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The Taguchi method is applied to optimize the design of a micro-prism diffusion film in such a way as to maximize the luminance uniformity of a slim-type bottom-lit backlight unit illuminated by cold cathode fluorescent lamp (CCFL) light sources. The effects on the luminance uniformity of the filling density, apex angle and aspect ratio of the prism structures and the placement angle of the diffusion film are systematically examined using a three-level Taguchi orthogonal array. The signal-to-noise (S/N) ratio and analysis of variance (ANOVA) results show that the interaction between the filling density, apex angle and aspect ratio have a significant impact on the luminance uniformity. It is shown that the optimal design parameter settings are a filling density of 100%, an apex angle of 100 , an aspect ratio of 5:1 and a optical film placement angle of 20 . The optimized micro-prism optical film design not only results in a luminance uniformity of 97.8%, but also enables a significant reduction in the thickness of the bottom-lit backlight unit.