How do infrared thermometers work?

All matter- liquid, solid, or gas--constantly exchanges thermal energy in the form of electromagnetic radiation with its surroundings.  If there is a temperature difference between the object in question and its surroundings, there will be a net energy transfer in the form of heat.  This means that a colder object will be warmed at the expense of its surroundings, and a warmer object cooled.  If the object in question is at the same temperature as its surrounding, the net radiation energy exchange will be zero.  In either case, the characteristic spectrum of the radiation depends on the object and its surroundings' absolute temperatures which is a relative to absolute zero (0 K, –273.16°C, –459.69°F).  Infrared thermometers take advantage of this "radiation dependence" on temperature to produce a value for the targeted object and displays the results for the operator to read.