Lies, Damn Lies and Thermal Images

Last week someone took a thermal camera and tried to create a scandal over energy waste by the Big Six energy companies. They did this by taking thermal images, properly known as thermographs, of energy company office buildings purporting to show them wasting heat. They then sent these images to picture agency SWNS who distributed them to the national newspapers. Many of the national dailies ran with the story. So far I’ve seen versions in the Telegraph, The Mirror and The Daily Mail along with The Plymouth Herald and a handful of online news sites.

Now the problem with putting a thermal camera in the hands of someone who doesn’t know how to use it is that it is an instrument of measurement, not a simple camera. The thermograph is created in false colour and in order to interpret it you need to know the temperature sensitivity and range settings used and the emissivity of the surface being imaged. Clearly the ignorant user of this particular camera just tweaked the settings until he or she got a nice bright red building against a dark blue sky intended to make us thing that the buildings were very hot.

Now I am going to reproduce the images here for purely educational purposes, to demonstrate the fallacy of thermal imagery like this, and not for any kind of commercial gain (please take note SWNS if you happen to be looking at this page).


In the image above you will notice the strident reds and oranges, intended to make you think there is a lot of heat leaking from the building. However the colour range is just a representation and can be adjusted to cover any temperature range that the thermographer chooses, simply by adjusting the upper and lower limits of the sensitivity range. Look at the photo below and then back at the thermograph. Now you should immediately notice that the surface of the carpark in front of the building is showing up as the same temperature as the first floor and that the trees to the left and right of the shot are the same temperature as the ground floor. Now either these are very hot trees or ….


Now glass itself is a tricky material for thermographers. At the near infra red glass is pretty much transparent, but becomes less so at longer wavelengths so it is pretty much opaque at environmental temperatures. Glass also has a high emissivity, which means that it is very good at absorbing and emitting radiation. Thus in windows and facades the glass is generally treated to reduce its emissivity in order to cut down on the transmission of heat. However, depending on whether the concern is heat loss from the building or heat gain from the sun, the emissivity could vary at different wavelengths. When something has low emissivity, by definition it has high reflectivity. Glass further complicates the issue by having high surface specular reflection. So without extensive checking the thermographer cannot necessarily determine what portion of the infra red detected by the camera is a result of the surface temperature of the glass and what is merely a reflection of the temperature of the surroundings.

Any reputable thermographer would ensure that the emissivity of the materials was properly accounted for and publish the temperature scale along with the image as in the one below that I produced some years ago for a well known client (you might be able to guess).


So, if you are a building owner and someone offers to undertake a thermography survey for you then please do question their credentials. If you are a newspaper reader and you see a thermograph without a reference temperature scale then do not believe your eyes.

Now I don’t have any particular love for the Big Six. But trying to create a scandal by falsifying thermal images like this is not on, and shame on agencies and newspapers who don’t even check with their science editors before publishing such rot.

3 thoughts on “Lies, Damn Lies and Thermal Images

  1. Doug

    Good to educate people on the common smoke and mirrors use of thermal imaging.

    However as someone admittedly only vaguely familiar with building physics, I have to disagree with you about IR at the wavelengths sensed by a thermal imaging camera passing through glass, it doesn’t.

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