The impact of different lens designs on thermal imaging images under the same specifications

The impact of thermal imaging lenses with the same specifications but different designs on image quality is mainly reflected in the differences in optical design details. Although specifications (such as focal length, aperture, field of view, resolution, etc.) may be similar, the following design factors can cause significant differences in actual imaging results:



1. Aberration correction capability

Differences in optical structure: The number, shape and arrangement of lenses (such as the use of aspherical mirrors and diffractive elements) will affect the correction effect of aberrations (spherical aberration, coma, astigmatism, etc.). Designing more complex lenses can better suppress edge distortion and chromatic aberration, thereby improving center-to-edge consistency.

Thermal stability: Thermal imaging lenses are mostly used in environments with a wide temperature range. Differences in thermal expansion coefficients of different materials (such as germanium, chalcogenide glasses, and chalcogenide compounds) can cause image quality to drift as temperature changes. Optimally designed lenses will reduce thermal defocus through material matching or mechanical compensation.

2. Coating and transmittance
Anti-reflection coating: Different coating processes will affect the transmittance and stray light suppression in the infrared band (such as 8-14μm). High-quality coatings can improve the signal-to-noise ratio, reduce "ghosting" and glare, and make images clearer, especially near strong heat sources (such as flames and high-temperature equipment).
Material absorption characteristics: Even if the specifications are the same, differences in lens material purity or doping processes may cause transmittance to decrease at certain wavelengths, thus affecting image contrast.

3. Stray light and noise control
Aperture and inner wall treatment: The position and shape of the aperture and the matte treatment of the inner wall of the lens barrel (such as thread structure and black coating) will affect the path of stray light. Poorly designed lenses can cause halos or artifacts around hot targets, reducing detail resolution.
Narcissus effect: In some designs, the detector's own low-temperature radiation is reflected back to the detector by the lens, forming dark spots. Optimized designs can reduce such artifacts through lens curvature or coatings.

4. Mechanical structure and durability
Focus and zoom mechanism: The accuracy of manual/auto focus and sealing (dustproof and waterproof) will affect the stability of the lens in long-term use. For example, poor sealing can cause lenses to fog up, reducing image quality in humid environments.
Shockproof: In industrial or field applications, the way the lens set is fixed will affect the shockproof performance. Long-term vibration may cause the optical axis to shift, affecting clarity.

5. Actual resolution and MTF curve
Nominal vs. Measured Resolution: Within the same "resolution" specification, differences in modulation transfer functions (MTF) may result in differences in actual imaging sharpness. For example, a lens whose MTF drops quickly at the edge of the field of view will cause edge details to be blurred when shooting at wide angle.
Detector matching: Whether the lens is designed for a specific detector (e.g. pixel size, cold screen matching) can affect actual resolution. An unoptimized design may result in vignetting or underutilization of resolution.

6. Adaptability to application scenarios
Close-range imaging: Some designs are optimized for close-range focus (such as industrial inspection), while others may be targeted at long-range observation (such as security monitoring), resulting in differences in imaging quality between close-range and far-range scenes under the same specifications.
Dynamic range processing: For highly dynamic scenes (such as the presence of extremely high and extremely low temperature targets at the same time), vignetting control or detector response matching of different lenses may affect the ability to restore levels.

Selection suggestions
Refer to the MTF curve and actual measured data: specification parameters cannot fully reflect actual performance and need to be combined with laboratory testing (such as thermal contrast test, noise equivalent temperature difference NETD).
Environmental adaptability: Choose a coating process and a design with better sealing according to the usage scenario (high temperature, high humidity, vibration).
Compatibility Verification: Make sure the pixel size and cold screen F-number of the lens match the detector to avoid wasted resolution or vignetting.

In short, the design details of the thermal imaging lens will significantly affect the final image quality, especially in complex environments or high-precision applications. The selection needs to be combined with measured performance rather than simply relying on specifications.