View Full Version : Thermal imaging and water...

06-18-2003, 07:51 PM
My group of friends and myself always have completely pointless debates and arguments over the dumbest things.

So we had a debate that mutated from "Big Foot" sightings to running a manhunt in the wilds of British Columbia (actually Vancouver Island).

Anyway we were wondering if a guy was fleeing a manhunt, could they hide in a body of water (stream, pond, lake) and not be detected by thermal imaging?

I say unless his body temperature is the same as the water no way. If the water is cooler or warmer he would stick out. BUT what I don't know is how deep can that thermal imaging go?


06-18-2003, 08:14 PM
Thermal imaging doesn't have any more depth than normal optical imaging. It works on emitted rather than reflected heat. Just as with normal imaging there are objects that are transparent, but the first non-transparent surface between the sensor and the target area is what the imager records. Water isn't IR transparent, and so would be the returned source. If a person were to lie in a large body of water they would be more or less invisible. However if the equipment were sensitive enough it could potentially detect the heat from the exhaled air.

Of course all this is a bit much trouble to go to. Simply lying under dense vegetation would be a lot easier, far more pleasant and just as effective.

06-18-2003, 08:57 PM
You'd think if the guy had his head above the water (to breath) that would show up. If you got totally submerged with just a breathing tube, especially in moving water, I doubt the camera could see you, since the water wouldn't heat up enough to show much of a difference in temp, and it gets washed away anyways.

I do remember seeing a news interview with some detective/cop when the Washington Sniper was on the loose. This guy was pissed off that the media were spreading all kinds of theories as to how he/they might be eluding the law, thinking they might be giving the sniper/s even more info on how to get away. The dumb thing was he then proceeded to give a long list of examples of what he meant, himself giving out more tricks of the trade than I'd heard in all the media reports combined... didn't seem too bright an idea. Anyways he mentioned that you might want to throw a blanket over yourself when the helicopter gets close to disappear from the camera for a short while. Don't know if it would work or not.

06-19-2003, 01:18 AM

I'm not certain about this, but I think that a human's thermal emission spectrum is dominated by the emissions from warm water (we are mostly water, after all). And if water can emit [IR] light at a given wavelength, it must also absorb water at that wavelength. If this is right, it would mean that water must be opaque to the wavelengths used in thermal imaging cameras, and that a person under water would be invisible to a thermagraph, just like a person hiding in a lake full of paint would be to visible light.



06-19-2003, 02:50 AM

Oops, I look like such an idiot. In my previous post, read 'absorb [IR] light' for 'absorb water'.



06-19-2003, 03:05 AM
Yes, Infrared Imaging cameras that are sensitive to the spectrum of body heat can't see through water very well. I worked on two different infrared systems, and they were tuned to see slightly different frequency bands. One was very good at picking up body heat, but wasn't very good at seeing through fog(water). The other wasn't as good at seeing body heat, but could see through fog very well.

Since I've never tried this ( and I no longer hold that job so I can't in the future), I think the second camera mentioned above just might be able to pick up someone who was underwater as a slightly warmer looking blob, provided they were close to the surface, and the water was cold. The second used a frequency closer to visible light.

One related factoid- Mirrors reflect infrared light just fine, and glass is opaque, it acts as a mirror.

08-17-2003, 09:27 PM
Well, I saw something to revive this thread with today at the Lethbridge airshow. The HAWC1 (http://gov.calgary.ab.ca/police/inside/technologyf.html) helicopter and it's pilots were sitting there, and remembering this thread I asked one of them this same question (could your camera see a person hiding in/under water?). He said it could, and that they can even see submerged dead bodies if they are reasonably fresh (ie warm); they can direct divers where to go to find it... which seems to go against the general WAG consensus we've come up with here.

Now either the guy was lying to me (entirely possible - never trust a cop to give you the straight dope on any kind of question that could show their own weaknesses),


I'm not so up to speed on how IR radition detection works and the difference between IR and body heat. Obviously these cameras don't pick up the same physical heat I would feel by putting my hand on a person... so uh, maybe I'll bump this thread and someone else could fill in more of the gaps we have here...

08-17-2003, 10:13 PM
Some cameras are more sensitive than others, and it greatly depends on the stillness of the surroundings. A man standing inside a house against an exterior wall can be seen by IR, but only if he stays there for a long time and the wind is not cooling the outside of the house. A body could be imaged underwater, but it would have to heat the water around it; fast flowing water would tend to spread the heat over such a large and distorted area as to be undetectable/unrecognizable.

Probably the best bet for someone trying to hide would be to find some place where the readings would be expected. If you were really gutsy you could wait until the pattern of search got right on top of you, and then move with it in formation. Any thermal search would just turn up a slightely skewed march pattern, and you would end up right on the edge of the search area.

08-17-2003, 10:32 PM
OK. Here's a manufacturer's web site:


The machine shown on that page operates at wavelengths of 2-5 μm.

On this page:


You can find the absorption spectrum of liquid water. At those wavelengths, water is pretty much opaque. The light gets attenuated by a factor of around 100 for every cm of water it passes through.

08-17-2003, 11:01 PM
Not entirely on topic, but not totally off topic, either. I had a teacher in high school who was in the Naval reserves whose job for the Navy was studying satellite imagery. I asked him about seeing objects underground. He said that the thermal cameras on sats could detect anything underground that raised the surface temp of the ground by as little as 3 degrees. He claimed that they could watch magma as it rose up in volcanos. He had low-level security clearance, so who knows what the guys with high levels of security clearance got to see?

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