Graduated Neutral Density Filter Reflectance Control
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Solution Overview
Problem
Existing graduated neutral density (GND) filters face challenges in maintaining low reflectance across the optical plane due to variations in transmittance, leading to issues like flare and ghost artifacts, as the change in reflectance is difficult to control with traditional absorption layers.
Innovation Solution
The optical element incorporates a substrate with a surface layer comprising multiple thin films and an absorption layer with a varying thickness, where the surface layer includes films with specific refractive indices and an extinction coefficient of no more than 0.5, allowing for controlled transmittance and reduced reflectance changes across the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a metal absorption layer with large extinction coefficient is used to achieve transmittance distribution, then the transmittance control is improved, but the reflectance becomes position-dependent and cannot be reduced to sufficient level
Solution Approach 1:
The patent changes the extinction coefficient parameter of the absorption layer from large (metal) to small (0.01-0.5 range), which fundamentally alters the optical behavior. This parameter change allows the absorption layer to provide transmittance distribution through thickness variation rather than through high absorption, thereby maintaining low reflectance across the entire filter surface.
Solution Approach 2:
The patent employs a composite structure combining multiple thin films with different refractive indices (first film with n=1.05-1.4, second film with n>1.4, third film with n<1.4) and an absorption layer. This composite structure enables independent optimization of reflectance control and transmittance distribution functions.
2Adaptability or versatility
If the thickness of the absorption layer is varied to achieve transmittance distribution, then the graduated neutral density effect is improved, but the reflection condition changes noticeably at different positions
Solution Approach 1:
By changing the extinction coefficient to a small value (0.01-0.5), the patent enables transmittance distribution to be achieved primarily through thickness variation of the absorption layer without causing significant changes in reflection conditions. The small extinction coefficient ensures that the absorption layer does not strongly affect the reflected light, maintaining stable reflection characteristics across different positions.
Solution Approach 2:
The patent introduces intermediate thin films with specific refractive indices between the substrate and the absorption layer. These intermediate films act as mediators that decouple the thickness variation effect from the reflection condition, allowing the absorption layer thickness to vary for transmittance control while the intermediate films maintain consistent optical interfaces for uniform reflection.
3Object-generated harmful factors
If a low refractive index film is provided on the outermost surface to reduce reflectance, then the overall reflectance is reduced, but the reflectance change with transmittance variation cannot be sufficiently controlled
Solution Approach 1:
The patent uses a composite structure with multiple thin films having different refractive indices (first film: n=1.05-1.4, second film: n>1.4, third film: n<1.4) on the outermost surface. This composite structure provides both low overall reflectance and precise control over reflectance changes with transmittance variation by leveraging the different optical properties of each layer.
Solution Approach 2:
The patent applies different refractive index characteristics to different regions of the surface layer structure. The first film has low refractive index (n=1.05-1.4) to reduce overall reflectance, while the second and third films have higher and lower refractive indices respectively to control the variation of reflectance with transmittance. This local differentiation of optical properties enables simultaneous optimization of both objectives.
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 configuration effectively reduces the change in reflectance associated with transmittance variations, minimizing flare and ghost artifacts, and allows for a more uniform optical performance across the GND filter.
Implementation Method 1
an absorption layer that is provided between the surface layer and the substrate and has transmittance that varies depending on a position on the substrate
Implementation Method 2
The surface layer includes a first film disposed at a position farthest from the substrate, and the first film has a refractive index of no less than 1.05 nor more than 1.4 at a wavelength of 550 nm
Data Source
AI summary
An optical element includes a substrate, a surface layer including thin films disposed on the substrate, and an absorption layer provided between the surface layer and the substrate and having transmittance that varies depending on a position on the substrate. The surface layer includes a first film disposed farthest from the substrate and having a refractive index of no less than 1.05 nor more than 1.4 at a wavelength of 550 nm. The surface layer includes a second film disposed closer to the substrate than the first film and having a higher refractive index than the substrate at a wavelength of 550 nm and a third film disposed adjacent to the second film and having a lower refractive index than the second film at a wavelength of 550 nm. The absorption layer has an extinction coefficient of no more than 0.5 at wavelengths of from 400 nm to 700 nm.