Thermogram of a telephone receiver. Thermograms show the emission of infrared radiation, displaying it as colour-coded surface temperatures. The range shown is from 23 degrees Celsius (purple), through blue, green and yellow, up to 35 degrees Celsius (red). The hand is gripping the telephone receiver very tightly. The fingers have less blood flow as a result and are therefore cooler (green/yellow) than the rest of the arm. The telephone receiver is also hot (red) indicating that the plastic has absorbed heat from the hand.