Motion sensing lights can work with a variety of different technologies. Which motion lights will work best for a particular residence will depend on the surroundings and what the lights are mean to detect. In general, there are two types of motion sensing technology used for such lights: active and passive. Active motion sensors are the more traditional type of system, and use either sound or some type of electromagnetic energy to detect an alteration in the surrounding environment. Normal light beams are sometimes used, such as lasers, but these typically need a sensor point located somewhere else, and motion detecting lights are designed to be more open-ended. In systems that use radar, radio waves are sent out toward a certain area until they meet objects and are reflected back. Some type of sensor is located on the light and is used to pick up the returning radio waves. Like military radar systems, the sensor can measure the returning waves and detect how far they have travelled, allowing the system to judge the distance to the ground or walls. When radio signals return with different information, suggesting that a sizeable object has moved into the space normally free for radio waves to travel, this information triggers the light. Sonar systems are also used in a similar manner, but use sound waves instead of radio waves.
Passive systems, on the other hand, are almost always set up to detect infrared radiation. Infrared waves are how heat travels through the air, which is why infrared systems can be used to see in the dark, detecting warm and cool areas instead of normal light reflections. While there is always a certain amount of infrared radiation all around (the Earth itself is a primary source of infrared energy), some objects emit much more energy than others--living bodies, for example. The human body is very hot compared to its surroundings, and releases infrared waves measured from 9 to 10µm (micrometers).
Infrared systems use a pyroelectric sensor to pick up a specific range of infrared energy. The sensor is made of a crystal that generates a light electrical charge based on the infrared radiation that strikes it. The changes in electrical energy can be measured by using a magnifying lens, an amplifier and a separate sensing device. If the infrared energy falls within the human level, the light is triggered to turn on. This is the most widespread system for motion detecting lights, since it is far less likely to be activated by moving leaves or other inanimate objects that do not produce as much heat.