Security device and method of manufacture thereof

A method of forming a security device comprising:

• forming a diffractive structure in a carrier layer, wherein
• the diffractive structure comprises a plurality of grating elements each having a principal component of orientation within the plane of the device substantially parallel with a first axis lying in the plane of the device,
• the diffractive structure having a first area comprising a plurality of regions arranged as a first contiguous pattern, wherein the grating elements within a region have either (i) a constant pitch or (ii) a constant spacing along a second axis orthogonal to the first axis,
• the pitches or spacings of the plurality of regions of the first area increase from one region to the next between a first region having a grating element pitch or spacing of less than or equal to 0.6 microns and an end region having a grating element pitch or spacing of greater than or equal to 5 microns such that,
• upon illumination and viewing along a first viewing direction substantially orthogonal to the first axis, the device exhibits a first optical effect in that at substantially any angle of tilt about the first axis at least one region exhibits a diffractive colour, and
• each region comprises a plurality of sub-regions having respective grating element orientations within the plane of the device, wherein the orientations of the grating elements of the plurality of sub-regions are such that the diffractive colours exhibited by the plurality of sub-regions within a region are perceived to be the same upon tilting of the device about the second axis, and wherein the grating element orientations of the sub-regions within a region have a repeating arrangement whereby the first optical effect is the same and perceived to be replayed at more than one angle of tilt of the device about the second axis.