Choosing the Right Optical Solution
In the current political climate, sourcing Germanium has become increasingly challenging. Even when material is available, sharp price increases and extended lead times are common.
As Crystran manufactures high-precision Germanium optics — including lenses and windows — we have experienced this acutely over the past 18 months.
This situation presents an important opportunity to explore alternative infrared (IR) materials.
Infrared systems rely on specialised optical materials to transmit and focus energy beyond the visible spectrum (>700 nm, and typically >2 µm for IR optical applications). From thermal imaging to spectroscopy, overall system performance begins with selecting the correct material.
In this guide, we compare Germanium with other commonly used IR materials, including Zinc Selenide (ZnSe), Zinc Sulphide (ZnS), Silicon (Si), and speciality glasses.
Why Material Choice Matters in IR Optics
Different IR applications demand different material characteristics — including:
- Transmission range
- Refractive index
- Mechanical robustness
- Thermal behaviour
- Cost and manufacturability
Whether you are designing an ATR crystal, a thermal imaging lens, or a high-power CO₂ laser window, understanding real-world material performance is essential.
Best Used For
- LWIR thermal cameras
- High-resolution IR lenses
- Systems requiring high refractive power in compact form
Germanium (Ge)
Key Strengths
- High refractive index (~4.0)
Enables compact lens designs with strong optical power. - Excellent LWIR transmission
Very high transmission from approximately 2–23 µm, making it ideal for thermal imaging and FLIR systems. - Low dispersion
Reduces chromatic aberration in IR imaging systems. - Coating compatibility
Suitable for AR and DLC coatings, allowing use in harsh environments.
Limitations
- Limited availability
Germanium remains difficult to source, resulting in price volatility and long lead times. - High density
Heavier than many alternatives — important for aerospace and handheld systems. - Temperature sensitivity
Transmission reduces at elevated temperatures. The refractive index changes rapidly with temperature, and the material becomes opaque above ~350 K as the bandgap fills with thermally excited electrons.
Zinc Selenide (ZnSe)
Key Strengths
-
Broad transmission (0.5–20 µm)
Covers visible, MWIR, and LWIR bands. -
Ideal for CO₂ lasers
Extremely low absorption at 10.6 µm makes ZnSe the industry standard for high-power laser optics.
Limitations
-
Lower refractive index (~2.4)
Requires larger radii and thicker elements to achieve the same optical power as Germanium. -
Relatively soft and brittle
More prone to chipping; careful handling is required.
Best Used For
- High-power CO₂ laser optics
- Mid-IR imaging systems
- Broadband IR spectroscopy
Best Used For
- Multispectral optics (visible + IR transmission)
- Ruggedised systems
- Defence and aerospace platforms
Zinc Sulphide (ZnS) – Multispectral (MS) & FLIR Grades
Key Strengths
-
Wide transmission band
- ZnS MS: ~0.4–13.5 µm (visible through IR)
- ZnS FLIR: ~1–13.5 µm
-
Improved mechanical strength
Generally tougher than ZnSe with good environmental resistance. -
Emerging AR/DLC capability
Developments in AR/DLC coatings for both MS and FLIR grades are progressing, and Crystran expects to offer these options shortly.
Limitations
- Narrower transmission than ZnSe or Germanium
-
Lower laser resistance
Not as suitable for high-power CO₂ laser applications.
Silicon (Si)
Key Strengths
- Excellent thermal conductivity
Performs reliably in high-temperature and high-power environments. - Good mechanical strength
Durable and robust, making it suitable for demanding optical assemblies. - Cost-effective
Typically one of the most economical infrared materials available for MWIR applications. - AR / DLC compatible
Suitable for anti-reflection (AR) and diamond-like carbon (DLC) coatings, making it ideal for harsh or exposed environments.
Limitations
- Transmission range: 1.2–8 µm
Strong transmission in the MWIR region but unsuitable for LWIR (8–12 µm) applications. - High refractive index (~3.4)
Similar optical design considerations to Germanium, including higher Fresnel reflection losses, but with a more limited spectral range.
Best Used For
- MWIR lenses
- Harsh or high-temperature environments
- Cost-sensitive IR designs
Chalcogenide Glass
Chalcogenide glass can be used as an alternative to Germanium, particularly in mid-IR applications.
However, many chalcogenide formulations contain arsenic, which introduces health, safety, and environmental considerations during manufacture, handling, and disposal.
In line with Crystran’s material policy and compliance requirements, chalcogenide glass is not offered by Crystran.
Material Comparison at a Glance
| Material | Transmission Range | Refractive Index (n) | Relative Hardness | Typical Applications |
|---|---|---|---|---|
| Germanium | 2–23 µm | 4.0 | Medium | LWIR imaging, compact optics |
| ZnSe | 0.5–20 µm | 2.4 | Soft | CO₂ laser optics, broadband IR |
| ZnS (MS / FLIR) | 0.4–13.5 µm | 2.2 | Tough | Multispectral systems, rugged IR optics |
| Silicon | 1.2–8 µm | 3.4 | Hard | MWIR imaging, high-temperature applications |
Choosing the Right Material for Your Design
When selecting an IR optical material, consider:
✔ Spectral requirements – What wavelength band must your system operate in?
✔ Thermal environment – Will the optic see high power or temperature variation?
✔ Mechanical demands – Does your application require durability or lightweight performance?
✔ Cost and manufacturability – Some materials are easier and more economical to fabricate into precision optics.
At Crystran, we manufacture precision IR optics in a wide range of materials and offer custom fabrication to tight tolerances. Our technical team can help you balance performance, availability, and budget.
Need Help Selecting the Right IR Material?
Contact our team at sales@crystran.com
or visit our Materials & IR Optics section for detailed datasheets and expert guidance.
Why Choose Crystran
Choosing Crystran as your optical components supplier ensures access to world-class expertise and state-of-the-art manufacturing facilities. Crystran specializes in fabricating high-quality IR and UV materials, including exotic crystals and innovative glasses, all processed under stringent quality controls. With ISO9001:2015 and ISO14001:2015 certifications, Crystran guarantees exceptional quality and environmental responsibility. Their technical sales team offers unparalleled support, ensuring quick, reliable service, whether you need a single optic or a high-volume production run. Crystran’s global reach and competitive pricing make them an ideal choice for optical solutions.
For more details, visit our About Us page.