Abstract: An infrared radiation ear thermometer has an optical system, an infrared detector, an ambient temperature sensor, and display unit, a signal processing section. Wherein, the infrared detector further includes an infrared sensor and a temperature reference sensor; the infrared sensor is deposition on the substrate and the temperature reference sensor is mount near the substrate of the infrared detector to convert the infrared signal into an electrical signal and sense the reference temperature separately. The ambient temperature sensor is set in the space near the optical system to detect the fast change of the ambient temperature. The signal processing section receives the signals from these temperature sensors to produce an offset by a mathematical algorithm. The offset is used to correct the temperature reading and maintain a high precision even though the ear thermometer suffers from an extreme temperature change.