Mobile telephone use. Thermogram of a woman using a mobile or cellular telephone showing the variation in temperature. The temperature scale runs from red (warmest), through yellow, green and cyan, to blue (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. Mobile telephones use low-power radio signals to transmit a signal calls to an antenna in the user's local area or cell. The call will then be connected to another mobile telephone, or, if necessary, a conventional telephone exchange.