Measuring Infrared Energy
Passive infrared sensors work by measuring incoming infrared energy. They do not emit energy themselves, which is why they are called "passive." Infrared energy is released when heat is created. Humans and animals both release infrared energy. Passive infrared sensors (sometimes called pyroelectric detectors) detect this energy and measure it against previous, or standard, levels. In this manner, they can monitor changes in the environment. On average, humans emit 9 to 10 micrometers of infrared energy.

Passive infrared sensors are used in the motion detectors that are commonly used for security reasons by businesses and residential home owners.
Photo Detectors
Passive infrared sensors are made to detect infrared energy emissions of 8 to 12 micrometers. The tool they use to detect these energy levels is called a photo detector. A photo detector takes incoming light and measures its wavelengths, or levels of infrared energy. A mirror within the sensor is what is used to capture the energy, which is then projected onto a computer chip. As the subject emitting the energy moves, the hot spot on the chip moves also, which allows the computer chip to monitor and measure the variations and speeds of the infrared energy being emitted.

The level amounts are then converted into an electrical current, which is sent through a minuscule computer contained in the sensor. The computer determines if these levels are standard levels for the supervised area or if they vary widely from normal levels.

If there are large variations in the levels, the computer may trigger an alarm, which is how many home security systems work. In fact, normal human movement can release enough variation in a supervised area to trigger such an alarm in most cases. Most passive infrared sensors will ignore any smaller emissions or slower variations in the emissions as those are usually caused by the movement of animals or by the slow rise of heat in the environment during the day.
Motion Detectors
It is common for passive infrared sensing systems to be combined with a photo-sensor detector. These motion detectors are often located near the entrance to many businesses. They consist of a light sensor in the form of a laser beam. When the beam of light is blocked, such as when a person walks through it, the infrared sensor will notice a drop in the light emission and alert a nearby control box. The control box will then react accordingly. This reaction can be something as simple as emitting a soft tone to alert shopkeepers that someone has entered the store. The reaction might trigger an alarm, as in cases where the light beam is used to monitor and protect an area such as in a museum.