All-dielectric optically variable pigments
View Patent ↗The light transmitted and reflected by all-dielectric optically variable pigments varies according to viewing angle. The color travel of an all-dielectric optically variable pigment depends on amplitude changes and wavelength shifts in reflectance peaks of the pigment. The width and center wavelength of reflectance peaks can be controlled by selecting the ratio of thicknesses between high-index and low-index layers in a thin film stack. Reflectance peaks can regenerate or become suppressed and shift with tilt angle, thus providing a wide variety of color trajectories.
1. A dichroic pigment flake comprising: a plurality of thin film layers forming an optical interference structure with a m th -order reflectivity peak, where m is an integer greater than 1, the m th -order reflectivity peak having a first reflectivity at a first viewing angle and a second reflectivity at a second viewing angle, the first reflectivity and the second reflectivity being expressed as a percentage of total reflectivity, wherein the first reflectivity is less than the second reflectivity by at least 20%, wherein the plurality of thin-film layers are arranged as (aH bL) n aH wherein n is an integer and a and b are multipliers, H representing thin-film layers of high-index material having a relative index of refraction not less than 2.0 and a quarter-wave optical thickness at a center wavelength λ400 nm to 2200 nm, and L representing thin-film layers of low-index material having a relative index of refraction not greater than 1.6 and the quarter-wave optical thickness at the wavelength, and wherein a ratio a:b is an integer ratio between 1:4 and 4:1.
2. The dichroic pigment flake of claim 1 wherein the wavelength is between 700 nm and 2000 nm.
3. The dichroic pigment flake of claim 1 wherein each of the H layers comprises a first high-index material and each of the L layers comprises a first low-index material.
4. The dichroic pigment flake of claim 1 wherein a first H layer comprises a first high-index material and a second H layer comprises a second high-index material.
5. The dichroic pigment flake of claim 1 wherein n is greater than one and a first L layer comprises a first low-index material and a second L layer comprises a second low-index material.
6. The dichroic pigment flake of claim 1 wherein a=b and the wavelength is between 900 nm and 1300 nm.
7. The dichroic pigment flake of claim 6 wherein the dichroic pigment flake has a characteristic color at a normal viewing angle.
8. A coating composition comprising a carrier having a refractive index n c , and plurality of dichroic pigment flakes suspended within said carrier, wherein the plurality of dichroic pigment flakes each comprise: a plurality of thin film layers forming an optical interference structure with a m th -order reflectivity peak, where m is an integer greater than 1, the m th -order reflectivity peak having a first reflectivity at a first viewing angle and a second reflectivity at a second viewing angle, the first reflectivity and the second reflectivity being expressed as a percentage of total reflectivity, wherein the first reflectivity is less than the second reflectivity by at least 20%, wherein the plurality of thin-film layers are arranged as (aH bL) n aH wherein n is an integer and a and b are multipliers, H representing thin-film layers of high-index material having a relative index of refraction not less than 2.0 and a quarter-wave optical thickness at a center wavelength λ400nm to 2200 nm, and L representing thin-film layers of low-index material having a relative index of refraction not greater than 1.6 and the quarter-wave optical thickness at the wavelength, and wherein a ratio a:b is an integer ratio between 1:4 and 4:1; and, wherein the refractive index n c of the carrier is less than 2.0.
9. A coating composition as defined in claim 8 , wherein the composition is an ink or paint.