Optical devices and methods of manufacture thereof

A method of manufacturing an optical device that exhibits a variable optical effect upon illumination, the method comprising:

• forming a diffractive structure in a carrier layer, the diffractive structure comprising:
• a set of first grating regions, wherein each first grating region comprises a plurality of first grating elements, each first grating element having a principal component of orientation within the plane of the device that is substantially orthogonal to a first direction;
• and
• a set of second grating regions, wherein each second grating region comprises a plurality of second grating elements, each second grating element having a principal component of orientation within the plane of the device that is substantially orthogonal to a second direction different to the first direction, wherein
• at least a subset of the first grating regions each exhibit a diffractive color such that a user viewing the device at a viewing angle along the first direction perceives a first diffractive color image,
• at least a subset of the second grating regions are achromatic grating regions, wherein each achromatic grating region comprises a plurality of grating sub-regions having different respective grating element pitches ranging from a coarse pitch to a fine pitch and arranged such that, for a plurality of viewing angles along the second direction, the diffractive colors exhibited by the plurality of grating sub-regions cooperate such that the achromatic grating region is perceived to exhibit diffractive whiteness,
• the set of first grating regions is interlaced with the set of second grating regions, and
• the plurality of grating sub-regions within each achromatic grating region comprises:
• a fine grating sub-region having a fine grating element pitch of less than or equal to 0.7 microns,
• a coarse grating sub-region having a coarse grating element pitch of greater than or equal to 3 microns, and
• one or more intermediate grating sub-regions each having a grating element pitch that is between the coarse grating element pitch and the fine grating element pitch, and wherein the grating element pitches of the one or more intermediate grating sub-regions are different.