Pre-processing system for pre-processing an image signal succession prior to identification

In a pre-processing system (45) for pre-processing a succession of image signals which are extracted from an object on an input surface and which are representative of a ridge pattern of the object, a center position of the ridge pattern is determined with reference to the image signal succession and is compared with a center zone of the input surface so as to decide whether or not the center position is placed at the center zone. The center position can be decided by monitoring occurrence of runs along each scanning line and by detecting a maximum one of the numbers of runs. Alternatively, the center position may be decided by monitoring ridges extended in a preselected direction with respect to the scanning lines and by detecting a maximum one of the numbers of the ridges extended in the preselected direction. The pre-processing system may detect whether the object is true or false by monitoring the image signal succession.

BACKGROUND OF THE INVENTION 
This invention relates to a pre-processing system for use in pre-processing 
a fingerprint prior to identification of the fingerprint. 
A fingerprint has uniquenss and unchangeability and therefore serves to 
identify each person in a criminal investigation. A recent attempt has 
been directed to application of identification of the fingerprint to a 
cash dispensing system and/or to a person identifying system of preventing 
an unauthorized person from entering an offlimits zone. 
In such a cash dispensing system or the like, a fingerprint is impressed by 
a person on an input surface, namely, a finger positioning surface after a 
preliminary input of an identification code assigned to the person and is 
collated with a memorized fingerpeint pattern corresponding to the 
identification code. Thus, both of the fingerprint and the identification 
code are used in the cash dispensing system or the like to quickly 
identify each person. 
In a copending U.S. patent application Ser. No. 755,810, filed July 17, 
1985, by Ko Asai et al, the present applicants, it is pointed out that 
each fingerprint is preliminarily processed prior to identification of the 
fingerprint. Such preliminary processing is carried out so as to extract a 
distinct and clear fingerprint impression and to distinguish between a 
true fingerprint impression extracted from an actual fingertip of each 
person and a forged fingerprint impression willfully placed on the input 
surface. 
Any other preliminary processing or pre-processing may preferably be 
carried out for each fingerprint to accurately identify each person and to 
reject the forged fingerprint impression. For example, it is preferable 
that each fingerprint is impressed on a center portion of the input 
surface. This is because a great deal of image signals can be extracted 
from the fingerprint and accurately collated with the memorized 
fingerprint pattern, when the fingerprint is impressed on the center 
portion. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a pre-processing system which 
serves to facilitate identification of a fingerprint. 
It is another object of this invention to provide a pre-processing system 
of the type described, which can detect whether or not the fingerprint is 
impressed in position. 
It is a further object of this invention to provide a pre-processing system 
of the type described, which is capable of distinguishing between a true 
fingerprint impression and a forged fingerprint impression in new manners. 
A pre-processing system to which this invention is applicable is for use in 
pre-processing a succession of image signals representative of a ridge 
pattern prior to identification of the ridge pattern. The ridge pattern is 
derived from an object placed on an input surface. The ridge pattern has a 
center pattern while the input surface has a center zone. According to 
this invention, the pre-processing system comprises position deciding 
means responsive to the image signal succession for deciding a position of 
the center pattern with reference to the image signal succession to 
produce a position signal indicative of the position and pattern detecting 
means responsive to the position signal for detecting whether or not the 
center pattern is placed at the center zone of the input surface to 
produce a detection signal representative of whether or not the 
identification is to be carried out.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 through 3, an identification system is for use in 
carrying out identification of a fingerprint after a preliminary input of 
an identification code assigned to each person. A keyboard or push buttons 
30 is included in the identification system to input the identification 
code. 
In FIGS. 1 and 2, an image input device 31 comprises a prism 32 of a 
right-angled isosceles triangle in section. The triangle has equilateral 
sides directed downwards of FIG. 2 and a hypotenuse directed upwards 
thereof. An input surface, namely, a finger positioning surface 33 is 
defined along the hypotenuse while incident and exit surfaces are defined 
along the equilateral sides, respectively, as will presently be described. 
A user brings his fingertip 34 in contact with the input surface 33. The 
fingertip 34 will be called an object or a true object and has a 
fingerprint to be identified by the identification system. Incident light 
35 is incident from a light source 36 onto the input surface 33 through 
the incident surface. Reflection takes place on the input surface 33 and 
reflected light 37 exits through the exit surface to an image pickup 
device 38, such as an industrial television camera, a CCD, or the like. 
When the fingertip 34 is actually brought into contact with the input 
surface 33, the reflected light 37 is subjected to optical modulation on 
the input surface 33 by the fingertip 34 to provide an optical image on 
the image pickup device 38 as schematically illustrated in FIG. 3. In FIG. 
3, such an optical image appears as a ridge pattern 39 on an image area 40 
of the input surface 33. The ridge pattern 40 is represented by a grey 
level and has a black level on ridges of the fingerprint and a white level 
along valleys thereof. This is because each ridge is an aggregation of 
sweat glands and sweat is laid between the input surface 33 and the ridges 
to refract the incident light 35. Therefore, the illustrated ridge pattern 
39 is representative of a fingerprint impression. 
The image pickup device 38 successively scans the image area 40 along 
scanning lines from the lefthand side of FIG. 3 towards the righthand side 
at every each horizontal scanning period to produce a succession of image 
signals. The image signal succession is therefore divisible into a 
plurality of partial sequences each of which lasts for each horizontal 
scanning period. 
In the interim, it is assumed that the image area 40 is divided along the 
horizontal lines into a center zone and a pair of peripheral zones laid on 
both sides of the center zone. In this event, the ridge pattern 39 is also 
divisible along the scanning lines into a center pattern and a side 
pattern laid on both sides of the center pattern. Under the circumstances, 
it is preferable that the center pattern of the ridge pattern 39 is placed 
in register with or is coincident with the center zone of the image area 
40. Otherwise, collation of the ridge pattern 39 might not be done because 
of lack of information derived from the ridge pattern 39. The center 
pattern will later be defined more exactly. 
The identification system illustrated in FIG. 1 comprises a pre-processing 
system 45 for detecting the center pattern of the ridge pattern 39 to make 
the center pattern coincide with the center zone of the image area 40 of 
the input surface. 
Referring to FIG. 4 together with FIG. 3, let the image area 40 be scanned 
along the scanning lines exemplified by L.sub.1, L.sub.2, and L.sub.3 in 
FIG. 3 with the ridge pattern 39 impressed on the image area 40. The 
exemplified scanning lines L.sub.1, L.sub.2, and L.sub.3 will be called a 
first, a second, and a third line, respectively. When the ridge pattern 39 
is scanned along the first line L.sub.1, a single long run of the black 
level appears. In other words, the ridge pattern 39 has a first partial 
pattern of a single long run-length along the first line L.sub.1. 
Therefore, the run-lengths and the number thereof have a relationship 
which can be shown by FIG. 4 (A). 
Similarly, when the ridge pattern 39 is scanned along the second line 
L.sub.2, a second partial pattern is extracted from the ridge pattern 39 
and specified by three runs which have different run-lengths. Therefore, 
the second partial pattern is characterized by a relationship illustrated 
in FIG. 4 (B). When the ridge pattern 39 is scanned along the third line 
L.sub.3, a third partial pattern is extracted from the ridge pattern 39 
and specified by seven short runs classified into three-kinds of 
run-lengths in the manner illustrated in FIG. 4 (C). In FIG. 4 (C), 
two-kinds of the run-lengths appear three times while the remaining 
run-length appears once. 
In general, it may be said that the numbers of the runs increase as the 
scanning lines approach a center portion of the ridge pattern 39. In the 
example being illustrated, a maximum one of the numbers of runs is 
detected to determine the center pattern of the ridge pattern. 
Specifically, a position of a scanning line corresponding to the maximum 
number is determined as a center pattern position for the center pattern. 
Thus, a feature of the center pattern can be extracted from the ridge 
pattern. On the other hand, the center zone of the input surface, namely, 
image area 40 can be determined by selecting a center one of the scanning 
lines. 
In FIG. 1, the identification system comprises a controller 42 operable in 
a manner to be described and a pre-processing system 45 for detecting the 
center pattern position from the ridge pattern 39 (FIG. 3) in the 
above-mentioned manner. 
Referring to FIG. 5 together with FIG. 1, operation is started in the 
identification system (FIG. 1) at a first step S.sub.1 (FIG. 5) and 
proceeds to a second step S.sub.2 for waiting for an input of an 
identification (ID) code assigned to each user. When the user manipulates 
the keyboard 30 to input his identification code, the controller 42 
delivers the identification code to a memory file 46 which stores pattern 
signals specifying features of fingerprints of all users, in addresses 
corresponding to identification codes. The memory file 46 may be a 
magnetic disk or the like and reads a specific one of the pattern signals 
out of one of the addresses corresponding to the delivered identification 
code. 
The specific pattern signal is sent from the memory file 46 to a collation 
processor 47 to be described later. Concurrently, the controller 42 
acoustically or visually informs the user of an impression of his 
fingerprint in a known manner. When the user impresses his fingerprint on 
the input surface 33 at a third step S.sub.3, the image input device 31 
scans the image area 40 (FIG. 3) at a fourth step S.sub.4 and produces a 
succession of image signals in the manner described in conjunction with 
FIG. 3. The image signal succession is sent through the controller 42 to 
an image memory 48 to be stored therein and to the pre-processing system 
45. 
The pre-processing system 45 comprises a run-length detector 51 for 
detecting each run-length of the runs at a fifth step S.sub.5. At a sixth 
step S.sub.6, the run-length detector 51 counts the numbers of runs for 
each scanning period. A total number of the runs is calculated at every 
scanning line in the run-length detector 51. A local memory 52 has a 
plurality of local addresses for the scanning lines, respectively. Each 
total number is successively stored in each local address of the local 
memory 52. 
A decision circuit 53 successively compares the total numbers read out of 
the local memory 52 to detect a maximum one of the total numbers. When the 
maximum number of runs is determined by the decision circuit 53, the 
scanning line corresponding to the maximum number of runs is determined as 
a center position of the center pattern of the ridge pattern 39 (FIG. 3) 
by the decision circuit 53 at a seventh step S.sub.7. In this event, the 
decision circuit 53 produces a scanning line number assigned to the 
scanning line of the maximum number of runs. The scanning line number is 
sent to a detection circuit 54 as a position signal representative of the 
center position of the center pattern. Thus, a combination of the local 
memory 52 and the decision circuit 53 is operable to monitor a 
distribution of the runs and to detect the position of the center pattern. 
The detection circuit 54 is supplied from a reference number generator (not 
shown) with a reference number signal indicative of the center zone of the 
image area 40 (FIG. 3). The reference number signal may be indicative of a 
scanning line number assigned to a center one of the scanning lines that 
is placed at a center of the image area 40. The detection circuit 54 
compares the position signal with the reference number signal and detects 
at an eighth step S.sub.8 whether or not the center pattern of the ridge 
pattern 39 coincides with the center zone of the image area 40. 
On coincidence between the center pattern and the center zone, the 
detection circuit 54 supplies the controller 42 with a coincidence signal 
representative of the coincidence. Responsive to the coincidence signal, 
the detection circuit 54 accesses the image memory 48 to read the image 
signal succession out of the image memory 48. At a ninth step S.sub.9, the 
collation controller 54 collates the image signal succession read out of 
the image memory 48 with the specific pattern signal sent from the memory 
file 46. 
On incoincidence between the center pattern and the center zone, the 
detection circuit 54 supplies the controller 42 with an incoincidence 
signal representative of the incoincidence. In this case, the eight step 
S.sub.8 is followed by the third step S.sub.3 so as to audibly or visually 
indicate to impress the fingerprint again. Thereafter, similar operation 
is repeated until coincidence is detected between the center pattern and 
the center zone. Practically, the center zone of the image area may be 
represented by a plurality of the scanning lines, although the center 
scanning line alone is selected in the above-described manner to specify 
the center zone. 
The coincidence and the incoincidence signals may be called a detection 
signal representative of whether or not the identification of the 
fingerprint is to be carried out by collating the image signal succession 
with the specific pattern signal. Each of the run-length detector 51, the 
local memory 52, the decision circuit 53, and the detection circuit 54 can 
be implemented by a known circuit and is therefore not described any 
longer. 
Referring to FIGS. 6 through 8, an identification system comprises similar 
parts designated by like reference numerals and a pre-processing system 
45a a according to a second embodiment of this invention. The 
pre-processing system 45a is for detecting each angle between each 
scanning line and each ridge of a ridge pattern exemplified in FIG. 7 to 
determine a center pattern of the ridge pattern. More specifically, let 
the ridge pattern illustrated in FIG. 7 be scanned along first and second 
scanning lines L.sub.a and L.sub.b. The first scanning line L.sub.a 
intersects two ridges of the ridge pattern at four points. The two ridges 
are substantially parallel to each other and cross the first scanning line 
L.sub.a at a crossing angle of 135.degree. at two points placed on the 
lefthand side of FIG. 7 and at a crossing angle of 45.degree. at two 
points placed on the righthand side thereof. Therefore, the crossing 
angles along the first scanning line L.sub.a have distribution as 
illustrated in FIG. 8 (A). 
Likewise, the second scanning line L.sub.b intersects eight ridges of the 
illustrated ridge pattern. Two rightmost ridges of the eight ridges 
intersect the second scanning line L.sub.b at a crossing angle of 
135.degree. while the remaining ridges intersect the second scanning line 
L.sub.b at a crossing angle of 90.degree.. Therefore, the crossing angles 
along the second scanning line L.sub.b have a distribution as shown in 
FIG. 8 (B). 
It has been found out that the crossing angles frequently becomes a right 
angle as the scanning lines approach a center portion of the ridge 
pattern. In other words, it is possible to detect a position of the center 
pattern of the ridge pattern by monitoring the crossing angles between 
each scanning line and the ridge pattern. 
Referring to FIG. 9 afresh and FIG. 6 again, the pre-processing system 45a 
(FIG. 6) comprises a direction detector 60 operable in cooperation with 
the image memory 48 which is loaded with the image signal succession from 
the image input device 31 through the controller 42. More specifically, 
the direction detector 60 serves to detect the crossing angles between 
each scanning line and the ridges by determining a direction of each 
ridge. 
In FIG. 9, the direction detector 60 successively monitors an element group 
of three-by-three elements stored in the image memory 48, as illustrated 
in FIGS. 9 (A) and (B). Such monitoring is possible by successively 
accessing nine of the addresses for the element group in a known manner. 
For convenience of description, the element group of the three-by-three 
elements will be called a detection mask having nine windows which 
correspond to the respective elements and which are divided into three 
rows and three columns. The three rows will be called upper, intermediate, 
and lower rows, respectively, while the three columns will be called left, 
center and right columns, respectively. In FIG. 9 (A), let the scanning 
line be placed along the intermediate row and a center one of the windows 
be indicated along the intermediate row. As shown in FIG. 9 (A), black 
levels appear in the center window, a left lower one of the windows, and a 
right upper one thereof. In this event, the ridge in question is inclined 
at an angle of 45.degree. with the scanning line, as readily understood 
from FIG. 9 (C). Therefore, the direction detector 60 detects that the 
crossing angle at the center window becomes equal to 45.degree.. In FIG. 9 
(B), the black levels appear in three windows of the center column. In 
this case, the ridge under consideration is perpendicular to the scanning 
line extending along the intermediate row, as readily understood from FIG. 
9 (B). 
At any rate, the direction detector 60 successively produces an angle 
signal representative of the crossing angle calculated in relation to each 
window. Responsive to a succession of the angle signals, a direction 
processor 63 processes the angle signal succession to monitor the angles 
and the numbers of the right angles. Subsequently, the direction processor 
63 determines a maximum number of the right angles together with a 
position of the scanning line which corresponds to the maximum number of 
the right angles and which is detected as a center position of the ridge 
pattern, namely, a position of the center pattern. The center position of 
the ridge pattern is produced as a position signal. 
The position signal is sent to a center position detector 65 which may be 
identical with the detection circuit 54 illustrated in conjunction with 
FIG. 1. The center position detector 65 compares the position signal with 
the center zone of the image area to produce, as the detection signal, 
either the coincidence signal or the noncoincidence signal in the manner 
described in conjunction with the detection circuit 54 illustrated in FIG. 
1. Thereafter, operation is carried out in a manner similar to FIG. 1. 
In order to detect the angles of the ridges with a high precision, a 
detection mask of, for example, five-by-five windows may be substituted 
for the detection mask of the three-by-three mask. 
Referring to FIG. 10, an identification system comprises similar parts 
designated by like reference numerals and a pre-processing system 45b 
according to a third embodiment of this invention. The illustrated 
pre-processing system 45b comprises a light source controller 71 and a 
grey level detector 72. In addition, the controller 42 serves as a part of 
the pre-processing system 45b. It is to be noted here that the 
identification code (ID) is directly delivered from the keyboard 30 to the 
collation controller 47. As a result, the collation processor 47 is 
directly driven by the keyboard 30 to access the memory file 46 and to 
extract a pattern signal corresponding to the identification code from the 
memory file 46. 
The pre-processing system 45b illustrated in FIG. 10 is operable to avoid 
an impression of a forged fingerprint. As known in the art, such a forged 
fingerprint can be skilfully formed by reproducing or printing a 
fingerprint on a medium, such as a plastic film, a rubber sheet, or the 
like. The reproduced medium may be called a false medium or a false 
object. When the false medium is closely brought into contact with the 
input surface 33 (FIG. 2) with liquid, such as water, interposed between 
the false medium and the input surface 33, the forged fingerprint may 
unfavorably be identified as a correct fingerprint. Such unfavorable 
identification should be avoided in the identification system. 
The forged fingerprint may be illuminated by an external optical system 
(not shown), such as an external source, an external optical system. In 
this connection, the light source 36 shown in FIG. 2 may be called an 
internal light source. In FIG. 10, the internal light source 36 is 
repeatedly and intermittently lit and darkened with a preselected period 
so as to make it difficult to input the forged fingerprint. 
Referring to FIG. 11 together with FIG. 10, operation is started at a first 
step Sa.sub.1. The control section 71 at first supplies the light source 
controller 71 with a sequence of clock pulses having the preselected 
period. Supply of the clock pulse sequence is readily possible by the use 
of a usual pulse generator. The light source controller 71 lights the 
internal light source 36 during presence of each control pulse and darkens 
the internal light source 36 during absence thereof. 
While the internal light source is darkened, the image signal succession is 
delivered at a second step Sa.sub.2 in FIG. 11 from the image input device 
31 through the controller 42 to the grey level detector 72. At a third 
step Sa.sub.3, the grey level detector 72 detects each grey level of the 
image signals for a dark duration of the internal light source 36. The 
grey level may be classified, for example, into 128, 256, or so. A 
detected grey level is sent back to the controller 42. 
The illustrated controller 42 comprises a control section 73 and a data 
memory 74 for storing a reference grey level for the dark duration. At a 
fourth step Sa.sub.4, the control section 73 detects whether or not the 
detected grey level is coincident with the reference grey level. 
Incoincidence between the detected and the reference grey levels indicates 
that the detected grey level may be lighter or brighter than the reference 
grey level and that the external light source may be used in place of the 
internal light source 36. In this event, the fourth step Sa.sub.4 is 
followed by a fifth step Sa.sub.5 at which an impression of the 
fingerprint is audibly or visually requested again under control of the 
control section 73 in the known manner. At a sixth step Sa.sub.6, the 
control section 73 judges whether or not the request for the impression 
reaches a predetermined number of times. 
When the request for the impression does not reach the predetermined times, 
the sixth step Sa.sub.6 is returned back to the second step Sa.sub.2. 
Otherwise, the sixth step Sa.sub.6 is followed by a seventh step Sa.sub.7 
at which collation is rejected between the image signal succession and the 
pattern signal. Thus, operation is completed. 
On the other hand, when the detected grey level is coincident with the 
reference grey level, the fourth step Sa.sub.4 is succeeded by an eighth 
step Sa.sub.8 at which the internal light source 36 is lit again. At a 
ninth step Sa.sub.9, the control section 73 judges whether or not the 
internal light source 36 is lit a prescribed number of times. When the 
internal light source 36 is lit the prescribed number of times at the 
ninth step Sa.sub.9, the control section 73 makes the collation processor 
47 collate the image signal succession with the pattern signal read out of 
the memory file 46 at a tenth step Sa.sub.10. 
On collation of the image signal succession with the pattern signal, the 
image signal succession may be produced at the tenth step Sa.sub.10 by the 
image input device 31 with the internal light source 31 being lit and may 
thereafter be delivered from the control section 73 to the image memory 
48. Alternatively, the image signal succession may previously be sent from 
the image input device 31 to the image memory 48 prior to the tenth step 
Sa.sub.10. Thus, operation is completed at an eleventh step Sa.sub.11. 
Referring to FIGS. 12 and 13, an identification system comprises similar 
parts designated by like reference numerals and a pre-processing system 
45c according to a fourth embodiment of this invention. The pre-processing 
system comprises an image input device 31a illustrated in FIG. 13 and a 
forged input detector 75, as shown in FIG. 12. 
In FIG. 13, the illustrated image input device 31s is similar to that 
illustrated in FIG. 2 except that first and second optical band-pass 
filters 76 and 77 are placed along the incident and the exit surfaces of 
the prism 32, respectively. In the example being illustrated, the first 
and the second optical band-pass filters 76 and 77 entirely cover the 
incident and the exit surfaces with gaps left therebetween, respectively. 
The first optical band-pass filter 76 has a passband partially superposed 
on that of the second optical band-pass filter 77. For example, the 
passband of the first optical band-pass filter 76 may be wider than that 
of the second optical band-pass filter 77. 
In this structure, the incident light 35 from the internal light source 36 
is incident onto the input surface 33 through the first optical band-pass 
filter 36 and reflected on the input surface 33 to be sent through the 
second optical band-pass filter 77 to the image pickup device 38. 
When a fingertip is actually brought into contact with the input surface 33 
as a true object in the manner illustrated in FIG. 2, the image pickup 
device 38 picks up a ridge pattern, as mentioned in conjunction with FIG. 
3. The ridge pattern is surrounded by a true background image as 
illustrated in FIG. 3. Herein, a grey level of the true background image 
is determined by the passbands of the first and the second optical 
band-pass filters 76 and 77. 
Herein, attention will be directed to a forged fingerprint printed on a 
transparent film 78. It is assumed that the transparent film 78 is brought 
into close contact with the input surface 33 with liquid laid between the 
input surface 33 and the transparent film 78 and is illuminated through an 
external prism (unnumbered) by an external light source 79 which emits 
external light falling within the same band as the incident light 35 
emitted from the internal light source 36. 
Under the circumstances, the external light reaches the image pickup device 
38 through the second optical band-pass filter 77 alone to provide a false 
fingerprint image and a false background image. Such a false background 
image has a grey level different from that of the true background image 
because the external light passes through the second optical band-pass 
filter 77 alone. 
The forged input detector 75 is for discriminating between the true and the 
false background images by detecting a difference of grey levels between 
the true and the false background images. 
Referring to FIG. 14 together with FIG. 12, the forged input detector 75 
comprises a control section 81 operable in a manner to be presently 
described and a memory section 82 which stores a reference background 
level corresponding to the grey level of the true background image. 
When operation is started at a first step Sb.sub.1, the image signal 
succession is sent from the image input device 31a through the control 
section 81 to the image memory 48. The control section 81 derives from the 
image memory 48 preselected ones of the image signals that are placed at a 
background portion of an image area and which will be called background 
images. At a second step Sb.sub.2, the control section 81 detects a grey 
level of each background image. Subsequently, the control section 81 reads 
the reference background level out of the memory section 82 and judges at 
a third step Sb.sub.3 whether or not the grey level of each background 
image is coincident with the reference background level. 
If the grey level of each background image is coincident with the reference 
background level, the control section 81 makes the collation processor 47 
collate the image signal succession with the pattern signal at a fourth 
step Sb.sub.4 in a manner similar to that mentioned in conjunction with 
FIG. 10. 
Otherwise, the third step Sb.sub.3 is followed by a fifth step Sb.sub.5 at 
which another impression of the fingerprint is audibly or visually 
requested. At a sixth step Sb.sub.6, the control section 81 judges whether 
or not the request for the impression is repeated a predetermined number 
of times. 
If iteration of the request reaches the predetermined times, the sixth step 
Sb.sub.6 is succeeded by a seventh step Sb.sub.7 for rejecting collation 
between the image signal succession and the pattern signal. Otherwise, the 
sixth step Sb.sub.6 is succeeded by the second step Sb.sub.2 so as to 
repeat the above-mentioned operation. 
Thus, it is possible to discriminate between the true and the false objects 
by monitoring the grey level of the background image of the image signal 
succession. 
In FIG. 13, the first and the second optical band-pass filters 76 and 77 
may consist of thin films and may be coated on the incident and the exit 
surfaces, without any gaps left therebetween. In addition, each of the 
first and the second optical band-pass filters 76 and 77 may partially be 
coated on each surface of the prism 32. In this event, a part of the 
incident light 35 passes through the first and the second optical 
band-pass filters 76 and 77 while the remaining incident light does not 
pass therethrough. The forged input detector 75 can discriminate between 
the true and the false objects by monitoring a ratio between the part and 
the remaining part of the incident light. 
It is preferable that each of the first and the second optical band-pass 
filters 76 and 77 is interchangeable in case that each filter is stolen. 
Referring to FIG. 15, a grey level of a fingerprint impression, namely, a 
ridge pattern is varied with time when a fingertip is placed as a true 
object on an input surface in a manner as illlustrated in FIG. 2. More 
specifically, when the fingertip is brought into contact with the input 
surface at a time instant T.sub.a, the grey level of the ridge pattern is 
rapidly varied and reaches a first predetermined grey level G.sub.a at a 
time instant T.sub.b. Thereafter, the grey level is slowly changed from 
the first predetermined grey level G.sub.a and reaches a second 
predetermined grey level G.sub.b at a time instant T.sub.c. Such a 
variation of the ridge pattern results from sweat oozing through sweat 
glands. A time interval between the time instants T.sub.a and T.sub.b will 
be referred to as a first interval. 
On the other hand, when the forged fingerprint is placed as a false object 
on the input surface in the manner illustrated in FIG. 13, the grey level 
is never changed with time because of absence of sweat. 
Under the circumstances, it is possible to discriminate between the true 
and the false objects by monitoring the grey level for the first time 
interval between the time instant T.sub.a and the time instant T.sub.b. 
Moreover, it is possible to accurately identify the fingerprint, if the 
true object is scanned for a time interval between the time instant 
T.sub.b and the time instant T.sub.c to produce the image signal 
succession. The time instant between the time instants T.sub.b and T.sub.c 
will be referred to as a second interval. 
Referring to FIGS. 16 and 17 together with FIG. 12, a pre-processing system 
45d according to a fifth embodiment of this invention comprises an image 
input device 31 similar to that illustrated in FIG. 2 and a forged input 
detector 75 similar to that illustrated in FIG. 12. When an object is 
placed on the input surface of the image input device 31 at a first step 
Sc.sub.1, operation is started. At a second step Sc.sub.2, the image input 
device 31 produces the image signal succession in the form of a digital 
signal succession which may be called a preceding digital signal 
succession. Specifically, the grey level of each image signal is 
converted, for example, into 256 levels. Responsive to the preceding 
digital signal succession, the forged input detector 75 compares each 
preceding digital signal with a predetermined threshold signal 
corresponding to the first predetermined grey level G.sub.a. As a result, 
only black elements are selected from the preceding digital signal 
succession and represented by the grey levels. The grey levels of the 
black elements and their positions are stored in the image memory 48. 
After lapse of a preselected duration shorter than the first interval 
between the time instants T.sub.a and T.sub.b (FIG. 15), the image signal 
succession is again extracted as a succeeding digital signal succession 
from the object at a third step Sc.sub.3. The forged input detector 75 
selects the succeeding digital signals with reference to the positions of 
the black elements selected from the preceding digital signals. The grey 
levels of digital signals derived from the same positions are compared by 
the forged input detector 75 with the grey levels of the black elements 
derived from the preceding digital signals. 
At a fourth step Sc.sub.4, the forged input detector 75 detects whether or 
not a variation of the grey levels occurs between the preceding and the 
succeeding digital signals. On occurrence of the variation, the forged 
input detector 75 judges that the object is true. As a result, the fourth 
step Sc.sub.4 is followed by a fifth step Sc.sub.5 at which a further 
image signal succession is derived from the object within the second 
interval between the time instants T.sub.b and T.sub.c (FIG. 15) and is 
collated with the corresponding pattern signal read out of the memory file 
46. 
When the variation of the grey levels does not occur between the preceding 
and the succeeding digital signals, the fourth step Sc.sub.4 is succeeded 
by a sixth step Sc.sub.6 at which a further impression of the fingerprint 
in question is requested again. At a seventh step Sc.sub.7, the forged 
input detector 75 judges whether or not the request for the impression is 
repeated a predetermined number of times. If repetition of the request 
does not exceed the predetermined times, operation is returned back to the 
second step Sc.sub.2. Otherwise, the forged input detector 75 rejects 
collation of the fingerprint in question at an eighth step Sc.sub.8. Thus, 
the forged input detector 75 judges that the object is false. 
After either the fifth step Sc.sub.5 or the eighth step Sc.sub.8, operation 
is completed at a ninth step Sc.sub.9. 
Referring to FIG. 16 again and FIG. 18 afresh, the pre-processing system 
45d illustrated in FIG. 16 may be operated in a manner to be presently 
described. In this event, the ridge pattern is subtly moved with time when 
the true object, namely, fingertip is actually placed on the input 
surface. This means that positions of black levels are slightly changed on 
an image area as shown in FIG. 3 with time. Under the circumstances, it is 
possible to discriminate between the true and the false objects by 
monitoring variations of the grey levels at fixed positions determined on 
the image area. In the example being illustrated, the fixed positions are 
of all of picture elements arranged on the image area. 
More specifically, when operation is started at a first step Sd.sub.1, the 
image signal succession is produced from the image input device 31 in the 
form of a succession of digital signals at a second step Sd.sub.2. The 
image signal succession is supplied as a succession of preceding image 
signals through the forged input detector 75 to the image memory 48. At a 
third step Sd.sub.3, a succession of succeeding image signals is sent from 
the image input device 31 to the forged input device 75 after lapse of a 
predetermined duration. The succeeding image signal succession may also be 
stored in the image memory 48. 
The forged input device 75 successively compares the succeeding image 
signals with the corresponding preceding image signals read out of the 
image memomry 48. At a fourth step Sd.sub.4, the forged input device 75 
judges whether or not variations of the grey levels occur between the 
preceding and the succeeding image signals. 
When the variations of the grey levels are detected by the forged input 
detector 75, the fourth step Sd.sub.4 is followed by a fifth step Sd.sub.5 
at which collation is carried out in the manner described in conjunction 
with FIG. 17. 
When variations of the grey levels are not detected at the fourth step 
Sd.sub.4, sixth, seventh, and eighth steps Sd.sub.6, Sd.sub.7, and 
Sd.sub.8 are successively carried out in the manner described in 
conjunction with the sixth, the seventh, and the eighth steps Sc.sub.6, 
Sc.sub.7, and Sc.sub.8 of FIG. 17. 
Referring to FIGS. 19 through 21, an identification system comprises 
similar parts designated by like reference numerals and a pre-processing 
system 45e according to a sixth embodiment of this invention. The 
illustrated pre-processing system is operable to quickly detect whether an 
object is true or false and to extract a distinct fingerprint impression 
from the object. 
The grey levels of the fingerprint are variable with time in the manner 
illustrated in FIG. 15. In this connection, detection of either a true 
object or a false object may be made within the first interval between the 
time instants T.sub.a and T.sub.b while extraction of a fingerprint from a 
true object detected may be made after lapse of the first interval. 
However, such variations of the grey levels have individual differences in 
dependency upon an amount of sweat. Accordingly, each user may feel 
unconfortable when it takes a long time to indicate a request for another 
impression as described in conjunction with the sixth steps Sc.sub.6 and 
Sd.sub.6 illustrated in FIGS. 17 and 18. 
In order to quickly detect either a true or a false object, the 
pre-processing system 45e illustrated in FIG. 19 comprises a forged input 
detector 75a for partially processing an optical image on an image area 40 
as illustrated in FIG. 3 and an image controller 81 for controlling a 
timing to extract the distinct fingerprint impression from the object. 
More specifically, the illustrated forged input detector 75a samples the 
image signal succession at sampling instants corresponding to sampling 
positions 82a illustrated in FIG. 21. The sampling positions 82a are 
regularly arranged on the image area 40 and are equally spaced from one 
another. From this fact, it is readily understood that the forged input 
detector 75a comprises a clock pulse generator (not shown) for producing a 
sequence of clock pulses and a controller (not shown also) for sampling 
the image signal succession by the clock pulse sequence. 
In FIG. 20, operation is started at a first step Se.sub.1 and the image 
signal succession is successively sent from the image input device 31 to 
the forged input detector 75a. The image signal succession is sampled in 
the manner illustrated in FIG. 21 by the forged input detector 75a to be 
produced as a succession of sampled signals. The sampled signal succession 
is stored as a succession of preceding signals in the image memory 48 at a 
second step Se.sub.2. After lapse of a predetermined duration, the image 
area 40 (FIG. 21) is scanned and sampled again in the manner illustrated 
in FIG. 21. As a result, a succession of succeeding sampled signals is 
produced by the forged input detector 75a and stored in the image memory 
48 at a third step Se.sub.3. 
At a fourth step Se.sub.4, the forged input detector 75a accesses the image 
memory 48 to compare the succeeding sampled signals with the preceding 
sampled signals and to judge whether the object is true or false. 
Responsive to a result of the judgement made at the fourth step Se.sub.4, 
the forged input detector 75a determined at a fifth step Se.sub.5 whether 
a request for another impression is to be produced or not. 
When the request for another impression is produced by the forged input 
detector 75a, the fifth step Se.sub.5 is followed by a sixth step Se.sub.6 
at which the forged input detector 75a judges whether or not an iteration 
of the request is within a prescribed number of times. If the iteration is 
within the prescribed times, operation is returned from the sixth step 
Se.sub.6 back to the second step Se.sub.2. Otherwise, identification of 
the object is not carried out any longer. 
When the request for another impression is not produced at the fifth step 
Se.sub.5, namely, the object is true, the image area 40 is further scanned 
under control of the image controller 81 to produce an additional image 
signal succession at a seventh step Se.sub.7. The additional image signal 
succession is sampled at either the sampling instants as illustrated in 
FIG. 21 or any other sampling instants. Anyway, the forged input detector 
75a supplies a succession of additional sampled signals at the seventh 
step Se.sub.7 to the image memory 48. 
The optical image or image area is sampled at a time instant controlled by 
the image controller 81. The time instant appears after the time instant 
T.sub.b illustrated in FIG. 15. 
At an eighth step Se.sub.8, the image controller 81 decides whether or not 
the image signal succession is to be extracted from the object so as to be 
collated with the corresponding pattern signal. The decision is made with 
reference to the first predetermined grey level G.sub.a illustrated in 
FIG. 15. The eighth step Se.sub.8 is succeeded by a ninth step Se.sub.9 at 
which the image controller 81 judges whether or not a further impression 
of the fingerprint is to be requested. When the request is to be repeated, 
a repetition of the request is measured by the image controller 81 at a 
tenth step Se.sub.10. If the repetition of the request is within the 
prescribed number of times, the tenth step Se.sub.10 is succeeded by the 
seventh step Se.sub.7. Otherwise, operation is completed. 
If any other impression is not necessary, the ninth step Se.sub.9 is 
followed by an eleventh step Se.sub.11 at which collation is carried out. 
In this event, the optical image is further scanned on the image area 40 
to produce the image signal succession derived from a whole of the input 
surface. The image signal succession may directly be sent from the image 
input device 31 to the image memory 48. 
Referring to FIG. 22, another example of a sampling position arrangement 
has sampling positions irregularly distributed or scattered on the optical 
image. With this example, the sampling positions can be determined by 
random numbers to sample or select the picture elements thereat. 
It is to be noted here that the fingerprint has a ridge arrangement 
comprising a lot of curved ridges arranged in order. An irregular 
distribution of the sampling positions serves to uniformly select the 
ridges and the remaining portions. Uniform detection of the fingerprint 
impression is therefore possible all over the optical image. 
Referring to FIGS. 23 through 25, sampling positions are distributed on the 
optical images, respectively. In FIG. 23, the sampling positions are 
radially and discretely arranged from a center portion of the optical 
image to a peripheral portion surrounding the center portion. In FIGS. 24 
and 25, the sampling positions are irregularly arranged. In any event, 
each distribution of the sampling positions is dense or thick at the 
center portion and is rough or thin at the peripheral portion. Anyway, the 
sampling positions are bidimensionally distributed on the optical images, 
namely, the input surface. 
While this invention has thus far been described in conjunction with 
several embodiments thereof, it will readily be possible for those skilled 
in the art to put this invention into practice in various other manners. 
For example, it is possible to pre-process an image signal succession 
derived from an object except the fingertip. In addition, decision of 
either a true object or a false object may be made after the center 
pattern of the optical image is detected in the manners illustrated in 
FIGS. 1 and 6.