Polymer Engineering Faculty Research


Birefringence, Anisotropic Shrinkage and Luminance in Light Guide Plates: Modeling and Experiment

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Publication Date

Fall 2007


Liquid crystal displays (LCDs) with edge-lit backlight system have been widely used in industry due to several advantages over traditional cathode-ray tubes (CRTs). The main component of the backlight system is the light-guide plate (LGP) which is designed to provide the highest possible degree of light concentration and luminance efficiency. However, the relationship between processing conditions in manufacturing and their optical performance have not been established. In addressing this issue, LGP moldings were made of optical grade polycarbonates (PCs) of low and high viscosity and a polystyrene (PS). The theoretical and experimental studies on the effect of the processing conditions on the anisotropic shrinkage, residual stresses and birefringence, and resulting luminance in the injection molded V-groove LGPs were carried out. The stress-optical coefficient and relaxation modulus functions of polymers were obtained by specially designed rheo-optical instrument. These functions were incorporated to the linear viscoelastic and photoviscoelastic constitutive equations to predict the thermal birefringence in constrained, freely quenched plates and LGP moldings. The flow-induced birefringence and anisotropic shrinkage of LGPs were simulated by using a combination of a CV/FEM/FDM technique, a nonlinear viscoelastic constitutive equation, and orientation functions. The residual normal and transverse birefringence in LGPs along with shrinkages were measured. The predicted total birefringence was obtained by a summation of the predicted flow- and thermally-induced birefringence. Numerical results were compared with measurements at various processing conditions indicating a good agreement in the anisotropic shrinkage and a fair agreement in the residual birefringence. The luminance as a function of a viewing angle on the injection molded LGPs at various molding conditions was measured by means of the device that was built for this purpose. Strong effects from the processing conditions on optical performance were found. The results indicated that the LGPs made of low viscosity PC exhibited the best light concentration and the highest luminance while LGPs made of PS showed the lowest optical performance. Also, it was found that the luminance of LGPs showed a strong correlation with the depth of melt filling of the V-grooves and some correlation with the frozen-in birefringence.



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