A directory shell references one or more directories. The repetitive prismatic structure includes optical path changing slopes aligned in a substantially constant direction so as to be inclined at an inclination angle of from 35 to 48 degrees with respect to a plane of the polarizer. The sections are located in substantially two planes which, each section is at an angle to a horizontal plane, which intersect along an intersecting line parallel to the conveying direction T. Other more complicated defect conditions can also be detected. Representative examples of such polymers may be, for example, trans 1,4-polybutadiene, 3,4-polyisoprene, natural cis 1,4-polyisoprene rubber, isoprenebutadiene copolymer having a high trans 1,4-polybutadiene component, polystyrene, styrenebutadiene rubber having a high trans 1,4-polybudadiene component and styrenebutadiene rubber having a high styrene content.