What is electronic article surveillance and how does it work?

Electronic article surveillance is a technological method for preventing shoplifting from retail stores. Special tags are fixed to merchandise or books. These tags are removed or deactivated by the clerks when the item is properly bought or checked out. At the exits of the store, a detection system sounds an alarm or otherwise alerts the staff when it senses active tags. For high-value goods that may be manipulated by the shoplifters, wired alarm clips may also be used.

Type of EAS

There are several major types of electronic article surveillance systems:

• Radio frequency
• Magnetic, also known as magneto-harmonic
• Acousto-magnetic, also known as magnetostrictive
• Microwave
• Video surveillance systems

Radio-Frequency Systems

Radio Frequency (RF) Systems are the most widely used systems in the United States today and RF tags and labels are getting smaller all the time. The RF EAS system works like this: A label -- basically a miniature, disposable electronic circuit -- attached to a product responds to a specific frequency emitted by a transmitter antenna (usually one pedestal of the entry/exit gate). The response from the label is then picked up by an adjacent receiver antenna (the other pedestal). This processes the label response signal and will trigger an alarm when it matches specific criteria. The distance between the two gates, or pedestals, can be up to 80 inches wide. Operating frequencies for RF systems generally range from 2 to 10 MHz (millions of cycles per second); this has become standard in many countries. Most of the time, RF systems use a frequency sweep technique in order to deal with different label frequencies.

Sometimes both the transmitter and receiver are combined in one antenna frame -- these are called mono systems and they can apply pulse or continuous sweep techniques or a combination of both. According to experts, mono systems could be effective for you if your store's entry is small. The mono system is used with hard labels, which are slightly more expensive than paper labels used with RF sweep techniques.

Antennas made by Checkpoint Systems, one of the largest manufacturers of EAS products, emit a low-energy RF pulse, which "listens" for the tag. This technology, known as digital signal processing, actually "learns" about its surroundings so that it can accurately distinguish between the tag signal and extraneous noise. Store employees love this because it virtually eliminates false alarms.

These tags are essentially an LC tank circuit that has a resonance peak anywhere from 1.75 MHz to 9.5 MHz. The most popular frequency is 8.2 MHz. Sensing is achieved by sweeping around the resonant frequency and detecting the dip. Deactivation for 8.2 MHz label tags is achieved by detuning the circuit by partially destroying the capacitor. This is done by submitting the tag to a strong electromagnetic field at the resonant frequency which will induce voltages exceeding the capacitor's breakdown voltage, which is artificially reduced by puncturing the tags.

There are many different ways to implement an RF system. The basic idea is that the tag has a helical antenna etched from thin aluminum bonded to a piece of paper. At the end of the antenna is a small diode or RC network that causes the tag to emit a radio signal in response to the radio signal it receives. To disarm the tag, a strong RF pulse (much stronger than the gates emit) blasts the tag and burns out the diode or RC components. Between the antennas, a burned out tag does not emit a signal so they let it pass without an alarm.

Magnetic Systems

These tags are made of a strip of amorphous metal (metglas) which has a very low magnetic saturation value. Except for permanent tags, this strip is also lined with a strip of ferromagneticmaterial with a moderate coercive field (magnetic "hardness"). Detection is achieved by sensing harmonics and sum or difference signals generated by the non-linear magnetic response of the material under a mixture of low-frequency (in the 10 Hz to 1000 Hz range) magnetic fields.

When the ferromagnetic material is magnetized, it biases the amorphous metal strip into saturation, where it no longer produces harmonics. Deactivation of these tags is therefore done with magnetization. Activation requires demagnetization. Due to the system being able to deactivate and re-activate this type of system is extremely suitable for libraries.

Magnetic systems are often referred to as "Electromagnetic" (or EM) systems. EM systems rarely appear in retail environments outside of Europe.

Acousto-Magnetic Systems

These are similar to magnetic tags in that they are made of two strips, a strip of magnetostrictive, ferromagnetic amorphous metal and a strip of a magnetically semi-hard metallic strip, which is used as a biasing magnet (to increase signal strength) and to allow deactivation. These strips are not bound together but free to oscillate mechanically

Amorphous metals are used in such systems due to their good magnetoelastic coupling, which implies that they can efficiently convert magnetic energy to mechanical vibrations.

The detectors for such tags emit periodic tonal bursts at about 58 kHz, the same as the resonance frequency of the amorphous strips. This causes the strip to vibrate longitudinally by magnetostriction, and will continue to oscillate after the burst is over. The vibration causes a change in magnetization in the amorphous strip, which induces an AC voltage in the receiver antenna. If this signal meets the required parameters (correct frequency, repetition etc.) the alarm is activated.

When the semi-hard magnet is magnetized, the tag is activated. The magnetized strip makes the amorphous strip respond much more strongly to the detectors, because the DC magnetic field given off by the strip offsets the magnetic anisotropy within the amorphous metal. The tag can also be deactivated by demagnetizing the strip, making the response small enough to that it will not be detected by the detectors.

These tags are thicker than magnetic tags and are thus seldom used for books. However they are relatively inexpensive and have better detection rates (fewer false positives and false negatives) than magnetic tags.

Microwave Systems

These permanent tags are made of a non-linear element (a diode) coupled to one microwave and one electrostatic antenna. At the exit, one antenna emits a low-frequency (about 100 kHz) field, and another one emits a microwave field. The tag acts as a mixer reemitting a combination of signals from both fields. This modulated signal triggers the alarm. These tags are permanent and somewhat costly. They are mostly used in clothing stores.

Source Tagging

Source tagging is the application of EAS security tags at the source, the supplier or manufacturer, instead of at the retail side of the chain. For the retailer, source tagging eliminates the labor expense needed to apply the EAS tags themselves, and reduces the time between receipt of merchandise and when the merchandise is ready for sale. For the supplier, the main benefit is the preservation of the retail packaging aesthetics by easing the application of security tags within product packaging. Source tagging allows the EAS tags to be concealed and more difficult to remove.

False Alarms

A major concern with these systems is when false alarms occur. A false alarm (or false positive) is when the alarms go off when a person passes through the gate without having stolen any merchandise. This most often is due to tags on merchandise not being properly deactivated.

In some older systems electronic devices have been known to set off alarms. RF systems can react even to a coil of wires (for example, a Cat5 cable carried by a forgetful network technician) because stray capacitance forms a LC circuit within the coil itself. It is also possible for alarms to go off without anyone crossing the gates. Such interference is often due to other nearby alarm systems.

Tag Orientation

Except for microwave, the detection rate for all these tags depends on their orientation relative to the detection loops. For a pair of planar loops forming a Helmholtz coil, magnetic field lines will be approximately parallel in their center. Orienting the tag so that no magnetic flux from the coils crosses them will prevent detection, as the tag won't be coupled to the coils. This shortcoming, documented in the first EAS patents, can be solved by using multiple coils or by placing them in another arrangement such as a figure-of-eight. Sensitivity will still be orientation-dependent but detection will be possible at all orientations.

Detaching

A detacher is used to remove re-usable hard tags. The type of detacher used will depend on the type of tag. There is a variety of detachers available with the majority using powerful magnets. Any store that uses an anti-shoplifting system and has a detacher should take care to have it secured so it cannot be removed. Some detachers actually have a security tag inside them to alert store personnel of them being removed from (or being brought into) the store.