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17 Jun 2002
A U.S. patent has been granted to Advanced PrecisionTechnologies for a miniature fingerprint sensor using atrapezoidal prism and a holographic optical element.
"Basically what we've come up with is a way of usingholographic optical elements to image fingerprints in such a wayto eliminate distortion," said Bruce Pastorius, the president andco-founder of Advanced Precision Technology.
The miniature fingerprint sensor has a holographic phasegrating glued to the base surface of a trapezoidal prism. Theprism operates on light which reflects internally from a finger-coverslipinterface, protecting the exterior surface of thegrating. The reflected light is diffracted to propagate backthrough the base surface and out the top parallel surface of theprism in a converging beam. A mirror folds the high-contrastconverging light beam and directs it into a camera system tocapture the detailed images of ridges, valleys, and pores of afinger-surface interface.
The holographic phase grating which diffracts reflectedlight into a converging beam eliminates distortion due todimensional compression and "hot spots" in the reduced image. The converging beam from the phase grating also allows shorteroptical lengths and significant dimensional miniaturization ofthe sensors.
Pastorius feels that the most significant benefit will bethe ability to transmit fingerprint data for quick reference.
"The problem has been getting a print which can match up,"said Pastorius, noting that transmission of fingerprints requiresthe same optical access.
The holographic system also eliminates the distortioninherent in ink-based prints. "If it goes through the lens itdistorts the print image," Pastorius said of the holographicprism option.
The prismatic convergence method also reduces the length ofthe necessary optical focal lines from 12 inches to about 2 to 3inches. "We're not only able to correct the distortion butprovide it in a very small package," notes Pastorius.
Advanced Precision Technology was founded in 1993 to developoptical processing and correlation technologies for biometricidentification. "We didn't know anything about fingerprinting. We just approached it from an optical imaging standpoint,"Pastorius remarked.
APT co-founder Dr. Joseph Barbanell had a 1992 U.S. patentfor a holographic credit card with automatic authentication andapproval, and APT contracted with San Jose State University tobuild an auto-correlator based on that patent. Upon completionof the proof-of-concept, the team constructed a working opticalcorrelation system which would match a fingerprint imagepresented on a prism to one pre-stored as a holographic filterfixed within the object beam upon an optical table. This systemwas completed in 1994.
Although demonstrations were successful, testing fordiscrimination effectiveness and false reject/accept raterevealed that they were not receiving the "pure" discriminationpossible from the matched filtering algorithm. The experimentsshowed that the problem was caused by the ink-based fingerprintused in making the filter.
APT and San Jose State developed a holographic phase gratingand placed it onto the capture surface. They were able toredirect the reflected light waves diffracting from the capturesurface and propagate them normally back into the right angleprism. They were reflected out the remaining right angle surfaceof the prism by a second total internal reflection at theinternal hypotenuse system of the prism. In addition toeliminating image distortion due to dimensional compression, thismethod also eliminated the necessity for optically correctingastigmatism in the image before capture.
The team later discovered that by designing the output ofthe holographic optical element to converge to a point source,the focal length could be shortened, reducing the size of theoptics block. Holographic prisms and a spatial light modulatorwere added to the system. In July 1997 APT contracted withStanford Research Institute to reduce package size and design formanufacture, and in 1998 prototypes for law enforcement andcommercial applications were delivered.
After feedback from the prototype users, APT madeenhancements to geometric accuracy, dynamic range, uniformillumination, and the ability to meet the highest federalstandards. In October 1998 APT contracted with NewportCorporation to improve the opto-mechanical design, insuring arugged optics block and allowing commercial consistency inmanufacture.
"It appears we're in the right place at the right time,"said Pastorius of the six-year effort. "It's now the right time."
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