Author Topic: Use of plastic wrap for MWIR window  (Read 649 times)

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Offline Joe99

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Use of plastic wrap for MWIR window
« on: May 02, 2020, 09:55:42 pm »
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.
 

Offline Fraser

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Re: Use of plastic wrap for MWIR window
« Reply #1 on: May 02, 2020, 11:48:43 pm »
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
« Last Edit: May 02, 2020, 11:53:53 pm by Fraser »
 

Offline Joe99

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Re: Use of plastic wrap for MWIR window
« Reply #2 on: May 04, 2020, 02:07:15 am »
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.
 

Offline Max Planck

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Re: Use of plastic wrap for MWIR window
« Reply #3 on: May 06, 2020, 04:41:01 pm »
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
 
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Offline adammunich

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Re: Use of plastic wrap for MWIR window
« Reply #4 on: May 17, 2020, 12:01:20 pm »
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.
 

Offline Fraser

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Re: Use of plastic wrap for MWIR window
« Reply #5 on: May 17, 2020, 12:42:30 pm »
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
« Last Edit: May 18, 2020, 11:29:53 am by Fraser »
 

Offline Fraser

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Re: Use of plastic wrap for MWIR window
« Reply #6 on: May 17, 2020, 12:57:27 pm »
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
« Last Edit: May 17, 2020, 12:59:39 pm by Fraser »
 

Offline Fraser

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Re: Use of plastic wrap for MWIR window
« Reply #7 on: May 17, 2020, 01:05:02 pm »
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
« Last Edit: May 17, 2020, 01:08:44 pm by Fraser »
 

Offline Uho

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Re: Use of plastic wrap for MWIR window
« Reply #8 on: May 18, 2020, 08:34:30 am »
I once made a film holder for a thermal imager. The film protected the lens during rain and snow.
 
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Offline adammunich

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Re: Use of plastic wrap for MWIR window
« Reply #9 on: May 19, 2020, 05:57:03 am »
I tried it, and it is dang-near completely clear. Same deal with zip-lock bags.
 


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