What can defeat it?

How deap will it penitrate when looking at water?

IR is light that humans can't see caused by
heat.
FLIR is not radar so it doesn't "penetrate".
If heat is radiated, the FLIR can detect it.

There are companies (like Barracuda) that make netting for vehicles and people that will defeat FLIR and thermal sights

Quoted: What can defeat it? How deap will it penitrate when looking at water?

They can't see through glass, for one.

I dunno about how much water they can see through.

Quoted: IR is light that humans can't see caused by heat. FLIR is not radar so it doesn't "penetrate". If heat is radiated, the FLIR can detect it.

So what will block the heat from say a human body?

If under water, how deep?

This is a tricky question for anyone to answer. Under perfect conditions, FLIR works really well. At any other time, it all depends on how much experience the operator has looking at the display.
Like ARDunstan said, it's not RADAR, but it still is Electro-magnetic Radiation.

Here's our Physics lesson for the night. Light doesn't technically reflect in the way you think it does. Light (EMR) is actually retransmitted, and that's what we see. Same thing with FLIR. The tuned EMR is retransmitted by the object the EMR illuminates. THe sensor then picks up this retransmission and software makes sense of it all for the operator to decide what he's looking at.
So, if you know what the frequency is, all you have to do is to make sure whatever it is you're wearing or hiding behind can't retransmit that frequency.
Like iamblades said, Glass cannot retransmit infra-red or ultra-violet (that's why glass houses are hot; since the glass cannnot generate enough energy to retransmit either IR or UV, it just gives off heat instead;) you can hide behind glass, and not be detected by an active infra-red system. But they have other things for that.

Quoted: Quoted: IR is light that humans can't see caused by heat. FLIR is not radar so it doesn't "penetrate". If heat is radiated, the FLIR can detect it. So what will block the heat from say a human body? If under water, how deep?

For humans, it would have to be some sort of enclosure because
clothing does not block IR.
Though camo BDUs do a halfway decent job of blocking IR, you still have the
exhalation of breath to deal with. Use a gasmask I guess.
Also, no exposed skin.

For water, read this

Quoted: This is a tricky question for anyone to answer. Under perfect conditions, FLIR works really well. At any other time, it all depends on how much experience the operator has looking at the display. Like ARDunstan said, it's not RADAR, but it still is Electro-magnetic Radiation. Here's our Physics lesson for the night. Light doesn't technically reflect in the way you think it does. Light (EMR) is actually retransmitted, and that's what we see. Same thing with FLIR. The tuned EMR is retransmitted by the object the EMR illuminates. THe sensor then picks up this retransmission and software makes sense of it all for the operator to decide what he's looking at. So, if you know what the frequency is, all you have to do is to make sure whatever it is you're wearing or hiding behind can't retransmit that frequency. Like iamblades said, Glass cannot retransmit infra-red or ultra-violet (that's why glass houses are hot; since the glass cannnot generate enough energy to retransmit either IR or UV, it just gives off heat instead;) you can hide behind glass, and not be detected by an active infra-red system. But they have other things for that.

So, if I read this correctly... it's old news?

I think that there was an old thread here about this very subject. The thread author was taking pictures of himself through an ir scope. I don't remember what most of the pictures looked like, but I do remember that those silver emergency thermal blankets worked very well for hiding from the scope.

No, it's still a good system. Everything has it's limitations, but I have to admit I never met anyone that successfully could move under thermal imaging when they went to the White Hot/Black Hot software. But even that has it's limitations.

Btt for the input of the day crew...

My experience:

Glass tends to reflect the ir image, just like a mirror.  

A standing body of water should give pretty decent protection against someone on shore looking in as you have to get beyond the angle of reflection before you can see in.