Tunable Filter Spectroscopy Parallel Ray Alignment
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Solution Overview
Problem
Existing spectroscopic devices using liquid crystal tunable filters (LCTFs) face issues with spectral ununiformity and bandwidth broadening due to field-position-dependence of the central wavelength, especially in wide field cameras or those with large numerical apertures, and are affected by polarization of incident light, making it difficult to distinguish spectral properties from polarization states.
Innovation Solution
The optical lens unit includes a variable wavelength filter with a depolarizer and polarizer configuration, where the optical tunable filter is placed in a space between lens elements to ensure parallel rays from off-axial object points, minimizing oblique angles and maintaining spectral uniformity, and a depolarizer is used to eliminate polarization effects, allowing for accurate wavelength and polarization information acquisition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a liquid crystal tunable filter (LCTF) is used to enable real-time wavelength tuning, then operational flexibility and compactness are improved, but spectral ununiformity and bandwidth broadening occur due to field-position-dependence of the central wavelength
Solution Approach 1:
The optical system is divided into multiple lens elements (first through fourth lens elements) that work together to control light paths. Each lens element has specific focal length relationships (f1, f2, f3, f4) that segment the optical function to achieve parallel light incidence on the LCTF across the field of view, resolving the spectral ununiformity issue while maintaining real-time tuning capability
Solution Approach 2:
The patent applies different optical properties to different parts of the system: the first lens element has positive focal length while the second has negative focal length, and similarly for the third and fourth elements. This local differentiation of optical characteristics ensures that off-axial light rays are properly directed to incident parallel to the optical axis on the LCTF, maintaining spectral uniformity across the field
2Difficulty of detecting and measuring
If the LCTF is configured as a polarizing filter to enable wavelength selection, then filtering capability is improved, but polarization information of the object is lost or degraded
Solution Approach 1:
A depolarizer is introduced as an intermediary component between the object and the LCTF. This depolarizer converts polarized light from the object into unpolarized light before it reaches the LCTF, allowing the LCTF to perform its wavelength selection function without being affected by the polarization state of the incident light, thereby preserving the object's polarization information
Solution Approach 2:
The polarization effect is extracted and removed from the light path by the depolarizer before the light reaches the LCTF. This separates the wavelength selection function (performed by the LCTF) from the polarization state (preserved by removing its interfering effect), allowing independent optimization of both functions
3Area of stationary object
If a wide field camera or large numerical aperture is used to increase field of view, then observational coverage is improved, but oblique angles of incident light increase causing spectral ununiformity and bandwidth broadening
Solution Approach 1:
The patent employs a multi-element lens system with curved surfaces that are specifically designed to redirect off-axial light rays. The spherical or aspherical surfaces of the lens elements bend light paths in a controlled manner, ensuring that even light rays from wide field angles are redirected to incident parallel to the optical axis on the LCTF, maintaining spectral uniformity across the entire field of view
Solution Approach 2:
The optical system uses asymmetric arrangement of lens elements with different focal lengths and positions. The first and second lens elements have different focal length relationships from the third and fourth elements, creating an asymmetric optical path that specifically addresses the asymmetric nature of off-axial light rays entering at different angles, thereby correcting the oblique incidence issue across the wide field
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 ensures spectral uniformity and minimal bandwidth broadening across a wide field of view, enabling accurate hyperspectral data acquisition without polarization interference, even with polarized light, and allows for precise control of polarization direction.
Implementation Method 1
By applying a set of voltages to the LCTF to adjust the retardation of the liquid crystals, the LCTF selectively extracts a specific transmitting band from the incident white light
Implementation Method 2
the LCTF selectively extracts a specific transmitting band from the incident white light
Implementation Method 3
a depolarizer is used to eliminate polarization effects, allowing for accurate wavelength and polarization information acquisition
Data Source
AI summary
A spectroscope device is provided to maintain the uniformity of the central transmitting wavelength in the field of view and to minimize the broaden of the bandwidth of the transmitting wavelengths in an optical lens using an optical tunable filter (variable wavelength filter), even with a wide field of view and/or a large numerical aperture. A space is defined in which, when each beam that is incident from each off-axial object point on the object surface toward the optical lens that includes a plurality of lens elements between an object surface and a conjugate real image surface reaches the optical tunable filter, the chief ray is maintained parallel to the optical axis. Therefore, if an optical tunable filter is disposed in this space, each beam is always incident normal to the filter, so only the narrow band components at the specific central wavelength can be transmitted.