Application of Photoalignment Materials in Optical Elements

Photoalignment of liquid Crystalline Materials Physics and Applications V. Chigrinov, V. Kozenkov and H.-S. Kwok 2008 John Wiley Sons, Ltd 

Quantitatively, the transmittance of light along two orthogonal polarizations through an absorptive polarizer is given by Beer s law 

Here a is the absorption coefficient and d is the absorptive film thickness. T /T can be defined as the extinction ratio e and T can be defined as the transmission efficiency rj. Of course, for a good polarizer e should be very small and rj should be as close to 100% as possible. 

Sometimes it is useful to define the contrast ratio of a polarizer as the inverse of the extinction ratio  

Thus for reasonable performance polarizers, the dichroic ratio has to be over 25. Current polarizers are based on iodine impregnation of stretched polymer films [4] (e.g. 30-50 tim polyvinyl alcohol). The dichroic ratio is over 50. There are two issues in getting high performance from polarizers based on anisotropic absorption. First, the intrinsic absorption anisotropy has to be large. Second, the absorber has to be well aligned in one direction. Both are difficult issues. 

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