Germanium wafer with AR coating D55mm

Properties of Germanium Windows

Germanium windows are characterized by their unique physical and optical properties. They have a high refractive index, which is why an anti-reflective coating (AR coating) is recommended to ensure optimal transmission. The transmission of these windows is temperature-dependent: as the temperature increases, the transmission decreases, a phenomenon known as “thermal runaway.” Therefore, germanium windows should be used at temperatures below 100°C.

Other important properties include a high density of 5.33 g/cm³, which should be considered when designing weight-sensitive systems, and a Knoop hardness of 780, which is about twice that of magnesium fluoride. This hardness makes germanium windows ideal for demanding infrared applications requiring rugged optics.

• Clear Aperture (%): 90.00
• Dimensional Tolerance (mm): +0.0/-0.1
• Coefficient of Thermal Expansion CTE (10-6/°C): 6.1
• Density (g/cm3): 5.33
• Index of Refraction nd: 4.002 @ 11µm
• Knoop Hardness (kg/mm2): 780.00
• Parallelism (arcmin): <1
• Poisson’s Ratio: 0.28
• Substrate: Germanium (Ge)
• Surface Quality: 60-40
• Thickness Tolerance (mm): ±0.1

Additional Coating Options

Germanium windows are available with three different anti-reflective coatings to cover specific application areas:

• 3–5 μm: Suitable for mid-infrared applications.
• 3–12 μm: Optimized for broadband multispectral applications.
• 8–12 μm: Specifically designed for thermal imaging.

The germanium window shown in the picture has a diameter of 55 mm, a thickness of 1 mm, and is coated on both surfaces with an AR coating in the range of 7–14 μm. The average transmission (Tavg) is over 95%.

Applications

Germanium windows are used in a wide range of high-tech applications. Typical use cases include:

• Infrared Optics: Due to their high transmission in the infrared range, they are ideal for IR spectroscopy and other IR-based systems.
• Thermal Imaging: Germanium windows enable precise thermal imaging in areas such as surveillance, medicine, and industrial inspections.
• Multispectral Applications: The broadband coating makes them a flexible choice for applications covering multiple wavelength ranges.

Advantages of Germanium Windows

Using germanium windows offers numerous advantages:

• High Transmission: Thanks to special AR coatings, they achieve a transmission of over 95% in the respective wavelength ranges.
• Durability: The high Knoop hardness of 780 makes them resistant to scratches and mechanical stress.
• Broad Application Range: With various coating options, they can be optimized for different wavelength ranges.
• Longevity: Germanium windows are durable and provide reliable performance in harsh environments.

In summary, germanium windows offer a combination of high optical performance, durability, and flexibility, making them a preferred choice for demanding infrared and thermal imaging applications.