Optical germanium : a complete guide

Germanium famously known as predicted by Mendeleev before its actual discovery in the end 19th century, is widely used in optics as a component material for IR applications.  It has also interesting semi-conductor properties that we will not discuss into details in this guide.

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Why is germanium a good optical material ?

While rather expensive (around USD/kg) Germanium offers interesting optical properties.

It has good transmission in the 8 to 14 µm range covering the thermal Infrared light, therefore it is a material of choice for thermal and night vision. It also shows a high refractive index which enable high open aperture angle optical component’s design.

Ge is also interesting for its low optical dispersion and therefore very low chromatic dispersion in it’s transmission spectrum.

Interestingly, due to its similar crystal structure to Diamond can be coated with a DLC layer (Diamond Like Carbon) which on top of reducing the reflection on it’s surface, adds a diamond strong protective layer. Finally Germanium is not transparent in the visible which can be useful to focus on IR light.

Unfortunately Germanium is sensible to high temperature with degradation of its optical properties over 100°C. For high temperature Mid-IR (8-13µm) alternative we recommend using BaF2 (! much lower transmission after 11µm) or ZnSe.

Germanium properties

Properties chart of optical germanium :

Transmission curve of germanium

Raw germanium have a relatively average transmission in the IR (less than 50%) but Anti-reflect or DLC treatments do improve a lot it’s transmittance.

What type of optical component can be made with germanium ?

Raw germanium material is rather hard but can still effectively be machined and polished into optical components:

• Optical windows : main usage of germanium with very good transmission in the IR and no transmission in the visible.  Often, germanium windows are strengthened with DLC coating on one face.
• Lenses : germanium lenses are common for usage in thermal and night vision objectives.
• Aspherical germanium lenses : designed and used for enhanced optical properties and for applications that need weight saving. Germanium can be diamond turned into high surface quality aspheres.
• Wafers : as a semi conductor material, like Silicon, it is widely used as a thin substrate for electronic component.
• Germanium is also frequently used as a dopant in optical fiber’s silica optical cores. Due to the high quantities of fibers used for data transfer in the world, this application amounts for 30% of the world’s total germanium consumption.

It is also worth noting that germanium is relatively more brittle that silicon and therefore should be handle with care during machining.

General specifications for windows and lenses

Above value are generic specifications for fabrication, specific better value may be available and need to be reviewed case by case.

Where to find a good germanium optical component supplier ?

Germanium optical components along with Silicon ones are made by precision optical suppliers usually focusing on IR applications. Due to high value of the material, specialization enables these companies less pressure on the stocks and lower risks of non-quality rejection.

Most of the raw material comes from China, Russia and USA being also important suppliers, quality material is available everywhere.

Don’t hesitate to ask SINOPTIX for more details or quote request for your component:

Getting into thermals can be confusing. Here are the SIX main considerations and specifications you want to watch out for and what they mean (ranked in order of most to least important):

• Device Format
• Core Size / Core Resolution
• Base Magnification
• Objective Lens Size
• NETD
• Pixel Pitch
  

Device Format

Thermal devices can be broken down into several major categories:

• Thermal Scopes
• Thermal Clip-Ons
• Thermal Monoculars
• Multi-Function Thermals
• Clip-On Thermal Imagers (COTI) & Fusion Devices

To determine which device format to consider, you will want to ask yourself the following questions:

• Do I need the ability to target and engage? If the answer is no, then consider a thermal monocular.
• Do I plan to have a dedicated rifle platform for a thermal scope? While many scope mounts claim to be return-to-zero, there can be some zero-shift when mounting and dismounting optics. If you want the ability to run your normal rifle platform but be able to quickly add or remove thermal capabilities on that platform, consider a thermal clip-on instead. These devices effectively "clip-on" to the front end-bell of your traditional day optic, giving you thermal capability. Some clip-ons are also available with a Picatinny base so you do not necessarily have to hang a thermal off your front end-bell.
• Do I want to head-mount my thermal? If the answer is yes, then consider a multi-function thermal imager.
• Do you want to add rapid thermal detection capability to your existing night vision device? Consider a Clip-On Thermal Imager. These devices are very intuitive and immediately highlight heat signatures but do not have the more sophisticated features stand-alone thermals have (eg: ability to zoom, multiple colour palettes, on-board recording, and are always 1x magnification).

Core Size / Core Resolution

99% of thermal devices are simply digital cameras with a fixed optical zoom. There are some devices that have an optical zoom (eg: Infiray RH50 Pro) but these are very rare. Zooming on a thermal is almost always done digitally. Because of this, you will want to start with the highest thermal core resolution as possible to give you the additional pixels required to zoom in. Most modern mid-to-high-end thermal devices in have a 640x512 core, with some new models with x cores released in . Some lower-end models will have 384 cores and below.

To visualize core size and why this is important, the graphic below depicts how each core size stacks up against each other.

As you can see, a 384x288 core is about 25% the size of a 640 core and therefore has less pixels to work with if you plan on doing a lot of digital zoom. 

A 384-core thermal isn't necessarily bad but the image could start to look pixelated sooner because there's less pixels to work with. Think about this like TVs - p vs 4K, or Standard Definition TVs vs. HD-TVs. The higher the core resolution, the more "clear" an image will look.

Here is a side-by-side of two devices (a 384-core on the left; and 640-core on the right) with the same base magnification, using 4x digital zoom:

A 384-core thermal device can still be very viable if you choose the correct base magnification that suits your needs. The images below show the same devices, at their base magnifications:

If you want to learn more, please visit our website Germanium Lenses.

Note that the 384-core thermal looks quite good in its native base magnification but the 640-core thermal will give you more image fidelity and flexibility when zooming in and out. If you do not require zooming flexibility and have a good understanding of the approximate ranges of the targets you are looking for, you could consider 384-core thermals as a viable cost-effective option if you choose the appropriate base magnification.

Base Magnification

Choosing the correct base optical magnification becomes more critical with lower-resolution (384-cores and below) because the image breaks down faster when zooming digitally past 2x. With the example above, you can see that the 640-core thermal still gives a very usable image at 4x on top of the base optical magnification. 

Multi-Function Imagers - Most multi-function devices (such as the Jerry-YM 2.0, PFalcon640+, etc), in order to be usable as a helmet-mounted dual-band solution, need to have a 1x base magnification. Otherwise, users will become quickly disoriented with their night vision monocular at 1x and their thermal at 2x or beyond. Because of this, multi-function thermals typically work best when paired with an LPVO (Low Powered Variable Optic) such as a Vortex Razor HD 1-6x or an ATAC-R 1-8x. When used as a thermal clip-on, the LPVO is effectively zooming in on the back of the screen on the thermal device. From our experience, the limit of usability is around 6x but could be pushed to 8x if the target size is sufficiently large. 

Here is the image of the PFalcon640+ V2 in front of an Eotech Vudu 1-6x24 LPVO:

Generally speaking, multi-function thermal imagers, because their base magnification is 1x has a maximum effective range of 300m.

Thermal Scopes and Clip-Ons - Most thermal scopes will have a base magnification between 2-4x and choosing the correct base magnification is important to get the most of your thermal. Generally speaking, using an example of spotting a coyote:

• 2x base magnification has a max effective range of about 400m
• 3x base magnification has a max effective range of about 500m
• 4x base magnification has a max effective range of about 600m

Note that effective range is defined as the ability to accurately target vitals and is not the same as raw detection range, which could be up to 2.6km for some devices. If your target size is larger, then the max effective range will increase accordingly.

Image below of coyote from a 4x base magnification 640-core thermal, target is approximately 150 yards away.

Objective Lens Size

Objective lens size is the single most important factor in determining detection range. The larger the objective lens, the farther you can detect.

Generally speaking, here are the most common lens size vs. detection ranges:

• 25mm - 1.3km
• 30mm - 1.6km
• 35mm - 1.8km
• 50mm - 2.6km

Note that DETECTION range is not the same as IDENTIFICATION range.

Here is a simple chart showing the industry standard definitions of Detection Recognition and Identification (DRI)

The larger the objective lens, typically the more expensive the device (due to the larger germanium lens required). As the lens size increases, as do the size, weight and cost of the device. You will want to consider the ranges and distances in your specific environment and choose the appropriate objective lens size accordingly. 

NETD

NETD or "Noise Equivalent Temperature Difference" is a measurement of how well a thermal core is able to distinguish between very small differences in thermal signature. This can be interpreted as how "noisy" a thermal image is. See image below:

The lower the NETD, the less noisy the thermal image will be. Today's thermal devices can range from 12mK to 40mK. Generally speaking, a difference of 5mK or less is near undetectable but larger differences can be more apparent, especially if you add today's advanced image processing algorithms.

Pixel Pitch

Pixel Pitch is the size of the pixels in the thermal core. Generally speaking, the larger the pixel, the more sensitive the thermal core is, with the industry standard (as of the time of this article - June ) being 17µm and 12µm. Many thermal manufacturers have moved to the 12µm cores because a smaller core requires a smaller objective lens, smaller housing, and a cheaper device, making them more attractive to the consumer. 

Pixel pitch is less important than other specs because most thermal device manufacturers are choosing the optimal pixel pitch size for the application and objective lens size. Most new thermal devices released today will have 12µm cores, for the reasons above.

Conclusion

Thermal terminology can be daunting at first but a rudimentary understanding of the concepts above will help to narrow down the search for the best thermal device for your application. Cold Harbour Supply stocks the largest assortment of thermal devices in Canada and are always here to assist you through the procurement process. First understanding each individual or group's requirements is the key to having the best thermal tool at-hand when the need arises. 

The company is the world’s best Germanium Optics supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.