Optical: systems and elements – Lens – With reflecting element
Reexamination Certificate
1999-08-04
2001-11-27
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C359S737000, C359S798000, C359S831000, C356S071000, C382S127000
Reexamination Certificate
active
06324020
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical acquisition apparatus for use with an image capturing and recognition system. In particular, the present invention includes an optical acquisition apparatus for reducing or substantially eliminating trapezoidal distortion in images of patterned objects and allowing such images to be more sharply focused.
BACKGROUND
Patterned object recognition systems are becoming common in industrial and commercial settings and have a variety of uses. For example, such systems can be used in scanners for the scanning of text, drawings, and photographs. Recently, manufacturers have been attempting to reduce costs associated with pattern recognition systems to make them more viable for consumer use. One such consumer application for pattern recognition systems includes fingerprint acquisition and recognition. Such a system is useful, for example, to enhance computer security by reading a potential user's fingerprint to compare with the fingerprints of users authorized to use the computer or access certain files or functions of the computer. Such a system could, for example, take the place of a security system that uses a login name and password.
The first thing such a fingerprint recognition system, or any pattern recognition system, must be able to do is to accurately acquire the fingerprint, or other pattern, for analysis. A number of mechanisms exist for such acquisition of pattern data. For example, U.S. Pat. Nos. 3,975,711; 4,681,435; 5,051,576; 5,177,435 and 5,233,404 all disclose apparatuses for acquiring an image of a patterned object.
FIG. 1
shows a schematic diagram of one such prior art optical fingerprint capturing and recognition system. In
FIG. 1
, an optical recognition system
108
includes a light source
112
, an optical triangular prism
110
, a lens assembly
114
, an image sensor
116
, and a storage and processing unit
125
. The prism
110
includes an imaging surface
118
, a light receiving surface
120
, and a viewing surface
122
. Imaging surface
118
is the surface against which a patterned object, such as a fingerprint, is placed for imaging. The light source
112
, which may, for example, be a light emitting diode (LED), is placed adjacent to light receiving surface
120
and generates incident light
124
that is transmitted to the optical prism
110
. The optical prism
110
is an isosceles right triangle, with the angle opposite the imaging surface
118
being approximately 90 degrees and the other two “base” angles (that is, the two angles of an isosceles prism that are equal) each being approximately 45 degrees.
Generally, incident light
124
strikes imaging surface
118
at an angle
126
with the incident surface normal line
115
. Angle
126
is greater than the critical angle
128
. In general, a critical angle is measured between an incident light ray and a normal line to a surface. If incident light strikes a surface at an angle greater than the critical angle, the incident light will undergo total internal reflection off the surface, if the incident light strikes the surface at an angle less than the critical angle, the incident light will substantially pass through the surface. Accordingly, critical angle
128
is the angle with the normal line to the imaging surface
118
above which incident light will totally internally reflect from imaging surface
118
and pass out of prism
110
as reflected light
130
through viewing surface
122
.
Reflected light
130
passes through lens assembly
114
located adjacent to viewing surface
122
. Lens assembly
114
may contain one or more optical lenses. Thereafter, light from lens assembly
114
is captured by image sensor
116
. Image sensor
116
, which may, for example, be a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, captures optical light images and converts them to electrical signals. Such image sensors are well known to those skilled in the art. The electrical signals are then transmitted to the storage and processing unit
125
.
Storage and processing unit
125
may include a memory unit, a processor and an analog to digital converter (not shown). The analog to digital converter converts the analog electrical signals from the image sensor
116
into digital data. The memory is used to store the digital data and algorithms for comparing a captured fingerprint image with a stored fingerprint image. The processor compares the captured digital data with data previously stored in memory based on an algorithm for comparing such data. The processor may also analyze the captured digital data for purposes different from comparison with stored data. Such storage and processing units are known to those skilled in the art and can include standard personal computers equipped with appropriate software. Algorithms for processing and comparison of image data are disclosed, for example, in U.S. Pat. Ser. Nos. 4,135,147 and 4,668,995 each of which is incorporated in its entirety by reference.
When a fingerprint is placed on the optical prism's imaging surface
118
, ridges
111
of the fingerprint contact imaging surface
118
, and valleys
109
of the fingerprint remain out of contact with imaging surface
118
. Thus, in fingerprint valleys
109
incident light
124
entering optical prism
110
from light source
112
undergoes total internal reflection at imaging surface
118
if the incidence angle of the incoming light exceeds the critical angle of the optical prism
110
. However, at ridges
111
of a fingerprint some of incident light
124
is absorbed and scattered off the fingerprint ridge. As used herein, the term “scattered” indicates light which, after striking an irregular surface, is radiated or irregularly reflected off the irregular surface in multiple directions.
As a result of this scattering and/or absorption, there is less than total internal reflection of incident light
124
at fingerprint ridges
111
. Thus, the intensity of reflected light
130
leaving prism
110
from the valleys
109
of a fingerprint is of greater intensity than reflected light
130
leaving prism
110
from ridges
111
. The lower intensity reflected light
130
from ridges
111
translate into darker regions to indicate the presence of an object at the point of incidence between the light beam and the fingerprinting surface. Conversely, higher intensity reflected light
130
, such as that which undergoes total internal reflection, translates into brighter regions to indicate the absence of an object at the point of incidence between the incident light
124
and the imaging surface
118
. This allows distinguishing the darker fingerprint ridges
111
from the relatively brighter fingerprint valleys
109
. Because absorption of incident light at fingerprint ridges
111
is primarily responsible for creating a fingerprint image, system
108
is referred to as an “absorption” imaging system.
The above described system allows capturing an optical fingerprint image and processing the electrical representation of the optical fingerprint image. However, in regions of fingerprint ridges
111
, incident light
124
still undergoes some total internal reflection and some scattering in a direction parallel to reflected light
130
. Thus, the difference in intensity between reflected light
130
from fingerprint valleys
109
and fingerprint ridges
111
can be relatively low. That is, the contrast between fingerprint ridges
111
and valleys
109
in the fingerprint image can be relatively low. This can make image acquisition, processing, and comparison relatively difficult.
Additionally, in optical recognition system such as optical recognition system
108
it can be desirable that the diameter of the first lens in lens assembly
114
be smaller than the image of a fingerprint on viewing surface
122
. This both allows optical recognition system
108
to be relatively small and can be less expensive to manufacture.
However, as shown in
FIG. 2
, in an absorption type system such as system
108
, if the diamet
Jo Sung-Chan
Teng Harry H.
Lester Evelyn A
Morrison & Foerster / LLP
SecuGen Corporation
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