How ZnS and ZnSe Are Replacing Germanium (Ge) in IR Optics
Due to recent exporting rules, the price of germanium on the global market is surging, and the delivery is much longer than before. Germanium is a predominant material used in IR optics due to its wide IR transmission band from 2000-16000nm, large refractive index (4.0), which is favored for IR lens design, and excellent hardness compared to other IR materials, which makes germanium suitable for manufacturing protective windows of thermal imaging cameras. However, as the cost of germanium has risen drastically due to the export control, it becomes essential for users and manufacturers of infrared optics to look for substitutes for germanium.
Infrared optics are used for imaging in the infrared band of non-visible light, and mainly work in the night vision system of the two atmospheric windows of 3~5μm and 8~14μm. Since the infrared optics work in the infrared band, they can passively form thermal images by using the radiation of the object itself. The infrared optical system is made of infrared optical materials, mainly special IR materials that are transparent to the infrared band, such as germanium, silicon, fused quartz, etc. Optical materials for infrared optical systems cannot use optical glass materials in the visible light band, but must use materials that transmit the infrared band. When designing infrared lenses, the most commonly used infrared materials include germanium, silicon, calcium fluoride, zinc selenide, zinc sulfide, fused quartz, etc.
Zinc Sulfide (ZnS) and Zinc Selenide (ZnSe) are increasingly used as substitutes for Germanium (Ge) in infrared (IR) optics due to several advantages in optical performance, weight, cost, and environmental resilience.
Below is a detailed comparison and explanation of how ZnS and ZnSe are replacing Ge (there are two types of zinc sulfide, the CVD ZnS, produced via the chemical vapor deposition process, and the multispectral ZnS, also known as Cleartran, which goes through a further hot isostatic press process) :
1. Optical Performance Comparison
CVD ZnS is widely used in thermal IR (8–12 µm), but is opaque to visible light.
Multispectral-ZnS is a post-processed version of CVD ZnS, polished via hot isostatic pressing to achieve optical transmission across visible to LWIR, enabling true multispectral capability.
ZnSe: The transmission range of zinc selenide is 0.5~20μm, slightly wider than zinc sulfide, and its refractive index is also slightly higher than that of zinc sulfide. Zinc selenide has the advantages of high purity and easy processing. The biggest advantage of zinc selenide is its good light transmittance, extremely small absorption coefficient, and low light transmission loss. It is the preferred material for high-power CO2 laser optical components. Zinc selenide has a uniform and consistent refractive index and is also used to protect windows in forward-looking infrared thermal imaging systems.
Ge: Its transmission spectrum ranges from 2 to 23 μm, covering mid-wave infrared and long-wave infrared. Ge has a high refractive index, reaching above 4.0. A large refractive index is beneficial to a small light deflection angle, thereby reducing the spherical curvature radius and facilitating aberration correction. Since the Abbe numbers of Ge at 4μm and 10μm are very different, germanium exhibits different characteristics at medium and long wavelengths. It can be used as a negative lens at medium waves, similar to the "flint glass" in achromatism; it can be used as a positive lens at long waves, similar to the "crown glass" in achromatism. Germanium has a small absorption at 10.6 μm and can be used as a CO2 laser lens.
2. Thermal and Mechanical Properties
CVD-ZnS has the advantages of high purity, moderate density, easy processing, high hardness, insolubility in water, low price, and good environmental adaptability.
MS-ZnS improves mechanical strength and scratch resistance over standard CVD ZnS.
ZnSe is softer and more susceptible to moisture; protective coatings are usually required.
Ge is hard but brittle, and its high thermal conduction can be both an advantage and a drawback depending on the system. Also, germanium has a large dn/dt, so an athermalized design is required in environments with large temperature differences.
3. Environmental and Practical Considerations
Ge is a relatively rare and expensive material, and its price is volatile.
ZnS and ZnSe are synthesized from more abundant raw materials, offering better cost control.
Cleartran, produced by hot isostatic pressing, has become more commercially viable and scalable.
4. Application Scenarios Replacing Ge
Infrared Domes and Windows
MS-ZnS transmits both visible and IR wavelengths, making it a suitable replacement for Ge in multi-spectral seeker systems.
Thermal Imaging Systems
ZnSe provides excellent performance in the 8–12 μm atmospheric window, replacing Ge in many thermal cameras.
High-Power Laser Windows
ZnSe is widely used in CO2 laser optics due to its higher laser damage threshold compared to Ge.
Aerospace Imaging Optics
ZnS is lightweight and environmentally robust, ideal for optical windows in defense and aerospace.
5. Limitations and Considerations
ZnS and ZnSe are softer than Ge, requiring more careful handling and protective coatings.
ZnSe is slightly hygroscopic, so AR coatings and sealing are recommended.
ZnS and ZnSe have a lower refractive index than germanium, which means optical designs might need to compensate for increased lens thickness or reduced reflectance.
Conclusion
ZnS and ZnSe are increasingly replacing Ge in infrared optics, particularly in multi-spectral imaging systems, lightweight platforms, high-power laser applications, and cost-sensitive projects. With proper design and coating strategies, ZnS and ZnSe offer performance that meets or exceeds Ge, often with added benefits in durability and affordability.
Shalom EO offers a comprehensive portfolio of Zinc Sulfide (ZnS) and Zinc Selenide (ZnSe) IR optics, including :
ZnS windows, ZnS lenses, and ZnS domes, both CVD zinc sulfide and multispectral zinc sulfide, are available
ZnSe windows, ZnSe lenses, and ZnSe domes
The optics are engineered to meet the needs of IR optical applications. We can also provide ZnSe focusing lenses for CO2 lasers. These ZnS and ZnSe IR optics are becoming the preferred alternatives to germanium (Ge) in many applications due to their superior transmission characteristics, lighter weights, and lower costs.
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