Liquid crystal achromatic compound retarder

An achromatic shutter comprising:

• a first linear polarizer oriented at an angle .theta..sub.1 ;
• a second linear polarizer oriented at an angle .theta..sub.2 ;
• a liquid crystal retarder unit, positioned between said first and second polarizers, said retarder unit having composite retardance .pi. at a design wavelength and having composite orientation electronically switchable between 5.pi./12 and 8.pi./12 with respect to .theta..sub.1 ;
• a first retarder, positioned between said first polarizer and said liquid crystal retarder unit, having retardance .pi. at said design wavelength and orientation .pi./12 with respect to .theta..sub.1 ;
• and
• a second retarder, positioned between said second polarizer and said liquid crystal retarder unit, having retardance .pi. at said design wavelength and orientation .pi./12 with respect to .theta..sub.1.

A reflection-mode liquid crystal achromatic compound retarder, comprising:

• a first passive retarder having retardance .GAMMA..sub.1.sup.0 at a design wavelength and orientation .alpha..sub.1 ;
• a reflector;
• and
• a liquid crystal retarder unit, positioned between said first retarder and said reflector, said retarder unit having composite retardance .pi./2 at said design wavelength and having composite orientation electronically switchable between .alpha..sub.2, in a first state, and .alpha..sub.2 ', in a second state;
• wherein .alpha..sub.1 and .alpha..sub.2 are such that said compound retarder is achromatic in said first state.