Steam iron. Thermogram of the heat distribution over a steam iron whilst in use. The temperature scale runs from white (warmest, 140 degrees Celsius) through yellow and orange to mauve and black (coldest, 34 degrees Celsius). Thermography is a technique for visualising the temperature of surfaces by recording their emission of long- wavelength infrared radiation. This heat radiation is detected electronically and displayed as colour variation, different colours representing different temperatures.