Four-element athermal lens for near-infrared gesture recognition
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Solution Overview
Problem
Camera modules designed for near-infrared applications, such as gesture recognition, face challenges in maintaining image clarity across varying temperatures due to the lack of effective athermalization in imaging lenses, which affects the accuracy of gesture recognition systems.
Innovation Solution
A four-element athermal lens system is developed, comprising coaxially aligned lenses with specific refractive indices and temperature dependencies, along with optimized radii of curvature and aperture stop placement, to minimize focal shift and aberrations across a temperature range of -40°C to 125°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional imaging lens is used in near-infrared camera modules, then the lens structure is simple and easy to manufacture, but the focal length shifts significantly with temperature changes, causing image blur and reducing gesture recognition accuracy
Solution Approach 1:
The imaging lens is divided into four separate lens elements (first, second, third, and fourth lenses) with different optical powers. The first and second lenses have positive optical power while the third and fourth lenses have negative optical power. This segmentation allows each element to contribute differently to the overall athermalization, enabling the system to compensate for thermal effects while maintaining a practical structure.
Solution Approach 2:
The patent utilizes temperature-dependent refractive index changes of lens materials to achieve athermalization. By selecting lens materials with specific refractive indices (n1, n2, n3, n4) and their temperature dependencies (dn1/dT, dn2/dT, dn3/dT, dn4/dT), the system compensates for thermal expansion and refractive index changes. The condition that the difference between maximum and minimum refractive indices is less than 0.05 ensures balanced optical performance across temperatures.
2Reliability
If lens elements with different refractive indices are used to achieve athermalization, then temperature compensation is improved, but the manufacturing precision requirements increase due to tight refractive index matching constraints
Solution Approach 1:
The patent employs controlled parameter changes by selecting lens materials whose refractive indices and temperature dependencies satisfy specific mathematical relationships. The condition |nmax - nmin| < 0.05 provides a practical manufacturing tolerance while ensuring adequate athermalization. The temperature dependency ratio constraint (2.05 ≤ (dn1/dT)/(dn4/dT) ≤ 2.85) further guides material selection to achieve thermal compensation without excessive manufacturing difficulty.
Solution Approach 2:
Each lens element is designed with specific local optical properties (refractive index, curvature radii, thickness) that differ from the others. The first and second lenses use materials with one set of optical properties while the third and fourth lenses use materials with different properties, allowing each element to perform its specific function in the athermalization process while maintaining overall system performance.
3Reliability
If the focal length is minimized to reduce focal shift, then temperature stability is improved, but the imaging capability and light gathering efficiency deteriorate
Solution Approach 1:
By dividing the imaging lens into four elements with alternating positive and negative optical powers, the system achieves focal length stability without requiring an excessively short overall focal length. The positive power elements (first and second lenses) provide light gathering capability while the negative power elements (third and fourth lenses) compensate for thermal expansion, allowing the system to maintain both imaging performance and thermal stability.
Solution Approach 2:
The imaging lens uses a composite structure with four different lens elements made from materials with different refractive indices and thermal properties. This composite approach allows the system to achieve athermalization through the combined optical paths of all elements, rather than relying on a single material property, thereby maintaining imaging capability while achieving focal length stability.
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
The athermal lens system ensures a focal length shift of less than 0.2 μm/°C, maintaining image clarity and reducing aberrations, thereby enhancing the accuracy and reliability of gesture recognition systems across extreme temperatures.
Implementation Method 1
imaging lens 100's being athermalized such that it forms a clear image on image sensor 132
Implementation Method 2
The first lens, second lens, third lens, and fourth lens having respective refractive indices n1, n2, n3, and n4, a difference between (i) the maximum of n1, n2, n3, and n4 and (ii) the minimum of n1, n2, n3, and n4 being less than 0.05 in a free-space wavelength range. Refractive indices n1, n2, n3, and n4 have respective temperature dependences
Data Source
AI summary
A four-element athermal lens includes four coaxially aligned lenses including a (i) first lens and, in order of increasing distance therefrom and on a same side thereof, (ii) a second lens, a third lens, and a fourth lens. The first lens and the second lens are positive lenses. The third and fourth lenses are negative lenses. The first lens, second lens, third lens, and fourth lens have equal respective refractive indices n1, n2, n3, and n4. A difference between (i) the maximum of n1, n2, n3, and n4 and (ii) the minimum of n1, n2, n3, and n4 being less than 0.05 in a free-space wavelength range. Refractive indices n1, n2, n3, and n4 have respective temperature dependencesΔn1ΔT,Δn2ΔT,Δn3ΔT,Δn4ΔT.Each ofΔn1ΔTandΔn2ΔTexceeds, in the free-space wavelength range, each ofΔn3ΔTandΔn4ΔTby a factor between 2.05 and 2.85, inclusive.