Germanium Free Infrared Optics: Feasibility Using Chalcogenide Glass to Replace Germanium in MWIR & LWIR Optical Design and Manufacturing
Application Note Serial No.: AN-SGYL-02-03-26
Author: Dr. Qu Yingli – R&D Director
Editor: Bryan Ng – Marketing Manager
Published on:
Last edited:
Germanium (Ge) has historically dominated infrared optical design due to its high refractive index, broad transmission in the long-wave infrared region, and relatively low chromatic dispersion. These properties have made it a preferred material for compact, high-performance thermal imaging and infrared sensing systems for decades. However, Ge also presents several limitations, including high raw material cost, significant density, strong temperature-dependent refractive index variation, and limited compatibility with high-volume, cost-sensitive manufacturing processes. It’s pronounced thermal sensitivity, particularly the large dn/dT, can lead to focus shift and performance drift in systems operating across wide temperature ranges, increasing the need for complex athermalization strategies.
In contrast, chalcogenide glass has emerged as a compelling alternative for modern infrared optical systems. These materials offer broad infrared transmission, tailored refractive index options, and comparatively lower thermal sensitivity, enabling more stable performance over temperature. Their lower density also contributes to lighter optical assemblies, which is increasingly important for compact, portable, and airborne platforms. From a manufacturing perspective, chalcogenide glass is well-suited for precision glass molding, supporting high repeatability, tight tolerances, and scalable mass production without extensive post polishing.
This application note demonstrates that chalcogenide glass is a fully feasible replacement for Ge from optical design, thermal performance, and manufacturing perspectives. With modern optical design optimization, advanced coating solutions, and precision molding technologies, chalcogenide-based systems can achieve equivalent or even superior imaging performance. At the same time, they enable significant cost advantages and improved scalability, making them highly attractive for next-generation infrared systems in automotive, security, industrial, and consumer applications.
1. Material and Optical Design Feasibility
Replacing Ge with chalcogenide glass does not require direct material equivalence in refractive index or dispersion. Instead, system feasibility is achieved through intelligent optical design optimization and manufacturing enablement. Modern infrared optical engineering no longer relies on one-to-one material substitution, but rather on leveraging additional degrees of freedom such as aspheric surfaces, diffractive optical elements, and multi-material combinations to meet performance targets. When combined with scalable precision glass molding, chalcogenide glass offers a fundamentally different cost-performance balance compared to traditional Ge-based systems.
The table below summarizes the key material and optical design trade-offs when transitioning from Ge to chalcogenide glass.
Table 1: Key material and optical design trade-offs when replacing Ge with chalcogenide glass
| Aspect | Ge | Chalcogenide Glass | Design Implication |
|---|---|---|---|
| Refractive Index | ~4.0 | 2.2 to 2.8 | Lower index can be compensated by optimized lens count, higher curvature, and aspheric surfaces |
| Dispersion | Very low | Moderate to high | Chromatic aberration managed via diffractive optical elements or multi-glass correction strategies |
| Lens Count | Low | Moderate | Slightly increased element count is acceptable due to improved manufacturing scalability |
| Aspheres | High cost, diamond turning required | Mold friendly | Aspheric surfaces become economically viable in volume production |
| Thermal dn/dT | High | Low | Enables passive athermalization and reduced focus shift over temperature |
While Ge benefits from its very high refractive index, enabling compact designs with fewer elements, chalcogenide systems compensate through design flexibility. The availability of moldable aspheric geometries allows aberration correction that would otherwise require additional elements in spherical Ge designs. Furthermore, although chalcogenide materials exhibit higher dispersion, modern infrared systems can effectively control chromatic effects using diffractive optical elements or hybrid material configurations.
From a system perspective, the slightly higher lens count in chalcogenide designs is offset by significant manufacturing advantages. Precision glass molding enables repeatable, high-volume production with lower per-unit cost compared to diamond turned Ge optics. Additionally, the lower thermal dn/dT of many chalcogenide glasses simplifies athermal design, reducing mechanical compensation requirements and improving environmental stability.
2. Optical Performance Equivalence
Published MWIR and LWIR optical design studies demonstrate that chalcogenide-based systems can achieve modulation transfer function MTF, distortion control, and field performance comparable to traditional Ge benchmarks. While Ge benefits from its very high refractive index and low dispersion, performance equivalence with chalcogenide materials is achievable through modest and well-controlled increases in optical complexity. Rather than direct material substitution, the approach relies on design re-optimization, the use of special surfaces, and strategic material pairing.
Table 2: Optical performance feasibility comparison between Ge and chalcogenide designs
| Design Type | Lens Count | Special Surfaces | MTF vs Ge | Feasibility Assessment |
| 2× Ge | 2 | None | Baseline | Reference |
| 2× ChG | 2 | None | Lower | Not sufficient |
| 2× ChG + DOE | 2 | DOE | Equivalent | Feasible |
| 3× ChG (2 glasses) | 3 | Optional asphere | Equivalent | Feasible |
| 4× ChG (1 glass) | 4 | Asphere | Near-equivalent | Feasible |
From Table 2, it can be seen that a two-element Ge optical system can be replaced with equivalent MTF performance using the same number of chalcogenide lenses by introducing one diffractive optical element DOE surface. The DOE provides additional chromatic correction capability that compensates for the higher dispersion of chalcogenide materials. Alternatively, equivalent performance can be achieved using one molded aspherical chalcogenide lens combined with two additional infrared materials, such as ZnSe or ZnS, enabling multi-material chromatic balancing without excessive system growth.
The table provides representative design pathways rather than strict rules. In many practical systems, especially hybrid designs where Ge is already combined with other infrared materials, Ge can be replaced by an equivalent number of chalcogenide elements with similar surface complexity. When the overall system is reoptimized rather than directly substituted, comparable field performance, distortion control, and thermal behavior can be maintained.
These examples illustrate that performance feasibility is fundamentally a design optimization problem rather than a material limitation. With appropriate use of DOEs, molded aspheres, and multi-glass combinations, chalcogenide-based MWIR and LWIR systems can match Ge benchmarks while offering advantages in cost, manufacturability, and thermal stability.
To illustrate more clearly, two optical designs with the same focal length and F number, one incorporating Ge and one fully based on chalcogenide glass, are compared using the parameters listed in Table 3. In this example, IRG refers to a chalcogenide glass material from SCHOTT. Both systems are designed for MWIR or LWIR imaging under identical system-level constraints, allowing a direct performance comparison.
Table 3. Parameters for two designs with 1 Ge + 3 IRG and 4 IRG
| Parameter | 1 Ge + 3 IRG | 4 IRG |
|---|---|---|
| EFL | 50 mm | 50 mm |
| F# | 1.0 | 1.0 |
| Detector | 1280 × 1024 – 12 µm | 1280 × 1024 – 12 µm |
| Wavelength | 8–12 µm | 8–12 µm |
| Distortion | ≤ 1% | ≤ 1% |
| Relative Illumination | ≥ 96% | ≥ 93% |
| TTL | 63.76 mm | 63.89 mm |
| Back Focal Length | 18.03 mm | 14.73 mm |
| ASP No | 2 | 2 |
| DOE No | 1 | 1 |
| Coating for First Lens | DLC / HD / AR | HD / AR |
From Table 3, it can be observed that, for this specific design case, the optical performance of the four-element chalcogenide system is essentially equivalent to the hybrid 1 Ge + 3 chalcogenide design. Effective focal length, F number, distortion control, total track length, and surface complexity are nearly identical. Modulation performance and field correction are maintained through comparable use of aspheric and diffractive surfaces.
The primary difference appears in relative illumination at the edge of the field, where the all chalcogenide design shows a slight reduction. This is typically attributable to differences in refractive index and pupil mapping, which may influence marginal ray control and vignetting behavior. However, the reduction remains within acceptable system-level tolerances and can often be further optimized through minor design adjustments.
Another difference lies in the back focal length, which is shorter in the fully chalcogenide design. While this may impose slightly tighter mechanical packaging constraints, it can also enable more compact module integration depending on system requirements. From a coating perspective, the Ge-based first element typically requires a DLC protective layer in addition to high durability and anti-reflection coatings, whereas chalcogenide materials may not require the same level of mechanical protection, potentially simplifying coating architecture.
Fig 1 and 2 show the MTF and spot diagram for the 1Ge+3IRG and 4 IRG in (a) and (b), respectively. We can see that, with the same number of lenses, the two systems have similar performances, with 4 IRG being even slightly better
3. Thermal Feasibility and System Robustness
Thermal performance represents one of the most decisive advantages of chalcogenide glass over Ge in infrared optical systems. While Ge offers strong optical power due to its high refractive index, it also exhibits a relatively high temperature-dependent refractive index change, dn/dT. This means that even moderate temperature variations can lead to noticeable focal shift, degrading image quality unless active or mechanical compensation mechanisms are implemented.
Chalcogenide glass, by contrast, generally exhibits significantly lower dn/dT values. This intrinsic material stability enables passive athermal optical designs, where focus stability is achieved through material selection and optical balancing rather than mechanical refocus mechanisms. As a result, system complexity can be reduced while maintaining consistent imaging performance across wide operating temperature ranges.
Table 4: Thermal feasibility comparison between Ge and chalcogenide optical systems
| Metric | Ge System | ChG System | System Impact |
|---|---|---|---|
| dn/dT | High | Low | Reduced focus drift |
| Refocus Needed | Yes | No | Simpler mechanical structure |
| Thermal Darkening | Possible | None | Higher environmental robustness |
Due to its higher dn/dT, a Ge-based system often requires mechanical refocusing, floating lens groups, or temperature compensation structures to maintain optimal modulation performance across temperature variations. These solutions increase mechanical complexity, size, weight, and potential long-term reliability concerns.
In contrast, the superior thermal properties of chalcogenide materials allow lens assemblies to meet athermal performance requirements through optical design alone. By carefully pairing materials and distributing optical power appropriately, the focus position can remain nearly constant over a wide temperature range without adjusting the detector plane. This passive stability is particularly valuable in automotive, outdoor surveillance, industrial monitoring, and aerospace applications, where systems must operate reliably from low to high ambient temperatures.
Additionally, some Ge-based systems may exhibit thermal darkening effects under certain environmental conditions, whereas chalcogenide glass generally demonstrates stable transmission characteristics in typical operating ranges. Overall, the improved thermal stability of chalcogenide lenses translates into simpler mechanics, reduced system weight, improved reliability, and lower total system cost, while maintaining consistent optical performance.
4. Manufacturing and Scalability Feasibility
Precision glass molding is a key enabler that transforms optical feasibility into economic feasibility for chalcogenide-based infrared systems. While advanced optical designs may be theoretically achievable, practical adoption depends on the ability to produce components consistently, at scale, and at a reasonable cost. Chalcogenide glass is particularly well suited to precision molding, allowing complex optical surfaces—including aspheres and freeform geometries—to be manufactured with high repeatability and minimal post processing. This capability significantly lowers per-unit costs as production volume increases, making high-performance infrared optics economically viable for mass market applications.
Table 5: Manufacturing feasibility comparison
| Manufacturing Aspect | Ge | Chalcogenide Glass | Impact |
|---|---|---|---|
| Fabrication Method | Diamond turning | Precision molding | Scalable, high-throughput manufacturing |
| Asphere Cost | High | Low incremental | Complex surfaces become economically feasible |
| Design Freedom | Limited by machining constraints | High | Enables use of aspheres, DOEs, and hybrid designs |
| Volume Scaling | Poor | Excellent | Mass-market production achievable |
Ge optics are typically fabricated using diamond turning or grinding/polishing, which provides excellent surface quality but is labor-intensive, slow, and expensive, especially for aspheric or multi-element designs. The cost per unit remains high, and scaling to large production volumes is challenging.
In contrast, chalcogenide glass is compatible with precision molding processes, where molten glass is shaped directly in a mold to form aspheric and freeform surfaces with minimal post-polishing. The incremental cost of adding aspheric or diffractive surfaces is small, enabling sophisticated optical designs without prohibitive expense. Molding also supports high-throughput production, excellent dimensional repeatability, and consistent optical quality across large batches.
By combining optical design flexibility with scalable manufacturing, chalcogenide glass makes it possible to deliver high-performance MWIR and LWIR optics at volumes and costs suitable for commercial and mass-market applications. This approach bridges the gap between technical feasibility and economic viability, making chalcogenide lenses an attractive alternative to germanium in modern infrared systems.
5. Our Existing Germanium-Free Infrared Lenses
We have developed a certain number of Germanium-Free Infrared Lenses, where the Ge is replaced by chalcogenide glass, as shown in tables 6 to 8, for fixed focus lenses, zoomed lenses, and athermal lenses, respectively.
Table 6: Wavelength Opto-Electronic Germanium-Free Fixed-Focus Infrared Lenses
| PN. | Focal Length | Wavelength | Detector |
| IRG6.81.0-17A | 6.8mm | 8um -12um | 640×480 ,17um |
| IRG9.61.0-17A | 9.6mm | 8um -12um | 640×480 ,17um |
| IRG111.0-17A | 11mm | 8um -12um | 640×480 ,17um |
| IRG1001.0-12A | 100mm | 8um -12um | 1024×768, 12um |
| INFRA-MW133.6-10 | 13mm | 3.6 µm – 4.2 µm | 640×512 ,10um |
| INFRA-MW253.6-10 | 25mm | 3.6 µm – 4.2 µm | 640×512 ,10um |
| INFRA-MW503.6-10 | 50mm | 3.6 µm – 4.2 µm | 640×512 ,10um |
| INFRA-MW13.72.0-15V2 | 13.7mm | 3um -5um | 640×512 ,15um |
| INFRA-MW62.0-15V2 | 6mm | 3.7um – 4.8um | 640×512 ,15um |
Table 7: Wavelength Opto-Electronic Germanium-Free Zoomed Infrared Lenses
| Opto-Mechanic Property | Specification |
| Focal Length | 18-180 |
| F# | 0.8/1.2 |
| Wavelength | 8-12um |
| Detector | 640×512_15um |
| Focus Range | 3.m~∞ |
| Back Focus Distance | – |
| Back Working Distance | – |
| Dimensions | – |
| Focus Type | Auto Focus |
We have developed more than 100 types of athermal LWIR lenses with Chalcogenide Glass and other non-Ge materials, with focal length ranges from 0.9 mm to 110 mm. Table 7 gives examples of the designs.
Table 8: Table list of some of our Athermal LWIR fixed focus lenses without Ge
| Focal length | F# | Wavelength Range | BWD | Sensor pixel number-pixel size |
| 0.9 mm | 1.1 | 8-12 um | 2.4 mm | 120*90-12 um |
| 1.6 mm | 1 | 8-14 um | 2.36 mm | 120*90-12 um |
| 3.2 mm | 1 | 8-12 um | 2.53 mm | 256*192-12 um |
| 5 mm | 1 | 8-12 um | 6.1 mm | 384*288-12 um |
| 10 mm | 1 | 8-12 um | 8.1 mm | 384*288-17 um |
| 16.8 mm | 1 | 8-12 um | 8 mm | 640*512-12 um |
| 25 mm | 1 | 8-12 um | 10 mm | 640*512-12 um /384*288-12 um |
| 35 mm | 1 | 8-12 um | 10 mm | 1280×1024-12 um |
| 50 mm | 1 | 8-12 um | 11 mm | 1280*1024-12 um |
| 55 mm | 1 | 8-12 um | 10 mm | 640*512-12 um |
| 75 mm | 1 | 8-12 um | 8.075 mm | 1280*1024-12 um |
| 110 mm | 1.3 | 8-12 um | 10.29 mm | 640*512-17 um |
7. Conclusion
Wavelength Opto-Electronic combines extensive infrared optics expertise with end-to-end manufacturing capabilities to be an ideal partner for Germanium-free infrared optics. The company’s integrated operations cover the entire value chain from in-house chalcogenide glass development and purification to optical design, precision fabrication, coating, and assembly, enabling full production of infrared lenses and systems without relying on Ge. Wavelength’s specialized R&D in chalcogenide materials (including proprietary purification & growth processes) and significant production capacity (~10 tons of chalcogenide glass per year) ensure it can deliver high-performance MWIR/LWIR optics at scale. With decades of infrared lens design experience (spanning athermal imaging to continuous zoom systems) and advanced technologies like durable DLC coatings tailored for chalcogenide optics, Wavelength is well-positioned to support system designers and manufacturers in deploying scalable, high-performance Germanium-free infrared optics and systems.
Replacing Ge with chalcogenide glass is not only feasible but strategically advantageous for modern MWIR and LWIR optical systems. Enabled by optical design freedom and precision molding, chalcogenide glass delivers performance parity, thermal robustness, and scalable manufacturability.
