Use of plastic wrap for MWIR window

[Joe99]

I've seen that plastic bags and plastic wrap (e.g. Saran wrap) are transparent in the IR, but am wondering over what wavelength band.  Is it only the SWIR?  I have a 3-5 micron camera and and want to look at a black surface with temperatures varying between 20C and 30C and be able to resolve small temperature differences.  Than camera can do this when looking directly at the surface, but I need to do it through a window with area of about 40 cm x 20 cm.  There would be a slight pressure difference across the window, so I can't just cut a hole to look through, but the pressure is small enough that a thin sheet of plastic would be enough.  Would Saran wrap or something like it work in this band?  What about a thicker sheet of plastic?  I'm trying to do this without spending a lot on an IR window.  I don't have access to the camera right now to just try it, which would be simple if I could.

[Fraser]

Polyolefin film is used at LWIR in some lens protectors.

Polyolefin film is also known as shrink wrap and is used in many industries to wrap products. It is easily purchased and comes in different micron thicknesses. I use 12micron for most tasks but also have some 20micron. I find it to be very strong and is not to be confused with the wrapping materials that are stretchy like cling film. Another advantage of this film is that it can be gently stretched over a wood frame and then warmed with a hair dryer to tighten its surface. This polyolefin film is also used in this exact manner for cheap secondary glazing in houses without double glazing.

I know you asked about MWIR and I have not tested a polyolefin film with my MWIR cameras yet, but it should work well. If using the film for measurements you can place a piece across the lens when viewing the intended temperature of target and get some idea how much effect the film will have on your calibration. It will not be exact, but a fair indicator of the transmission loss that you will introduce into the measurement path. Also remember that you are effectively inserting a window sitting at ambient temperature into the path. Some cameras allow you to enter a window temperature and transmission into the measurement menu.

Fraser

[Joe99]

Thanks very much for this.  I had not thought about shrink wrap, but it sounds promising.  From what I was able to find, it looks like it has an absorption peak at 3-3.5 microns, but the absorption is low outside that band.  With a thin film, it looks like it could have good transmission in the MWIR.  The strength should also be better for the application than cling film.  The camera does allow entering a window temperature and transmission, and we have used it for a similar application with a smaller CaF2 window.

[Max Planck]

You could use any polyester film, for example a binding cover.

http://abbe.net.au/products/binding-machines-and-consumables/jastek-gloss-binding-covers/

Because of a slight infrared radiation attenuation in the MWIR band, it will affect the measurements but it would be easy to determine a correction factor.

Max

[adammunich]

I have several MWIR cameras, this is a good idea to protect the lens. I'll try it and report back on how well it worked.

[Fraser]

The official FLIR “disposable” lens protectors are designed to have a replaceable polyolefin film. The assembly comprises 3 parts. A ‘barrel’ that mounts on the cameras lens threads, or supplemental lens bayonet mount, the highly transmissive Polyolefin film, and a film retaining ring that slips over the barrel to lock the film in place. The film be tightened using a hair dryer to remove any ripples in its surface. This type of lens protector could be made using a 3D printer but I recommend using ABS or other heat resistant filament if you intend to use a hair dryer to tighten the film (low melting point filaments like PLA could distort due to the heat). Alternatively a commercial aluminium photographic lens tube may be modified for the task. The film retaining ring may be replaced with an elastic band if desired.

Note that a key benefit of the Polyolefin film is its availability in very thin, yet strong types. The 12 Micron type is excellent and minimises transmission losses. A thicker film or binding cover may be sub optimal in some cases as higher transmission losses are introduced to the optical path. As with most optics, lower losses and higher transmission is a desirable. If a material has an absorption characteristic ‘in-band’, as in the case of Polyolefin at MWIR wavelengths, the thinner the film the batter as the impact of the material absorption Wavelength is greatly reduced. This is how Silicon lenses may be used at LWIR despite Silicons poor spectral Transmission response in that band. It has to be a thin lens, as in the FLIR Lepton.

The fact that Polyolefin film may be tightened using heat is an added bonus

Fraser

[Fraser]

For anyone needing ballistic or high temperature protection for your camera lens, I recommend sourcing a Germanium window of the appropriate diameter and coating for the band being used. Modern Fire fighting cameras commonly employ such protection for the lens. Such windows can be expensive but nowhere near as expensive as a new objective lens on a professional Thermal camera. Thermal viewing ports are also available and could be adapter to protect a thermal camera. Some of the window materials can be made from unusual materials however so study the specifications and handling requirements if taking this path.

Fraser

[Fraser]

A picture of one of the ‘non-disposable’ lens protectors offered by FLIR. The protection window is made from monocrystaline fluoride as found in equipment cabinet inspection windows. Note that the protection element is tilted inside the barrel to reduce undesirable optical effects.

https://www.flir.co.uk/products/t197343_protective-window/

Fraser

[Uho]

I once made a film holder for a thermal imager. The film protected the lens during rain and snow.

[adammunich]

I tried it, and it is dang-near completely clear. Same deal with zip-lock bags.

[KarlaRoeser]

Saran wrap or a similar thin plastic sheet could potentially work in this wavelength band. However, for better results, you might have considered a thicker sheet of plastic for enhanced durability and performance.

[TimFox]

"Polyolefin" is a general term:  popular examples include polyethylene and polypropylene.
Which ones are used in this application?

[Fraser]

This one…..

Polyolefin heat shrink is a type of plastic that is made from a polymer of ethylene and propylene. It also has a wide operating temperature range and is resistant to chemicals, oils, and fuels.

[TimFox]

Yes, both polyethylene and polypropylene are polyolefins.
I prefer that type of heat shrink tubing to PVC.
My question was:  which polyolefin is used in MWIR windows?

[Fraser]

FLIR do not detail the chemical composition of the Polyolefin film used in the disposable lens protectors. It is just a heat shrinkable film, as has been detailed in this thread. The material needs to have excellent transmission with no absorbance band within the cameras passband. Whilst Polyolefin shrink film is normally used on LWIR cameras, I have also used it on MWIR without noting any difference in performance. I use the 12 micron film to reduce transmission losses and shrink the film onto the holders frame for a taught window.

If exact transmission loss plots and absorbance figures within the MWIR band are important, sadly you move into the realm of buying specific films used for science and so manufactured for IR transmission. I will stick with what is basically a product packing material that exhibits excellent transmission in my use cases and is very inexpensive. My only refinements have been to select the thinnest practical film and to buy from suppliers who offer well made film with a nice transparent and smooth surface.

Fraser

[TimFox]

Did you intend to attach a link after your "this one ..." comment?
(I had asked a serious question.)

[Fraser]

Nope. The identity of the film is only given as “Polyolefin film” by FLIR and I detailed what was meant by FLIR by Polyolefin.

Beyond that, it will be for you to determine whether common Polyolefin heat shrink film as detailed in this thread will meet your specific needs. You will not find an exact specification of film used by FLIR so it is a case of buying some films and testing them. Not being flippant but the chemical make up of the material is of little to no interest to me, or many users, beyond how the film performs in a given situation. That said, if you have specific needs then regrettably you will have to do quite a bit of research into which polymer films will meet your needs. There are some research papers on Polyolefin film transmission and absorption plots. Sadly some are behind pay walls

Can you say why the materials chemical composition is important in your use case please ? The properties that I needed were as follows:

1. A thin film that may be placed over a frame to mount in front of a lens
2. Excellent transmission at sensible film thicknesses
3. No serious absorption bands from the material within the required band of use
4. Robust enough for a lens protector
5. Low cost
6  Easily obtainable
7. Heat shrinking an advantage but not essential characteristic

When I asked my friends at FLIR about their disposable lens protectors, they told me that they use Polyolefin film similar to that used in the food packaging industry. I searched for food packing Polyolefin film and discovered that it is very common, very nice surface quality, heat shrinkable and very transmissive at LWIR and MWIR wavelengths. It looked perfect for my application and no, I did not delve into what it’s chemical make-up was as it just worked and met my needs. That said, I did step down from thicker films to the 12micron and 15micron that I now use. This decreased the transmission loss through the film without the lens protector becoming too fragile.

There are other plastic films that are transmissive at LWIR and MWIR but Polyolefin heat shrink food and product packaging is such a great fit for my needs that I have not tested other materials. So much depends upon your specific needs. In some cases you cannot avoid buying a very expensive non-disposable lens protector that uses a Germanium (LWIR & MWIR) or Sapphire (MWIR) PL-PL AR coated window. You have to sit down when asking FLIR for the price though !

If looking for some of the film, just go on eBay and search for “Polyolefin shrink film” and you will find the film on spools and in bag form. It is used for food wrapping so is very common. It can be hard to buy in smaller quantities. I found a small company willing to provide sensible quantities at low cost some years ago….. a little goes a long way when dealing with small diameter lens protectors !

Hope this helps

Fraser

[CatalinaWOW]

I will double down on what Fraser says.  The transmission depends of the details of the plastic composition in many ways.  As a specific example I would expect the polyolefin shrink wrap to have different transmission characteristics before and after it has been heat shrunk.  Around 30 years ago I did see a couple of vendors marketing plastic materials for windows/lenses in the MWIR and LWIR bands, but they were small players in a small market and as far as I know they didn't survive.  So the films that you can buy have compositions that are optimized for some other purpose and/or for easy of manufacture and packaging.  Modifiers are added to the base material to affect hardness, flexibility over temperature and any number of other properties.

The attached link to a paper has transmission characteristics for a number of common plastic families that may guide you in selection but there is no guarantee that the published curves will match any material you buy.  Note that this paper uses wavenumbers, but the range you are interested in is covered. 

https://library.uoh.edu.iq/admin/ebooks/45621-dke291_ch07.pdf

There is no substitute for testing the materials you end up using.  I went into the kitchen and found that the cling wrap on the shelf there met my needs but your mileage can vary widely.  If you care about actual temperatures you will have to calibrate through the window you select, but merely detecting small differences is much less stressing.