Heat ALWAYS moves from a hotter to colder object.
Heat is transferred by 3 different mechanisms: radiation, convection, and conduction.
- Conduction – heat transfers because one physical body is in contact with another. You put an ice pack on your aching knee, and heat flows from your warmer knee into the cooler ice pack. There has to be physical contact between the two objects.
- Convection – heat transfers because the TINY layer of air next to a wrmer object gets heated up. This can be literally just a few molecules of oxygen and nitrogen, less than a micron thick next to your body. Heat flows from your body to the air, which takes almost no heat to warm the thin layer up to your body temp. BUT: if/when that thin layer is stripped away, a cool layer replaces it, and takes a tiny bit MORE heat from you. Picture how much cooler you are with just a small breeze or fan blowing air against you. Each moment the “boundary layer” of air next to your body (that you have warmed up from room temp to your body temp) is replaced with cold air and you perceive the cooling effect against your body. Convection can occur by an external air flow (fan, wind, etc) or simply because the warmer air rises and cooler air sinks (natural convection).
- Radiation – heat transfers “line of sight” literally by light waves (visible or invisible spectra, such as IR, visible, UV, etc); think of standing IN FRONT of a hot fire and feeling the fire on your skin. This is radiant transfer of heat. EVERY physical object radiates heat energy when line of site with a cooler object.
Radiation works because of light waves (electromagnetic spectrum emission) which are radiated at a wavelength that is proportional to temperature. Lower temperature objects radiate BELOW the visible spectrum in the infrared range (IR). As they get hotter, the radiation moves into the visible spectrum. Picture a piece of metal heating up that begins to glow a dull red. If it gets really hot, we call it “white hot”, right? That is because the radiation is in shorter and shorter wavelengths, moving into “white light”.
IR cameras/sensors respond to wavelengths in a certain range that correspond to temperatures in our day-to-day routine. Human bodies, for instance. So a passive infrared sensor (PIR) in a ‘motion detector’ is triggered by radiation coming from a body temp object. In your house, that might trigger an alarm if you had a sensor like that. In your driveway, the heat of a body or car may turn on a light.
With these basic principles, lets talk about how this works. In the example given previously, when you put your hand on a wall that is colder than you, heat flows (CONDUCTION) from your hand to the wall. The area directly in contact with your hand becomes warmer than the area around it. The RADIATION from the wall is a shorter wavelength in the shape of the hand and a longer wavelength where the wall was untouched (remember: EVERY object radiates heat). FLIR is simply an array of IR sensors that translates the infrared light into an image. So you can see the handprint as a different wavelength than the surrounding area. It remains visible until that area returns to the same temp as the rest of the wall. It cools by these same mechanisms (radiation to cooler object, convection by air molecules stripping heat away, and conduction by contact to the surrounding and underlying structure).
Let’s imagine you are standing in the woods in your super ninja camo ghillie suit. Any temperature difference between the surrounding woods and your outer surface can be detected by FLIR. Radiation occurs ONLY from the most outer layer that is line-of-sit. You cannot see THROUGH a body like an xray as IR does not penetrate solid objects; REALLY short wavelength light does. Now, if you had left your super ninja suit in the woods and put it on JUST before they turned the FLIR in your direction (and had your face COMPLETELY covered), it would be the same temp as the background and so all of the radiation would be identical = you are invisible.
However you are not likely to have JUST put on your ninja suit at that moment. As you wear the ninja suit all three of those heat transfer mechanisms are working. Your body directly conducts heat into the suit where it touches you (and the layers of the suit each conduct heat to the next cooler layer) so it warms up. The radiation from the ninja suit is no longer identical to that of the surrounding woods (as it is a different temperature) so your cover is blown.
To avoid detection, you have two options.
- Be the same temperature as your surroundings – On the surface, this seems like a great idea…but my ENTIRE GOAL in spending so much time to write this is to help you avoid becoming room temperature. If you have become room temperature, you are dead (and have been dead for a few hours) You want to be body temp, not room temp.
- Block the IR ‘signature’ or shape of warmer object – this is our true goal
What you must do is have an object at the same temp as your surroundings (and it must STAY at the same temp) that completely blocks the line of sight between you and the FLIR unit. ANYTHING can block the radiation, the trick is that it must stay the same temp as the surroundings.
An earth banked barrier, foxhole, etc works fine IF there is no heat ‘seeping around the edges’. IR cameras are used around homes to look for heat loss. Think about what ‘lights up’: doors, windows, a wall where someone inadvertently left out the insulation, that sort of thing. So if you had a sheet of mylar that you were hiding behind, the mylar reflects almost all of your radiation back to you (so it does not heat up appreciably) and you would not be seen. But if the air at the edge of the mylar is warmer (as your body has warmed the surrounding air by convection) then IT will show as a hotter outline against a colder rectangle (the mylar sheet). Bang, you are found. If you have a nice cave, but heat is flowing out at the entrance, it will light up. What is needed is that the heat must be diffused and the IR blocked.