Germanium is used widely for lenses and windows in thermal imaging and FLIR applications. Its high index of refraction (about 4) makes it of particular interest. Useful transmission range of Germanium windows is from 2 to 13mkm. Germanium is opaque in the visible. Germanium has the property of thermal runaway, meaning that the transmission decreases as temperature increases. As such, Germanium window should be used at temperatures below 100°C. The Knoop Hardness of Germanium (780) is approximately twice that of Magnesium Fluoride, making it ideal for IR applications requiring rugged optics. Germanium’s high density (5.33 g/cm^3) should be considered when designing for weight-sensitive systems.
Custom germanium windows and germanium lenses are available in any size and configuration. Anti-reflection coatings can be applied for 2-14µm or 8-12µm (BBAR coating or DLC/BBAR) depending on your custom application.
| Germanium properties | |
| Chemical Formula | Ge |
| Crystal Form | Poly or Single Crystal |
| Crystal Class | Cubic |
| Resistivity, Ohm*cm | 5-40 |
| Lattice Constant, Å | 5.66 |
| Molecular Weight | 72.60 |
| Density, g/cm3 at 300 K | 5.33 |
| Dielectric Constant for 9.37 x 109 Hz at 300 K | 16.6 |
| Melting Temperature, K | 1210 |
| Thermal Conductivity, W/(m K) at 293 K | 59 |
| Thermal Expansion, 1/K at 298 K | 6.1 x 10-6 |
| Specific Heat, cal/(g K) at 273-373 K | 0.074 |
| Debye Temperature, K | 370 |
| Bandgap, eV | 0.67 |
| Solubility in water | None |
| Knoop Hardness, kg/mm2 | 800 |
| Mohs Hardness | 6.3 |
| Young’s Modulus, GPa | 102.66 |
| Shear Modulus, GPa | 67.04 |
| Bulk Modulus, GPa | 77.86 |
| Poisson’s Ratio | 0.278 |
Refractive index of Germanium
| µm | No | µm | No | µm | No |
| 2.058 | 4.102 | 2.153 | 4.0919 | 2.313 | 4.0786 |
| 2.437 | 4.0708 | 2.577 | 4.0609 | 2.714 | 4.0562 |
| 2.998 | 4.0452 | 3.303 | 4.0369 | 4.258 | 4.0216 |
| 4.866 | 4.017 | 6.238 | 4.0094 | 8.660 | 4.0043 |
| 9.720 | 4.0034 | 11.04 | 4.0026 | 12.00 | 4.0023 |
| 13.02 | 4.0021 |
