Liquid Crystal Variable Attenuator

With crossed polarizers, light transmission is maximized by applying the correct voltage to achieve half-wave retardance from the LC cell. Half-wave operation rotates the incoming polarization direction by 90°, so that light is passed by the second polarizer. Minimum transmission is obtained with the retarder operating at zero (or a whole number of) waves.

Transmission decreases as the applied AC voltage amplitude increases (half- to zero-waves retardance). The relationship between transmittance T and retardance (in degrees) for crossed polarizer configuration is given by: T(Θ) = 1/2 [1 – cos(Θ)] Tmax, where Tmax is the maximum transmittance when retardance is exactly one-half wave (or 180°).