Optical film, polarizing plate, and liquid crystal display device

The optical film of the present invention contains a cycloolefin resin. A surface layer region (Sa) is defined as a region from one surface of the optical film to a depth of 30% of the thickness, a surface layer region (Sb) is defined as a region from the other surface of the optical film to a depth of 30% of the thickness, and an inner layer region (C) is defined as a region between the surface layer region (Sa) and the surface layer region (Sb). The ratio As/Ac between the absorption coefficient As of light having a wavelength of 9.6 [mu] m in at least the surface layer region Sa and the absorption coefficient Ac of light having a wavelength of 9.6 [mu] m in the inner layer region C is 1.1-20, and the absorption coefficient of light having a wavelength of 9.6 [mu] m in the optical film is 1.5 * 10-5/[mu] m or more. 本发明的光学膜包含环烯烃树脂。将从光学膜的一侧的面至厚度的30％的深度为止的区域设为表层区域Sa，将从另一侧的面至厚度的30％的深度为止的区域设为表层区域Sb，将表层区域Sa和表层区域Sb之间的区域设为内层区域C时，至少表层区域Sa的波长9.6μm的光的吸收系数As与内层区域C的波长9.6μm的光的吸收系数Ac之比As/Ac为1.1～20，光学膜的波长9.6μm的光的吸收系数为1.5×1