Monday, 10 April 2017

I want a heat camera!

More technically it's called a thermograph, but I thought the Greek may put half the readers off. It probably has done now! I borrowed one before, and here are some examples of the photos.

Some sushi. Getting warm, Eat up!
There are two problems with these cameras. First they are really expensive. Thousands of dollars in fact. This has been somewhat remedied now that we're all walking around with cameras built into the phones in our pockets, but even the FLIR for android costs over 300 dollars. There has been a kickstarter project for a while to deliver cheaper thermographs, but I'm not sure if anything came of it. 

The other problem is working out exactly what they are showing you. Unlike a regular camera, which seeks to show different colours at brightnesses in rough proportion to how your eye would see them in the real world, thermographs seek to show the temperature of objects. The camera doesn't actually know the temperature of the objects, it just knows how much heat they are radiating in the infrared spectrum, and will show this value using a different colour. Bodies radiate heat depending partly on their temperature, but also on their emissivity. I find it very difficult to fully understand the idea of emissivity and will need to write more about it later.

A neighbour's house: hotter downstairs
Heat radiates in proportion to the fourth power of the absolute temperature of an object, and something called the Stephan-Boltzmann constant. It also depends on the emissivity, which is 0 for a shiny mirror and 1 for a black body. Mirrors confuse me, because when you look at them it seems like they are radiating your image back to you. I have trouble making the mental leap that this means they are not radiating anything themselves. But that is the situation. I also wonder, when a thermograph looks at a window how much if is seeing the temperature of the glass rather than the temperature of what is beyond, or a reflection of the temperature around you. I guess on a very cold clear night, the amount of radiation reflected from the outside sky will be negligible.

 I also have trouble remembering how many m's and s's there are in emissivity.

Black body?
Everything else has an emissivity between 0 and 1, and most organic, oxidised or painted surfaces have an emissivity of around 0.95. A further complication is that the emissivity is different depending on the wavelength of the radiation, so some materials may reflect more radiation at lower frequencies. Thermographs must work this out, ideally allowing you to choose what frequency radiation you are looking at.

Thermographs don't know the emissivity of an object, so they must be told. Here is a thermograph of our cat. She has a black body, but in the picture it comes out yellow, and her face is white hot. If she had been male we could have called him Stephan Blotzmann. 

In fact I don't really want a heat camera, I want a temperature camera, but a thermograph would be a nice toy.  

Raytekjapan has more data on emissivity of several materials.