Radiator, thermogram footage. Thermography records the temperature of surfaces by detecting long-wavelength infrared radiation. The colours show variations in temperature. The scale runs from red (warmest), through orange, yellow, green, and light blue to dark blue (coldest). A colour-coded temperature scale is at right, ranging from 20 degrees Celsius (dark blue) to 56 degrees Celsius (red). In this footage, the radiator is rapidly filled with hot water, and then cools to room temperature.