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I'm competing in a simple robotics competition where most of the participants use reflected infrared light to detect their opponent. I'd like to make my own robot as difficult to see as possible. What common materials (fabric, plastic, paint, etc) absorb most infrared light, and reflect as little of it as possible? I first thought any flat-black substance might do, but I've learned that what appears dark in the visible spectrum might still appear bright to infrared.

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    $\begingroup$ I'm reasonably sure that if you paint something such that it's black in the visible spectrum, it will be black at whatever IR wavelength your competitors are using. Your intuition to use flat black is good as this will have low reflection. You actually may need to worry most about heat sources on your robot. Can you get an IR camera to fool around with this? $\endgroup$ – DanielSank Nov 13 '14 at 4:05
  • $\begingroup$ @DanielSank paint the inside of the robot black too? Then only the black body radiation will be leaving it, a bit higher than the surroundings , but the heat sources (motor?) will be masked. $\endgroup$ – anna v Nov 13 '14 at 4:31
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    $\begingroup$ @annav: I think the real issue is whether or not heat sources in the robot will lead to warm spots on the exterior. This might cause discernible images for the other robots to track. I think the only way to really tell is buy a Flir IR camera or something like that and take a look. Actually, presumably OP has some kind of IR camera for the robot itself... Anyway, not sure if painting inside surfaces would matter. I guess not but I haven't thought about this too carefully. $\endgroup$ – DanielSank Nov 13 '14 at 5:11
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    $\begingroup$ @DanielSank if the exterior were metal then the internal sources would be reflected around and not be point like. Maybe dress the inside with aluminum foil? $\endgroup$ – anna v Nov 13 '14 at 5:16
  • $\begingroup$ @annav: I was thinking more about the possibility that something like a motor mechanically mounted somewhere in the robot would conduct heat to the outside. $\endgroup$ – DanielSank Nov 13 '14 at 5:27
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The IR sensors that OP is talking about work by emitting IR from an LED and then measuring IR intensity reflected back from an object close enough (scroll to Sharp GP2Y0A21YK IR Proximity Sensor). Addressing the comments: the temperature of the robot and the resulting blackbody emission in the IR is small enough to be irrelevant to the question. IR is used specifically because objects near room temperature don't produce much conflicting signal.$^\dagger$

Q1: Will materials that are black to visible light also be "black" to infrared?

The answer is "not necessarily," as you stated in the question. Here is a neat video of a person's hand visible in IR through a black/opaque plastic bag, and here are some pictures of the same:

visible light bag ir light bag

Image source: NASA/Caltech

From a typical IR proximity sensor datasheet it indeed appears that there is no difference in ability to sense between reflection of IR off a white piece of paper vs. a gray one:

Voltage vs. Distance

Q2: Which materials absorb infrared?

A quick search did not prove very fruitful. Glass is not transparent to IR, as you can see in the man's glasses in the picture above, as well as at 0:20 in this video. However, at 0:52 in the same video you can see that glass does reflect IR to some extent (when the hand is on the same side as the glass). As we've seen, the absorbance of light depends very much on the wavelength, so first you would need to figure out which wavelengths the opponent's IR LED would output. It may be hard to find a common material that absorbs through that entire IR spectrum, so don't forget there are many ways to skin a cat! Alternatives to making your bot out of an IR absorbent material:

  • Reflect the incoming IR at angles away from the opponent's detector
  • Make your bot out of material transparent to IR
  • Produce additional IR in order to interfere with the opponent's detector (TV remote would work)

Happy bot smashing!


$^\dagger$The sun does produce a lot of IR, hence a precaution in the sensor datasheet: "When the detector is exposed to the direct light from the sun, tungsten lamp and so on, there are cases that it can not measure the distance exactly. Please consider the design that the detector is not exposed to the direct light from such light source."

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  • $\begingroup$ If heat is the main factor, I wonder if making the surface a little colder than room temperature would work, or at least confuse the sensor a bit? e.g. keep the external cowling in a freezer and then pop it on right before the match. $\endgroup$ – Cerin Nov 14 '14 at 14:57
  • $\begingroup$ Hi @Cerin, as I said in my answer, the comments are misleading and heat is not a factor. The IR proximity sensor you are using does not work by detecting hot objects. It comes with an IR flashlight (LED) that is very bright compared to the IR heat of the object, so even an extremely cold object which doesn't produce much of its own IR will be detected as long as it reflects the IR of the LED. $\endgroup$ – pentane Nov 14 '14 at 15:14
  • $\begingroup$ @Cerin have I answered your question, or is there more you would like to know? $\endgroup$ – pentane Dec 1 '14 at 13:38
  • $\begingroup$ It would have been great to see the same IR images of a hand painted by Vanta black which absorbs 99.85% of incident IR. See here. $\endgroup$ – not2qubit Sep 18 '18 at 17:30
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Apparently a 180-nm-thick layer of vanadium dioxide on a sheet of sapphire will absorb almost 100%...

http://www.photonics.com/Article.aspx?AID=52427

Correct, but likely not helpful, answer. More practical might be this film that you can actually buy...

http://www.edmundoptics.com/lab-production/general-tools/acktar-light-absorbent-foil/3634/

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Although an answer has been selected, I have to weigh in. Don't try to absorb the transmit beam. Instead, cover the exterior of the robot with mirrors angled 10 degrees or so from the vertical. Because you are not using an imaging sensor, the robot will look just like the surrounding walls, since that is what it will be seeing.

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  • $\begingroup$ This is an excellent point. I just edited my own answer to note the same thing. A few months ago, I tested a few mirrors, and found these to work much better than any matte material. $\endgroup$ – Cerin Dec 16 '16 at 14:41
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Uvita SME 3811 added to thin one mil plastic film will absorb MIR and FIR The FIR heat radiation is absorbed making the object invisible to all IR military scopes and IR cameras

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The common materials I found that best absorbed IR light are rubbery matte-black substances. Spray-on rubbery substances like Plastidip and Flexidip also worked well. But like pentane's answer explains, even these still reflect quite a lot.

Edit: Although it wasn't exactly what I was looking for, I found that, as far as IR sensors are concerned, most mirrors I tested seem to reflect most IR light. So if the goal is to confuse IR sensors, a mirror positioned at a 45 degree angle is better at effectively "absorbing" light than a matte black finish.

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