The impact of lenses of the same specification but different designs on the image
The impact of thermal imaging lenses with the same specifications but different designs on image quality is mainly reflected in the differences in the details of the optical design. Although the specifications (such as focal length, aperture, field of view, resolution, etc.) may be similar, the following design factors will lead to significant differences in the actual imaging effect:
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.). Lenses with more complex designs may better suppress edge distortion and chromatic aberration, thereby improving the consistency between the center and the edge.
Thermal stability: Thermal imaging lenses are mostly used in a wide temperature range environment. The difference in thermal expansion coefficients of different materials (such as germanium, chalcogenide glass, and chalcogenide) may cause image quality drift when the temperature changes. Lenses with optimized design will reduce thermal defocusing through material matching or mechanical compensation.
2. Coating and transmittance
Anti-reflective coating: Different coating processes 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 "ghost images" 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 the purity or doping process of the lens material may cause the transmittance of certain wavelengths to decrease, affecting the 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 threaded structure and black coating) will affect the path of stray light. Poorly designed lenses may cause halos or artifacts around high-temperature 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 reduce such artifacts through lens curvature or coating.
4. Mechanical structure and durability
Focus and zoom mechanism: The accuracy of manual/auto focus and the sealing (dust and water resistance) affect the stability of long-term use. For example, poor sealing may cause the lens to fog up, reducing the image quality in a humid environment.
Anti-vibration: In industrial or field applications, the way the lens group is fixed affects the anti-vibration performance. Long-term vibration may cause the optical axis to shift and affect the sharpness.
5. Actual resolution and MTF curve
Nominal vs measured resolution: Under the same "resolution" specification, the difference in modulation transfer function (MTF) may lead to different actual imaging sharpness. For example, a lens with a faster MTF drop at the edge of the field of view will blur the edge details when shooting at a wide angle.
Detector matching: Whether the lens design is targeted at a specific detector (such as pixel size, cold screen matching) will affect the actual resolution. Unoptimized designs may cause vignetting or underutilization of resolution.
6. Adaptability to application scenarios
Close-range imaging: Some designs optimize close-range focusing (such as industrial inspection), while others may be targeted at long-range observation (such as security monitoring), resulting in different image quality for close or distant scenes under the same specifications.
Dynamic range processing: For high-dynamic scenes (such as the presence of extremely high and low temperature targets at the same time), vignetting control or detector response matching of different lenses may affect the ability to restore layers.
Selection suggestions
Refer to MTF curve and measured data: Specifications cannot fully reflect actual performance and need to be combined with laboratory tests (such as thermal contrast test, noise equivalent temperature difference NETD).
Environmental adaptability: Select a design with better coating process and sealing level according to the usage scenario (high temperature, high humidity, vibration).
Compatibility verification: Ensure that the pixel size and cold screen F number of the lens match those of the detector to avoid resolution waste or vignetting.
In short, the design details of thermal imaging lenses will significantly affect the final image quality, especially in complex environments or high-precision applications, and the selection needs to be combined with measured performance rather than simply relying on specifications.
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