Measurement of mechanical properties of optical diffuser films by bugle test and finite element method

The optical diffuser films have the excellently optical properties, but the mechanical properties are very important information. Especially, the more lamps used to enhance brightness in direct type backlight unit let more heating to be created. This side thermal effect makes optical diffuser films to occur warps and wrinkles via thermal stress and it leads to poor uniform performance in brightness. Therefore, the mechanical properties of optical diffuser films are the significant parameters for design. In this study, we employ the bulge test on the optical diffuser films to obtain the Young’s modulus, Poisson’s ratio, and residual stress. Meanwhile, the phase shifting electronic speckle pattern interferometry (PS-ESPI) was used to measure the out-of-plane displacement of optical diffuser films. Furthermore we use the experimental datum into finite element analysis software (ANSYS) to prove that the experiment is reliable. Then, we study the temperature effect on the changes of the mechanical properties of diffusers, which used in cell-phone, notebook, and LCD TV products, in order to avoid warps and wrinkles phenomenon happened in optical films. An experimental method which combining the finite element method and the bulge test is developed in this paper to measure the mechanical properties of the optical diffuser film. The influence of temperature on the mechanical properties was also investigated. The reasonableness of the developed method was verified through the well agreement between the FEA simulation result and ESPI fringe pattern. Furthermore, the temperature does have the significant effect on the Young’s modulus of the optical diffuser film and less influence on the Poisson’s ratio and residual stress. In practical application, the optical diffuser film is baked by a cold cathode light tube and the temperature of optical diffuser film is not uniform. Consequently, the Young’s modulus will vary with the location and this may cause easily the optical diffuser films wrinkle such that the display quality would be degraded from this phenomenon. The performance of accuracy and advantages on the mechanical properties are determined by evaluation the optical diffuser films.