Optics: measuring and testing – Document pattern analysis or verification
Patent
1997-01-13
1998-06-09
Font, Frank G.
Optics: measuring and testing
Document pattern analysis or verification
G06K 974
Patent
active
057643474
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an optical imaging system for imaging an object, mainly for a fingerprint reading apparatus, which comprises a total reflection prism with a total reflection surface that constitutes an object plane for receiving the object, the total reflection prism has a first transmitting surface for receiving light to illuminate the object when placed on the object plane through the interior of the prism, and a second transmitting surface for transmitting the light reflected from the total reflection surface; optical means with an optical axis and an input and output pupil, the input pupil receives the reflected light transmitted through the second transmitting surface and the optical means forms the image of the object plane in an image plane which is perpendicular to the optical axis; and an electronic image detector means located in the image plane.
Such a fingerprint identifying optical system is described in U.S. Pat. No. 5,187,748. In this patent a semi-transparent mirror is placed in slant position in the path between the total reflexion prism and the optics, the mirror divides the light coming from the prism, therefore the image will pass not only to the image detector but it will also enable direct observation. The total reflexion prism is arranged in the optical axis and its object plane is in a slant position relative to that axis. This slant position of the object plane is characteristic to all systems using a total reflexion prism.
In EP 0 585 141 A2 the image of a finger is provided by using a total reflection prism, an imaging optics and a CCD camera. The object plane is placed on the optical axis of the imaging optics and the consequence of this arrangement is that the image is distorted just as in case of the previously cited publication.
When a slant object plane is imaged to an image plane extending normal to the optical axis, owing to the path difference between the rays arriving from opposing edges of the object, distortions and an unacceptable decrease in image resolution will take place that can be compensated only by use of a sophisticated lens system and simultaneously by selecting a large object-to-optics distance. The large distance increases the size of the device and renders the handling more difficult or even excludes some applications, where the size is critical. The complexity of the lens system is a factor that increases costs.
A partial compensation for the path difference can be learned from EP 0 361 987 A1, wherein in the light path between the total reflexion prism and the optics a further prism and between the optics and the image detector a pin hole was inserted. Although by this solution the picture distortion was decreased below 5%, owing to the increased number of optical elements and to the presence of the pin hole a decreased sensitivity was obtained, i.e. a very high illumination is required for obtaining an image of acceptable brightness.
The object of the invention is to provide an optical imaging system of the kind defined hereinabove which, in spite of the slant object plane relative to the optical axis, can produce images with acceptable distortion and good resolution and has sufficient sensitivity, furthermore which has a geometrical length substantially smaller compared to known systems. The reduced dimensions are crucial for constructing a compact fingerprint reading apparatus.
According to the invention it has been recognized that the above problems can be decreased if the total reflexion prism is offset relative to the optical axis in normal direction so that the center of the object plane will not lie on the optical axis any more. From this offset arrangement it follows that the image detector i.e. a CCD matrix detector should also be placed in a position offset from the optical axis. This position corresponds to the one when the object plane is imaged in the image plane. The offset arrangement enables an easier dimensioning of the optics, the geometric distortion due to the geometrical differences in the path length decreases to an accep
REFERENCES:
patent: 3850510 (1974-11-01), Baker
patent: 4783167 (1988-11-01), Schiller et al.
patent: 5175593 (1992-12-01), Kumagi et al.
patent: 5222152 (1993-06-01), Fishbine et al.
Kallo Peter
Kiss Imre
Podmaniczky Andras
Talosi Janos
Dermo Trade Gyarto Es Kereskedo Rt.
Font Frank G.
Merlino Amanda
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