The present invention provides an authentication apparatus operative to determine the authenticity of a polymer film, comprising: an optical response modifier operative to modify a first effect influenced by said birefringence characteristic of said to a modified first effect; and an optically-based birefringence measuring arrangement operative to measure said modified first effect; and wherein said apparatus is operative to: compare a value, or range of values, representative of said modified first effect as measured with a value, or range of values representative of a specified first effect as modified by a same optical response modifier and corresponding to a predetermined birefringence characteristic of an authentic polymer film; and output an authenticity signal indicative of authenticity or otherwise of said film based upon said comparison. There are also provided one or more methods of determining the authenticity of a polymer film.