Abstract: A copy preventing sheet-form screen comprises a lenticular screen formed by a sheet of transparent plastic material having, a series of adjacent lens portions with major optical axes perpendicular to the front face and a series of light blocking striae located spaced apart underlying respective lens portions. The striae extend across a major part of the major optical axes so that when covering a sheet printed with meaningful information, the screen enables the information to be easily seen when viewed obliquely in ambient white light but prevents a meaningful copy thereof being made by conventional, perpendicularly aligned, phototransference techniques. The screen can be attached to the printed sheet by adhesive or formed as an adhesive tape to cover only preselected areas of printing. The screen may also form one or both sides of a security envelope enclosing the printed sheet and sealed by a security label carrying an authorized signature.