Motion detectors are devices that contain either a physical mechanism or an electronic sensor that detects motion within a specific field of view. Common motion detectors include passive infrared (PIR) and active ultrasonic and microwave. Less common types of motion detectors also include tripwires and video surveillance cameras. Passive infrared motion detectors sense body heat. Ultrasonic detectors send out acoustic pulses and then measure the sound reflecting back from any moving objects. Microwave detectors work in a similar fashion, using microwave pulses and measuring the reflections that come back from any moving objects. Ultrasonic and microwave detectors sense motion through doppler shifts in the reflected signal, similar to how a radar gun can determine how fast a car is traveling. Passive infrared motion detectors contain pyroelectric sensors. Pyroelectric sensors are made of crystal materials that produce a surface electric charge when exposed to infrared radiation. The electric charge is measured using a sensitive electronic device built into the sensor. These detectors also include a focusing element such as a Fresnel lens and a protective, transparent polyethylene covering to eliminate false readings caused by wind or rain. Passive infrared sensors are calibrated to detect human body temperature as measured from the surface of the skin, typically around 93 degrees Fahrenheit. The human body also radiates heat or infrared energy within a particular range, usually between 9 and 10 micrometers. Passive infrared or pyroelectric motion detectors are typically sensitive to ranges from 8 to 12 micrometers. PIR motion detectors sense a rapid change in the heat energy coming from an object, such as a person in motion. The detectors do not pick up slower changes in heat energy, such as pavement cooling off after the sun sets or components of an engine cooling off after shutting down. Many motion detectors contain more than one type of sensor. These detectors, called dual-technology motion detectors, typically contain both passive infrared sensors and an active type of sensor, such as microwave. Dual-technology motion detectors reduce the likelihood of false alarms. These detectors also frequently come with functions such as pet-immune features, which will ignore the movement of animals up to a certain weight. Motion detectors are used in a variety of applications, such as indoor and outdoor security systems, surveillance, and occupancy detection. Motion detectors that function as occupancy sensors detect whether or not people are occupying offices or warehouses and use that information to control heating, cooling, and lighting systems. Occupancy sensors allow companies to save substantial amounts of energy by turning on heating and lighting systems only when spaces are occupied. Motion detectors used for security or surveillance applications can trigger an alarm, turn on lights, or begin the recording of video.