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This paper describes an innovative approach for fabricating a seamless roller mold based on cylindrical photolithography, isotropic chemical etching, and electropolishing. Hexagonally arrayed concave microstructures with a near-semispherical profile are directly and seamlessly fabricated on a metal roller. The fabricated roller mold is then used for continuous roller imprinting of microlens array (MLA) on a thin polyethylene terephthalate (PET) substrate. The fabricated ball-shape microlenses have a footprint diameter of 54 and a sag height of 20.5 and are hexagonally close packed with a filling ratio larger than 91%. This MLA/PET optical film can serve as a brightness enhancement film in flat panel backlighting, and its optical performance is experimentally verified. A 30% increase in the forward on-axis luminance is observed.