There are many technologies used to make motion detectors, some are active in that they emit energy such as microwave or radar and some are passive in that they absorb energy such as heat or sound.
A passive infrared detector can sense temperature changes within its field of view as low as a human walking past. Unfortunately they can also detect an animal and any change in temperature that may occur which limits its effectiveness when used externally. In order to overcome this some manufacturers have incorporated signal processing and a second detection technology to work in conjunction with the passive infrared. These detectors are referred to as dualtech detectors.
Motion detectors are best suited to internal environments where detection zones are easily controlled. When using them externally it is important to ensure that nuisance alarms are minimized by installing them in confined areas such as adjacent to buildings or across gateways. Even with a high quality detector it is not practical to use motion detection for wide open spaces when there are many other forms of detection available.