Infrared is the invisible radiation lies between the Visible and Microwave portions of the electromagnetic spectrum. The term Infra means below the ability to detect visually and the term Red means colour of the light with low energy level. The Near infrared is close to the red portion of visible light and the Far infrared is close to the microwave region. Far infrared is thermal and we feel it as heat by the temperature sensors in our skin. Short and Near infrared rays cannot be detected since these are invisible as well as cool. Active infrared rays are produced from semiconductor sources but natural objects like animals including human beings emit infrared rays from the body heat. Humans at normal body temperature emits Infrared rays at the wavelength of 10 microns.PIR Sensors are used in surveillance systems to detect moving persons in protected areas.