Some homeowners and store owners are not aware that motion sensors can be created in a number of ways. Usually, they are only aware of the most common type, which is the one that projects a beam of light across the room near its entry point. This is paired with a photosensor that will detect any variation in the amount of light that is being projected, which cause an alarm to be triggered.
Automatic doors also make use of some type of sensor that will detect motions within its vicinity that will cause the doors to automatically open. This involves simple radar, which relies on microwave radio energy. A box is usually positioned above the door that detects when energy is reflected and bounced back to it from the floor to trigger the door mechanism automatically. The drawback is that these sensors may set off radar detectors based on the same technology.
These types of active sensors rely on the injection of energy such as sound, microwaves, or even UV light to determine changes made in the immediate environment it is monitoring. For most systems using lights, they make use of a passive technology that will detect infrared energy known as Passive Infrared (PIR) detectors. To detect the presence of a human being, the sensors should be sensitive to human body temperatures, which is normally in the 8 to 12 micrometers wavelength range.
You may have observed that there are instances wherein the sensors are sensitive to the motion, but not to a human being standing still. The reason for this is that the sensors are calibrated to detect rapid changes instead of slower changes in the monitored environment. Otherwise, it will be equally sensitive to and will be triggered by environmental factors like the cooling of the sidewalk at night.
The motion sensor light is designed to have a wide field of view. This is attributed to the lens that is used to cover the sensor. Keep in mind that infrared energy is a type of light, which means that it can be focused and bent using plastic lenses. Normally, one to two sensors are used in devices to effectively detect infrared energy changes in the environment that is being monitored.
You also must understand the limitations of the technology. For example, since infrared energy does not see glass as something completely transparent, it cannot easily detect a human being watching through the window. This is the same technology applied to greenhouses wherein the infrared energy is trapped inside to help heat the inside of the structure. This is the explanation why motion detectors do not see beyond glass windows.
Steven Blanger is employed as a security consultant who specializes in the use of technology in protecting the properties of his clients.