Conventional metal detectors rely on mutual inductance or eddy current detection and make use of a coil to generate an alternating magnetic field. The range of a metal detector is typically limited to between one and two meters depending on coil size and operating frequency. Increasing the range requires larger coil diameters. Typical metal detection systems used in security applications use this technique to detect the presence of metal objects moving through an interrogation zone. Common examples of such systems are used at security checkpoints at airports or court houses. Such metal detection systems need to be calibrated in order to maintain the correct detection threshold for the operating environment.
Retail stores have long made use of “electronic article surveillance” or EAS, which is used to detect shoplifters. EAS systems commonly in use today employ magnetic or electromagnetic fields to detect tags placed on items in the stores. There has been a recent movement toward using radio frequency identification (RFID) for retail EAS. When an item is paid for the tag on that item is deactivated or removed at the point of sale. The EAS system at the exit detects the presence of active tags as they leave the store, indicating the presence of an item which has not been purchased and is being stolen. When the EAS system detects a tag, an alarm is activated for the retailer's loss prevention team.
Some shoplifters construct shopping bags lined with layers of aluminum foil to block the magnetic or radio frequency (RF) fields from detecting the tags on shoplifted items as they exit the store. As a countermeasure to foil-lined bags, some retailers are placing metal detection systems as described above at the entry/exit points of the store to detect the presence of the foil-lined bags. These metal detection systems are functionally placed side-by-side with the existing RFID systems, although these independent systems may use the same mounting hardware, stanchions or alert systems.
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