A pressure sensitive adhesive obtainable by free-radical polymerization, characterized in that

• at least 65% by weight of the adhesive is based on at least one acrylic monomer from the group of the compounds of the following general formula: ##STR1##
• where R₁=H or CH₃ and the radical R₂=H or CH₃ or is selected from the group consisting of branched or unbranched saturated alkyl groups having 2-20 carbon atoms,
• the average molecular weight of the adhesive is at least 65,000,
• applied to a carrier, the adhesive possesses a preferential direction, where the refractive index measured in the preferential direction n_MD is greater than the refractive index measured in a direction perpendicular to the preferential direction n_CD, the difference n=n_MD-n_CD being at least 110^-5.