Optical tagging tracks suspicious vehicles
Police who want to follow suspicious cars or military commanders who need to distinguish friend from foe may be able to do so with optical tagging, using systems like those reported by John Coulter of Aerospace Corp. (Chantilly, VA) and James Jafolla of Surface Optics Corp. (San Diego, CA) at Aerosense (15 April; Orlando, FL; paper #4708-87). Taking advantage of the fact that license plates are retroreflectors, Coulter applied quarter-wave plates designed for 820-nm operation to the license-plate surface. Although clear in the visible spectrum, the waveplates show a clear mark to the appropriate detector when illuminated with near-IR light.
For proof of concept, Coulter used small rectangles of quarter-wave plate (Edmund Industrial Optics; Barrington, NJ). For a detector, he used a commercial camcorder. The silicon CCD detectors in camcorders are usually sensitive to about 900 nm, Coulter says, and manufacturers install filters to block near-IR wavelengths. The Sony camera he used offers the option of removing the filters and using near-IR LEDs to take shots in the dark.
Instead of the LEDs, Coulter performed his test with linearly polarized diode lasers emitting 30 mW at 830 nm to provide greater illumination. He tilted one laser to +45° from the horizontal to produce output polarized at 45° to horizontal and the other to 45°; beam switches allowed him to select one or both. He put a polarizing filter over the camera lens that could also be set at +45° or 45°. Setting the laser to +45° and the filter to 45° gave the best signature, he said, noting that the tags stood out clearly on the license plate at 200 ft. A narrowband filter on the camera prevented the license-plate lamps from washing out the signal. He is now experimenting with a 130-mW laser and may also try focusing the beam more narrowly in order to increase the distance at which the tags can be spotted.
"What we are trying to do is come up with a solution that is extremely low cost so that it could be used by as many law-enforcement agencies as possible," said Coulter. Though some agencies have expressed interest, he said they want the tag to be completely undetectable to the naked eye, rather than merely unnoticeable. He is investigating methods for achieving that goal.
Another way to tag vehicles would be to paint a clear coating on a window. Auto glass has a wide reflectance band centered around 9.5 µm. Researchers from Surface Optics Corp. (San Diego, CA) used spectral measurement software to identify a varnish with lower reflectivity in the 9- to 11-µm band, which provided a sharp contrast to the emissivity of unvarnished glass. "Anyone with a broadband imaging contraster will see that," said Jafolla. One possibility would be to use an inexpensive, uncooled microbolometer as a detector. It would be preferable, Jafolla said, to have a coating that stood out only in a very narrow band so that only people with the correct narrowband filter would be able to see it. He declined to discuss further details.
Antonio Cantu, chief scientist for the Forensic Services Division of the U.S. Secret Service, chaired the Aerosense session on forensic science and was interested in Coulter's presentation. Since the terrorist attacks of last September, research into surveillance technologies has picked up. "There's a lot of projects like that that were on the back burner and have now moved up to the front burner," he said.
Optical tagging technologies, such as inks in currency that fluoresce under UV light, are already in use, and others are being developed, though Cantu would not go into details for security reasons. "There's a big interest in trying to do the tagging remotely, but that is very difficult," he said.
