Key Takeaways
- Ultra-wide aperture LWIR lens design must balance aperture size, compactness, thermal stability, and image quality—often conflicting goals.
- Two lenses (20 mm F/0.85 and 40 mm F/1.0) achieve strong MTF performance, low distortion, and stable imaging from −40 °C to 80 °C via passive athermalization.
- Aspherical elements and optimized materials enable compact, high-performance optics. Careful control of illumination and chief ray angle ensures detector compatibility.
- Designs support applications from UAV imaging to surveillance, demonstrating scalable, manufacturable solutions for high-performance infrared systems.
In long-wave infrared (LWIR, 8–12 μm) optical systems, achieving a balance among large aperture, compact form factor, thermal stability, and high imaging performance remains a fundamental design challenge. These requirements are often mutually constraining, particularly in applications demanding long-range detection, wide field coverage, and reliable operation across extreme environmental conditions. Conventional design approaches frequently encounter limitations in simultaneously optimizing aberration correction, athermalization, and manufacturability.
This technical note presents two high-performance fixed-focus LWIR lenses—a 20 mm F/0.85 ultra-wide aperture lens and a 40 mm F/1.0 high-resolution lens—designed to address these challenges. The designs demonstrate advanced performance across key parameters including modulation transfer function (MTF), distortion, relative illumination, and chief ray angle (CRA), while maintaining compact mechanical configurations and passive athermalization over a wide temperature range.
1. Core Performance Comparison
Parameter | 20 mm F/0.85 | 40 mm F/1.0 |
Focal Length | 20 mm | 40 mm |
F-number | 0.85 | 1.0 |
Field of View | 43.85° × 36.04° | 11.56° (Diagonal) |
Detector Compatibility | Φ16.85 mm | — |
Optical Length | 32.5 mm | 47.51 mm |
Back Focal Length | 7.17 mm | 11.81 mm |
Athermal Range | -40°C to 80°C | -40°C to 80°C |
Relative Illumination | 66% | 81.92% |
Distortion | 4.65% | -2.96% |
CRA | 3.52° | 6.27° |
Spatial Resolution | — | 0.5 mrad |
Minimum Object Distance | 5 m | 10 m |
Max Diameter | 25 mm | 40 mm |
Total Length | 35 mm | 48 mm |
2. 20 mm F/0.85 Lens: Ultra-Large Aperture in a Compact Architecture
Optical Performance
The lens maintains stable MTF performance across the full field of view at three representative temperatures (20°C, -40°C, and 80°C), ensuring consistent contrast under varying environmental conditions. The defocus response exhibits smooth behavior, indicating adequate depth of focus and facilitating tolerance management during assembly.
At a 5 m object distance, near-field MTF performance remains robust, supporting applications requiring both imaging and radiometric measurement.
Aberration control is achieved through effective energy concentration in diffraction spots across the field, while distortion is maintained at 4.65%, a competitive level for wide-angle systems with such a large aperture.
Relative illumination reaches 66% at F/0.85, indicating effective control of vignetting. The CRA (3.52°) is well-matched to typical uncooled detector microlens arrays, minimizing shading and ensuring uniform signal response.
Mechanical Design
The optical system is integrated into a compact housing with a maximum diameter of 25 mm and total length of 35 mm. The form factor supports integration into size- and weight-constrained platforms such as UAVs, handheld thermal imagers, and vehicle-mounted systems. Interface configurations can be adapted to specific system requirements.
3. 40 mm F/1.0 Lens: High-Resolution Medium Telephoto Design
Optical Performance
The 40 mm lens achieves near-diffraction-limited MTF performance across the operational temperature range. At Nyquist frequency (25 lp/mm), high contrast is preserved, enabling reliable high-resolution detection.
At a 10 m object distance, MTF stability supports medium-range recognition applications. Aberrations—including spherical aberration, astigmatism, and field curvature—are effectively suppressed.
Distortion is limited to -2.96%, enabling accurate measurement and compatibility with computer vision and AI-based image analysis workflows.
Relative illumination exceeds 81%, ensuring uniform brightness across the field. The CRA (6.27°) is optimized for efficient coupling with standard infrared detectors.
Mechanical Design
With a maximum diameter of 40 mm and a total length of 48 mm, the lens balances optical performance with mechanical robustness and thermal management. The design is compatible with standard infrared camera interfaces and supports deployment in demanding environments such as surveillance systems, automotive platforms, and airborne payloads.
4. Technical Considerations
4.1 Large Aperture and Miniaturization
The designs employ multiple aspherical surfaces in combination with low-absorption infrared materials, including germanium (Ge), zinc selenide (ZnSe), and chalcogenide glasses. Global optimization techniques and tolerance sensitivity analysis are applied to maintain high imaging performance within compact geometries.
4.2 Passive Athermalization
Athermal performance is achieved through the coordinated use of materials with differing thermo-optic coefficients, combined with mechanical compensation strategies. This enables stable imaging without active refocusing over a temperature range of -40°C to 80°C.
4.3 Illumination and CRA Optimization
Pupil aberration control and vignetting optimization contribute to improved edge illumination. CRA is carefully matched to detector microlens structures to prevent non-uniformity and radiometric artifacts.
4.4 Manufacturing and Assembly
The implementation relies on sub-micron precision aspherical machining and high-transmission infrared coatings. Precision mechanical design and active alignment processes ensure repeatability and consistency in volume production. Environmental validation includes thermal cycling, vibration, shock, salt spray, and particulate exposure.
5. Typical Applications
20 mm F/0.85
- UAV-based reconnaissance
- Handheld thermal imaging systems
- Automotive night vision
- Industrial thermography
- Short-range detection and monitoring
40 mm F/1.0
- Border and perimeter surveillance
- Medium-range target detection
- Infrared measurement systems
- AI-assisted vision systems
- Robotics and automation
6. Customization Capability
Customization options include:
- Focal length, F-number, and field of view
- Mechanical interface and back focal distance
- Spectral band optimization (e.g., 5–8 μm, 8–14 μm)
- Optional active focus mechanisms (stepper motor or voice coil motor)
- Production and qualification test planning
7. Conclusion
The presented LWIR lens designs demonstrate a balanced integration of ultra-wide aperture performance, passive athermalization, compact mechanical structure, and manufacturability. These developments reflect a systematic approach to overcoming key limitations in infrared optical design, enabling robust imaging performance across a wide range of applications and environmental conditions.
GREAT ARTICLE!
Share this article to gain insights from your connections!