An optical device that exhibits a variable optical effect upon illumination, 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; 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 colour such that a user viewing the device at a viewing angle within a first viewing plane that intersects the optical device along the first direction perceives a first diffractive colour image; and wherein the set of first grating regions is interlaced with the set of second grating regions; the optical device being characterised in that 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 within a second viewing plane that intersects the optical device along the second direction, the diffractive colours exhibited by the plurality of grating sub-regions cooperate such that the achromatic grating region is perceived to exhibit diffractive whiteness. 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; 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 colour such that a user viewing the device at a viewing angle within a first viewing plane that intersects the optical device along the first direction perceives a first diffractive colour image; and wherein the set of first grating regions is interlaced with the set of second grating regions; the optical device being characterised in that 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 within a second viewing plane that intersects the optical device along the second direction, the diffractive colours exhibited by the plurality of grating sub-regions cooperate such that the achromatic grating region is perceived to exhibit diffractive whiteness.