best way to quantify anti reflective coatings DIY - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: March 13, 2025, 04:34:24 am
content: So I was doing some experiments with 'blackening' metals with chemical mixtures (i.e. copper) and the result seems pretty impressive to me. I found alot of different formulations for different metals.I believe some parts in optics have a anti reflective coating, one that comes to mind is putting a 'shroud' over a laser to get rid of 'sidelobes' or whatever you want to call it. Another use may be a iso-thermal chamber for calibrating IR sensors, its a blackened sphere that is heated with a hole in it.This is the closet thing to a optics forum so I was wondering if you can build something to evaluate these coatings. Blackening with acetylene soot seems very powerful too, but its not durable, these oxide coatingsaredurable.Like if you have a small coated sample (say 1x1cm), is it possible to build something from optics elements to measure it reliably? I do have some plans to buy some prisms and such for beam correction.Things I have1) stable current source2) optics plate (1/4 inch aluminum 16x16 inches)3) respectable laser diode holder (but no diodes yet) for TO-type diodesI got my optics mixed up after all these years, but I do recall something about attaching a surface to the back of a certain type of prism?

author: EE-digger
author_rank: Frequent Contributor
timestamp: March 14, 2025, 04:47:50 pm
content: I'm currently on a quest for high emissivity, high temperature coating for thermal camera black body target.Responding here because you mentioned chemical blackening and thought I'd share. Birchwood Casey makes many products for the firearm market including bluing (really black on steel) and aluminum black. The aluminum black is exothermic and a small piece will get hot. If you let it sit too long, it also creates a soot which to me looks as good as candle soot, but is also not durable.I see you have laser diode on your list. You also mention AR coatings, which can get fairly complex. If it is anti reflection measurment you're after, I would lean toward a white light or visible spectrum source for your testing. You can bounce it off your coating and measure with an optical power meter, or for DIY, set up a photodiode and transimpedance amplifier (one chip, few resistors).Emissivity is a different animal. To compare, I would deposit my sample coatings or materials side by side on a temperature controlled plate, then compare temperature readings with a thermal camera.added - you may want some optics to collimate your beam on the source side and to capture it on the reflected side.All over the place here, don't know if this helped or notBut one last comment. If you want to evaluate AR coatings it can get complex as they are not only wavelength dependent but also sensitive to angle of incidence. A beamsplitter is a good example of this and an Agilent or Keysight scope or analyzer AR window is another. In the display window case, angle of incidence will be around zero and for a beam splitter, around 45 degrees (or some special angle).

author: coppercone2
author_rank: Super Contributor
timestamp: March 14, 2025, 08:10:41 pm
content: Well I think there is confusion because there are translucent AR coatings and just absorber AR coatings. I just essentially want to test black objects made with different formulations. I.e. for copper black I found at least 5 different chemical mixtures that workI will mix my own formulations based on book stuff though. For something scientific, specifying a commercial product that might stop being manufactured is no good IMO, thats a boondogle the manufacturing industry can deal with

author: Bill W
author_rank: Super Contributor
timestamp: March 15, 2025, 11:12:54 pm
content: I did some crude tests on a few materials by using a flat surface as a mirror, reflecting a soldering iron.The better the 'black' the less hot the soldering iron measures at.

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 11:14:00 pm
content: Oh yeah especially if I polish it first that might actually be pretty good. I always loved that when you setup thermal imager the soldering iron is like a light stick and you can actually illuminate stuff with it lol. It makes me feel like ridick

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 15, 2025, 11:23:52 pm
content: Doing a search on BDRF measurement should give some ideas. As with all measurements the concept is not too difficult, but the devil is in the details.Because the BDRF is a vector measurement, not a scalar defining "best" requires some application knowledge. Trying to reduce the background from clouds is different that blocking the sun.

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 11:25:02 pm
content: I also have a lock in amplifier, could that help design a test?

author: IR_Geek
author_rank: Regular Contributor
timestamp: March 16, 2025, 06:30:34 pm
content: This is an interesting project and wish you the best of luck. Can you clarify where you want to use this surface coating? Depending on the geometry of the setup and the waveband of interest will better determine what is 'good' for your application. i.e. how much characterization you need to do ... spectral bands, angles of incidence, surface durability, etc...If you are looking to create a cavity blackbody for calibration then the shape of the cavity is usually more important than the coating. When done correctly you can get a high emissivity and spectrally flat over a large spectral band.If done for a large flat surface blackbody then you have to worry about BRDF (Bidirectional Reflectance Distribution Function) along with the above.Trying to minimize stray light in an optical path is similar to a surface blackbody. This depends if you want to trap energy internally or reflect it back out of the system.Lots of good sources from NIST on radiometry that could prove useful to your experiments. Most are open source. Lots more behind paywall at SPIE / OE. The old IR Handbooks can be found in PDF online and fairly cheap if you want hard copy. Authors like Palmer and McCluney are good starts on radiometry.

author: coppercone2
author_rank: Super Contributor
timestamp: March 26, 2025, 12:57:09 pm
content: I don't know where I want to use it, I got the capability to blacken copper (for gluing) and it looks like there is a ton of different formulations and methods to black metals and I find the chemistry part interesting and I was wondering if I can use it for something more advanced then just glue or decoration, and my fledgling optics equipment collection might be able to do something with it.I got some stuff to make silver mirrors also (electroless deposition reactants) and it might be related to this too.I like wet chemistry, not so big a fan of the vacuum stuff, even though it seems to dominate. I do wonder though if this is because chemical phobia, because the vacuum stuff seems like maybe a tiny bit cleaner and forgiving (well I actually have no idea, it might be really fucking dangerous, but I am saying it looks cleaner to the casual observer (though, my knowledge of nano particles makes me think perhaps vacuum chambers are not quite as nice as they look... even if some chrome ions in a beaker freak you out in comparison.. but I noticed at works the dry nano-labs have way stronger safety systems compared to the wet labs! they were scared to turn the hoods off like a year after work was finished)).. but that's just a separate rant.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 26, 2025, 01:59:58 pm
content: A big component of blackness is physical structure of the surface. You might Google "Gouffe" for some insight into why. They all involve forming cavities, cracks or other structures which cause incident light to make multiple reflections before leaving the surface. Many different approaches to achieving the appropriate characteristics, and also several approaches on what is the appropriate structure. Mechanical removal (sanding, machining, etc.), chemical removal (etching, lithography, etc.), mechanical deposition (textured paints, sputtering and others) and chemical addition (crystal growth and the like) are all used in one or another application.

author: bap2703
author_rank: Regular Contributor
timestamp: March 28, 2025, 02:28:09 pm
content: Quote from: coppercone2 on March 14, 2025, 08:10:41 pmIn optics we only don't really usually use the term AR coating to describe a better absorption.Do you want to test your black samples in the thermal IR range?If yes, remember than in thermal radiation black and white are the same thing: a good absorber is a good emitter.So you could just heat your sample (goal is just to get room temperature different than that of the sample) and measure it's temperature using a thermal camera and a contact thermometer. The closer the values are, the blacker the coating.Problem is when you're looking at black samples you usually work in <5% reflectivity and then it becomes quite tricky to measure things accurately.

author: coppercone2
author_rank: Super Contributor
timestamp: March 28, 2025, 11:35:31 pm
content: What if you glue thermocouples to them and shoot it with a substantial laser beam at some angle to warm it up a little?If they are small and the thermocouple is sensitive you might not need that much power to get a measurable result?

author: _Wim_
author_rank: Super Contributor
timestamp: March 29, 2025, 07:37:39 am
content: Do you just want to compare your mixtures relative to each other, or are you looking to measure them so they can be compared with online data of existing coatings?If the first, I would just thermally attach the coated copper sheets to a thermo-electric cooling element, shine a led/laser/light bulb on the surface and measure the voltage of the thermo-electric cooling element over time. This is how thermal laser power sensors work. Be careful to ensure your light sources are always at the same angle and distance. If you modulate the light source (light chopper wheel or modulated current source), you can use the lock-in to measure the output voltage of the thermo-electric cooling element even below the noise floor of the setup and remove influences from any other non modulated light that hits the copper sheet. Thermal coupling between the thermo-electric cooling element and coated copper sheet might prove difficult to get repeatable results, so I would first attach and remove the same copper sheet several times and repeat the measurement to check reproducibility of the setup first. One reproducible, you can start comparing with different coatings.

author: _Wim_
author_rank: Super Contributor
timestamp: March 29, 2025, 07:47:00 am
content: This excellent video from the signal path shows a setup where a lock-in amplifier was used to test a led in "photo diode" duty:In your case that led would be replaced by a thermally coupled sandwich of a peltier cooler and your coated copper sheet under test...

author: _Wim_
author_rank: Super Contributor
timestamp: March 29, 2025, 08:01:22 am
content: Forgot to add, modulation must be much slower (<1 Hz) compared with the Signal Path example, as the sensor has "lots" of thermal inertia.