Infra-red and heat

Here’s a bit of physics that’s coming up in my lectures – what’s the connection between heat and infra-red?  You’ve probably seen imagery from ‘thermal imagers’ or infra-red (IR) cameras, usually on police shows, taken from a helicopter as it follows a suspect fleeing down some alley-way at night.  You’ll see that ‘hot’ things (like the bonnet of a car or a person) will be emitting strongly, whereas colder things emit less. Click here for a more cute example.

Infra-red is part of the electromagnetic spectrum, and is the region with wavelengths a bit longer than visible light (about 0.7 micrometres to about 14 micrometres). Our eyes aren’t sensitive to it, but it is not so difficult to build equipment that is. We ‘see’ because our eyes pick up light that has been reflected from an object (outdoors in the daytime, it will be sunlight that has been reflected). An infra-red camera will ‘see’ because it picks up infra-red light that has been emitted by an object AS WELL AS light that has been reflected (e.g. from the sun).  The sun emits a truck load of infra-red light, just as it does visible light. 

Why do everyday objects emit IR but not visible?  It’s because of their temperature. A blackbody (an object that doesn’t reflect light) will emit energy across a broad spectrum, and just where this spectrum is centred depends on its temperature. Hotter objects have a higher fraction of their emissions at shorter wavelengths than cooler objects. The sun (about 6000 Kelvin) emits a lot of energy in the visible spectrum (400 – 700 nanometre wavelengths), but an object much cooler (such as me) will emit virtually zero energy in the visible spectrum – instead it will be all in the infra-red region (and longer wavelengths still). An object somewhere between the two in temperature (e.g. a log on a fire) will  emit a bit of visible light, but this will be at long-wavelengths, so will look red or perhaps orange – it will never get hot enough to start looking green or blue.  

So an infra-red camera used at night will tend to see warm or hot things. It’s also perfectly possible to use it during the daytime; it will still ‘see’ warm objects, but interpreting the image will be complicated by the fact that the sun also irradiates the scene with infra-red. So in that sense, IR is not quite synonomous with heat – a cold object that happens to be quite reflective and have the sun on it could look quite bright to an infra-red camera.

Also, bear in mind that there are other forms by which heat can move. With infra-red (and visible) we are talking about radiation – remember that heat can pass by convection (e.g. circulation of warm air) and conduction (flow through a solid such as the bottom of your frying pan). And, while we’re at it, remember that visible (and ultraviolet) light carries heat energy that can heat an object as well – bathe yourself in u.v. and you’ll start burning nicely. So we need to be a bit careful using the words infra-red and heat synonomously.


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