EEVblog Electronics Community Forum
Products => Thermal Imaging => Topic started by: Logan on December 27, 2020, 09:52:52 am
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Here I found some charts as attached pictures:
(https://i.postimg.cc/2jQ82k4Q/5-E3-C5-F87-E9-AF-4-D25-975-D-A989-DB606-D6-F.jpg)
(https://i.postimg.cc/vTtYBhQq/9-E1-E842-C-1-F51-47-F2-98-D1-8626-E7-DB6639.jpg)
(https://i.postimg.cc/BQJZzrgd/EE7-BD3-A2-EA7-D-4-D5-B-BF11-6811-F9-E7-FC3-F.jpg)
As you can see, they all have good transmission rate, but I never see them used as TIC lens, why?
I remember some of them will be damaged by moisture, is that the main reason?
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You are right, these are all salts: they dissolve in water.
They also have lower refractive index than germanium so it's not as easy to build the very high aperture lenses that you need to collect more light and fight diffraction.
They are also transparent in the visible, a property you might want as a first filter against everything not in the LWIR spectral window.
There might be other practical considerations like molding/tooling ability/brittleness.
However such salts are used for windows when you want something cheap, or broadband: KBr is used extensively together with FTIR spectroscopy.
In case of bare salt windows (without an anti moisture coating) you can even rework their surface with a wet tissue.
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An interesting page detailing several IR transmission materials. It includes prices :-+
Many look very promising for use in thermal imaging cameras but Monocrystal Germanium has been the material of choice for many years thanks to its relatively robust structure and optical performance. ZnSe and ZnS are to be found in thermal imaging camera lenses where they are deemed an advantage over a totally Germanium lens set.
https://eksmaoptics.com/optical-components/uv-and-ir-optics/?&gclid=Cj0KCQiA_qD_BRDiARIsANjZ2LBSt5tqlvC7-Ob8Vh84oZkJ22tC0jKsxz-WV3kuBm6UZUiG-AnNLdEaAnBsEALw_wcB
Some of the alternatives are fragile in nature and may have limited operational life. If you google them I feel sure the materials characteristics and vulnerabilities will be detailed. There is also their optical performance to consider. Broadband transmission does not mean that they are the best choice for a lens. The internal structure of the material can heavily influence imaging performance. This was the case with early Chalcogenide IR Glass materials used to make experimental IR lenses. The early material suffered masses of micro cracks within its structure and this adversely effected imaging performance. Development of the manufacturing process solved the micro fracture issue and we now have affordable, mouldable Chalcogenide IR glass lenses. Efforts have been ongoing for many years to reduce the production complexity and cost of IR lenses suitable for thermal imaging. All suitable materials were tested but Chalcogenide IR Glass lenses became the preferred option. Such ‘synthetic’ material lenses that could be moulded have been a real breakthrough in bringing affordable thermal imaging to the masses. As far as I know, Chalcogenide IR glass lenses ‘rule the roost’ in the budget end of the thermal imaging market and other materials are considered ‘inferior’ in terms of production cost or other drawbacks relating to material structure and longevity etc.
It is worth researching the invention of the Chalcogenide IR glass material for thermal imaging. The articles will likely provide an insight into the experiments and efforts to produce a material that is better than that available at the time.
In my view, Mono Crystal Germanium is still the King of thermal imaging lenses BUT it does have its issues ! Namely cost of production, relatively poor transmission unless AR coated, susceptibility to ambient temperature influence on transmission (as temperature rises above a certain point, transmission reduces relatively) It does offer an amazing refractive index though :)
You will find other materials used in IR inspection Windows and this may be due to production cost and the ability to dictate the use environment more than with a general use thermal camera. Such Windows may also be easily changed if degraded, unlike thermal camera lens elements.
Fraser
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An interesting read regarding Chalcogenide IR glass and what it offers the optical designer in the thermal imaging industry.
https://www.nature.com/articles/s41598-018-33824-x (https://www.nature.com/articles/s41598-018-33824-x)
Fraser
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More useful reading :)
https://www.sciencedirect.com/science/article/pii/S1631070517300361 (https://www.sciencedirect.com/science/article/pii/S1631070517300361)
Fraser
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Germaium has one (if not the) of the highest refractive index. It's great for this purpose. It has to be grown in single crystal and than machined. Not molded. You can even see the marks of that in the attached image.(will add later)
Isn't Cs radioactive? So it would cause more hot pixels?
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And now for something completely different ...... flat lenses !
https://www.pnas.org/content/116/43/21375 (https://www.pnas.org/content/116/43/21375)
Fraser
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Choosing the material for IR optics.......
https://www.photonics.com/Articles/Choosing_a_Material_for_Use_in_the_Infrared/a44935 (https://www.photonics.com/Articles/Choosing_a_Material_for_Use_in_the_Infrared/a44935)
As seen in the conclusion..... the selection of the materials used is often down to the lens element designer having a favourite material for the specific role.That preference is, of course, influenced by cost against delivered performance in the areas detailed.
Fraser
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Some of the known issues with materials that appear suitable for a thermal imaging lens..... based on transmission figure alone...... but may not be suitable for the role due to unfortunate vulnerabilities......
https://www.iriss.com/articles/what-type-of-lens-materials-are-used-in-infrared-inspection-windows/ (https://www.iriss.com/articles/what-type-of-lens-materials-are-used-in-infrared-inspection-windows/)
In the case of hygroscopic materials.... who wants their expensive thermal camera to need repair or recalibration if it gets damp ! That is a real possibility if a hydroscopic lens element is used in the optical block. The lens element would absorb moisture from the atmosphere over time and it’s transmission figure would change :--
Fraser
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And some more interesting reading for you :)
https://core.ac.uk/download/pdf/268638811.pdf
Fraser
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Thank you!
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A picture from my archive showing IR transmissive materials......
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Shining a light at the rear of the giant front element and having the camera near the focal point. It shows the tool marks(and other scratches).
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Isn't Cs radioactive? So it would cause more hot pixels?
There are radioactive Cs isotopes, but natural Cs is low radiation. It is more that potassium (the natural K40) is slightly radioactive. Same with thorium and uranium found in some special glasses with extra high refractive index in the optical range.
A high refractive index has advantages when a large aperture with a single lens is needed, but it also needs a good coating to reduce reflections. The higher the refraction the thinner the lens can be and thus more absorption can be tolerated. For a window a low refractive index is an advantage.
The alternative not used very much is reflective optics.
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Germaium has one (if not the) of the highest refractive index. It's great for this purpose. It has to be grown in single crystal and than machined. Not molded.
Some of the mouldable materials have some germanium content, others are Ge free.
Common mixes are various proportions of Ge / As / Se / Sb / Te
The refractive index is around 2.7
http://www.lightpath.com/wp-content/uploads/2015/11/Investigation-of-As40Se60-chalcogenide-glass-in-precision-glass.pdf (http://www.lightpath.com/wp-content/uploads/2015/11/Investigation-of-As40Se60-chalcogenide-glass-in-precision-glass.pdf)
Also contains a fair bit of background about moulding IR lenses
Bill