Convex Cylindrical Dichroic Plate for Parasitic Image Suppression
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Solution Overview
Problem
Current spectral splitting components for mono-pupil multispectral optronic systems face challenges in minimizing parasitic images, aberrations, cost, and manufacturing complexity when dealing with convergent beams, particularly in the MWIR band, while maintaining modulation transfer function and optical compactness.
Innovation Solution
A spectral splitting component with a planar dichroic front face and a convex cylindrical back face, oriented such that the back face is less inclined than the front face, effectively shifting the parasitic image under the diffraction spot of the main image and compensating for aberrations introduced by prismaticity, using materials with high refractive indices to reduce thickness and deformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a planar dichroic plate is used to split convergent beams, then spectral separation is achieved, but a visible parasitic image is created in the MWIR band
Solution Approach 1:
The patent applies curvature to the dichroic plate by making it slightly convex on the incident beam side. This curvature modifies the optical path of reflected MWIR beams, causing them to converge at a different location than transmitted beams. The convex shape creates a focal shift that moves the parasitic MWIR image away from the focal plane where the useful image is formed, thereby eliminating visible parasitic images while maintaining spectral separation functionality.
Solution Approach 2:
The patent changes the geometric parameters of the dichroic plate by introducing a specific curvature radius (R) that is much larger than the plate diameter. This parameter modification allows the plate to function as both a spectral splitter and a parasitic image suppressor. The curvature parameter is optimized to create sufficient focal shift for MWIR beams while maintaining minimal impact on visible and NIR beam paths.
2Ease of manufacture
If the dichroic plate is made thinner to reduce parasitic image separation, then manufacturing is easier, but deformation during operation increases
Solution Approach 1:
The convex curvature of the plate serves a dual purpose: it suppresses parasitic images and simultaneously increases structural rigidity. The curved geometry distributes mechanical stresses more effectively across the plate, reducing deformation during operation. This allows the use of thinner plates without compromising stability, as the curvature provides inherent structural reinforcement.
3Object-generated harmful factors
If the plate is inclined to the optical axis to separate useful and parasitic beams, then parasitic image separation is improved, but aberrations (coma and astigmatism) increase
Solution Approach 1:
The convex curvature of the plate compensates for aberrations introduced by inclination. The curved surface modifies the wavefront of reflected MWIR beams in a way that counteracts coma and astigmatism. By optimizing the curvature radius, the patent achieves a balance where the plate can be inclined sufficiently to separate parasitic images while the curvature corrects the resulting aberrations, maintaining manufacturing precision.
4Object-generated harmful factors
If multiple multispectral components are used upstream to form an afocal system, then no parasitic images are produced, but device complexity and cost increase
Solution Approach 1:
The patent makes the single dichroic plate perform multiple functions: spectral separation (reflecting visible/NIR, transmitting MWIR), parasitic image suppression (via curvature-induced focal shift), and aberration correction (via curvature optimization). This multi-functionality eliminates the need for multiple separate components upstream, reducing device complexity while maintaining the advantage of no visible parasitic images.
Solution Approach 2:
The patent merges the functions of spectral splitting and parasitic image suppression into a single dichroic plate component. By integrating the curvature feature directly into the dichroic plate, the design combines what would traditionally require separate components (afocal system elements) into one unified element, simplifying the overall device architecture.
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 solution reduces parasitic images, aberrations, and manufacturing costs, maintaining high modulation transfer function and optical quality, while being easier to produce and integrate into multispectral optronic systems.
Implementation Method 1
The dichroic front face reflects the short portion of the spectrum (visible or NIR or SWIR) and transmits the long portion (MWIR)
Implementation Method 2
the back face is less inclined than the front face, so that, the image comprising a main image taking account of diffraction and a parasitic image formed by double reflection
Implementation Method 3
the parasitic image is shifted back under the diffraction spot of the main image
Implementation Method 4
split the incident beam into beams that converge, just downstream of the focal point of the multispectral head objective
Data Source
AI summary
A spectral splitting component is provided, having two faces, a planar front face comprising a dichroic treatment and a back face. It is intended to be placed downstream of a convergent objective. The back face is convex and forms a cylindrical surface defined by a generatrix of fixed direction moving perpendicularly along a circular arc comprising two ends, the plane passing through these two ends and parallel to the generatrix of the cylindrical surface forming a dihedral with the plane of the front face, the generatrix of the cylindrical surface being parallel to the edge of the dihedral.