Spectrum defects such as hot band and rainbow spectra on back light unit (BLU) composed of prism film and diffusing film have been examined by optical analysis. Hot band and rainbow phenomenon could be resulted from rapid luminance decrease and fluctuation in spectra at viewing angles in the range of 30-40 degrees and 50-70 degrees, respectively. By optical simulation with modification in prismatic structure, it is suggested that spectrum defects could be diminished or eliminated in both calculated simulation and practical prism pattern when the plane structure of prism is changed from straight to slow curved.