Image analysis – Applications – Personnel identification
Patent
1997-10-06
2000-07-18
Boudreau, Leo H.
Image analysis
Applications
Personnel identification
382108, 374 43, 374141, G06K 928, G01B 734
Patent
active
060918373
DESCRIPTION:
BRIEF SUMMARY
SENSOR FOR ACQUIRING A FINGERPRINT IMAGE BASED ON HEAT TRANSFER
The invention relates to a method and an apparatus for measuring patterns in a partially heat conducting surface, preferrably a fingerprint.
Identification by the use of fingerprints has lately come to the fore as a result of the increasing danger of the forging of credit cards, as well as the greatly increased availability of pattern recognition algorithms. Some systems for recognition of fingerprints have already been made available on the market. The technique used to register the fingerprint varies.
Some of the previously known solutions are based upon optical technology using light with one or more wavelengths. These are sensitive to dirt and contamination, both in the fingerprint and on the sensor surface, and thus cleaning is necessary for both.
Another alternative is pressure measurements. This, however, has the disadvantage that the sensor surface becomes sensitive to mechanical wear and damage, as the sensor has to have an at least partially compliant surface.
Since these kind of sensors may be exposed to long term use in varying and sometimes demanding conditions the sensor needs to have a robust surface, to be as insensitive to pollution in the fingerprint and on the sensor as possible, and to be able to be screened electrically in order to avoid interference from outside and electromagnetic discharges that can harm the electronic circuits in the sensor. It must be capable of reading most fingerprints without being disturbed by latent prints from earlier use. It must also be capable of reading worn fingerprints in which the pattern is no longer visible. In some cases, e.g. in credit cards, it would also be advantageous if the sensor could be made compact.
In the view of costs there is also a demand for simplicity and minimizing of the number of parts.
An interesting alternative is to measure the pattern in the fingerprint using the difference in temperature between the "valleys" and the "ridges". This, however, demands very sensitive detectors, and is also sensitive regarding variations in temperature due to different circumstances. This kind of sensors is known from Norwegian patent 153,193 (corresponding to U.S. Pat. No. 4,582,193 and U.S. Pat. No. 4,429,413.
It is an object of the present invention to provide a sensor being easy to produce, making them cheap in production, and also having such small dimensions that they may be integrated in identification cards and credit cards or the like. It is an additional object to make a sensor being as insensitive to pollution in the fingerprint and on the sensor as possible, and which may be used without maintenance for long periods of time.
According to the present invention the problems related to the known solutions are solved using a method as being characterized in claim 1, and a sensor device as defined in claim 7.
The invention is related to detection of thermal structures by measuring differences in heat conductivity at the surface, preferrably a fingerprint, touching the sensor. This makes the measurements independent of the temperature of the surroundings.
Dirt and contamination will be less critical for the measurements of the sensor than in other, corresponding methods. If a relatively smooth, thin layer of contamination is present on the sensor it will, to a certain degree, affect the contrast in the picture, but the print may still be read. Larger quantities of dirt affecting the measured heat conduction may give measuring errors.
The heat conductivity is measured by heating the sensor being touched by the object to be measured, and by measuring the resulting change in temperature (relative temperature) in relation to the provided energy. The better heat conductivity the measured object has, the less the local change in temperature will be. The heat conductivity can be measured as a function of time, integrated over a period of time or at a chosen time after the heating is started or stopped. A plurality of measurements, or a continuous temperature control at each measurin
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Boudreau Leo H.
Werner Brian P.
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