Understanding the infrared thermometers’ working principle, technical specifications, working conditions, operation and maintenance is the basis for users to correctly select an infrared thermometer or use it. An infrared thermometer consists of optical system, photoelectric detector, signal amplifier, signal processing and display output. The optical system of infrared thermometer accumulates the target infrared energy in its field of view. And the size of field of view is determined by the optical components and location of the thermometer. The infrared energy is focused on the photoelectric detector and converted to corresponding electrical signal. The signal is corrected by the amplifier and signal processing circuit according to the algorithm inside the instrument and the target emissivity and then is converted to the temperature value of measured target.