Flats. Thermogram of blocks of flats by a canal. A thermogram shows variation in temperature on the surface of an object, measured by the long-wave infrared radiation it emits. The temperature scale is colour-coded and runs from dark blue (coldest, -41 degrees Celsius) through green, yellow and orange to red (warmest, 3 degrees Celsius).This imaging technique can be used to check the energy efficiency of buildings. The areas which appear hotter in a thermogram are where heat is being wasted.