Title: Method and apparatus for discriminating latent fingerprint in optical fingerprint input apparatus
Abstract: A method and an apparatus for distinguishing a latent fingerprint is provided to prevent a fingerprint recognition apparatus from mis-recognizing a latent fingerprint as a fingerprint of a biomass due to a fingerprint residual on an imaging surface of an optical fingerprint input apparatus. According to the present invention, there is provided a backlight control device for controlling switching on and off of the backlight; an image acquisition device for acquiring a fingerprint image without illuminating the backlight onto the imaging surface; a fingerprint detection device for detecting the existence of a fingerprint from the image acquired by the image acquisition device; and a device for determining that the fingerprint, if detected by the fingerprint detection device, is a latent fingerprint detected due to an external light.
Patent Number: 6,989,547 Issued on 01/24/2006 to Lee, et al.
| Inventors:
|
Lee; Hwi-Seok (Sungnam, KR);
Jung; Soon-Won (Seoul, KR);
Jun; Jae-Hyun (Sungnam, KR)
|
| Assignee:
|
NITGEN Co., Ltd (Seoul, KR)
|
| Appl. No.:
|
067792 |
| Filed:
|
February 28, 2005 |
Foreign Application Priority Data
| Apr 29, 2000[KR] | 10-2000-0023151 |
| Current U.S. Class: |
250/556; 382/124; 340/5.83 |
| Current Intern'l Class: |
G06K 11/00 (20060101); G06K 5/00 (20060101) |
| Field of Search: |
250/556
382/124
340/583
283/78
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Porta; David
Assistant Examiner: Sohn; Seung C.
Attorney, Agent or Firm: G W i P S
Parent Case Text
This is a divisional of Ser. No. 10/258,455, filed Oct. 24, 2002, now U.S. Pat.
No. 6,885,017, which is a 371 of PCT/KR01/00708, filed Apr. 27, 2001.
Claims
What is the claimed is:
1. A method of discriminating a latent fingerprint residual from that of a biomass
on an imaging surface of an optical fingerprint input apparatus, which acquires
a fingerprint image by a backlight illuminated onto a fingerprint and reflected
there from, the method comprising the steps of:
acquiring an image while switching on and off the backlight onto the imaging surface;
detecting an existence of a fingerprint with respect to the image part acquired
under an "on" state of the backlight;
detecting the existence of a fingerprint with respect to the image part acquired
under an "off" state of the backlight; and
determining that the fingerprint, if detected from both image parts acquired
under the "on" state and "off" states of the backlight, is a latent fingerprint
detected due to an external light.
2. The method of claim 1, wherein the step of acquiring an image is performed
by receiving either one of:
an entire screen of an image,
only a part of an image from the tip thereof or
only a part of an image by using a windowing function of an image sensor.
3. The method of claim 2, wherein the step of detecting the existence of a fingerprint
comprises the sub-steps of:
calculating a sum of differences between gray levels of adjacent pixels of either
one of the acquired images on an X-axis or a Y-axis of a coordinate;
comparing the calculated sum of the gray levels with a pre-set reference value; and
determining that a fingerprint has been detected if the sum of the gray levels
is greater than the reference value.
4. The method of claim 2, wherein the step of detecting the existence of a fingerprint
comprises the sub-steps of:
calculating a sum of differences between gray levels with respect to either one
of the acquired images;
comparing the calculated sum of the gray levels with a pre-set reference value; and
determining that a fingerprint has been detected if the sum of the gray levels
is less than the reference value.
5. The method of claim 2, wherein the step of detecting the existence of a fingerprint
comprises the sub-steps of:
calculating an average value and a dispersion value of gray levels with respect
to either one of the acquired images; and
determining that a fingerprint has been detected if the calculated average value
is less than a first reference value and the calculated dispersion value is greater
than a second reference value.
6. An apparatus for discriminating a latent fingerprint residual from that of
a biomass on an imaging surface of an optical fingerprint input apparatus, which
acquires a fingerprint image by a backlight illuminated onto a fingerprint and
reflected there from, the apparatus comprising:
a backlight control means for controlling switching on and off of the backlight;
an image acquisition means for acquiring an image while switching on and off
the backlight onto the imaging surface;
a fingerprint detection means for detecting an existence of a fingerprint with
respect to the image parts acquired under an "on" state of the backlight and under
an "off" state of the backlight, respectively; and
a means for determining that the fingerprint, if detected from both image parts
acquired under the "on" and "off" states of the backlight, is a latent fingerprint
detected due to an external light.
7. The apparatus of claim 6, wherein the image acquisition means receives either
one of;
an entire screen of an image,
only a part of an image from the tip thereof or
only a part of an image by using a windowing function of an image sensor.
8. The apparatus of claim 7, wherein the fingerprint detection means comprises:
a calculating means for adding differences between gray levels of adjacent pixels
of either one of the acquired images on an X-axis or a Y-axis of a coordinate;
a comparing means for comparing the added value with a pre-set reference value; and
a means for determining that a fingerprint has been detected if the added value
is greater than the pre-set reference value.
9. The apparatus of claim 7, wherein the fingerprint detection means comprises:
a calculating means for adding gray levels with respect to either one of the
acquired images;
a comparing means for comparing the added value with a pre-set reference value; and
a means for determining that a fingerprint has been detected if the added value
is less than the pre-set reference value.
10. The apparatus of claim 7, wherein the fingerprint detection means comprises:
a calculating means for calculating an average value and a dispersion value of
gray levels with respect to either one of the acquired images; and
a means for determining that a fingerprint has been detected if the calculated
average value is less than a first reference value and the calculated dispersion
value is greater than a second reference value.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and an apparatus for distinguishing a latent
fingerprint from a fingerprint of a biomass. Particularly, the method can prevent
a fingerprint recognition device from mis-recognizing a latent fingerprint as a
fingerprint of a biomass due to a fingerprint residual on the imaging surface of
an optical fingerprint input apparatus.
2. Description of the Prior Art
A fingerprint recognition device can be utilized on a wide scale as a device
for
comparing and recognizing fingerprints between an inputted fingerprint and a pre-registered
fingerprint. A fingerprint recognition device is used as a locking mechanism of
a door or a safe, access control to a gate, attendance control of employees, access
control to a computer, or various other controls. The fingerprint input apparatuses
for receiving fingerprints to perform fingerprint recognition are mainly classified
into two types: an optical type and a non-optical type. The fingerprint recognition
device employing an optical fingerprint input apparatus is a device that initially
illuminates a fingerprint laid on a prism, interprets the fingerprint image reflected
according to the shapes of valleys or ridges of the fingerprint and formed on an
image sensor, and then compares the interpreted image with a pre-stored fingerprint.
The optical fingerprint input apparatuses are mainly classified into an absorption
type and a scattering type.
FIG. 1 is a schematic diagram illustrating an operational principle of a fingerprint
input apparatus of an absorption type, which comprises a backlight
112,
a triangular prism
110, a lens
114, an image sensor
116, and
an image processor
125. The backlight
112 uses a plurality of LED
aligned. The triangular prism
110 is a prism of a right triangular shape
that generates a total reflection inside an imaging surface when no fingerprint
is inputted. The image sensor
116 is an element outputting electric signals
corresponding to an amount of inputted light, such as a CCD or a CMOS sensor, well
known to those skilled in the art. The inclined surface of the triangular prism
110 is an imaging surface, while an internal plane of the imaging surface
118 is a total reflection surface causing the total reflection.
Under no input of a fingerprint to the imaging surface
118, the light
originating from the backlight
112 is totally reflected from inside the
imaging surface of the triangular prism
110, and is incident on the image
sensor
116 through the lens
114. If a finger is laid on the imaging
surface, the light illuminated onto the valleys of the fingerprint is totally reflected
from the internal surface of the imaging surface
118 and reaches the image
sensor
116 because the valleys of the fingerprint are not in contact with
the imaging surface. By contrast, the light illuminated onto the ridges of the
fingerprint is not totally reflected from the internal surface of the imaging surface
118 but rather, only a part of the reflection reaches the image sensor
116.
Accordingly, the amounts of light incident on the image sensor
116
differ between the valleys and the ridges, and as a consequence, the image sensor
116 outputs electric signals of different levels depending on a pattern
of a fingerprint. The image processor
125 formulates the output values of
the image sensor
116 into digital signals so as to recognize a fingerprint pattern.
FIGS. 2A and 2B are schematic diagrams illustrating an operational principle
of a fingerprint input apparatus of a scattering type.
The fingerprint input apparatus in FIG. 2A comprises a backlight
212,
a prism
210, a lens
214, and an image sensor
216 with a similar
construction to the one in FIG. 1. However, the prism
210 is of a ladder
shape rather than a triangular shape. Unlike the absorption type shown in FIG.
1, the light is incident on the imaging surface
218 of the prism
210
from the backlight
211 at an angle far smaller than the right angle or a
critical angle. Therefore, the light illuminated onto the valleys of the fingerprint
not in contact with the imaging surface
218 penetrates the imaging surface
218 and does not reach the image sensor
216. Meanwhile, the light
illuminated onto the ridges of the fingerprint is scattered by the ridges. The
scattered light is incident on the lens
214 and is sensed by the image sensor
216.
FIG. 2B is a schematic diagram illustrating an operational principle of the
fingerprint input apparatus of another scattering type. As in the case of FIG.
2A, the light illuminated onto the valleys of a fingerprint penetrates the imaging
surface
318 and does not reach the image sensor
316. The light illuminated
onto the ridges of the fingerprint is scattered by the same principle. However,
the difference lies in using a prism of an isosceles triangular shape and changing
the position of the backlight
312.
In case of the fingerprint input apparatus of an absorption type, the light is
absorbed at the ridges of a fingerprint. Therefore, the image of the fingerprint
appearing on the image sensor is dark at the ridges and bright at the valleys.
In case of the fingerprint input apparatus of a scattering type, however, the light
is scattered at the ridges of a fingerprint. Therefore, the image of the fingerprint
appearing on the image sensor is a bright image at the ridges and dark at the valleys,
thereby reflecting a comprehensively contrary image to that of the fingerprint
input apparatus of an absorption type. To facilitate processing the fingerprint
image as well as to avoid an inversion of bright and dark images of a fingerprint
appearing on a monitor of a computer depending on the input methods, an inversed
image is displayed on the monitor of a computer in the case of the fingerprint
input apparatus of a scattering type. To be specific, although the actual fingerprint
image appearing on the image sensor is bright at the ridges and dark at the valleys
of the fingerprint, the gray level in the course of processing the fingerprint
image has a low value at the ridges and a high value at the valleys as in the case
of the fingerprint input apparatus of an absorption type.
In case of the optical fingerprint input apparatus, however, sebum or a contaminated
material leaves a latent fingerprint on the fingerprint recognition apparatus due
to contact with a person's finger. If a light is incident on the imaging surface
from an external light, rather than from a backlight, at a particular angle, the
image sensor is apt to sense a latent fingerprint. Thus, if the image sensor senses
any latent fingerprint, the fingerprint recognition apparatus mis-recognizes the
latent fingerprint as a fingerprint of a biomass. This causes a problem in that
an unauthorized user may be authenticated for access by using the latent fingerprint
left on the fingerprint recognition apparatus instead of inputting his or her own fingerprint.
FIG. 3A shows an image of a normal fingerprint of a biomass, and FIG. 3B shows
a clear image of a latent fingerprint, which is quite similar to the one in FIG.
3A. FIG. 3C shows a vague image of a latent fingerprint.
To solve the problem of mis-recognizing a latent fingerprint, the conventional
art uses a method of storing the most recently inputted fingerprint of a person,
comparing the stored fingerprint with the currently inputted fingerprint, and distinguishes
the newly inputted fingerprint from the fingerprint of a biomass as a latent fingerprint
if the two fingerprints are quite similar (i.e., when the positions of a particular
point of the two fingerprints coincide with each other or when comprehensive patterns
of the two fingerprints overlap with each other).
However, this method still poses a problem in that a pattern of the latent
fingerprint read by the image sensor is variable due to a change of the external
light, and due to other factors, and the stored pattern may be distinguished as
different from the latent fingerprint. The image sensor therefore may fail to discriminate
a latent fingerprint from a biomass fingerprint accurately.
SUMMARY OF THE INVENTION
The present invention was conceived by the insight that a latent fingerprint
can be detected due to an external light even when the backlight is off and not
illuminated onto the fingerprint in the fingerprint input apparatus of an optical type.
It is, therefore, an object of the present invention to provide a method and
an
apparatus for discriminating a latent fingerprint from that of a biomass. The present
invention enables one to discriminate between fingerprints as either being latent
or biomass by acquiring an image without illuminating a backlight onto an imaging
surface and identifying it as a latent fingerprint detected due to an external light.
To achieve the above object, this invention provides a method of determining a
latent fingerprint in an optical fingerprint input apparatus. The method comprises
the steps of: acquiring an image without illuminating a backlight onto an imaging
surface; detecting the existence of a fingerprint from the acquired image; and
determining a detected fingerprint is a latent fingerprint detected due to an external light.
The present invention also provides an apparatus for detecting a latent fingerprint
in an optical fingerprint input apparatus. The apparatus comprises: a backlight
control means for controlling on/off of the backlight; an image acquisition means
for acquiring a fingerprint image without illuminating a backlight onto an imaging
surface under a control by the backlight-control means; a fingerprint detection
means for detecting the existence of a fingerprint from the image acquired by the
image acquisition means; and a latent fingerprint discrimination means for discriminating
a fingerprint detected by the fingerprint detection means as a latent fingerprint
detected due to an external light.
In the method and apparatus for detecting a latent fingerprint, a method of acquiring
an entire frame of an image can be considered as a method of acquiring an image.
When the backlight is in the "off" state, an entire frame of an image is acquired
and stored in memory so that the existence of a latent fingerprint can be detected
for the stored image.
However, use of the above method of acquiring an image wastes time by requiring
an entire frame of an image to detect a latent fingerprint. Therefore, time can
be saved if a latent fingerprint is detected by acquiring a partial frame of a
fingerprint image without illuminating the backlight and by using the partial frame
of the image.
Here, in cases where a finger is small or when a fingerprint is in contact
with a lower part of the imaging surface, it is desirable to acquire a fingerprint
image of about ¼ to ½ from the tip of the fingerprint image.
Meanwhile, a windowing function is provided for receiving an image at
only a partial area of an image sensor depending on the type of image sensor. Therefore,
it is possible to detect a fingerprint by selectively acquiring a middle part of
an image clearly reflecting the fingerprint when using an image sensor of this kind.
Detection of the existence of a fingerprint is made through the fingerprint
detection means. The fingerprint detection means comprises: a calculating means
for adding all the sums of the differences between gray levels of adjacent two
pixels in an X direction or a Y direction of a coordinate for the acquired image;
a comparing means for comparing the added value with a pre-set reference value;
and a discriminating means for determining that a fingerprint has been detected
when the added value is greater than the reference value.
To be specific, if a fingerprint is clearly detected when an image has been acquired
when the backlight is in an "off" state, the gray levels on the X axis (or on the
Y axis) is varied in a wide range, and detection of a fingerprint is determined
based on that variation. A detected fingerprint is determined to be a latent fingerprint
detected due to an external light. The method of detecting a fingerprint image
by adding the differences in gray levels of adjacent pixels is a technology that
can be easily carried out by one skilled in the art.
In addition to the above method, another method is also applicable for detecting
the existence of a fingerprint by adding all the gray levels of each pixel, comparing
the added value with a pre-set reference value, and determining that a fingerprint
has been detected when the added value is less than the reference value.
Another method is still applicable for detecting the existence of a fingerprint
by calculating an average value of the gray levels of pixels of an image and a
dispersion value, and determining that a fingerprint has been detected when the
average value is less than a first reference value and when the dispersion value
is greater than a second reference value. Other methods, including this method,
are also well known to those skilled in the art.
When the above method is used, however, an image is acquired without a backlight
illuminated onto the imaging surface, and a user is unable to ascertain whether
or not the fingerprint input apparatus is in operation. Furthermore, it is possible,
though rare, for the latent fingerprint discrimination means to first determine
that the fingerprint is not a latent fingerprint in the absence of an incident
external light but also to mis-recognize the latent fingerprint as a fingerprint
of a biomass when an external light is incident at the moment of illuminating an
backlight and acquiring the fingerprint image.
To resolve this problem, another method has been suggested to acquire an image
by illuminating a backlight, and when a fingerprint has been detected, switch off
the backlight and acquire the image. If a fingerprint is detected under this state,
it is determined to be a latent fingerprint. Otherwise, it is determined to be
a fingerprint of a biomass.
In this case, the method of discriminating a latent fingerprint from that of a
biomass comprises the steps of: illuminating a backlight to the imaging surface
and acquiring an image; switching off the backlight and acquiring an image when
a fingerprint has been detected from the acquired image; and determining the detected
fingerprint is a latent fingerprint detected due to an external light when a fingerprint
is detected when the backlight is in the "off" state.
In that case, the apparatus for discriminating between a latent fingerprint and
a biomass fingerprint comprises: a backlight control means for controlling the
turning on and off of a backlight; an image acquisition means for illuminating
the backlight to an imaging surface under a control by the backlight control means
to acquire an image; a fingerprint detection means for detecting the existence
of a fingerprint from the acquired image; wherein, if the fingerprint detection
means has detected a fingerprint, the image acquisition means re-acquires the image
without illuminating the backlight onto the imaging surface; and a latent fingerprint
determining means for determining a fingerprint, if detected by the fingerprint
acquisition means from the re-acquired image, to be a latent fingerprint detected
due to an external light.
The methods for acquiring an image in the above method and apparatus can be performed
by a method similar to the method used for acquiring an image while the backlight
is in the "off" state. The available methods in this regard are to acquire an entire
screen of an image or a partial image when a backlight is either illuminated or
not illuminated.
The existence of a fingerprint within the acquired image can also be detected
by the same method as the one used for detecting a fingerprint from an acquired
image as described above.
Also, an image acquired by using a method of receiving an image while switching
"on" and "off" the backlight in the course of acquiring the image contains a part
acquired under illumination of the backlight and a part acquired while the backlight
is in its "off" state in turn. A latent fingerprint can be detected using this
method of each part of the image. In other words, it can be determined to be a
latent fingerprint if a fingerprint image is detected from the part of the image
acquired under illumination of the backlight and a fingerprint image is detected
from the part of the image acquired while the backlight is off as well. In the
opposite case, it can be determined to be a fingerprint of a biomass if a fingerprint
image is detected from the part of the image acquired under illumination of the
backlight and no fingerprint image is detected from the part of the image acquired
while the backlight is in its "off" state.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention
will become more apparent from the following description when taken in conjunction
with the accompanying drawings, in which:
FIG. 1 is a schematic diagram illustrating an operational principle of a fingerprint
input apparatus of an absorption type;
FIGS. 2A and 2B are schematic diagrams illustrating operational principles
of a fingerprint input apparatus of an absorption type;
FIGS. 3A to 3C are exemplary patterns of a normal fingerprint and a latent fingerprint;
FIG. 4 is a flow chart illustrating a method of discriminating a latent fingerprint
from a fingerprint of a biomass according to the present invention;
FIG. 5 is a flow chart illustrating a process of determining whether or not
a fingerprint exists according to a first embodiment of the present invention;
FIG. 6 is a flow chart illustrating a process of determining whether or not
a fingerprint exists according to a second embodiment of the present invention;
FIG. 7 is a flow chart illustrating a process of determining whether or not
a fingerprint exists according to a third embodiment of the present invention;
FIG. 8 is an exemplary pattern of a partial image acquired from the tip of a
latent fingerprint;
FIG. 9 is an exemplary pattern of a partial image acquired from a latent fingerprint
by using a windowing function of an image sensor;
FIG. 10 is a flow chart illustrating a method of discriminating a latent fingerprint
from a biomass fingerprint according to a fourth embodiment of the present invention;
FIG. 11 is an exemplary pattern of a fingerprint image acquired by switching
a backlight on and off; and
FIG. 12 is a block diagram illustrating a function of an apparatus for discriminating
a latent fingerprint from that of a biomass according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described below with reference to the accompanying
drawings. In the following description, well-known functions or constructions are
not described in detail since they would obscure the invention in unnecessary detail.
FIG. 4 is a flow chart illustrating a method of discriminating a latent fingerprint
from that of a biomass according to the present invention, and FIG. 12 is a block
diagram illustrating a function of an apparatus for discriminating a latent fingerprint
from that of a biomass according to the present invention. For the sake of convenience,
the description of a method will be made along with an apparatus according to the
present invention.
If a person to be authenticated for access touches an imaging surface with one
finger, the backlight control means
503 switches off the backlight (S
101).
The image acquisition means
505 then acquires an image of the fingerprint
(S
103), and the fingerprint detection means
507 detects the existence
of a fingerprint (S
105). As described above, if a fingerprint has been detected
during the "off" state of the backlight, the latent fingerprint discrimination
means
509 determines that a latent fingerprint has been detected due to
an external light (S
109). If no fingerprint has been detected during the
backlights "off" state, the latent fingerprint discrimination means
509
determines the inputted fingerprint to be a fingerprint of a biomass (S
109).
In step (S
105) of detecting a fingerprint, the fingerprint can be detected
by adding gray levels of the image as described above. To be specific, differences
in gray levels between adjacent pixels of the acquired image on the X-axis or the
Y-axis of a coordinate are added, as shown in FIG. 5 (S
106).
As the next step, the latent fingerprint discrimination means determines whether
the added value is greater or less than a pre-set reference value (S
107).
If the added value is greater, this means that a fingerprint has been detected
while the backlight was in its "off" state, and it is determined that the acquired
fingerprint image is a latent fingerprint (S
108). If the added value is
less than or equal to the pre-set reference value, this means that a fingerprint
has not been detected (S
113), and it is determined that the acquired fingerprint
image is a fingerprint of a biomass.
FIG. 6 is a flow chart illustrating a process of determining whether or not
a fingerprint exists according a second embodiment of the present invention. A
fingerprint image is acquired by switching off the backlight, and all the gray
level values of the pixels of the image are added (S
206). If the added value
is less than the pre-set reference value, it means that a fingerprint has been
detected (S
208), and it is determined that the acquired image is a latent
fingerprint image.
FIG. 7 is a flow chart illustrating a process of determining whether or not
a fingerprint exists according to a third embodiment of the present invention.
A fingerprint image is acquired by switching off the backlight, and an average
value M as well as a dispersion value D of the gray levels of the pixels of the
image is calculated (S
306). If it is determined that M is less than a first
reference value and that D is greater than a second reference value (S
307),
it means that a fingerprint has been detected (S
308). Therefore, it is determined
that the acquired image is a latent fingerprint.
The above embodiment represents a case of distinguishing a latent fingerprint
from that of a biomass by acquiring an entire image of a fingerprint. As mentioned
above, time is wasted while receiving a frame of the image in this case and time
can be saved by acquiring only a part of the image, rather than the entire image,
to detect a latent fingerprint while the backlight is in an "off" state.
FIG. 8 is an exemplary pattern of a half of the entire image acquired from a
latent fingerprint. A latent fingerprint can sufficiently be discriminated from
that of a biomass with only about half of the entire fingerprint image. The same
method as described above is applied for discriminating a latent fingerprint from
that of a biomass.
FIG. 9 is an exemplary pattern of a mid-part image acquired from a latent fingerprint
by using a windowing function of an image sensor. When using such a windowing function
of the image sensor for receiving only a part of an image, the time consumed for
receiving the fingerprint image will be reduced as the width W of the window is
narrow, and consequently, the time consumed for determining the existence of a
latent fingerprint will also be reduced.
FIG. 10 is a flow chart illustrating a method of discriminating a latent fingerprint
from that of a biomass according to a fourth embodiment of the present invention.
Image acquisition means
505 switches on the backlight under a control by
backlight control means
503 (S
401) to acquire an image (S
402).
As the next step, the fingerprint detection means
507 determines whether
or not a fingerprint has been detected (S
403). In the affirmative, the image
acquisition means
505 switches off the backlight under a control by the
backlight control means
503 (S
404) to acquire an image (S
405).
The fingerprint detection means
507 then determines whether or not a fingerprint
has been detected while the backlight was in its "off" state (S
406). In
the affirmative, discrimination means
509 determines the detected fingerprint
to be a latent fingerprint (S
407). In the negative, the discrimination means
509 determines the detected fingerprint to be a fingerprint of a biomass
(S
408). The same method as shown in FIGS. 5 to 7 is used for detecting a
fingerprint according to the fourth embodiment, as in case of the third embodiment.
In the fourth embodiment shown in FIG. 10, an image is acquired while the backlight
is in an "on" state to detect the existence of a fingerprint, and the existence
of a fingerprint is detected by acquiring an image while the backlight is in an
"off" state when a fingerprint has been detected. An image acquired using a similar
method of receiving an image while switching on and off the backlight in the course
of acquiring the image contains a part acquired under an "on" state of the backlight
and a part acquired under an "off" state of the backlight that appear in turn.
FIG. 11 is an exemplary pattern of a fingerprint image acquired by switching
on and off a backlight. Part "A" represents an image acquired under an "off" state
of the backlight, while part "B" represents an image acquired under an "on" state
of the backlight. As described above, it is determined that an image was acquired
from a fingerprint of a biomass because a fingerprint has not been detected in
the part "A", which was acquired while the backlight was in the "off" state, but
has been detected in the part "B", which was acquired while the backlight was in
the "on" state.
It is well known to those skilled in the pertinent art that the switching on
and
off of the backlight is controlled by software, and that the image part acquired
while the backlight was in the "on" state is distinguished from the image part
acquired while the backlight was in the "off" state by calculation.
While the invention has been shown and described with reference to certain
embodiments to carry out the invention, it will be understood by those skilled
in the art that various changes in form and details may be made without departing
from the spirit and scope of the invention as defined by the appended claims. The
technical concept of the present invention lies in acquiring an image while the
backlight was in the "off" state and discriminating a latent fingerprint from a
fingerprint of a biomass based on the fingerprint image detected from the acquired image.
*
