Information associated with objects is typically collected using information carrying devices affixed to the objects. Information carrying devices include bar code symbols or other optically read symbols. A laser scanner, optical imager or other device scans or images the bar code symbol and decodes information encoded within the symbol.
One disadvantage of bar code symbols is that they may not be altered once printed. For example, if a bar code symbol encodes information about objects contained within a box, and then one of the objects is removed from the box, the bar code symbol may not be updated. Instead, a new bar code symbol must be printed and affixed to the box. Another disadvantage of bar code symbols is that they typically must be visible to the scanner or imager. If they are obscured (e.g., within a box), the scanner/imager may not read the symbol.
Radio frequency (RF) tags overcome these limitations of bar code symbols. Certain RF tags may be electronically rewritten with data, thereby overcoming the permanency of bar code symbols. Additionally, RF tags may be interrogated or polled through opaque surfaces, such as through boxes, to exchange data therewith.
One shortcoming of RF tags, however, is that they are expensive to manufacture. Often, RF tags are constructed using a small semiconductor chip with an associated antenna, both of which may be expensive to manufacture.
U.S. Pat. Nos. 5,204,681, 5,291,205, and 5,581,257 describe radio frequency automatic identification systems that overcome some shortcomings of RF tags. These patents describe radio frequency automatic identification systems that initially detect targets having numerous radio frequency resonators, such as quartz crystals. The quartz crystals may be made by a process of heating quartz to soften it and cutting crystals of approximate size and resonant frequency. The resonators are then produced by a process where resonance is measured to determine actual resonant frequency, and then the crystals are sorted based on certain predetermined frequency windows. The resonators may then be incorporated into a variety of objects, such as in paper. Information is attributed to the target (e.g., the paper), under the RF response characteristics of the target, such as the resonant frequencies of the resonators present, and/or the spatial locations of the resonators within the target.
In addition to quartz crystals, thin dipoles may be employed, which may be metallizations on a plastic film substrate. Information may be attributed to a target by fabricating the target with the resonators disposed at locations to encode information under a predetermined encoding system. Readers then read the radio frequency response characteristics of a target in a field near thereto of a radiating aperture that is activated by a radio frequency source.