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The two major types of infrared, or IR detectors, are active and passive systems. Active systems use both an infrared light source and a sensor to sense reflected IR energy. Passive systems use only a detector that senses the difference in IR energy from a person or warm object and the room itself. Infrared is a frequency of light below the range of human eyesight that is emitted or sent out by any warm object or person.
Within these two major classifications, there are additional differences for active and passive IR detectors. A range of commercial detectors are available to sense different IR wavelengths depending on the application. These sensors can be either photonic or light-sensing, or thermal or heat-sensing devices.
A large building or room may require active IR technology to successfully monitor for intruders. The ability of an active IR system to send infrared energy into the room improves the sensing range. Passive systems are excellent for use in in homes and smaller rooms. These systems, however, can miss parts of a room depending on design of the sensor lens. More than one sensor may be needed to cover a room.
Flame or heat sensing for fire protection and laboratory uses is another use of IR. Heat sensors are often incorporated into home or business alarm systems. This technology is generally passive, with the sensor monitoring temperature changes above normal room conditions. Fiber optic systems need to be monitored for service interruptions and signal quality, and IR detectors can be used to monitor changes in the optical signal and send alerts as required.
Moisture can be measured or monitored with infrared technology. Water molecules absorb IR energy in certain wavelengths. An active IR system can scan vegetation or crops and determine moisture levels from differences in the signal. Infrared can also be used for crop sorting by scanning the different wavelengths of crops, and debris such as dirt or rocks.
Temperature sensing is a common use of these systems. Foundries and metal fabricators can use IR detectors calibrated as thermometers to measure high temperatures at a distance. The detectors can be calibrated for different metals, because metals create different frequencies when heated.
Personnel detectors are often used by law enforcement and the military. IR can detect human and animal body heat at night against a dark landscape. Detection systems can be hand-held, or deployed in aircraft for search and rescue or law enforcement.
Gas stream analysis can be performed with IR sensors. Infrared energy is absorbed differently for the components of a gas mixture. These sensors are used for automotive emissions testing, power plant exhaust gas monitoring and laboratory gas analyzers.