Tomographic Motion Detection (TMD) vs. Competing Technologies

Passive infrared (PIR) motion sensors

PIRPassive infrared (PIR) sensors are the most commonly used motion sensors available today. They work by detecting coarse changes in thermal energy within the view of the sensor. A human body is a different temperature than a typical background environment, and when a person moves, the change in the thermal scene is detected by the sensor.

Although PIR sensors are very inexpensive, they suffer from significant false alarm and detection vulnerabilities. Changes in temperature will cause the sensor to trip. HVAC vents, outdoor air flow, sunlight, headlights, insects, and many other forms of harmless environmental changes will cause false triggering in a PIR. Xandem TMD is unaffected by all of these causes of false triggering.

PIR sensors also have severe detection weaknesses. Lenses deteriorate over time (especially in dirty environments) until the sensor will not function properly, and intruders can use chemical sprays to mask the sensor. Glass, foam, and other insulating materials can allow an intruder to easily pass without being detected. These vulnerabilities were recently demonstrated by the Discovery Channel television show "MythBusters" in an episode entitled "Crimes and Mythdemeanors." Xandem TMD on the other hand is virtually impossible to defeat when installed properly.

Finally, PIR sensors are unsightly. High-end residential and commercial clients prefer Xandem TMD due to the fact that it can remain completely hidden from view - no more distracting and unsightly white boxes in the corners of every room. With TMD, the walls and furniture themselves can be the motion detection sensor!

Photoelectric Beams

Photoelectric Beam DetectionPhotoelectric beams and lasers use rays of light that travel from a source to a detector. When a person or object crosses the path of light, the light beam is blocked and the sensor detects the obstruction.

Beams are problematic in areas where objects are moved frequently because they cannot see through obstructions. For example, boxes, equipment, or vehicles may be accidentally placed in locations that block the beams. This is very typical in busy commercial environments like warehouses, shops, industrial areas, and storage facilities. Xandem TMD sees right through obstructions, so people are free to use the area as they want without worrying about sensor blockage.

Beams only cover a single line-of-sight, so they leave the detection area largely uncovered. It is surprisingly common for an intruder to enter an area through walls or ceilings, thus completely bypassing a beam detection system. If an intruder is able to enter the area without crossing the line-of-sight of the sensor, he is free to move about the area without being detected. Xandem TMD covers entire areas or volumes, not just lines. It doesn't matter how someone enters the area, TMD will detect the movement.

Beams must be carefully installed an maintained on a regular basis. If the beam emitter or detector gets bumped, it may require re-calibration and manual adjustment to get it pointing back to the detector. If the detection area is prone to dirt, dust, soot, or smoke, regular maintenance must be performed on the beam (lens cleaning) to avoid sensor malfunction. Xandem TMD can be bumped or moved and the sensing will remain completely functional. TMD is not affected by dirt, dust, soot, or smoke.

Dual-tech (hybrid) motion sensors

Dual-tech (hybrid) sensors combine multiple forms of motion detectors to try to reduce the probability of a false alarms. The most typical configuration is a microwave and PIR sensor both housed in the same unit. For an alarm to occur, both sensors must detect motion. If an HVAC vent turns on, for example, the microwave will not detect motion. If motion happens on the other side of the wall, the PIR sensor will not activate, thus no false alarm. Some forms of motion are still problematic. For example, when an HVAC turns on, the air is blown by a nearby metal fan which may cause the microwave detector to trip at the same time.

By attempting to mitigate false alarms in dual-tech sensors, vulnerabilities are introduced. A dual-tech sensor is only as strong as its weakest link. For example, if one blocks the sensor with a piece of foam or glass, the PIR sensor will remain untriggered. Even though the microwave sensor is triggered, the system as a whole does not react. Thus, each time a new sensor is combined in a hybrid configuration, the detection capabilities are weakened. Xandem TMD is not a hybrid solution, it is a single form of motion sensing that powerfully detects while remaining immune to the leading causes of false triggering.

Find out more about detection vs. false alarm trade-offs by reading about "receiver operating characteristics."


Video cameraCameras excel in applications where a high level of visual detail is required, but they suffer from significant false alarm issues in many applications. Even a small fly landing on a camera lens can cause a false alarm, and changes in lighting (sunlight, headlights) are problematic. Xandem TMD remains completely unaffected by these minimal amounts of harmless motion, and can be combined with cameras when visual recording is needed.

Analytic software exists that mitigates false alarms through video processing algorithms. This software is generally very expensive, requires a computer to process the video, and is very cumbersome to tune parameters correctly. Simply put, cameras are the wrong tool for motion detection - they are like using a saw when you need a knife.

Xandem TMD can be combined with camera systems for a powerful integrated system. When TMD reliably detects motion, the system can begin camera recording and alert security personnel. Many resources will be saved because TMD is a much more reliable form of motion detection.