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What are Germanium lenses and, if a piece of equipment you own requires them, where can you buy replacement lenses when you need them?
What are Germanium lenses? ' These are a special type of lens that look like they are completely black to the human eye. This is because they are made from a special infrared material that does not allow specific rays to pass through it.
They also tend to be some of the most expensive lenses available. This is due to the material needing to be polished to a high level, which can be quite expensive to do.
These types of lenses must also be covered with a coat of anti-reflection agent before use, as they are highly reflective.
Who sells Germanium lenses? ' Many lens manufacturers make them, but they do differ widely in the prices they charge. If you need to order custom Germanium lenses, the prices can be quite high.
The most popular place to buy Germanium lenses in recent years has been China. This is due to some Chinese lens manufacturers making extremely high-quality lenses. Due to the labor costs in China, however, and the cost of raw materials, this can often be done at much lower prices than a comparable western manufacturer could provide.
Buying affordable Germanium lenses ' If the cost of buying these lenses is important to you, then you should definitely look into buying them from a Chinese company.
There are several large lens companies in China with a strong Internet presence. Look for their websites, read about the germanium lens they sell and then contact two or three of them for quotes.
All the larger companies have English-speaking staff, and can often provide you with custom Germanium lenses at low prices and in time frames much faster than comparable western companies.
Germanium (Ge) is an IR material with physical properties that make it ideal for aerospace and defense applications, among others. Using advanced crystal growth methods, Germanium boules can be produced, from which germanium lens and domes are manufactured. Because Germanium is an expensive material, it is imperative to generate blanks from the boules in a cost-effective manner.
By utilizing OptiPro 5-axis optical grinding machines, the method of scoop-grinding can effectively remove large pieces of material when creating dome blanks that would then be recycled. Once scoop-ground, the domes are processed using the same technology for spherical lenses: spherically generating the surface to the desired radius, then CNC high speed polishing the surface to desired irregularity and surface roughness.
Below is the process for manufacturing germanium hemispherical domes out of a boule sized 4.5' wide by 9.0' tall (see figure 1).
CNC scoop machining, or scoop-grinding, was developed with recycling and material conservation in mind (see figure 2).
This machine operation enables you to remove bulk pieces from a raw cylinder of material in a manner that allows you to recycle the material being removed. What remains after removing the bulk pieces of material is a complete near-net dome blank (see figures 3 and 4).
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After scoop-grinding germanium dome blanks, the scrap pieces can then be recycled to save on costs and maximize return on investment. To examine the recyclable material net yield, it is necessary to measure and record the combined weight of all the scrap pieces and compare that to the weight of an unprocessed germanium boule. As shown in Figure 5, a digital scale is used to weigh the scrap material. With this information, the percentage of material that was recovered for recycling can be calculated.
Prior to high speed polishing the germanium domes, the surfaces are spherically generated on a 5-axis optical grinding machine (see figure 6). This step is crucial for the following reasons:
Once the germanium dome is spherically generated to the desired specifications, the convex and concave surfaces can be high speed polished on a machine such as the PRO 160P+ using a known recipe of consumables and polishing parameters (see figure 7).
The finished germanium dome surfaces can be seen below in figures 8 and 9.
To ensure that each germanium dome has met the required specifications, several different metrology tools are required.
The following instruments were used to measure the finished dome surfaces:
OptiPro's UltraSurf' is a 4 or 5-Axis non-contact metrology system capable of measuring everything from symmetric, asymmetric and freeform optical components. With the availability of numerous compatible optical sensors, the UltraSurf' can measure a variety of materials including metals, ceramics, crystals and optical glasses.
OptiPro's PRO Tower' is a vertical interferometer system for measuring both the radius and wave front error of spherical and plano optics of up to 300mm in diameter. With an optional CGH (Computer Generated Hologram) stage available, the PRO Tower' can accommodate aspheric optics as well.
Final metrology of two germanium domes high speed polished at OptiPro are shown in the table below and in figures 10, 11, 12, 13. The PRO Tower' was used with an F0.65 objective to measure the concave (R1) dome surfaces. This allowed capture of the full clear aperture of the concave surface. The UltraSurf 4X 300 was used to capture 100% of the polished convex (R2) surfaces.
Dome # R1 Radius (mm) R2 Radius (mm) R1 Irregularity (FR) R2 Irregularity (FR) CT (mm) 1 44.458 50.796 3.285 2.688 6.361 2 44.470 50.797 3.001 2.311 6.349
Multiple surface roughness measurements were performed on both of the finished domes showing an average value of 24Å RMS. Figures 14 and 15 shows a couple examples of the surface roughness on each dome.
OptiPro has developed a turnkey solution to manufacture hemispherical germanium domes utilizing the latest optical manufacturing technology. By using the scoop-grinding method in the manufacturing process of germanium domes, a large portion of the germanium material that is typically lost in the conventional manufacturing process would be subsequently salvaged and recycled to save on cost.
OptiPro's CNC optical grinding machines can generate convex and concave spherical surfaces to the desired radius with minimal subsurface damage. To achieve optimal irregularity and surface roughness, high speed polishers like the PRO 160P+ are ideal solutions when a known recipe of consumables and parameters are utilized.
Once the dome surfaces are polished, it is critical to use reliable metrology instruments to generate accurate measurement data. OptiPro's PRO Tower vertical interferometer and UltraSurf non-contact metrology system are industry-standard methods to measure irregularity, surface roughness, and other important specifications that will ensure optimal performance. Because germanium lenses and domes are often utilized in critical applications such as aerospace and defense, performance is paramount.
The same OptiPro technology utilized to generate and polish germanium hemispherical domes can be applied to germanium lenses as well. If you have a job that involves prototype through high volume production of a germanium lens or dome, reach out to OptiPro today and let us optimize your manufacturing process.
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