Split-element liquid crystal tunable optical filter

A double split-element optical filter comprising one or more stages optically coupled in series wherein at least one stage comprises:

• a first polarizer;
• a second polarizer, oriented parallel or perpendicular to said first polarizer;
• a center retarder oriented at .+-..pi./4 to said first polarizer, and positioned between said first and second polarizers;
• a first inner-split-element retarder, oriented parallel or perpendicular to said first polarizer, and positioned between said first polarizer and said center retarder;
• a second inner-split-element retarder, of equal retardance to said first inner-split-element retarder, oriented parallel or perpendicular to said first polarizer, and positioned between said center retarder and said second polarizer;
• a first outer-split-element retarder, oriented at .+-..pi./4 with respect to said first polarizer, and positioned between said first polarizer and said first inner-split-element retarder;
• and
• a second outer-split-element retarder, of equal retardance to said first outer-split-element retarder, oriented at .+-..pi./4 with respect to said first polarizer and positioned between said second inner-split-element retarder and said second polarizer.

A reflection-mode split-element filter comprising:

• a polarizer;
• a reflector;
• a center retarder unit positioned between said polarizer and said reflector, and comprising one or more retarders and having a composite retardance oriented parallel or perpendicular to said polarizer;
• and
• a split-element retarder unit, positioned between said polarizer and said center retarder unit, and comprising a split-element retarder, oriented at .+-..pi./4 with respect to said polarizer;
• wherein at least one of said retarder units comprises a liquid crystal retarder, said liquid crystal retarder having a fixed retardance and having an optic axis rotatable between a first and a second orientation.

A liquid crystal split-element optical filter comprising one or more stages optically coupled in series wherein at least one stage comprises:

• a first polarizer;
• a second polarizer oriented parallel or perpendicular to said first polarizer;
• a center retarder unit positioned between said first and second polarizers, and comprising one or more retarders and having a composite retardance oriented parallel or perpendicular to said first polarizer;
• a first split-element retarder unit, positioned between said first polarizer and said center retarder unit, and comprising a first split-element retarder, oriented at .+-..pi./4 with respect to said first polarizer;
• a second split-element retarder unit, positioned between said center retarder unit and said second polarizer, and comprising a second split-element retarder of equal retardance to said first split-element retarder and oriented at .+-..pi./4 with respect to said first polarizer;
• and
• a liquid crystal means, positioned between said second split-element retarder unit and said second polarizer, for adding a half-wave of retardance and thereby reversing the locations of transmission maxima and minima;
• wherein at least one of said retarder units comprises a first liquid crystal retarder, said first liquid crystal retarder having a fixed retardance and having an optic axis rotatable between a first orientation and a second orientation.