EEVblog Electronics Community Forum
Products => Thermal Imaging => Topic started by: thermel on October 27, 2023, 03:27:31 pm
-
I thought germanium lens is a default/standard part in every thermal camera. Why this thermal camera need additional germanium lens?
https://www.pulsar-nv.com/glo/products/33/thermal-imaging-monoculars/helion-version-2/ (https://www.pulsar-nv.com/glo/products/33/thermal-imaging-monoculars/helion-version-2/)
Please explain, thank you.
-
There is an objective lens at the end, and a much smaller focusing lens near the sensor. The distance between the sensor and the focusing lens and the distance between the focusing lens and the objective lens can be adjusted for different magnification while still keeping the image in focus. (It is likely there are more than two lens elements in there, just with the objective lens assembly a separate, replaceable element. The out-facing ones do get scratched and whatnot in real-world use.)
As far as I understand, this is similar arrangement to any optical camera within the visible spectrum. As ordinary glass is not transparent to infrared, germanium glass (GeO₂) which is (and has suitable mechanical properties for lenses), is used instead.
-
The brochure you linked mentions interchangeable Ge lenses: this is similar to a normal SLR optical camera.
What are you referencing about "doesn't use Germanium lens"?
A camera typically uses only an objective lens to focus the object plane onto the sensor or film, but a telescope or binoculars uses two lenses (objective and eyepiece).
-
Germanium lenses are pure Ge and not GeO2 glass. It is well possibly that they use 2 lenses, one close to the sensor and one external. This can help to get away with less material, though 2 additional surfaces.
There are also alternative materials (e.g. ZnS) to germanium, though not without there problems (e.g. less robust, absorbtion, lower refractive index). This is mainly for cost savings as germanium is relative expensive.
-
Germanium lenses are pure Ge and not GeO2 glass.
GeO₂ (aka germanium dioxide, aka germanium glass oxide) lenses do exist (https://www.vitalchem.com/product/germanium).
Whether the lenses referred to in this thread are germanium or germanium dioxide, I have no idea. The main difference is that GeO₂ is also transparent to visible light, whereas pure Ge (also called reduced Ge) is opaque to visible light. Both are transparent to infrared, and used in lenses.
That said, for a thermal camera (as opposed to IR-sensitive cameras), a pure Ge lens sounds more likely, as then visible light would not affect the image.
-
There are many materials used infrared cameras and instruments. An ideal material would be cheap, resistant to abrasion, immune to moisture, have zero dispersion, low transmission losses at all wavelengths, and several other properties which may depend on application. Ge for example has a very high index of refraction which allows compact lenses of low curvature, but dramatically increases reflection losses. The reflection losses can be mitigated but not eliminated by coatings.
No material checks all these boxes so each application chooses a material best suited to it. Ge, ZnS, ZnSe and polyethylene are common choices. Less common choices include NaCl and diamond. There are many more that have been.researched but not commercialized.
-
There are no known great materials for making thermal camera lenses. Germanium is generally considered best, but costly. However, even Germanium sucks, as it doesn't need to get very warm before it becomes cloudy at thermal frequencies. This can make it entirely useless for lenses in things like fire fighting equipment, where the equipment is definitely going to get warm when its needed the most. For something like a helicopter or jet fighter thermal camera the critical bits are going to be cooled by liquid nitrogen or some type of refrigeration, so this can cool the lens while its also cooling the sensor.
-
https://www.tydexoptics.com/materials1/for_transmission_optics/germanium/ (https://www.tydexoptics.com/materials1/for_transmission_optics/germanium/)
This link provides a good list of most the properties of germanium lenses the are found important in optical applications. This vendor also deals in a pretty wide variety of other optical materials.