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 it 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.

Energy efficient homes will 'boost economy'

News from the BBC here about Scottish investment in housing energy efficiency, which will pay itself off for years.

They include this stock photo to symbolise an energy efficient house.



What does the photo tell us?

On a superficial level, it's a thermograph, which tells us that houses are giving off heat, and the fact we've taken a picture of the house means that we care about heat. So, it says low energy building.

A brief analysis of the photo tells a different story. The different colours indicated different temperatures, going from black for the coldest part of the picture (deep space high in the sky) to white for the hottest part of the picture, which is the parts of the upstairs wall away from the lintels.

The windows are colder than the walls.

Does this mean that the windows are doing a better job at insulating than the walls? I guess this is possible for an old house where nice new double-glazed windows have been installed, but no effort has been made to insulate the walls. Or are we seeing through the windows into the room inside, which is colder than the outside walls and the roof?

The gable end is cooler too.

Does that mean the gable end is better insulated than the front of the house? This would be a good idea as end terraces have a lot more external surface area, and need more insulation to reach the same energy efficiency as the rest of the terrace. But I thought there was no insulation in the front wall?
The front gate looks pretty warm too. Interesting. Is it heated?

The house next door seems to be equally red along the wall, and along the roof, except for an area going up into a point on the roof. Could that be the shadow of a tree?

Just a guess, but this picture was probably taken on a sunny afternoon. All the heat it shows has come from the sun, hence the warm gate and south-facing walls. The east-facing gable end has been in the shade for a while, as has the neighbour's wall and roof in the shade of the tree. The windows are cooler because a lot of the heat is going through them into the house rather than warming them up or reflecting into the camera. The bushes and trees in the garden are cooler still, because they do an even better job at absorbing the heat. Also the trees, and probably the windows too, have lower emissivity, so even if they are hotter, they'll radiate less and the thermograph won't know about it.

A thermograph taken in the day time will tell you almost nothing about the energy efficiency of a house. You need to take the picture on a cold night, when the heating inside is turned up high. Even then it's not obvious what the picture is telling you.