Optical devices and methods of manufacture thereof
An optical device includes: first grating regions each having a plurality of first grating elements each having a principal component of orientation substantially orthogonal to a first direction; and second grating regions each including a plurality of second grating elements each having a principal component of orientation substantially orthogonal to a second direction. At least a subset of the first grating regions each exhibit a diffractive colour such that a user viewing at a first viewing angle perceives a first diffractive colour image. At least a subset of the second grating regions are achromatic, each including a plurality of grating sub-regions having different respective grating element pitches and arranged such that, for a plurality of second viewing angles, the diffractive colours exhibited by the sub-regions cooperate such that the achromatic grating region is perceived to exhibit diffractive whiteness. The sets of first and second grating regions are interlaced.
1 . 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; 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 e 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.
2 . The device of claim 1 , wherein each of the second grating regions are achromatic grating regions.
3 . The device of claim 1 , wherein the arrangement of grating sub-regions and their respective pitches within an achromatic grating region is such that:
a first group of grating sub-regions comprises grating sub-regions corresponding to a first set of color channels when viewed at a first viewing angle which cooperate such that the achromatic grating region exhibits diffractive whiteness at said first viewing angle; and
a second group of grating sub-regions comprises grating sub-regions corresponding to a second set of color channels when viewed at a second viewing angle different to the first viewing angle which cooperate such that the achromatic grating region exhibits diffractive whiteness at said second viewing angle.
4 . The device of claim 1 , wherein each achromatic grating region comprises at least four intermediate grating sub-regions.
5 . The device of claim 1 , wherein the grating sub-regions within an achromatic grating region are arranged in a substantially contiguous manner.
6 . The device of claim 1 , wherein the grating elements of at least two grating sub-regions within an achromatic grating region have different orientations within the plane of the device.
7 . The device of claim 1 , wherein the first and second directions are offset by an angle greater than or equal to 45 degrees.
8 . The device of claim 1 , wherein a first subset of the first grating regions each exhibit a diffractive color and a second subset of the first grating regions are achromatic grating regions that are perceived to exhibit diffractive whiteness when viewed at a plurality of viewing angles along the first direction.
9 . The device of claim 1 , wherein a first subset of the second grating regions are achromatic grating regions and each of a second subset of the second grating regions exhibits a diffractive color such that a user viewing the device at a viewing angle along the second direction perceives a second diffractive color image.
10 . The device of claim 9 , wherein a first subset of the first grating regions each exhibit a diffractive color and a second subset of the first grating regions are achromatic grating regions that are perceived to exhibit diffractive whiteness when viewed at a plurality of viewing angles along the first direction, and wherein the positions of the achromatic grating regions of the first grating regions correspond to the positions of the achromatic grating regions of the second grating regions.
11 . The device of claim 1 , wherein the dimensions of each grating region are such that they are not discernible to the naked human eye.
12 . The device of claim 1 , wherein each grating region has a maximum dimension of 70 microns.
13 . The device of claim 1 , wherein each grating sub-region has a maximum dimension of 25 microns.
14 . The device of claim 1 , wherein the set of first grating regions is interlaced with the set of second grating regions in a grid pattern, such that the first grating regions are interlaced with the second grating regions along two directions.
15 . The device of claim 1 , wherein the first grating regions and the second grating regions are arranged in a substantially contiguous manner.
16 . The device claim 1 further comprising a set of non-diffractive regions, wherein each non-diffractive region comprises an absence of grating elements.
17 . The device of claim 1 , wherein the optical device is a security device.
18 . A security article or a security document comprising a security device according to claim 17 .
19 . 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.