Frequently, DLC coatings are often paired with Broadband AR Coatings (BBAR) to not only up the mechanical performance but also the transmission properties. Typically, a window is coated on one side with DLC and the other side with BBAR (typically in the 7-14μm range to boost both of these properties in tandem. This combination can increase the transmission to well over 85% in the target wavelength range. Optical windows treated in this manner will pass a variety of tests including MIL-C-675-C, MIL-STD-810 and APCM-01071.00001 and can survive salt water, humidity, high and low temperatures and hard smacks.

While most microscope objectives are designed to work with air between the objective and cover glass, objectives lenses designed for higher NA and greater magnification sometimes use an alternate immersion medium. For instance, a typical oil immersion object is meant to be used with an oil with refractive index of 1.51.

One of the more common applications we see at Firebird Optics for germanium windows is in low power CO2 laser systems. With a Laser Induced Damage Threshold (LIDT) of 10 J/cm2, germanium windows are not suited for high power or continuous wave (CW) lasers. Part of the reason for this is higher powered lasers cause temperature increases, dramatically dropping transmission properties over 100ºC and eventually damaging the substrate itself once temperatures near 600ºC are reached.

There are some important specifications and terminology you’ll want to be aware of when designing a microscope or ordering microscope objectives. Here is a list of key terminology.

Refractive objectives are so-called because the elements bend or refract light as it passes through the system. They are well suited to machine vision applications, as they can provide high resolution imaging of very small objects or ultra fine details. Each element within a refractive element is typically coated with an anti-reflective coating.

One common application for germanium is in night vision goggles. For these types of applications, both the inside and outside surfaces of the germanium is polished to a mirror finish and coated with multi-layered thin film filters to reduce any reflection of IR light. This enables optimal IR transmission essential to night vision/thermal imaging. As these windows will have excellent sensitivity at long range, this enables warfighters to retain long stand-off distances increasing safety and mission-effectiveness. Germanium is a literal life-saver.

The parfocal length of a microscope is defined as the distance between the object being studied and the objective mounting plane.

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…

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 1150 it is also harder and less brittle. Silicon also costs less making it a better candidate in several applications.

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Numerical aperture NA denotes the light acceptance angle. Where θ is the maximum 1/2 acceptance ray angle of the objective and n is the index of refraction of the immersive medium, the NA can be denoted by

The field of view (FOV) of a microscope is simply the area of the object that can be imaged at any given time. For an infinity-corrected objective, this will be determined by the objective magnification and focal length of the tube lens. Where a camera is used the FOV  also depends on sensor size.

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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.

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

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.

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.

A microscope is an optical device designed to magnify the image of an object, enabling details indiscernible to the human eye to be differentiated. A microscope may project the image onto the human eye or onto a camera or video device.

A basic compound microscope could consist of just two elements acting in relay, the objective and the eyepiece. The objective relays a real image to the eyepiece, while magnifying that image anywhere from 4-100x.  The eyepiece magnifies the real image received typically by another 10x, and conveys a virtual image to the sensor.

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.

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

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.

At Avantier we produce high quality microscope objectives lenses, ocular lenses, and other imaging systems. We are also able to provide custom designed optical lenses as needed. Chromatic focus shift, working distance, image quality, lens mount, field of view, and antireflective coatings are just a few of the parameters we can work with to create an ideal objective for your application. Contact us today to learn more about how we can help you meet your goals.

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In modern microscopes, neither the eyepiece nor the microscope objective is a simple lens. Instead, a combination of carefully chosen optical components work together to create a high quality magnified image. A basic compound microscope can magnify up to about 1000x. If you need higher magnification, you may wish to use an electron microscope, which can magnify up to a million times.

The working distance of a microscope is defined as the free distance between the objective lens and the object being studied. Low magnification objective lenses have a long working distance.

A reflective objective works by reflecting light rather than bending it. Primary and secondary mirror systems both magnify and relay the image of the object being studied. While reflective objectives are not as widely used as refractive objectives, they offer many benefits. They can work deeper in the UV or IR spectral regions, and they are not plagued with the same aberrations as refractive objectives. As a result, they tend to offer better resolving power.

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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.

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.

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.

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There are two major specifications for a microscope: the magnification power and the resolution. The magnification tells us how much larger the image is made to appear. The resolution tells us how far away two points must be to  be distinguishable. The smaller the resolution, the larger the resolving power of the microscope. The highest resolution you can get with a light microscope is 0.2 microns (0.2 microns), but this depends on the quality of both the objective and eyepiece.

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.

On the other hand, an AR-coated germanium will feel right at home in a low power pulsed laser setup. One particularly noteworthy application is in quantum cascade lasers (QC), which is used in high-end materials science. Quantum cascades are used by such institutions as the Max Planck Institute of Quantum Optics in Garching to produce ribbons and strip structures as well as to produce new materials for use in medical applications. Pretty cool stuff!

Most microscopes rely on background illumination such as daylight or a lightbulb rather than a dedicated light source. In brightfield illumination (also known as Koehler illumination), two convex lenses, a collector lens and a condenser lens,  are placed so as to saturate the specimen with external light admitted into the microscope from behind. This provides a bright, even, steady light throughout the system.

In fact, at an eye-popping Knoop Hardness of 2000kg/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.

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.

Both the objective lens and the eyepiece also contribute to the overall magnification of the system. If an objective lens magnifies the object by 10x and the eyepiece by 2x, the microscope will magnify the object by 20. If the microscope lens magnifies the object by 10x and the eyepiece by 10x, the microscope will magnify the object by 100x. This multiplicative relationship is the key to the power of microscopes, and the prime reason they perform so much better than simply magnifying glasses.

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Microscope objective lenses are typically the most complex part of a microscope.  Most microscopes will have three or four objectives lenses, mounted on a turntable for ease of use. A scanning objective lens will provide 4x magnification,  a low power magnification lens will provide magnification of 10x, and a high power objective offers 40x magnification. For high magnification, you will need to use oil immersion objectives. These can provide up to 50x, 60x, or 100x magnification and increase the resolving power of the microscope, but they cannot be used on live specimens.

While a magnifying glass consists of just one lens element and can magnify any element placed within its focal length, a compound lens, by definition, contains multiple lens elements. A relay lens system is used to convey the image of the object to the eye or, in some cases, to camera and video sensors.

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

An microscope objective  may be either reflective or refractive. It may also be either finite conjugate or infinite conjugate.

get wrecked! damage done to germanium windows by high power lasers. source: http://www.mdpi.com/2076-3417/10/10/3578/htm

Although today’s microscopes are usually far more powerful than the microscopes used historically, they are used for much the same purpose: viewing objects that would otherwise be indiscernible to the human eye.  Here we’ll start with a basic compound microscope and go on to explore the components and function of larger more complex microscopes. We’ll also take an in-depth look at one of the key parts of a microscope, the objective lens.

An uncoated germanium window is a decent IR generalist but specific optical coatings can really kick things up a notch. Diamond-Like Carbon (DLC) coatings can make this already hard material even more resistance to severe abrasions, environmental damage, mechanical strikes, thermal shock and whole other host of adversities.

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The optical performance of an objective is dependent largely on the optical aberration correction, and these corrections are also central to image quality and measurement accuracy. Objective lenses are classified as achromat, plan achromat, plan semi apochromat, plan apochromat, and super apochromat depending on the degree of correction.

The eyepiece or ocular lens is the part of the microscope closest to your eye when you bend over to look at a specimen. An eyepiece usually consists of two lenses: a field lens and an eye lens. If a larger field of view is required, a more complex eyepiece  that increases the field of view can be used instead.

As you’d imagine, since germanium tags along with warfighters, it needs to be durable in all adverse conditions from desert, sea, high altitude, etc. The DLC coatings can take a serious beating with temperatures ranging from -80ºF to 160ºF, 24 hours of continuous sea spray, ocean immersion for over 24 hours and being immersed in a sandstorm. Additionally, the coatings can withstand heavy mechanical damage and chemical attacks.

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!

A basic achromatic objective is a refractive objective that consists of just an achromatic lens and a meniscus lens, mounted within appropriate housing. The design is meant to limit the effects of chromatic and spherical aberration  as they bring two wavelengths of light to focus in the same plane. Plan Apochromat objectives can be much more complex with up to fifteen elements. They can be quite expensive, as would be expected from their complexity.

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.

Germanium windows are optical windows that are completely impermeable to UV and VIS light giving them a dark, metallic appearance to the naked eye. However, when it comes to the IR range this is where germanium truly shines with an excellent, broad transmission range from 2-16μm making it an ideal candidate for Mid-Wave-IR (MWIR) and Long-Wave IR (LWIR) applications. The sharp transmission cut-off before 2µm also enables germanium to be used as a long pass filter, only transmitting wavelengths in excess of 2µm and fully blocking everything before it.

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.

Historically microscopes were simple devices composed of two elements. Like a magnifying glass today, they produced a larger image of an object placed within the field of view. Today, microscopes are usually complex assemblies that include an array of lenses, filters, polarizers, and beamsplitters. Illumination is arranged to provide enough light for a clear image, and sensors are used to ‘see’ the object.

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.