Minimizing IR reflection - Page 1
Forum section: electronics / general

author: luky315
author_rank: Regular Contributor
timestamp: December 20, 2022, 09:34:24 am
content: I have a bit of an unusual question, but it is metrology related: I am working with very weak infrared signals (1100 nm to 1630nm range and we are trying to detect only a few 100 photons) and there are problems with reflections from other sources. I am suspecting the milled aluminium housing and mounting fixture, but also the "standard" PCB with "standard" green soldermask.Does someone have some data on the reflection coefficient of aluminium and green soldermask in the IR region? And even more important: Which coating is effective (high absorption, loww reflection) in the 1100-1630nm region? Is (black) anodization a viable method? This would also be a very durable surface. I was not able to get reliable informations, the informations used for IR camaras is not usable, because they work in the 7.000nm–21.000nm region.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2022, 09:51:51 am
content: Aluminium and copper are reflecting very well in the Ir range. Chances are the green solder mask can also be transperant, but it can depend on the brand. The properties of plastics in the IR range can be a bit surprising. As an example I found that a black "Edding" brand permanent marker was quite transparent at 950 nm. Similar pigments could be used in other paints too.For the absorbing material one may have to do tests on your own, but chances are some black ink paint is actually carbon based and well absorbing.

author: nfmax
author_rank: Super Contributor
timestamp: December 20, 2022, 09:55:19 am
content: 'Black' anodized aluminium is highly reflective in the near IR - I speak from experience! There are specialist paints available, but I did find an article that claims blackboard (chalkboard) paint is suitable, with the addition of a roughening filler:https://www.gerdneumann.net/english/instrument-building-parts-teile-fuer-den-fernrohrbau/totmatte-schwarze-optikfarbe-deep-black-optical-paint.html. No personal experience though.Also from experience, take great care with the design and use of baffles. They can help enormously to control stray light within your optical assembly.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 20, 2022, 10:31:53 am
content: For your application, I guess a lot of the methides used with thermographic inspections apply.The main reason for faulty measurements are shiny surfaces. During my training we were always told to do the measurements on coated, non shiny surfaces of metallic objects.My recommendation:Isolate the sensor as much as possible from anything that can influence the measurements.Coat all shiny surfaces with a black mat coating because this will give the least amount of reflections.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 20, 2022, 11:00:27 am
content: I didn't try, though modern materials like Vanta Black should work for IR, too.Vanta Black is very expensive. There are other materials with similar properties and affordable prices (from competitors).

author: luky315
author_rank: Regular Contributor
timestamp: December 20, 2022, 11:06:03 am
content: Thanks all, I know that I have to cover all reflective surfaces. But the problem is that for me the reflection / absorption in the 1100nm-1630nm region is important and the properties in this part of the spectrum can be very different from the visible spectrum (380nm to 700nm) or the infrared light used by IR cameras (7.000nm–21.000nm)

author: RoGeorge
author_rank: Super Contributor
timestamp: December 20, 2022, 11:10:05 am
content: Vantablack is very, very black from 13um IR up to UV.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 20, 2022, 12:48:28 pm
content: The advice on coatings given above is good. You will also probably want to pay attention to shapes of all the surfaces in the field of your sensor, and to what the sources of energy are. There are specialized ray tracing programs designed to track this sort of stuff. A couple of decades ago APART from Breault Associates was a leader in the field. I don't know if they retain this reputation or what other options might be available.

author: nfmax
author_rank: Super Contributor
timestamp: December 20, 2022, 01:54:51 pm
content: Lurking in my files I found a simple one-page précis of the baffle design process, for a basic lens-detector system. Enjoy!

author: Fraser
author_rank: Super Contributor
timestamp: December 20, 2022, 03:19:21 pm
content: I recommend Krylon Ultra Flat Black camouflage paint. It is used in optical system related circles and it worked well at SWIR for me.https://www.tactical-kit.co.uk/krylon-black-spray-paint-10918-p.aspDo not let the word camouflage concern you. It is just a very flat black and there are cans of it used in Industry that are identical except the word camouflage is not used. Inexpensive and very effective paint. NASA tested it and showed it to have excellent performance compared to the more expensive and harder to source coatings.Fraser

author: berke
author_rank: Frequent Contributor
timestamp: January 13, 2023, 07:52:31 pm
content: This linkhttps://instrumentation.tamu.edu/instruments/reflectance_plots/gives ~.25-2.5 µm reflectance plots for some common materials. As has been said above, "black anodize" aluminum is not black at all in the SWIR... You can get Acktar foil/adhesive tape from Edmund/Thorlabs, prices are not too ridiculous (as far as prices for opticsal components go.) Acktar can also coat your delicate optics for you. At grazing angles nothing will save you.