1. A method for authenticating documents by using moiré patterns, the method comprising the steps of: a) superposing a document with a base layer comprising patterns and a revealing layer comprising a revealing line grating, thereby producing moiré patterns and b) comparing said moiré patterns with reference moiré patterns and depending on the result of the comparison, accepting or rejecting the document, where successive lines of the revealing line grating sample within the base layer different instances of the patterns and where the produced moiré patterns are a transformation of base layer patterns, said transformation comprising at least an enlargement.