Drying off. Thermogram of a man drying himself in front of a radiator, using a towel. The colour- coding shows variations in temperature. The scale runs from white (warmest), through red, yellow and green to blue (coldest). Thermography records the temperature of surfaces by detecting long- wavelength infrared radiation. The white colour of the radiator indicates that it is hot, while the man is also warm after his bath or shower. The towel, by contrast, is blue and green, because it provides insulation and does not transmit heat efficiently.