Curved Interference Filter for Narrow Passband Control
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Solution Overview
Problem
Ambient radiation interference reduces the quality of images in electronic imaging systems by introducing spurious features and reducing contrast, particularly in 3D imaging systems where ambient background radiation is collected at wavelengths outside the target range, leading to a broadening of the interference filter's passband and increased rejection of target wavelengths with higher numerical apertures.
Innovation Solution
The implementation of a multi-element objective lens system with a narrowband interference filter positioned at a location where rays are collimated or nearly collimated, and the use of curved or flat interference filters with coatings on optical surfaces to minimize the range of incident angles, maintaining a narrow passband and high rejection of out-of-band radiation while allowing target wavelengths to pass efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a narrowband interference filter is used to reject ambient radiation, then the passband is narrowed to improve image quality, but the rejection of out-of-band radiation decreases at higher numerical apertures due to angular broadening
Solution Approach 1:
The patent applies curvature to the interference filter surface, designing it as a concave surface with a specific radius of curvature. This curved geometry ensures that rays from the aperture stop incident on the filter maintain a limited angular range, preventing the angular broadening that would otherwise occur with flat surfaces. The concave shape focuses the rays in a way that preserves the narrow passband characteristics while maintaining effective rejection of out-of-band radiation across the full numerical aperture range.
Solution Approach 2:
The patent changes the geometric parameters of the interference filter by specifying a particular radius of curvature and positioning it at a calculated distance from the aperture stop. These parameter adjustments are designed to control the angular spread of rays incident on the filter, ensuring that the passband remains narrow (less than 4% of center wavelength) while maintaining rejection performance. The specific parameter values are optimized to balance passband width and rejection characteristics.
2Productivity
If the numerical aperture is increased to improve light collection efficiency, then more light reaches the image sensor, but the passband broadens due to increased angular spread of rays
Solution Approach 1:
The concave curvature of the interference filter surface counteracts the angular spread caused by high numerical aperture. The curved surface geometry is specifically designed to constrain the angular range of rays incident on the filter, ensuring that even at high numerical apertures, the passband remains narrow. This allows the system to achieve both high light collection efficiency and narrow passband characteristics simultaneously.
Solution Approach 2:
The curved interference filter acts as an intermediary element between the aperture stop and the image sensor. It mediates the angular spread of rays by using its concave surface to focus and constrain the ray angles. This intermediary function allows the system to maintain a narrow passband while still permitting sufficient light collection efficiency at high numerical apertures.
3Ease of manufacture
If a flat interference filter is used, then the manufacturing is simpler, but the range of incidence angles is larger causing passband broadening
Solution Approach 1:
The patent transitions from a flat filter surface to a concave surface with a specific radius of curvature. This curvature is designed to limit the angular spread of rays incident on the filter, thereby preventing passband broadening. While curved surfaces are slightly more complex to manufacture than flat surfaces, the concave geometry provides significant performance benefits in maintaining narrow passband width across the full field of view and numerical aperture range.
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 maintains a narrow passband of less than 2% of the center wavelength, effectively rejecting ambient radiation while preserving high resolution and efficient light collection, even at high numerical apertures, thereby enhancing image quality by minimizing the broadening of the filter's passband due to angular effects.
Implementation Method 1
An interference filter, which has a center wavelength and a passband no greater than 4% of the center wavelength, includes a coating formed on one of the optical surfaces
Implementation Method 2
an optical assembly, which has a specified numerical aperture and is configured to focus optical radiation via an aperture stop onto the image sensor
Implementation Method 3
a first, curved surface through which the optical radiation enters the assembly, a final surface through which the rays exit the assembly toward the image sensor, and at least two intermediate surfaces between the first and final surfaces
Data Source
AI summary
Optical apparatus includes an image sensor and an optical assembly, which is configured to focus optical radiation via an aperture stop onto the image sensor. The optical assembly includes a plurality of optical surfaces, consisting of a first, curved surface through which the optical radiation enters the assembly, a final surface through which the rays exit the assembly toward the image sensor, and at least two intermediate surfaces between the first and final surfaces. An interference filter, which has a center wavelength and a passband no greater than 4% of the center wavelength, and includes a coating formed on one of the optical surfaces. All rays of the optical radiation passing through the aperture stop are incident on the coating over a range of incidence angles with a half-width that is no greater than three fourths of the numerical aperture of the optical assembly.