Optical Security Device with Textured Substrate for Rotation-Dependent Color Shift
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Solution Overview
Problem
Current optical security devices lack a feature that exhibits a controllable and identifiable color change upon rotation, making them difficult to authenticate and susceptible to counterfeiting, especially in applications like banknotes and security documents.
Innovation Solution
An optical device with a substrate featuring groove structures and a color-shifting coating, where the substrate is textured with pyramidal or rod-shaped structures smaller than human eye resolution but larger than 2 microns, allowing for a color shift when rotated in its plane, without causing diffraction of visible light, and a transparent cover layer can enhance the color shift effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If color-shifting pigments and foils are used in optical security devices, then the devices become difficult to counterfeit, but the devices lack a controllable color change upon rotation making authentication difficult
Solution Approach 1:
The patent applies the dynamics principle by creating an optical device that exhibits dynamic color changes when rotated. The groove structures with specific orientations (first groove orientation in first area, second groove orientation in second area) cause different color shifts when the device is rotated, transforming a static color-shifting material into a dynamic authentication system that requires rotational movement to verify authenticity.
Solution Approach 2:
The patent applies local quality by creating different groove structures in different areas of the substrate. The first area has grooves oriented in a first direction while the second area has grooves oriented in a second direction, causing each area to exhibit different color shift characteristics upon rotation. This local differentiation enables controlled, identifiable color changes that aid authentication while maintaining anti-counterfeiting properties.
2Reliability
If microlens arrays are used to provide floating image effect, then security features are enhanced, but the device thickness increases causing banknote stacks to tilt and fall
Solution Approach 1:
The patent applies the thin films principle by using a substrate with groove structures and color-shifting coating instead of thick microlens arrays. The device achieves security functionality through a thin optical structure that can be integrated into banknotes without causing thickness issues, allowing the banknote to remain flat and suitable for stacking while still providing enhanced security features through color shift upon rotation.
3Illumination intensity
If reflective thin film stacks are used to achieve color shifting, then high chromaticity is obtained at few layers, but the color remains constant when the device is rotated in its own plane
Solution Approach 1:
The patent applies asymmetry by introducing groove structures with specific orientations that break the rotational symmetry of the thin film stack. The grooves are oriented at specific angles (first groove orientation in first area, second groove orientation in second area) causing the optical properties to vary with rotation angle. This asymmetric structure enables the device to exhibit different color shifts when rotated, adding versatility to the high-chromaticity thin film stack.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The device provides a visible color shift upon rotation, enhancing authentication and making it difficult to counterfeit, while maintaining a thickness suitable for integration into documents without causing them to tilt or become thicker, thus maintaining practicality.
Implementation Method 1
Color shifting pigments and colorants exhibit a property of changing color upon variation of an angle of incident light, or upon variation of a viewing angle of the observer. Color shifting pigments and foils are taught in US Patent 5,135,812 by Phillips et al.
Implementation Method 2
The substrate is textured with pyramidal or rod-shaped structures smaller than human eye resolution but larger than 2 microns, allowing for a color shift when rotated in its plane
Implementation Method 3
Color shifting foil and inks exhibit a shift of perceived color due to a phenomenon of light interference in thin films. Reflective thin film stacks are particularly useful as they exhibit high chromaticity at only a few thin film layers in the stack.
Data Source
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AI summary
An optical device exhibiting a color shift upon rotation is disclosed. The optical device has a textured surface having a relief structure finer than a human eye resolution but large enough not to exhibit diffraction effects. The textured surface is coated with an interference thin film that exhibits a color shift with tilt. A uniform color seen at one angle of rotation changes to another uniform color when the optical device is rotated in its own plane. A method of manufacturing of such an optical device, as well as the use of the optical device as an optical security and authentication element, is also disclosed.