PURPOSE: To suppress optical strain by projecting a light ray vertically to a film, transmitting the light through the film to the rear and controlling the difference in luminance between the transmitted light rays at the places a specified distance apart within a specified range of the luminance exhibited at the place on the low-luminance side.

CONSTITUTION: A light ray is projected vertically to an optical film consisting of a transparent polymeric material and transmitted through the film to the rear. The luminances of the transmitted lights at the optional places 1cm from each other are compared, the difference in luminance between the transmitted lights is controlled within ≤30% of the luminance exhibited at the place on the low-luminance side. Polycarbonate, polystyrene, etc., are exemplified as the polymeric material used for the optical film. The light ray from a relatively high-luminance source such as mercury lamp and halogen lamp is converged into parallel rays with a lens having a relatively long focal distance, etc., to irradiate the film vertically, the gradation picture projected on a screen, for example, is taken by a CD camera, etc., and the film is evaluated by the gradation level of the picture signal.