Birefringence dispersion of polymer films as applied to optical compensation in LCD

Minimizing color shift effects in a liquid crystal display (LCD) requires close attention to the optical dispersion of compensation layers used to increase the contrast ratio and viewing angle of the display. This paper describes our attempt to systematize and characterize the birefringence dispersion of solvent-cast polymer films and relate it to the "native" out-of-plane birefringence (Δn_th) of the polymer and its structure. Results for a wide range of polymers show general correlation between the dispersion parameter DP_Δn [=Δn(450)/Δn(590)] and Δn_th. For all positively birefringent polymers, DP _Δn is always > 1, suggesting "normal" dispersion at any level of Δn_th, while for negatively birefringent polymers DP_Δn > 1 only when |Δn_th| > ∼0.003. Below this critical value, DP_Δn < 1, i.e., the polymer is reverse-dispersive. The results are explained in terms of the Cauchy coefficients for the corresponding polymers, and they suggest that the dispersion of birefringence cannot be controlled independently of the birefringence of the polymer film.