A system comprising: an optical surface (2,3) having a diffractive image generating structure disposed thereon, the diffractive image generating structure comprising a layer of reflective material incorporating a plurality of grooved diffractive elements each having a periodic wave surface profile, the periodic wave surface profiles having a groove alignment direction; a source of incident electromagnetic radiation arranged to illuminate the diffractive elements at an angle of incidence substantially normal to the plane of the surface of the diffractive elements; means (6,10) for polarising the radiation from the source, and means (6,10) for polarising radiation reflected from the diffractive elements, wherein: the diffractive elements are configured such that, in use, polarisation conversion of the incident radiation takes place; the diffractive elements are disposed in a two dimensional array of pixels to represent an image; the means for polarising is arranged to pass incident radiation having a polarisation state of approximately 45° azimuth to the groove alignment direction, and is arranged to select a polarisation, using the means for polarising the radiation reflected from the diffractive elements, and to pass radiation of the selected polarisation to a detection point; the surface profile is a rectangular, square or pulsed waveform having a mark to space ratio M; and wherein for each respective surface profile at least one parameter thereof is chosen to provide a particular colour response, the at least one parameter being selected from a list comprising the pitch G, depth d, mark, mark to pitch ratio, mark to space ratio M, Fourier harmonic content of the surface profile cross-section, permittivity of the layer of reflective material and permittivity of any optional protective coating layer. an optical surface (2,3) having a diffractive image generating structure disposed thereon, the diffractive image generating structure comprising a layer of reflective material incorporating a plurality of grooved diffractive elements each having a periodic wave surface profile, the periodic wave surface profiles having a groove alignment direction; a source of incident electromagnetic radiation arranged to illuminate the diffractive elements at an angle of incidence substantially normal to the plane of the surface of the diffractive elements; means (6,10) for polarising the radiation from the source, and means (6,10) for polarising radiation reflected from the diffractive elements, wherein: the diffractive elements are configured such that, in use, polarisation conversion of the incident radiation takes place; the diffractive elements are disposed in a two dimensional array of pixels to represent an image; the means for polarising is arranged to pass incident radiation having a polarisation state of approximately 45° azimuth to the groove alignment direction, and is arranged to select a polarisation, using the means for polarising the radiation reflected from the diffractive elements, and to pass radiation of the selected polarisation to a detection point; the surface profile is a rectangular, square or pulsed waveform having a mark to space ratio M; and wherein for each respective surface profile at least one parameter thereof is chosen to provide a particular colour response, the at least one parameter being selected from a list comprising the pitch G, depth d, mark, mark to pitch ratio, mark to space ratio M, Fourier harmonic content of the surface profile cross-section, permittivity of the layer of reflective material and permittivity of any optional protective coating layer.