or 90°;
• a first liquid crystal half-wave retarder switch (LC1) with slow-axis orientation α1;
• a second liquid crystal half-wave retarder switch (LC2) with-slow axis orientation α2;
• a passive negative half-wave A-plate retarder with slow-axis orientation α3;
• a passive negative C-Plate half-wave retarder;
• wherein the liquid crystal switches are electrically driven out of phase, such that LC1 is an A-plate when LC2 is a C-plate (State 1), and LC1 is a C-plate when LC2 is an A-plate (State 2);
• and
• wherein (α3−α1)≠±90°.

A two-stage digital neutral density (DND) filter, comprising:

• a first neutral polarizer with absorption-axis orientation θ1;
• a second neutral polarizer with absorption-axis orientation θ2, wherein the angle between θ2 and θ1 is β1 giving transmission cos2β1;
• a third neutral polarizer with absorption-axis orientation θ3, wherein the angle between θ3 and θ2 is β2 giving transmission cos2β2, wherein β2 is different from β1;
• a first liquid crystal achromatic rotator switch (LCAR1) between the first and second neutral polarizers;
• and
• a second liquid crystal achromatic rotator switch (LCAR2) switch between the second and third neutral polarizers;
• wherein LCAR1 has zero in-plane retardation in State 1, and polarization rotation β1 in State 2;
• wherein LCAR2 has zero in-plane retardation in State 3, and polarization rotation β2 in State 4;
• wherein each of the four voltage states can be independently selected to give four unique transmission levels.