LWIR Imaging Lens High-Index Silicon Segmentation
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Solution Overview
Problem
Current thermal imagers for the long wavelength infrared (LWIR) region face challenges in using materials with high absorption in the 7.5-13.5 μm waveband, limiting the options for optical lenses, and as designs shrink, there is a need for materials that offer low absorption and manufacturability advantages.
Innovation Solution
The development of imaging lenses using high-index materials like silicon, with specific optical element configurations and diffractive elements, that have optical power on multiple surfaces and a design that keeps the maximum clear aperture close to the sensor image diagonal, reducing sag height and absorption, while maintaining high refractive index and low absorption in the visible spectrum.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If high-index materials with high absorption in LWIR band are used, then refractive index is improved, but absorption loss increases
Solution Approach 1:
The lens is divided into multiple optical elements (at least two) with different functions. The first optical element provides positive optical power while the second optical element provides negative optical power, allowing each element to be optimized for lower absorption in the LWIR band while maintaining overall high refractive index through the use of high-index materials.
Solution Approach 2:
The patent employs composite lens designs combining different high-index materials (such as silicon, germanium, or chalcogenide glasses) with complementary properties. This allows the system to achieve both high refractive index and reduced absorption loss by selecting materials optimized for specific wavelength ranges within the LWIR band.
2Volume of moving object
If lens size is reduced for smaller thermal imagers, then device compactness is improved, but optical performance deteriorates
Solution Approach 1:
The patent employs aspheric surfaces on one or more optical elements to achieve better optical performance in a compact form factor. The aspheric curvature allows for improved aberration correction and focusing capability while maintaining reduced lens volume, enabling high optical performance in smaller thermal imager designs.
Solution Approach 2:
The patent optimizes key parameters including the ratio of clear aperture to image circle diameter (keeping it at 30% or less), the spacing between optical elements, and the refractive indices of materials used. These parameter optimizations enable compact lens designs that maintain reliable optical performance across the LWIR band.
3Loss of energy
If clear aperture is increased to improve light gathering, then imaging capability is improved, but sag height and absorption increase
Solution Approach 1:
By segmenting the optical system into multiple elements with distributed optical power, the patent reduces the sag height requirement for any single surface. This allows for effective light gathering with a managed clear aperture while keeping individual element thicknesses and sag heights optimized to minimize absorption loss in the LWIR band.
4Illumination intensity
If F-number is reduced to improve brightness, then illumination intensity is improved, but manufacturing precision requirements increase
Solution Approach 1:
The use of aspheric surfaces enables the achievement of low F-numbers (high brightness) while maintaining relaxed manufacturing precision requirements. The aspheric curvature provides superior aberration control that allows for effective light gathering with achievable surface tolerances, making low F-number designs manufacturable with standard precision capabilities.
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 allows for the use of materials like silicon, which are highly absorptive in the LWIR band but offer advantages such as low thermal expansion and manufacturability, enabling efficient LWIR imaging with reduced manufacturing and cost issues, and improved optical performance.
Implementation Method 1
The first and second high-index materials may have a refractive index greater than 2.2 in the operational waveband
Implementation Method 2
At least two surfaces of the first and second optical elements may be optically powered surfaces
Data Source
AI summary
An imaging lens for use with an operational waveband over any subset of 7.5-13.5 μm may include a first optical element of a first high-index material and a second optical element of a second high-index material, that may have a refractive index greater than 2.2 in the operational waveband, an absorption per mm of less than 75% in the operational waveband, and an absorption per mm of greater than 75% in a visible waveband of 400-650 nm. Optically powered surfaces of the imaging lens may include a sag across their respective clear apertures that are less than 10% of a largest clear aperture of the imaging lens. Respective maximum peak to peak thicknesses of the first and second optical elements may be similar in size, for example within 15 percent of each other. Ratios of maximum peak to peak thickness to clear aperture and, separately, to sag are also provided.