Security Devices with Polarisation-Dependent Zero-Order Outputs
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Solution Overview
Problem
Current security devices that utilize diffraction gratings for authentication lack additional covert features to enhance their security against copying, particularly in terms of polarisation-dependent zero-order outputs.
Innovation Solution
The implementation of at least two embossed, reflective metal diffraction gratings with different grating periods, exhibiting distinct zero-order outputs when viewed through different polarisation filters, which provide a secondary covert diffraction feature in addition to the primary overt first-order diffraction feature, ensuring a striking colour contrast between polarisations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If diffraction gratings are used in security devices to create first-order diffraction outputs, then the visible image and basic authentication feature are provided, but the security against copying is insufficient due to lack of covert features
Solution Approach 1:
The diffraction grating is segmented into multiple functional regions: a first region with first grating period for primary authentication, and a second region with second grating period for covert security features. This segmentation allows different regions to serve different security functions simultaneously.
Solution Approach 2:
Different regions of the diffraction grating are assigned different grating periods and optical properties. The first region has first grating period for visible first-order diffraction, while the second region has second grating period for zero-order polarisation-dependent outputs, creating local differentiation for enhanced security.
2Reliability
If additional covert diffraction features are added to enhance security, then resistance to copying is strengthened, but the complexity of the diffraction grating structure increases
Solution Approach 1:
The diffraction grating structure is designed to perform multiple functions: the first region provides primary authentication through first-order diffraction, while the second region provides covert security through zero-order polarisation-dependent outputs. Both functions coexist in a single grating structure without requiring separate components.
Solution Approach 2:
The grating period parameter is varied between different regions of the diffraction grating. The first region uses a first grating period optimized for first-order diffraction visibility, while the second region uses a second grating period that creates polarisation-dependent zero-order outputs, achieving different security functions through parameter variation.
3Reliability
If polarisation-dependent zero-order outputs are implemented, then covert security features are provided, but the manufacturing precision requirements increase
Solution Approach 1:
The second region of the diffraction grating is designed with specific local properties: a second grating period that differs from the first, creating polarisation-dependent zero-order outputs. This local differentiation allows covert security features to be implemented without requiring the entire grating to meet extremely tight precision specifications.
Solution Approach 2:
Instead of requiring uniform high precision across the entire diffraction grating, the invention applies enhanced precision requirements only to the second region where polarisation-dependent features are needed. The first region can have relaxed precision requirements since it only needs to provide visible first-order diffraction.
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
This approach significantly enhances the security device's authenticity verification by introducing polarisation-dependent zero-order colour effects that are only observable through orthogonal polarisation filters, thereby strengthening its resistance to copying and improving authentication.
Implementation Method 1
Diffraction gratings may be used in security devices to strengthen the security device against copying. Diffraction gratings exhibit at least first order diffraction outputs characterised by colours that change with viewing angle.
Implementation Method 2
the zero-order output of the first diffraction grating comprises different coloured first and second sub-outputs for respective first and second polarisations parallel and perpendicular to the first diffraction grating
Implementation Method 3
at least first and second embossed, reflective metal diffraction gratings in respective regions
Data Source
AI summary
A security device, including at least first and second embossed, reflective metal diffraction gratings in respective regions: wherein the first diffraction grating exhibits, in incident white light, a zero-order output over a first area of substantially uniform grating period, wherein the zero-order output of the first diffraction grating comprises different coloured first and second sub-outputs for respective first and second polarisations parallel and perpendicular to the first diffraction grating; wherein the second diffraction grating exhibits, in incident white light, a zero-order output over a second area of substantially uniform grating period; wherein the zero-order output of the second diffraction grating comprises third and fourth sub-outputs for respective first and second polarisations parallel and perpendicular to the second diffraction grating; and wherein the first and second diffraction gratings exhibit, for incident white light, substantially the same first order diffraction efficiency over the first and second areas.