1. Core Materials and Characteristics Analysis

1.  Silicon ( Si ) Lens

Optical Performance:

Transmission wavelength range concentrated at 1.5-8 μ m (covering part of the mid-infrared range), refractive index about 3.42 (at 4 μ m ), high reflectivity to infrared light (about 35% ), requires anti-reflective coating (such as magnesium fluoride coating) to increase transmittance to 90% or above.

Extremely high thermal conductivity (about 148 W/(m ・ K) ), one of the best heat dissipation materials among infrared materials, effectively preventing lens thermal deformation caused by temperature changes.

Physical Properties:

High hardness (Mohs hardness 7 ), strong wear resistance, not easily scratched; low density ( 2.33 g/cm ³), suitable for lightweight lens design; good chemical stability, insoluble in water and most acids and bases, no additional moisture-proof treatment required.

Processing Characteristics:

High purity silicon wafers can be obtained by single crystal growth (such as the Czochralski method), then cut, ground, and polished into lenses. Since silicon is a semiconductor material, static interference must be avoided during processing, and surface damage layers easily form during polishing, which must be removed by precise chemical etching.

Limitations:

Narrow transmission wavelength range, unable to cover 8-12 μ m (far-infrared atmospheric window), and at high temperatures ( >300 ℃) impurity ionization occurs, causing transmittance to decrease, thus not suitable for high-temperature applications.

2.  Germanium ( Ge ) Lens

Optical Performance:

Transmission wavelength range 2-14 μ m (covering mid-infrared 3-5 μ m and far-infrared 8-12 μ m two core atmospheric windows), extremely high refractive index ( 4 μ m about 4.0 ), uncoated reflectance as high as 50% or above, requiring multilayer anti-reflective coatings (such as zinc sulfide-germanium composite coatings) to increase transmittance to +  85% or above. or above.

Highly temperature sensitive: when temperature exceeds 100 ℃, germanium's lattice vibrations intensify causing increased infrared absorption and sharp transmittance decline (e.g., at 200 ℃ transmittance is only 50% of room temperature), thus must be used with temperature control design.

Physical Properties:

Mohs hardness 6.0 , slightly lower than silicon, moderate wear resistance; density 5.323 g/cm ³, heavier, not conducive to lightweight lens design; chemically stable but susceptible to corrosion by concentrated nitric acid and hydrofluoric acid.

Processing Characteristics:

Germanium single crystals must be grown by zone melting method (purity requirement 99.999% or above), brittle during processing, prone to chipping during grinding, requiring diamond wheel low-speed cutting. Due to high refractive index, spherical lens design must strictly control curvature to avoid excessive spherical aberration.

Typical Applications:

Core lens material for military thermal imagers (such as tank sights, missile seekers), covering the 8-12 μ m wavelength range (natural object thermal radiation mainly concentrated in this range), also commonly used in security night vision lenses.

3.  Zinc Selenide ( ZnSe ) Lens

Optical Performance:

Extremely wide transmission wavelength range ( 0.6-20 μ m ), one of the few materials that can transmit visible light, mid-infrared ( 3-5 μ m ) and far-infrared ( 8-12 μ m ) with refractive index about 2.4 (at 4 μ m ), uncoated transmittance about 70% , after multilayer anti-reflective coating can be increased to 95% or above.

Extremely low absorption for CO ₂ laser ( 10.6μm ) ( <0.001 cm ⁻ ¹ ), is CO ₂ the preferred window material for laser systems.

Physical Properties:

Mohs hardness only 2.5-3.0 , the material is relatively soft (similar to “soda-lime glass” in glass), surface is easily scratched, must rely on surface coating (such as diamond-like carbon film DLC ) to improve wear resistance; density 5.27 g/cm ³, weight close to germanium.

Chemical stability is good, but long-term exposure to humid environments may cause slow deliquescence, requiring sealed structures.

Processing Characteristics:

Mostly prepared by chemical vapor deposition ( CVD ) (divided into “infrared grade” and “multispectral grade,” the latter having higher light transmission uniformity), can be processed into aspheric lenses (reducing the number of lens elements and improving imaging quality). Due to the softness of the material, polishing requires very fine abrasives (such as nano-alumina) to avoid surface damage.

Typical Applications:

Widely used in multispectral imaging systems (such as drone-mounted infrared cameras that need to capture both visible and infrared images), medical infrared spectrometers (detecting infrared features of human tissues), industrial laser cutting /  and welding equipment window lenses.

4.  Zinc sulfide ( ZnS ) Lens

Optical Performance:

Transmission wavelength range 3-13 μ m (covering mid-infrared and far-infrared), refractive index about 2.2 (at 4 μ m ), uncoated transmittance about 75% , after coating can reach 90% or above.

Compared to zinc selenide, it is less sensitive to humidity (almost no deliquescence), and has better transmittance stability at high temperatures ( <300 ℃).

Physical Properties:

Mohs hardness 3.5-4.0 , wear resistance better than zinc selenide; density 4.09 g/cm ³, lighter than germanium and zinc selenide, suitable for large-aperture lens design (such as infrared windows with diameter > 100mm ).

Processing Characteristics:

Divided into “hot-pressed” and “chemical vapor deposition ( CVD )”: hot-pressed type is low cost but has poorer light transmission uniformity, suitable for low-end windows; CVD type (multispectral grade) has uniform transmittance, suitable for high-precision imaging lenses.

Typical Applications:

Used as infrared radomes (such as missile nose wave-transparent covers that must withstand aerodynamic heating and impact during high-speed flight), aerospace infrared windows (such as satellite infrared detector windows), and civilian infrared thermometers lenses.

5.  Sulfur-based glass lenses

Optical Performance:

Transmission wavelength range covers 1-14μm (full range covering three atmospheric windows: 1-3μm 、 3-5μm 、 8-12μm ), refractive index about 2.4-2.8 (varies with composition), temperature refractive index coefficient ( dn/dT ) extremely low (about - 1×10 ⁻⁶ / ℃, only 1/8 of germanium's 1/8 ), almost unaffected by temperature changes, is the core material of “ athermal lenses” (can maintain imaging clarity without additional temperature control devices).

Physical Properties:

Mohs hardness 2.0-3.0 , relatively soft texture, but hardness can be improved by doping metal elements (such as gallium, indium); density about 4.5-5.5 g/cm ³, between zinc sulfide and germanium.

Good chemical stability, insoluble in water, but long-term exposure to strong acids will cause corrosion.

Processing technology:

The biggest advantage is precision molding: heating the glass to the softening point (about 200-300 ℃), then directly pressing with high-precision molds (including aspheric, freeform and other complex shapes), production efficiency is more than 10  times that of traditional grinding and polishing, and cost is reduced by 30%-50% , very suitable for mass production (such as mobile phone infrared lenses, vehicle night vision lenses).

Typical Applications:

Mainly used in consumer-grade infrared devices (such as mobile phone infrared thermal imaging accessories, outdoor sports night vision devices), smart home (such as human presence sensor lenses), also used in industrial online inspection (such as circuit board thermal defect detection lenses), due to its athermal characteristics, performs excellently in outdoor environments with large temperature differences (such as forest fire monitoring).

2. Key Performance Comparison

3. Impact of Processing Technology on Performance

Single Crystal vs Polycrystal : Silicon and germanium are usually single crystal materials (with extremely high light transmission uniformity), while zinc sulfide and zinc selenide are mostly polycrystalline ( CVD prepared by the method, requiring control of grain size to avoid light scattering).

Coating Technology : All infrared lenses require coating, which besides anti-reflection, can also apply anti-reflective coatings (to enhance transmittance in specific bands), protective coatings (such as DLC coatings to increase hardness), and filter coatings (allowing only specific bands to pass, such as 3-5 μ m ).

Aspheric Processing : Zinc selenide and sulfur-based glass can be made into aspheric lenses through precision molding or diamond turning, reducing the number of lens elements (for example, traditional spherical lenses require 3-4 elements, while aspheric lenses only need 1-2 elements), significantly improving image clarity and reducing weight.

4. Summary

The selection of different infrared lenses must strictly match the application scenarios:

Military thermal imaging ( 8-12 μ m ) prioritizes germanium or sulfur-based glass;

Multispectral imaging (visible light +  infrared) must choose zinc selenide;

High temperature /  harsh environments (such as missile radomes) prefer zinc sulfide;

Consumer-grade mass products (such as mobile phone accessories) favor sulfur-based glass due to cost and efficiency advantages.

The material's transmission wavelength range, temperature stability, and processing cost are the three core decision factors, while coating and aspheric technologies are key methods to enhance lens performance.