Radio frequency identification (RFID) technology has gained tremendous popularity as a device for storing and transmitting information. RFID technology utilizes a tag transponder, which is placed on an object, and a reader, also referred to herein as an interrogator, to read and identify the tag. RFID technologies are broadly categorized as using either “active” tags or “passive” tags. Active tags have a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. Active tags have a longer signal range. “Passive” tags, in contrast, have no internal power source. Instead, passive tags derive power from the reader, and the passive tag re-transmits or transponds information upon receiving the signal from the reader. Passive tags have a much shorter signal range (typically less than 20 feet).
Both categories of tags have an electronic circuit that is typically in the form of an integrated circuit or silicon chip. The circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna that communicates with the reader from the tag's own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power. The chip then becomes energized and performs the communication function with the reader.
A chipless RFID tag has neither an integrated circuit nor discrete electronic components, such as the transistor or coil. This feature allows chipless RFID tags to be printed directly onto a substrate at lower costs than traditional RFID tags. These devices, which operate in a “read only” mode are entirely passive and rely on the resonances created when patterns of specific length are constructed with conductive materials. The tags are “queried” with a broadband, polarized microwave pulse and the reirradiated signal observed in the orthogonal polarization. The power spectrum of the reirradiated signal show decreases in intensity at those frequencies corresponding to the conductive resonant structure.
Optical sensors can be desirable for a variety of applications. For example, optical sensors can be useful for transporting or storage of goods, such as determining whether perishable goods sensitive to radiation are exposed to an unacceptable amount of radiation during transport or storage. Other applications include sensing radiation exposure of light sensitive documents or other light sensitive objects, such as photographic film. Remotely queriable optical sensors most generally rely on chipped RFID or near field communication (NFC) technologies coupled with standard optical detection methodologies. This means that, while effective, such sensors are generally expensive, costing several dollars to several tens of dollars apiece, thus limiting the range of applications in which they are used.
Novel techniques for reducing the cost of optical sensors would be considered a welcome advancement in the art.