One of the most common applications of RFID technology is access control. Many buildings have embedded RFID transponders in identity cards. Each time a card is issued, a company’s security department indicates which areas of a building that individual is allowed or not allowed to enter. So controlling access to buildings (or to specific rooms within a building) is easy.
Using an ID tag to detect a person in a forbidden area is more problematic, however. Most access-control systems employ low-frequency (LF) or high-frequency (HF) transponders. These have a relatively short read range of approximately three feet (1 meter) or so. That’s insufficient to detect someone in a fairly large room, unless he or she must go through a doorway.
Ultrahigh-frequency (UHF) tags have a longer read range—upwards of 20 or 25 feet—but the human body absorbs RF energy at UHF frequencies (because the body is mostly composed of water), thus making it difficult to read tags through a person’s body. So it might be difficult to detect an individual in an area with any reliability, unless he or she had to walk through a doorway.
It is possible to embed an active tag in an ID badge, though its battery would require the tag to be a bit thicker than normal badges (as thin-film batteries catch on, this might no longer be an issue). Active tags broadcast a signal enabling them to be read consistently over distances of 300 feet (100 meters) or more. But active talks would be more expensive, and require that a tag be changed before the battery loses its charge.
So it is possible—but the reliability of such a system would depend on how that system was designed, as well as the technology chosen.
—Mark Roberti, Editor, RFID Journal
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