Optical device and method of manufacture thereof

A method of forming an optical device that, upon illumination, exhibits one or more diffractive images dependent on viewing angle, the method comprising:

• forming a diffractive structure in a carrier layer, the diffractive structure comprising a plurality of grating regions, each grating region corresponding to a component of a respective diffractive image, wherein:
• each grating region of the diffractive structure comprises a plurality of grating elements arranged along a respective first direction, each grating element having a principal component of orientation within a plane of the optical device that is substantially orthogonal to the respective first direction;
• the grating elements within each grating region have a constant respective pitch and substantially the same respective orientation such that each grating region, upon illumination, exhibits a diffractive colour such that the corresponding diffractive image is exhibited;
• the diffractive structure comprises first and second grating regions that are both elongate along a common first direction, the first and second grating regions being adjacent along the common first direction, and wherein the pitch and/or orientation of the grating elements of the first grating region are different from the pitch and/or orientation of the grating elements of the second grating region;
• and
• at least one grating region is elongate along the respective first direction by a ratio of at least 2 to 1 such that a ratio of a dimension of the at least one grating region along the respective first direction to a dimension of the at least one grating region along a direction orthogonal to the respective first direction is at least 2 to 1.