This stems primarily from historical factors related to materials, cost, and system requirements:

• Material Limitations: The range of materials transparent in the infrared spectrum—particularly the mid-to-long wave infrared (MWIR: 3–5μm, LWIR: 8–14μm)—is far more limited than in the visible spectrum. Common materials like germanium (Ge), zinc sulfide (ZnS), and zinc selenide (ZnSe) exhibit poor dispersion characteristics, and few anomalous dispersion materials are available. This makes correcting chromatic aberration (especially secondary spectrum) extremely difficult, often resulting in significant residual chromatic aberration.

• Cost Considerations: Infrared materials (such as Ge and ZnS) and detectors are very expensive. Therefore, to control overall costs, system designs often prioritize the use of minimal lens elements (e.g., 1 to 3). Fewer lenses mean fewer degrees of freedom for correcting aberrations, resulting in systems that typically retain more aberrations like spherical and comatic aberrations.

• Larger Detector Pixel Size: Early infrared detectors typically had larger pixel sizes (e.g., 25μm or larger). Even with significant optical aberrations, the resulting circle of confusion might still be smaller than a single pixel. Consequently, system performance was determined by the detector pixel size rather than the optical diffraction limit (a scenario termed a “detector-limited” system). In such cases, investing heavily to achieve diffraction-limited optics was impractical, tolerating some aberrations significantly reduced costs.

• Cold Aperture Matching Requirement:Most high-performance infrared detectors are housed in a Dewar and cooled to cryogenic temperatures (e.g., 77 K) to reduce noise. This cold detector itself possesses an aperture known as the “cold aperture.” To minimize stray radiation from the lens barrel itself, the exit pupil of the optical system must precisely align with this cold aperture—a requirement known as “cold aperture matching.” This constraint severely limits optical design flexibility, sometimes necessitating trade-offs in aberration correction to achieve cold aperture matching.

Based on these factors, many traditional, cost-sensitive infrared applications (such as thermal imagers and pyrometers) indeed employ relatively simple optical designs and tolerate significant aberrations.

However, technological advancements and rising performance demands have changed this landscape:

Application Requirements: For high-end applications like military reconnaissance, infrared guidance, space remote sensing, and scientific research, systems require the highest possible resolution and sensitivity. This necessitates optical systems approaching the diffraction limit, where aberrations are corrected to extremely low levels.

Design Capabilities: Modern optical design software and manufacturing techniques, including diamond-turned aspheric and diffractive optical elements (DOE) are exceptionally powerful. Utilizing aspheric and diffractive surfaces (frequently etched directly into Germanium lenses) allows for major aberration correction—especially for spherical aberration and chromatic aberration—using very few lenses.

Detector Advancements: With pixel sizes in modern infrared detectors now as small as 5μm, the optical system's circle of confusion must be minimized accordingly. This requirement directly fuels the advancement of optical designs with minimal aberrations.