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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 (Δnth) of the polymer and its structure. Results for a wide range of polymers show general correlation between the dispersion parameter DPΔn [≡Δn(450)/Δn(590)] and Δnth. for all positively birefringent polymers, DPΔn is always > 1, suggesting “normal” dispersion at any level of Δnth, while for negatively birefringent polymers DPΔn > 1 only when |Δnth| > ∼0.003. Below this critical value, DPΔn < 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. |
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