Specifications:
| Materials | Optical grade germanium single crystals | Diameter Range | ~300mm |
| Diameter Tolerance | -0.01/-0.03mm | Thickness Tolerance | +/-0.03mm |
| Surface Quality | 60/40 S/D | Frings (N) | 3 |
| Irregularity (delta N) | 1 | Centration | 3' |
| Chamfer | 0.1-0.3mmx45 degree | Coatings | AR/AR@7-14micro DLC/AR@7-14micro BBAR/BBAR@3-12 micro See coating curves below |
Physical and Optical Properties:
| Transmission Range | 1.8 to 23 μm | Refractive Index | 4.0026 at 11 μm (1)(2) |
| Reflection Loss | 53% at 11 μm (2 surfaces) | Absorption Coefficient | <0.027 cm-1@ 10.6 μm |
| Reststrahlen Peak | n/a | dn/dT | 396 x 10-6 /°C (2)(6) |
| dn/dμ = 0 | Almost constant | Density | 5.33 g/cc |
| Melting Point | 936 °C (3) | Thermal Conductivity | 58.61 W m-1 K-1 at 293K (6) |
| Thermal Expansion | 6.1 x 10-6/°C at 298K (3)(4)(6) | Hardness | Knoop 780 |
| Specific Heat Capacity | 310 J Kg-1 K-1 (3) | Dielectric Constant | 16.6 at 9.37 GHz at 300K |
| Youngs Modulus (E) | 102.7 GPa (4) (5) | Shear Modulus (G) | 67 GPa (4) (5) |
| Bulk Modulus (K) | 77.2 GPa (4) | Elastic Coefficients | C11=129; C12=48.3; C44=67.1 (5) |
| Apparent Elastic Limit | 89.6 MPa (13000 psi) | Poisson Ratio | 0.28 (4) (5) |
| Solubility | Insoluble in water | Molecular Weight | 72.59 |
| Class/Structure | Cubic Diamond, Fd3m |
Germanium lenses (Ge lenses) are commonly used in IR imaging systems operating in the 2 µm to 16 µm spectral range, covering the LWIR (8-12μm) and MWIR (3-5μm) thermal imaging applications. Germanium (Ge) is a relatively hard, high-density, IR transmitting material that blocks UV and VIS wavelengths but transmits IR spectrum greater than 2µm. Ge has the highest refractive index of commonly available IR transmitters and exhibits slight chromatic aberration due to low optical dispersion, this makes it desirable in aspects of lens design where its refractive index allows otherwise impossible specifications to be realized. Germanium can be Diamond or DLC coated to be utilized as the front optics in the lens group with surpassing toughness and reliability. Germanium is more rugged than other IR materials, but caution should be taken for high-temperature conditions where the material will become opaque in the IR realm as the temperature rises. Besides the generally spherical surface lenses, Hangzhou Shalom EO also provides the aspherical surface lenses made by SPDT (Single Point Diamond Turning) technique.
The optical transmission rate of Germanium is over 45% between 2-14μm at temperatures up to 45⁰C, with optimal transmission spectrum residing between 2-12μm, and after the anti-reflection coating process, average transmission could be leveraged to above 95%. However, the transmission of Ge lenses starts to degrade gradually at 100⁰C, then plummets more rapidly above 200⁰C. Exposure to higher temperatures can lead to catastrophic failure in the material so Germanium is unsuitable for use in high-temperature conditions. Additionally, its relatively high density should be considered where weight is an issue. Germanium has a Knoop Hardness of 780, slightly higher than GaAs, this trait contributes to the rugged nature of Germanium Lenses.
Hangzhou Shalom EO suppliers a series of custom Germanium Lenses including plano-convex, plano-concave, double convex, double concave, and meniscus Ge lenses, with the spherical and aspherical lens surface. Coating Options encompass Anti-reflection (AR), Broad Band AR (BBAR), and Diamond (DLC) coatings, which are selectable upon customers' request. The shapes, dimensions (diameters, up to 300mm), and coatings could all be tailored upon request.
Plano-convex Germanium Lenses have a positive focal length and are designed for light collimation. Plano-concave Ge Lenses are for expanding collimated lights and could be incorporated into beam expanders. Double Convex and Double Concave Lenses have two convex or concave planes. Meniscus Ge Lenses have one concave front and one convex front, and are excellent for reducing spherical aberrations. Aspheric Ge Lenses have a more complex profile and the advantage of eliminating spherical aberrations, therefore using an aspheric lens could even be cheaper than using lens assemblies. Hangzhou Shalom EO fabricates aspherical Ge lenses using SPDT (Single Point Diamond Turning) technique, which is an alternate manufacturing process exquisite for IR optics, in which a computer-controlled lathe uses a diamond tip to directly cut the desired profile.
Applications for Germanium Lenses:
Curves:
1.Transmission curve1, transmission of Ge windows with no coating
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2. Transmission curve for Ge windows with coating AR/AR of 3mm thickness
