Image-Plane Diffraction Grating to Reduce Night-Vision Retroreflection
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Solution Overview
Problem
Night vision systems suffer from retroreflection, where part of the input light is reflected against the focal plane array and redirected back to the illumination source, compromising covert operation, particularly in military applications.
Innovation Solution
Incorporating a diffraction grating between the aperture stop and the focal plane array (FPA) to transmit input light and deviate reflections outside the aperture stop, with a blazed grating design concentrating at least 75% of the diffracted energy in a single order other than the zeroth order to prevent retroreflection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a diffraction grating is added to the imaging system to deviate reflections, then retroreflection is reduced, but device complexity increases
Solution Approach 1:
A diffraction grating is introduced as an intermediary optical element positioned between the aperture stop and the focal plane array. This grating acts as a mediator that selectively deviates reflected light paths while allowing incident light to pass through, thereby reducing retroreflection without requiring fundamental changes to the imaging system architecture.
Solution Approach 2:
The diffraction grating is designed with specific parameters including a spatial periodicity between 20 l/mm and 2000 l/mm, and in some embodiments as a blazed grating with groove depths and angles optimized to direct at least 75% of diffracted energy into a single non-zeroth order. These parameter optimizations ensure effective retroreflection suppression while maintaining acceptable device complexity.
2Object-affected harmful factors
If a blazed grating is used to concentrate diffracted energy in one order, then retroreflection is further reduced, but manufacturing precision requirements increase
Solution Approach 1:
The patent specifies precise parameter ranges for the blazed grating including spatial periodicity between 20 l/mm and 2000 l/mm, groove depths, and blaze angles that concentrate at least 75% of diffracted energy into a single non-zeroth order. These optimized parameters balance manufacturing feasibility with effective retroreflection suppression.
Solution Approach 2:
This principle is not applicable to this patent as it deals with optical diffraction gratings rather than pneumatic or hydraulic systems.
3Volume of moving object
If the grating is positioned close to the FPA, then the system remains compact, but the grating must be co-integrated monolithically increasing manufacturing complexity
Solution Approach 1:
The patent describes an embodiment where the diffraction grating and focal plane array are co-integrated monolithically, merging two components into a single integrated structure. This approach minimizes the distance between the grating and FPA, keeping the system compact, but necessarily increases manufacturing complexity compared to using separate components.
Solution Approach 2:
The patent also describes an alternative embodiment where the grating and FPA are distinct separate components, allowing independent manufacturing and assembly. This segmentation approach simplifies manufacturing but requires more space and precise alignment during assembly.
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
Effectively reduces retroreflection, ensuring covert operation by minimizing light return to the illumination source, enhancing the system's ability to maintain operational secrecy.
Implementation Method 1
a diffraction grating disposed between the aperture stop and the FPA, wherein the grating is configured to transmit input light passing through the aperture stop towards the FPA
Implementation Method 2
the grating is configured to deviate reflections arising from the FPA in response to the input light outside the aperture stop
Implementation Method 3
reflections arising from the FPA
Implementation Method 4
at least 75% of energy of light diffracted by the blazed grating is focused on one diffraction order other than a zeroth diffraction order
Data Source
AI summary
Designs for mitigating retroreflections in night vision systems are described. The designs described herein use diffraction gratings. A diffraction grating diffracts input light into several beams with different directions. A grating may be configured to transmit input light passing through an aperture stop towards the focal plane array (FPA), and to deviate reflections arising from the FPA in response to the input light outside aperture stop. Deviation of light may be obtained by designing the grating so that the majority of the total diffracted energy is concentrated in a single diffraction order while the energy in the other orders is limited. The diffraction order in which the energy is concentrated may be any diffraction order other than the zeroth order (the un-diffracted order), including for example the first order, the second order, the third order, etc. A grating may be formed as a discrete component or monolithically with the FPA.