Germanium lenses are optical lenses made of polished germanium. Germanium Optics are rugged and resistant to corrosion, and are ideal for harsh environments and applications where there is constant exposure to the elements.

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At Shanghai Optics we manufacture a wide variety of germanium lenses with various anti reflection coatings. Spherical plano-concave, p-convex, concave-convex and aspheric lenses are all available in a wide variety of sizes. We are also able to accommodate special orders should you need a size, shape or AR coating.

Our germanium lenses have a high refractive index—the highest refractive index among readily available IR transmitters. It also exhibits minimal chromatic aberration due to low dispersion. Because of these properties, Ge lenses are ideal for use in infrared (FLIR and FTIR) field applications. Other applications include thermal imaging systems and precision analytic instruments which may be used in harsh environments.

It is the special hardness of germanium that gives it its unique ruggedness; Ge has a Knoop hardness of 780. It is a high density material, at 5.33 g/cm3, and allowances must be made for this weight if you are designing a weight sensitive system.

Germanium is also subject to thermal runaway. What this means is that the transmission will decrease as temperature increases, and the lens becomes opaque. Because of this thermal runaway effect, germanium lenses should only be used at temperatures below 100°C.

Germanium has optimal transmission (over 45%) in the 2-12 μm band, which makes it most useful for mid-infrared light.  With anti-reflective coatings, transmission can be brought up to about 95%.

Types of Germanium Lenses

Shanghai Optics manufactures custom spherical plano-concave, p-convex, concave-convex and aspheric germanium optics.

Our spherical plano-concave germanium lenses are primarily used to diverge a beam from a collimated light source, or to increase the focal length of an optical system. They may be used in beam expanders, and can also be used to balance out aberrations which were introduced by other lenses in an optical arrangement.

Plano-convex Ge lenses are flat on one side and convex on the other. These lenses have a positive focal length, and are often used for light collimation. They can also be used in focusing applications, and have widespread use in various fields, from science to industry to defense. We recommend these lenses be oriented so that the flat side faces the desired focal plane.

Our concave convex germanium lenses have one concave side (depressed inward) and one convex side (bulging outwards).

Our aspheric lenses have a more complex front. The curvature gradually changes as you move from the center of the lens out to the edge. Our aspheric Ge lenses are most often used to eliminate spherical aberration in an optical system. Adding an aspheric lens into an optical assembly has the potential to greatly simplify the system, and even the addition of relatively heavy germanium lens may actually decrease the systems weight.

Germanium (Ge) is a shiny, hard element with a Knoop Hardness of 780kg/mm2 making it, along with its far stronger sister sapphire, the default candidate for applications where the environment is likely to put a beating on the material. For a full breakdown of how to wade through all of the major optical window choices check our optical window guide.

In fact, at an eye-popping Knoop Hardness of kg/mm2, one might wonder, why wouldn’t I just use sapphire windows instead of germanium? While sapphire is the undisputed champion of robust optics in the UV/VIS and mid-IR with a wavelength range that dips into the far UV range at 150nm, it can only be used up to 4.5µm. This leaves germanium as the best and only choice for brutally tough IR applications. This is typically why you will see germanium windows and lenses serving in places as inhospitable as outer space, battlefields and in the middle of high-powered CO2 laser systems. More on this later.

Another consideration is germanium’s density. At 5.33g/cm3 it does not float like a butterfly and sting like a bee. It’s heavy. Quite heavy and this will need to be considered when designing weight-sensitive systems.

One cautionary detail to note is that your germanium window will not appreciate being exposed to high temperatures. In fact, there is an inverse relationship to temperature and transmission when it comes to this material. As your temperature goes up, the transmission properties will drop precipitously, a property known as thermal runaway. Anything over 100ºC is not recommended. By the time it reaches 200ºC it is nearly opaque at all wavelengths. If you need a high temperature window, you’re better suited to stick with a material like MgF2, YAG or our old friend sapphire.

Since germanium windows have a high index of refraction of approximately 4.0 in the range of 2-16μm, transmission with minimal refraction is guaranteed but without any additional coatings only around 50% of the beam is able to pass through. Those are rookie numbers and in most applications we’ll need to boost our window signals via various coatings. While this may be a disadvantage for optical windows this property comes in handy for novel lens designs and ATR optics where refraction is desired.

Optical Coating Options

If high transmission is the only consideration, you’d be best served to double coat your windows with BBAR over particular wavelength ranges and you can see transmissions all the way up to 99%. Now we’re talking!

Various BBAR coatings typically requested are 3-5µm, 3-12µm, 2-14µm and 8-12µm though we can do even more customization. You can fill out our custom request form and see what Firebird Optics can do for your application.

Firebird Optics manufactures these types of windows in various geometries, coatings and can also produce lenses, prisms and various other optical components from germanium. Now back to your regularly scheduled programming…

Why Germanium and not Silicon?

We’ve already explored why you would pick germanium over sapphire but why would you pick germanium over silicon or vice versa?

Silicon (Si) is germanium’s next door neighbor who frequently borrows sugar. Both have an atomic number of 14, absorb visible light and have sharp cut-offs making them also function as long pass filters. In addition, there is similarity with their transmission ranges with silicon transmitting from 1.2-7µm. However, this is where they begin diverging. Silicon is far lighter with a density of 2.33g/cm3, making it ideal for weight-sensitive applications and less than half the density of germanium. Moreover, it is more thermally resistant than germanium and with a Knoop Hardness of it is also harder and less brittle. Silicon also costs less making it a better candidate in several applications.

Germanium holds the high ground when it comes to transmission range getting that additional coverage from 7-16µm, which silicon lacks. On top of this, germanium has higher electrical conductivity making it a more suitable candidate as an optical component in laser systems.

Summary of Advantages and Disadvantages of Germanium

Advantages

• No water solubility- advantage over salty windows such as KBr and NaCl that are extremely hygroscopic.

• Good spectral properties in the IR range.

• Sharp transmission cut-off at 2µm enables use as optical filter.

• Excellent mechanical and chemical resistance.

• High refractive index enables excellent performance for lenses, ATR optics or beamsplitters. For optical windows, AR coatings can be added to boost performance.

• Relatively low cost compared to similar materials.

Disadvantages

• Dense, heavy material not ideal for light-weight applications.

• Low resistance to high temperatures.

• High index of refraction makes an uncoated window less than ideal for transmission.

Cleaning Germanium Windows

You definitely don’t want to breathe in germanium dust so care must be taken while handling these optics. Using gloves whenever possible and washing hands should be part of any standard operating procedure.

The best way to clean the windows is to use either ethanol, isopropyl alcohol, methanol, reagent-grade acetone or lint-free lens cloths. You can also use nitric acid but this material, while okay for germanium can corrode other optics or mounts that may be nearby in your system.

Alternatively, you can use liquid CO2 from a specialized nozzle for cleaning though this requires additional cost in terms of equipment. While you will get a better, more controlled cleaning and can be used for stubborn stains, care must be taken to work in a controlled moisture-free environment and we do not recommend this for typical end-users.

When not in use, we recommend storing your germanium windows wrapped in lens tissue with humidity below 30% and between 15 and 25ºC. While germanium is pretty tough stuff, these are the ideal conditions that will prolong the life of your window.

Germanium Window Applications

Now for some fun stuff. We hear from plenty of our customers where their germanium windows are seeing action. Here are some highlights:

Aerospace

Germanium ends up in some interesting places including defense aircraft navigation, reconnaissance and surveillance systems soaring in the stratosphere on an airplane or even as part of a satellite.

One very common scenario we see germanium utilized is inside high and low speed wind tunnels for jet propulsion studies. Typically, these windows are much larger than stock configurations reaching sizes in excess of 190-200mm.

Germanium makes a good electromagnetic interference (EMI) shielding material and can shield IR systems on planes from other nearby signals that would render the system ineffective. This effectively creates an IR Faraday cage or as we like to refer to it, an aerospace tin foil hat. Typical resistance for EMI-grade germanium is approximately 4 Ohm per cm but this depends on the required level of spurious signal suppression. A germanium window made to these specs can effectively short out any errant signals and keep the IR system running well.