Method of using mask film to form relief images

A mask-forming film has a transparent layer between the imageable layer and the carrier sheet, which transparent layer has a refractive index that is lower (by at least 0.04) than that of the carrier sheet or any immediately adjacent layer between it and the carrier sheet. This lower refractive index layer modifies the path of incident radiation during mask image transfer so as to provide steeper shoulder angles in the relief image solid areas. This mask film is used to form a relief image such as in a flexographic printing plate.