LWIR-transparent thick plastic? - Page 1
Forum section: electronics / general

author: Ultrapurple
author_rank: Super Contributor
timestamp: October 29, 2019, 02:29:16 pm
content: Is there a plastic that's pretty-much transparent to LWIR and with roughly the strength and characteristics of Perspex?I'm considering the possibilities for a visible light blocking but LWIR transparent panel. I've done something similar with SWIR-transparent Perspex, which was very successful, but I wonder if there's anything out there that might replicate the trick at LWIR.Of course it wouldn't surprise me if the answer is "if you can invent something like that you could name your price for the patent rights"...

author: frogg
author_rank: Regular Contributor
timestamp: October 29, 2019, 09:47:36 pm
content: define "thick"?

author: Fraser
author_rank: Super Contributor
timestamp: October 29, 2019, 10:10:59 pm
content: Plastics do not have great transmission figures at LWIR. Very different to MWIR and SWIR. Most applications rely on reasonably transmissive plastics that are kept very thin to limit the transmission losses. You need a plastic polymer that contains Carbon and Hydrogen but does not contain oxygen.This company may have what you need or advice on the matter......http://www.kube.ch/The reason that Chalcogenide IR Glass was created was the lack of mouldable plastics suitable for thermal imaging lenses at LWIR. The thickness of the plastic needed for a lens becomes an unacceptable transmission loss. The best low cost solution became a material that, though less than perfect, was acceptable for LWIR lens use and was mouldable to save costs.Very thin plastic films can be used with tolerable transmission losses but they are fragile. Look at Polyolefin film as that is used in industry for LWIR lens protection. Polyolefin film is found commonly as heatshrink film used on food and product packaging. It is also easily purchased and can be made taught on a frame using a hair dryerDifferent thicknesses are available. I have a few that were bought cheaply on eBay from a seller called Bee Beautiful. Even the very thin Polyolefin film does have noticeable attenuation at LWIR.MWIR is much easier... you can use many plastics including Polycarbonate.Fraser

author: Ultrapurple
author_rank: Super Contributor
timestamp: October 30, 2019, 09:00:23 am
content: Thanks both@frogg - thickness of between about 1/8" and 1/4", sufficiently strong that an A4 size piece, reinforced at the edges if necessary, would be mechanically (and chemically) stable enough to be left out in the wind and rain in a semi-sheltered position in an area not known for hurricanes, monsoons or desert / arctic temperatures.@Fraser - thanks for the hint on Kube. It looks like they offer a maximum 1mm thick plate - but it only comes in a 1m x 2m size and costs 110 euro ex-works Switzerland. Perhaps I could persuade them to provide an A5-size sample as one of the four-for-€24 sample offerson their price list!

author: Fraser
author_rank: Super Contributor
timestamp: October 30, 2019, 10:21:29 am
content: They sound like a company that welcomes direct contact. It would be worth telling them that you are a hobbyist and your intended purpose for their product. Ensure you mention the World renowned EEVBlog thermal forum and the wide audience for your reports on their IR transmissive materialThey will hopefully provide exactly what you need at a reasonable cost.http://www.kube.ch/sample.phpFraser

author: Fraser
author_rank: Super Contributor
timestamp: October 30, 2019, 10:27:44 am
content: Data for the Kube plastic is here.....http://www.kube.ch/downloads/pdf/kube_plastics_properties.pdfhttp://www.kube.ch/downloads/pdf/kube_window_films.pdfNote the effects of the materials thickness on transmission ! The 1mm thick plastic has a transmission figure of only 41%. This is the drawback and penalty of using plastics for windowsFraser

author: Ultrapurple
author_rank: Super Contributor
timestamp: October 30, 2019, 03:32:49 pm
content: Thanks FraserOnce I find out whether my little nugget is trash or treasure - and work out what I'm going to do with it - I may well give them a prod. I have a feeling I've dealt with them before, buying some of their Fresnel lenses to see whether they'd form a usable image on a LWIR sensor. I wrote it up in another thread but the bottom line was that I only got very poor results with low definition and very limited sensitivity, though I did mention that my methodology would probably have benefited from improvement.

author: Fraser
author_rank: Super Contributor
timestamp: October 30, 2019, 03:48:55 pm
content: I remember you buying fresnel lenses but not where you bought them from.An issue you may have with these IR transmissive plastics used for a window is their possible negative effect on the image clarity. As with photographic windows and filters, if the material used is not decent optical quality, the image gets badly degraded. It will come down to the granularity, quality control and smoothness of the plastic sheet. They mention thermal imaging on their home page so should know whether the 1mm thick sheet is good enough for camera use. PIR sensors with their large pixel size and often only 4 pixels are far more forgiving of lens and window optical quality.It is interesting that IRISYS had to use proper Germanium lenses in their thermal People counters. Those counters had an ultra low resolution of 16 x 16 pixels yet they could not use a cheaper IR plastic lens of some sort. They were trying to keep the BoM as cheap as possible so such must have crossed their minds. The little Germanium lens that they used is only around 5mm in diameter and just a single lens element. You cannot get much simpler, or cheaper for thermal imaging. We have Chalcogenide IR Glass now of course. Early development of that glass did show it to have an awful Crystal grain and fracture structure that degraded the available resolution. Hence why monocrystal Germanium and materials like ZnSe remain in use in high end applications.I have rambled on long enough !Fraser

author: Ben321
author_rank: Frequent Contributor
timestamp: November 06, 2019, 06:20:36 am
content: Edmund Optics has plastic fresnel lenses that work in the thermal IR wavelength range.https://www.edmundoptics.com/f/infrared-ir-fresnel-lenses/12438/They come in various sizes. A 5-inch by 5-inch lens costs $30. The actual chemical name of the plastic (such as polyethylene) isn't given (probably a trade secret). Instead it is called by its brand name "Poly IR".

author: frogg
author_rank: Regular Contributor
timestamp: November 07, 2019, 07:34:05 pm
content: The most common/cost effective solution I have seen is a very thin LDPE or other plastic film sheet, dome, bubble, whatever.The mechanical structure is provided by a honeycomb or other perforated metal structure (sheet) onto which the thin plastic sheet is adhered (typically the film is placed on the inside of the perforated structure)The perforated metal structure/plastic sheet combination is placed sufficiently close to the FPA with a lens of sufficient focal length and sufficiently low numerical aperture such that its circle of confusion cannot adequately resolve the perforations themselves, except as a general darkening of the scene.This creates a strong and moisture impermeable structure that is cheap to manufacture. The penalty is that the overall brightness of the image is negatively affected by the perforations.This solution obviously doesn't work well with small wide angle lenses, obviously.

author: gerdir
author_rank: Newbie
timestamp: November 09, 2019, 05:30:32 pm
content: Si-HDPE:https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-25-2-1202&id=357440

author: defence2882
author_rank: Newbie
timestamp: December 20, 2019, 01:59:49 pm
content: While not total on topic, Si wafers used in semiconductor fabrication seem to exhibit reasonably LWIR transparency. I have obtained several 1mm thick 6" wafers, which I plan to use to make windows for external use.Obviously they are not IR coated, but neither the less a reasonable image is transferred. Also not sure, not sure of their longevity when subjected to the environment.In my searches I did also discover Ge wafers are available however the cost of such is unknown.

author: ArsenioDev
author_rank: Frequent Contributor
timestamp: December 20, 2019, 03:17:47 pm
content: Defence2882, where do you get the double side polished wafers, I've been trying to find small diameter (like 2 inches) for SWIR pass filtering on scanned aperture imagers

author: Fraser
author_rank: Super Contributor
timestamp: December 20, 2019, 03:24:50 pm
content: I bought a populated silicon wafer to test a while back. That did not work out so well in my case. Very degraded image detail.Silicon wafers do come up on eBay as curios but I think most will be populated production scrap. Silicon does have useful transmission in the band as evidenced by FLIR using it for the diffraction lens in the Lepton core. The thickness of the silicon has a direct effect on the transmission loss so thinner is better if being used as a window. There is plenty of information on the internet regarding silicon spectral transmission and the effect of thickness upon losses at a specific wavelength.Fraser

author: Bill W
author_rank: Super Contributor
timestamp: December 20, 2019, 11:15:42 pm
content: Silicon is also used for detector windows on lower cost sensors.Silicon also has a lower refractive index and less chromatic / thermal problems as well

author: Ben321
author_rank: Frequent Contributor
timestamp: December 22, 2019, 08:09:02 am
content: Quote from: Fraser on December 20, 2019, 03:24:50 pmWhat's the transmission spectral characteristics of doped silicon, compared to pure silicon? I found an n-type silicon wafer (doped with antimony) on eBay. It isn't populated with chips, but is one of the parts that a chip factory would start with in order to produce chips. Does the antimony doping significantly alter its LWIR spectral characteristics? If so, does it make it better or worse?

author: Max Planck
author_rank: Regular Contributor
timestamp: December 22, 2019, 07:33:04 pm
content: Long story short, it will depend on doping type and level, cristalline axis and surface finish (is there an antireflective coating).Max

author: Ben321
author_rank: Frequent Contributor
timestamp: December 31, 2019, 10:36:39 pm
content: Quote from: Max Planck on December 22, 2019, 07:33:04 pmConsidering doped silicon is usually used for making semiconductor electronics, rather than IR optics, I'm sure there's is no anti-reflection coating on it. Not sure what the crystalline axis is though. Is that standardized for the silicon used in manufacturing computer chips?