Office building, thermogram. 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 purple (coldest, -52 degrees Celsius) through blue, green and orange to red (warmest, 6 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.