Sleep. Thermogram of a person asleep in bed. The image shows variation in temperature. The temperature scale runs from red (warmest) through yellow, green, cyan and blue to mauve (coldest). 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 with different colours representing different temperatures. In this case variations on the surface of the bed and the person's face and hands are shown.