y of photonic technologies could drive down prices as they have done in the telecommunications market, which reduced the cost of optical fibers and lasers. 3.11 NAVIGATING THE REAL WORLD University of Rochester researchers have created a navigational assistant that can help inform a visually impaired person of his or her whereabouts. The technology could also be used to add new dimensions to museum navigation or campus tours for sighted people. The Navigational Assistance for the Visually Impaired (NAVI) system uses radio signals to gauge when someone is near a passive transponder, which can be as small as a grain of rice. The transponder might be located on the outside CONNECTIONS IN THE AIR-WIRELESS TECHNOLOGIES of a building, inside a hallway, or on a painting or other object of interest. The system works much like a retail security tag or the cards people wave at specially equipped gas pumps to make fast, automatic purchases. With such systems, a radio signal, beamed from a detector located near a door or gas pump, is picked up and returned by the tag. The security tag system, when acti- vated, simply sets off an alarm. Purchase tags, on the other hand, can encode information, allowing the reader to debit the user's bank or credit card account to complete the sale. With the NAVI system, researchers have decided to turn things around and have made the reader portable and affixed tags to stationary objects. The pro- totype reader uses a portable CD player that's programmed to play a partic- ular track through an earphone whenever a certain tag is detected. A particular track might contain a simple message, such as, "Mr. Smith's office door," an elaborate discussion concerning a particular piece of art, or the history of an entire building. Users can switch CDs for different purposes and locations. Production versions of the device could store information in solid- state memory-like the type used in MP3 players-that's instantly updated via a wireless link whenever the user enters a new building or exhibit area. 