# Limits and alternatives to IR **URL:** \ **Category:** Factual Questions\ **Created:** [February 29, 2008, 3:47pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645 "2008-02-29T15:47:36Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Dunderman](https://avatars.discourse-cdn.com/v4/letter/d/ecb155/32.png) [@Dunderman](https://boards.straightdope.com/u/Dunderman)\ **Post date:** [February 29, 2008, 3:47pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/1 "2008-02-29T15:47:36Z")
So I understand that you can’t use IR cameras to see through walls made from anything much sturdier than gauze, since this would require the wall to actually be warmer where a person is passing on the other side. So if you’re Robocop and looking for some cool technology to make tiny and implant into your eye that will give you some degree of looking-through-walls ability, what do you choose? ---
**Author:** ![hajario](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hajario/32/171_2.png) [@hajario](https://boards.straightdope.com/u/hajario)\ **Post date:** [February 29, 2008, 3:51pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/2 "2008-02-29T15:51:19Z")
[QUOTE=Priceguy] So I understand that you can’t use IR cameras to see through walls made from anything much sturdier than gauze, since this would require the wall to actually be warmer where a person is passing on the other side. [/QUOTE] IR camera can easily see things that are both warmer and colder than the scenes. The reason that IR cameras can’t see through walls is that the walls have too much thermal mass. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [February 29, 2008, 3:54pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/3 "2008-02-29T15:54:30Z")
**X- and gamma-rays** – They use these to look through the sides of metal soda and beer cans to measure the liquid level inside. Of course, they use a source to send x- or gamma rays through the target, something Robovcop won’t have the advantage of. If you don’t use ambient radiation, people aren’t going to like being irradiated. **patrticle vision** – intercept high-energy particles sent through your opaque medium. Same problems and requirements as the above **neutrino vision** – Need to come up with a good detector that can be used for imaging. There are different wavelengths of infrared, you know. Maybe you could find one that worked well enough for your purposes. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [February 29, 2008, 3:57pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/4 "2008-02-29T15:57:58Z")
The newest see-through walls technology is ultra wideband radar: [http://www.eurekalert.org/features/doe/2004-09/dlnl-etu091604.php](http://www.eurekalert.org/features/doe/2004-09/dlnl-etu091604.php) Not ready for prime time yet… ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 29, 2008, 4:03pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/5 "2008-02-29T16:03:55Z")
Backscatter X ray imaging is quite interesting - because it doesn’t require the placement of an x ray source behind the subject. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 29, 2008, 5:27pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/6 "2008-02-29T17:27:02Z")
[QUOTE=hajario] The reason that IR cameras can’t see through walls is that the walls have too much thermal mass. [/QUOTE] That’s got nothing to do with it. The reason is simply that (most) walls are opaque to the majority of the IR band. As the wavelength increases and approaches the microwave band, you’ll find a lot of materials are transparent but it’s also increasingly difficult and technically challenging to image these very longwave IR emissions. Imagers which see this longwave IR can, in fact, see through many walls to some extent. ---
**Author:** ![Spatial\_Rift\_47](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Spatial\_Rift\_47](https://boards.straightdope.com/u/Spatial_Rift_47)\ **Post date:** [February 29, 2008, 6:17pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/7 "2008-02-29T18:17:24Z")
[QUOTE=Q.E.D.] That’s got nothing to do with it. The reason is simply that (most) walls are opaque to the majority of the IR band. As the wavelength increases and approaches the microwave band, you’ll find a lot of materials are transparent but it’s also increasingly difficult and technically challenging to image these very longwave IR emissions. Imagers which see this longwave IR can, in fact, see through many walls to some extent. [/QUOTE] Longwave IR is also known as Terahertz radiation, and its imaging applications are only just beginning to be developed. Yes, it can see through walls. > **[Terahertz radiation](https://en.wikipedia.org/wiki/Terahertz_radiation)** > > Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the International Telecommunication Union-designated band of frequencies from 0.1 to 10 terahertz (THz), (from 0.3 to 3 terahertz (THz) in older texts, which is now called "decimillimetric waves"), although the upper boundary is somewhat arbitrary and has been considered by some sources to be One terah... ---
**Author:** ![hajario](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hajario/32/171_2.png) [@hajario](https://boards.straightdope.com/u/hajario)\ **Post date:** [February 29, 2008, 6:22pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/8 "2008-02-29T18:22:12Z")
[QUOTE=Q.E.D.] That’s got nothing to do with it. The reason is simply that (most) walls are opaque to the majority of the IR band. As the wavelength increases and approaches the microwave band, you’ll find a lot of materials are transparent but it’s also increasingly difficult and technically challenging to image these very longwave IR emissions. Imagers which see this longwave IR can, in fact, see through many walls to some extent. [/QUOTE] If something is hot and stable enough, you’ll “see” it through walls with even a low cost IR camera. IR cameras are, for example, used to check water or petroleum levels in tanks as well as by fire fighters to see if there is a fire behind a wall. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [February 29, 2008, 6:33pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/9 "2008-02-29T18:33:59Z")
> [@](#): > > If something is hot and stable enough, you’ll “see” it through walls with even a low cost IR camera. IR cameras are, for example, used to check water or petroleum levels in tanks as well as by fire fighters to see if there is a fire behind a wall. The IR camera isn’t really “seeing through” the container in those cases, though – it’s seeing the temoperature difference between the portion of the tank with liquid behind it (where the liquid acts like a big thermal mass) and where there’s only air behind it (so the temperature can change more rapidly). It works pretty well, too, as long as the liquid is pretty stable. We looked into building something similar for measuring the level in cans, but they tend to slosh around a lot on the assembly line, and there isn’t a well-defined level line. ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [February 29, 2008, 6:38pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/10 "2008-02-29T18:38:49Z")
[QUOTE=hajario] If something is hot and stable enough, you’ll “see” it through walls with even a low cost IR camera. IR cameras are, for example, used to check water or petroleum levels in tanks as well as by fire fighters to see if there is a fire behind a wall. [/QUOTE] But surely that’s still the temperature of the barrier that you’re seeing, which in those special cases is tied to the temperature of the thing right behind it. The IR radiation emitted _directly_ by the petroleum, or fire, isn’t getting through. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [February 29, 2008, 9:54pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/11 "2008-02-29T21:54:48Z")
[QUOTE=Spatial Rift 47] Longwave IR is also known as Terahertz radiation, [/QUOTE] Infrared means literally “below red”, or longer wavelength than red. Terahertz radation is by definition much, much higher than red or any of the other optical wavelengths. If someone’s actually calling it longwave IR I’d like to see a cite explaining why. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 29, 2008, 11:23pm UTC](https://boards.straightdope.com/t/limits-and-alternatives-to-ir/439645/12 "2008-02-29T23:23:04Z")
[QUOTE=Cosmic Relief] Terahertz radation is by definition much, much higher than red or any of the other optical wavelengths. [/QUOTE] 😕 Terahertz radiation occupies the band from 300 GHz to 3 THz. Red light peaks at about 700 nm, which is ~429 THz. You seem to be off by a couple orders of magnitude somewhere.