Note: Descriptions are shown in the official language in which they were submitted.
I 1 6125~
FINGER IDENTIFICATION
~ackground of the Invention
This invention relates generally to a finger
identification and finger image processing apparatus.
More particularly it relates to apparatus and method for
encoding finger image information into machine readable
language with apparatus that is simple, inexpensive and
reliable.
There are a number of systems that have been
proposed for the processing of identification information
based on the unique configuration of ridges and valleys in
an individual's finger. When such information is taken
from an ink impression of an individual's finger it is
normally called a fingerprint.
~, ..
-- 1 -- .4
5 0
The more sophisticated techniques that employ op~ical tech-
niques tend t~ provide a more refined, discriminate and
accurate identification image; which image applicant has
frequently called a fingerpress in order to distinguish it
from the more primitive ink fingerprint. However, since both
are based on the same uni~ue ridge and valley finger
characteristics, it should be understood that the term finger-
print is used ~y applicant generically while the term finger-
press is used to refer to the actual configuration of the
ridges and valleys of the finger when pressed against a surface
or to the fairly precise image of such that can he obtained
by using certain optical systems. In these optical systems,
the finger of the subject lndividual is placed against the
back of a transparent platen and the normally ~lat ~inger
image on the back surface of the platen is imaged through the
front of the platen and projected onto a receiving or
processing equipment. This recei~ing equipment may take the
form of a screen, of a camera or of an array of photocells.
For example United States Patent No. 3,138,059
discloses such a system. ~s described in the '059 patent,
the finger is pressed against a transparent platen and a light
beam is projected against the platen. Light is reflected
from the finger to a recorder in the form of a camera. United
States Patent No. 4t053,223, issued on October ll, 1977
discloses a holographic identification system. As described in
'~ $'0
the '228 patent, a collimated, coherent light beam is projected
against the front surface of a transparent platen. The light
is reflected from the back surface of the platen, against which
surface the subjectls finger is pressed. The reflected light
beam is modula~ed with the inger image and is correlated against
a hologram of the same fingerpress to provlde identification.
However, systems of this type suffer from a number of
disadvantages, formost among which is a high degree of inaccuracy.
That is, mismatching can easily occur between the hologram and
the image. Mismatching error is reduced by employing accurate
alignment procedures but this solution increases the cost and
complexity of the system. Aside from the question of alignment
apparatus, such systems are extremely expensive as they require
beam splitters, devices to change direction of the light beams,
focllsing lenses, devices to effect the necessary correlations,
etc. Additionally, these systems are difficult to maintain and
service because of the number of elements comprising the systems
and the fact that even the slightest vibration can knock` a lens
l or a mirror out of position.
¦ Accordingly, it is a major purpose of this invention
to provide a technique for processing a fingerprint or finger
image in a fashion that is simple and unambiguous, that avoids
undue messiness~ provides a high degree of reliakility in opera-
l tion and that can be implemented in equipment which is relatively
trouble free and that requires a minimum of maintenance.
It is a further object of this invention to provide
all of the above mentioned objects in a system that provides an
accurate and unambiguous finger image or fingerprint image which
5 Q
in turn is susceptible to being encoded into machine readable
signals.
In the holographic systems, stringent re~uirements
are placed on the platen. The surfaces of the platen must
be completely flat to minimize inaccuracies introduced into
the reflected light beam. In general, where a lot of light
is lost and where the contrast between the ridges and valleys
in the image is low, the platen must be an expensive
precision unit.
In addition, build up of finger oil introduces
inaccuracies into the system. Often, a latent image is fixed
to the platen by the finger oil residue on the platen. The
operator must maintain the platen clean by wiping it after
each use. However, even if the platen is clean, these
systems are sensitive to either too much or too little oil
from the finger. Either case may produce erroneous results.
Other problems occur when the platen is cold and a warm
finger is placed against it. This fogs the platen. While a
platen may be preheated to eliminat:e this problem, such a
solution is impractical.
Accordingly, a further object of this invention is
to provide a highly accurate and reliable finger identifi
cation and processing apparatus that includes a relatively
inexpensive platen.
To this end, the invention consists of fingerprint
processing apparatus comprising: a source of an interrogating
light beam, platen means positioned in the path of said
interrogating light beam to receive said light beam and
reflect light therefrom which is modulated with finger
information, said platen means adapted to receive a finger
thereon to provide a fingerprint object causing differential
scattering of said reflected light, the reflected light
5 ~
from zones corresponding to the fingex ridges scattering
substantially differently than the reflected light from
zones corresponding to the valleys of said fingerprint
object, linear displacement optical scanning means for
causing said interrogating light beam to scan said finger-
print object on said platen and for maintaining the angular
relationship between said interrogating beam and said
fingerprint object being scanned constant throughout the
optical scan, an array of photoelectric transducers
optically downstream from said platen, and lens means
positioned in said reflected light beam to provide at said
array a fin~gerprint image having differential intensity
representing valley and ridge zones respectively, said
array being arranged in a predetermined linear form for
receiving said modulated light beam and for converting the
fingerprint information carried by said modulated light
beam into a plurality of sets of information signals
representing a line by line scan of the fingerprint, and
electrical scanning means to electronically scan the
information signal outputs of said array and to synchronize
sald electronic scan to incremental positions of said
linear displacement scanning means to provide a relatively
faster electronic scan along said array and a relatively
slower optical scan along an axis transverse to said array.
The invention also consists of the method of
examining a fingerprint in which a finger is placed on the
platen to provide a fingerprint ob~ect comprising the steps
of: optically interrogating the fingerprint object by
scanning a light beam across said object by moving said
platen and said light beam relative to one another and
causing the angular relationship between said
- 4~
a~ 5~.
interrogating light beam and said fingerprint object being
scanned to remain constant throughout the optical scan,
modulating said interrogating light beam with said finger-
print object information by causing the reflected light
from zones corresponding to the ridges of said fingerprint
object to scatter substantially differently than reflected
light from the zones corresponding to the valley of said
fingerprint object and thus providing a reflected light
beam modulated with fingerprint information, imaging the
reflected light by converting the differential scatter of
the reflected light from the ridge and valley zones to
differential intensity representing ridge and valle~ zones
thus providing a fingerprint image, detecting said finger-
print image on a line by line basis to provide a set of
electrical signals representing light intensity along each
of said lines of said fingerprint :image, and electron-
ically scanning each of said detected lines, and providing
a relatively faster electronic scan simultaneously with
the relatively slower optical scan,.
~ - ~b -
1~61250
Brief Description
In brief, a laser provides an interrogating light beam
which is shaped by two cylindrical lenses into a slit-beam of
ligl~t. This beam is colIimated and scanned across a finger
placed against a platen. The finger is pressed against the back
surface of the platen and provides a -fingerprint object that is
constituted by a series of ridges and valleys. The beam is
directed towards the front surface of the platen, at a slight
angle to normal, and passes through the transparent substrate of
the platen to be reflected from the fingerprint object as a modu-
lated beam. The platen may have a deformable resilient layer
that conforms to the pattern of ridges and valleys and that
enhances the modulation of the light beam. The light beam is
scanned across the finger by a linear displacement scanning
technique that maintains the angular relationship between the
interrogating light beam and the plane of the platen throughout
the scan. At any position of the scan, the light bea~ is modu-
lated by the object being scanned to produce li~ht and dark spots
corresponding to finger valley and ridge zones. This reflected
modulated signal is projected on a linear array of photodiodes;
1,064 photodiodes in one embodiment. An imaging lens between
the platen and the photodiode array proJects an image of the
fingerprint at a plane in space that, depending on the choice
of platen, is either at or displaced from the plane of the photo-
diode array. Each photodiode responds to either relatively
light or dark spots by producing a corresponding signal. These
signals are serially interrogated to provide a digital output
that is a series of electrical signals representing fingerprint
_5~
1~ 250
information, An encoder is coupled to this scanning circuit and
to the circuit that interrogates the photodiodes to make sure
that the modulated slit-beam of light is interrogated at about
1,000 regular predetermined intervals during its scan across the
finger involved. By providing a mechanical optical scan in one
direction and an electronic scan of the diodes in the orthogonal
direction, a simple implementation is provided to proyide a two
dimensional scan with only a one dimensional displacement motion.
A preferred platen is composed of six layers. The
thickest layer is a glass substrate one surface of which forms
the front surface of the platen. On top of the glass substrate
is a highly deformable, highly resilient epoxy layer that pro-
vides a very flat back surface. On top of the epoxy layer, a
very thi.n (3,000 ~ thick) reflective silver layer is deposited.
The silver layer is flat because it is deposited on the flat
surface of the epoxy. The silver layer provides a mirrored sur-
face to reflect the incident col:Limated light beam. The fourth
layer is a fairly thin (for example, 0,~5 mm, thick~ layer of
the same epoxy material, The fifth layer is another very thîn
reflective silver layer. And finally a thîn (approxima-tely
0.025 mm. thick) lacquer layer is deposited on top of the silver
layer to protect the silver from mechanical wear. The subject's
fînger is applied to the ].acquer. When so applied, the epoxy
deforms sufficiently to conform to at least the ridges, The
result is a topographical map of the ridge and valley structure
at the reflective surface that differentially scatters the inci-
dent light. The ridges scatter the incident light substantially
more than do the valleys. This provides a modulated reflected
--6--
11~6125~
light beam. The use of the reflective layer provides a greater
light intensity than when the reflective layer is omitted. In
this multi layer sandwich, the touching surface is isolated from
the optically active surface and shields the optically active
surface from the effects of temperature changes. The lacquer
defining the back surface of the platen provides a durable pro-
tection to the delicate extremely thin silver reflective layer.
These various layers also smooth or filter the ridge breaks in
l the input finger to provide a more usable and useful topographic
¦ map of the finger in the optically active silver layer surface.
~ 2~
_rief Description of the Drawings
FIG. 1 is an optical and mechanical schematic drawing
of a first embodiment of a fingerprint processing apparatus con-
structed according to the present invention.
FIG. 2 is a block diagram illustrating the arrangement
for encoding the fingerprint information.
FIG. 3 is a schematic optical diagram of a second
embodiment of the apparatus constructed according to the present
invention.
FIG. 4 is a sectional view orthogonal to the plane of
FIG. 3, to an enlarged scale, of the movable platen portion of
the FIG. 3 apparatus.
FIG~ 5 and ~A are a detailed sectional view, in some-
what schematic format, of one platen that may be used in the
FIG.l and FIG. 3 apparatus with a finger impressed thereon.
FIG. SA is a very much enlarged view o the indicated portion of
FIG. 5. It whould be understood that FIG. 5A is even more sche-
matic than is FIG. 5 and only schematically represents what is
believed to be the relation between platen and finger ridges and
valleys.
FIG. 6 is a view comparable to that of FIG. 5A, illus-
trating a second embodiment of the platen that may be used in
the FIG. 1 and FIG. 3 apparatus. The FIG. 6 platen incorporates
a reflective layer 96.
FIG. 7 is a schematic cross sectional view of a third
platen illustrating a presently preferred embodiment having five
layers on top of a glass substrate.
FIG. 8 is a view comparable to that of FIGS 5A. and
6 illustrating a fourth platen having an anti-reflective coating
~6~5~
Description of the Preferred Embodiments
The various embodiments described below differ from
one another in two major respects. One distinction has to do
with what it is that is moved to effect the light beam scan.
The other distinction has to do with the type of platen employed
to provide the fingerprint or finger image that is interrogated.
In the FIG. 1 embodiment the interrogating light beam
30 is moved so that it scans across a stationary finger F. By
contrast, in the FIG. 3 embodiment a platen 64 and finger F on
the platen moves across a stationary interrogating light beam 61,
thereby effecting the scan.
The platen illustrated in FIG. 5 includes a deformable,
resilient layer 94. The platen of FIG. 6 includes a reflective
layer 96 under the deformable, resilient layer. The platen of
FIG. 7 includes a second deformable layer 97 as well as a second
reflective layer 98. The platen of FIG. 8 is a rigid glass
platen w;th an anti-reflective layer 101.
The term collimate is used herein to refer to a light
beam in which the light rays do not scatter. It is not essential
(except at its focal point).
that the light rays all be parallel to one another/ It is only
essential that they not cross over one another. Thus, a colli-
mated light beam as used herein migh~ be diverging, or parallel,
or converging. In one embodiment of this invention the collimatec
light beam converges at an angle of a few degrees. Accordingly
it should be understood that in the specification and claims,
"collimate", or "collimation" is used as defined above. The
laser 16 described provides a beam of coherent light. Coherent
light is required in a holographic system. However, in the non-
holographic embodiments described herein, the light beam does not
have ~o be coherent. What is important is that the light beam be
collimated in the broad sense of collimation as defined above.
_g_ ,
1161250
The FIG. 1 Embodiment
One embodiment of the apparatus constructed according
to the present invention is designated generally by the reference
numeral 10 in FIG. 1 and includes a linear photodiode array 12
mounted on a support 14. The photodiode array 12 is conventional
in construction and may, for example, comprise photodiode array
Model No. CCD 131 manufactured by the Fairchild Semiconductor
Division of Fairchild Camera & Instrument Co. J of Mountainview,
California. This particular array comprises 1,064 photodiodes
that extend in the longitudinal direction ~i.e., along a line
going into the paper in the configuration of FIG. 1). The diodes
are aligned in contact with one another and eaeh diode is about
0.02mm, on a side. Accordingly, the shape of the light receiving
opening of the array iQ in the form of a slit wherein the long
dimension of the slit corresponds to the longitudinal direction
of the array.
The source of the light beam is a laser 16. The laser
16 normally produces a circular light beam o~ relatively small
diameter in the order of approximately one millimeter. Cylindri-
cal lenses 18 and 20 are positioned in the path of the light beam
downstream from the laser 16 to convert the shape of the light
beam into a slit. ~s is conventional, the cylindrical lenses 18,
20 change the beam dimension along one axis and also collimate
the beam along this axis. In an actual embodiment, the beam was
stretched and collimated to about 2~ mm.along ~e axis. The trans-
verse axis of the beam remains one millimeter so that the shape
of the interrogating beam 30 has a slit format roughly conforming
to the ~ormat o~ the linear photodiode array 12.
-10-
1161ZSO
To aid in vizualization, the laser 16, the cylindrical f
lenses 18, 20 and the mirror 22 in FIG, 1 are rotated 90 to show
the elongated light beam in the plane of the ~IG, 1, In practice,
these elements have an orientation that is rotated 90 from that
shown to be consistent with the orientation of the rest of the
apparatus as shown in FIG, 1. In ~IG. 1, the interrogating light
beam 30, the reflected modulated light beam 54 and the diode
array 12 all have their long dimension perpendicular to the plane
of the FIG. 1.
¦ Although a laser beam is shown, the light source need
not be a laser not even a source of coherent light. It is impor-
tant however that the interrogating light beam 30 be collimated
to maximize the differential scatter în the reflected light beam
54. One benefit obtainable with.a laser is that there is little
or no divergence of the beam of light. Thus, since the laser
produces a light beam having a dimension of approximately o~e
millimeter along the small dimension, no lenses will be required
to change this portion of the beam and still maintain the slit
dimension approximately one mi~limeter, This reduces the overall
cost of the system and also eliminated the need for any complex
focusing arrangement.
In order to get a useful output out of the diode array
12, it is important that the level of light intensity incident on
the diode array be significantly above a threshold where a bright
spot of ligh~ is involved and significantly below that threshold
where a dark spot is involved; these spots of dark and light
representing points on the ridges and valleys of the finger.
By forming the light beam into a slit and then using that slit
light beam ta scan across the finger pressed against the platen,
I J B125û
the level of light intensity available is much greater than in
the arrangement shown, for example, in the U. S. Pat. No. 4,053,22 3
wherein the laser light beam is expanded so that it covers the
entire area of interest of ~he finger involved. In contrast with
this arrangement in U. S. Pat. No. 4,053,228 the shaping of the
light beam into the slit and the scanning of the finger with that
slit light beam provides sufficient intensity of light at the
diode array L2 to provide a signal level at the diode array 12
which permits a rapid interrogation of the diodes at each scanning
point.
The mirror 22 reflects the shaped light beam as beam
24 toward mirror 26. The mirror 26, affixed to a movable support
28, reflects the light beam 24 as beam 30 to platen 32. The
platen 32 is adapted to receive a finger F thereon with the
fingerprint (more accurately, a flattened pressed finger surface)
in contact with the upper surface of the platen. The platen 32
may be a transparent optical device the operation of which is
described more fully in my U. S, Pa~. No. 4,053,228.
The support 28 is movably mounted for reciprocating
movement on arms 34 positioned at each end of the support and
slidingly received in bores 35. The arms are supported by posts
37 upstanding from the support 14. Mounted on the top surface 36
of the support is a member 38 having side walls 40 and 42 which
terminate in shelves 41. In a preferred embodiment, the sidewalls
40 and 42 form an angle of 42.5 with respect to the horizontal,
for reasons notQd in greater detail hereinbelow, The mirror 26
is affixed to the wall 40 and a mirror 44 is affi~ed to the wall
42.
t~612~0
A spring 46 between the support 28 and a wall 48 biases
the support 28 toward the right, as taken in FIG. 1. An idler
roller 5~ is rotatably mounted adjacent the right-hand end of
the support 28, as shown in FIG. 1. A motor driven cam 52 is
S drivingly connected with the idler wheel 50. The cam is shaped
so that upon rotation of the cam by the motor the support 28 is
driven to the lift. As the cam rotates to the position shown in
FIG. 1, the spring 46 returns the support 28 to its right-hand
position.
The mirror 44 is in the pa~h of a reflected modulated
beam 54 from the platen 32 and reflects the light beam 54 to the
diode array 12. The beam 54,56 is modulated with fingerprint
information from the platen 32 as noted in detail below.
The angle o~ the mirror 26 causes the interrogating
light beam 30 to strike the platen 32 at an angle 5~ off normal.
That is, the angle of incidence of the beam 30 with respect to
a line perpendicular to t~e platen 32 is 5. Similarly, the
angle that the reflected beam 54 makes with the normal is also
5. This insures that ~he reflec~ed beam 54 will diverge from
the interrogating beam 30.
The fingerprint information is modulated onto the slit
ligh~ beam 30 when the finger F is pressed against the platen 32.
The mode of modulation contemplated involves differential scat-
tering of the light incident at the valley zones and ridge zones.
l~here the platen used has an anti-reflective coating of the back
surface thereof this mode of modulation may also incorporate
differential absorption and reflection from the ridge and valley
zones as explained in United States Patent No. 4,053,228, Other
-13-
3~25~
modulation implementations are explained in connection with the
discussion of the platen structures shown in FIGS. 5, 6 and 7.
More specifically, when a finger F is pressed against
the back surface of the platen 32, a surface object is created.
That portion of the incident light which is reflected from the
finger object is modulated by the ridges and valleys of the fin-
ger to provide a reflected light beam 54 that carries the finger-
print information. A lens 65 serves to project an image of the
finger object to an image plane downstream. Depending on the
platen used, the diode array 12 may be at the image plane or
displaced from the image plane. As a result, the modulated slit
light beam 56 striking the diode array 12 will contain light and
dark spots which are indicative of the fingerprint information.
This information is unique for each fingerprint and therefore
provides encoded fingerpring information which can be retrieved
or otherwise processed as desired by the operator.
The incident light beam 30 is scanned across the finger
(from right to left as taken in FIG. 1). The light beam infor-
mation is synchronized with the output from the diode array 12
by an encoder 55 which produces synchronizing signals that are
applied to scanning circuitry by lead 57. The encoder 54 is
conventional and produces a signal each time the support 28 moves
an incremental distance. In the array 12 each of the diodes are
about 0.02 mm. (about one mil.) on a side. The encoder 54 pro-
duces a synchronizing signal each time the support moves 0.02 mm.
While any type of encoder may be utilized, in practice an optical
linear encoder has been used ko generate synchronizing signals.
-14-
~12~
In operation, the finger to be examined is placed on
the back surface of the platen 32. The laser 16 is energized to
produce a slit light beam 30 that impinges on the finger pressed
against the back surface of the platen 32. This light is modu-
lated and reflected as light beam 54 7 56 to the diode array 12.
Simultaneously with the energization of the laser, the encoder
28 produces a synchronizing signal which is applied to a scan-
ning circuit 58 (FIG. 2) via the lead 57. The scanning circuît
58 is conventional in construction and is adapted to se~uentially
interrogate each one of the photodiodes comprising the array 12
in response to the synchronizing signal. Thus, the output of the
scanning circuit 58, which may comprise a train of pulses for
each scan line, is connected to the store or computer via a lead
60 so that the fingerprint info~ation can be processed.
The cam 52 is energized simultaneously with the laser
16 so that as the cam 52 rotates, the support 28 moves tow~rd the
left as taken in FI~, 1.
As the support 28 moves 0.02 mm. towards the left, the
slit light beam similarly mo~es 0,02 mm. and the modulation
of the reflected light beam 54 changes in accordance with the
ridges and valleys of the finger. The encoder 54 again produces
a synchronizing signal which causes the scanning circuit to again
interrogate each one of the photodiodes comprising the array 12
to produce a second train of pulses representative of the finger-
print information in the second scan line,
This operation continues until the entire fingerprint
or finger object has been scanned by the slit light beam 30. It
is to be understood that the interrogation of the diode array is
-15~
1t~2~0
accomplished electronically at a rate much faster than the rate
of movement of the support 28 so that all diodes will have been
interrogated before the slit light beam is indexed to the next
scan line.
One advantage of the scanning structure shown in that
it is a linear displacement scanner. Thus displacement along a
straight line of the mechanism, including the mirrors 26 and 44,
causes the interrogating slit light beam 30 to be displaced
without changing the angular relationship between the interrogat-
ing beam 30 and the finger object being scanned and thus without
changing the angular relationship between the reflected modulated
light beam 5~ and the finger object being scanned.
-16-
1 ~ 6~
The FIG~ 3 Embodiment
FIGS. 3 and 4 illustrate an arrangement in which the
finger F is moved rela~ive to the interrogating light beam 61 to
effect the Tnechanical scanning operation. This is by contrast
with the FIG. 1 embodiment where the interrogating light beam
30 is moved relative to the finger F. However both embodiments
employ the basic concept of (a~ a linear displacement scan be-
tween a slit light beam and a platen and (b) a synchronized
orthogonal electronic scan of a linear photodiode array 12.
The arrangement shown in FIG. 3 includes a fixed sup-
port 62 on which the optical platen 32 is movably mounted. The
support 62 has a central channel opening 66 through which the
interrogating light beam 61 passes to impinge on the platen 32
and the reflected modulated light beam 63 passes to be further
processed. The platen 32 is on a carriage 68 that rides on the
upper surface of the support 62 by means of roller bearings 70.
Affixed to the carriage 68 is an encoder 71 similar in construct-
ion to the encoder 54. Additionally, the carriage 68 is main-
tained in place by tracks 72 on the support 62 which are received
in appropriate recesses in the underside of the carriage.
As shown in FIG. 3, the upper surface of the suppor~
62 has a recess 74, the length of which is substantially longer
than the carriage 68. A rod 76 extends between one end of the
recess 7~ and the carriage 68 and is adapted to be slidingly
received in an opening 78 within the carriage. A speed reducing
device 80 is connected to the carriage 68 and receives the rod
7~ therethrough and is adapted to increase the coefficient o
friction between the rod and the carriage to limit the speed
I 1 61250
of forward movement of the carriage 68 within the recess 74.
For example, the device 80 may comprise a plurality of felt
washers that receive the rod therethrough in a tight frictional
fit.
In operation, the finger F is placed on the platen 32
with the tip of the finger in abutment with the end stop 82 of
the carriage. The slit beam 61 from the laser is positioned so
that it will impinge at the forward end of the finger when the
elements are in the position shown in FIG. 3. The encoder 71
produces a synchronizing signal that causes the array 12 to be
interrogated. Thereafter, the finger F exerts a continuous
pressure in the forward direction thereby causing the carriage 68
to move toward the right, as taken in FIG. 3. As the carriage
moves, its speed is limited by the device 80. Thus, as the
finger moves relative to the light beam, the entire fingerprint
or ~inger image is scanned in the manner noted above. The speed
limiting device 80 prevents generation of a synchronizlng signal
while the array is still being interrogated from the preceding
sc~ .
-18-
1161250
The Platen - In Gen`era7
The linear displacement scanning technique of thîs
invention has been described in connection with two embodiments
without specifying the detailed nature of the platen 32 employed.
A number of different platen devices have been developed to pro-
vide an improved and more useable image than hitherto has been
available. Four different platen arrangements are described in
connection with FIGS. 5, 6l 7 and ~ respectively~ An essential
part of the functioning of all four of these platens is that they
provide a means whereby the degree to which the reflected light
is scatter~d from under the ridge zones significantly differs
from the degree to which light i~ scattered from under the valley
zones. This differential scattering results in di~ferential pro-
cessing of the reflected beam by the imaging lens 65 to provide
differential intensity, at the array 12, representing ridge and
valley zones.
Imaging lens 65 is a simple lens and its ability to
/of the object on the platen
focus a re1ected light ray as part of the image/created down-
stream of the lens 65 depends upon the angle at which the reflec-
ted light ray is incident on the imaging lens 65. Reflected
light which remains collimated is received by the lens at an
angle essentially normal to the plane of the lens, Such light,
and any reflected light within a few degrees off normal to the
plane of the lens will be focused not only at the image plane but
also at substantial distances upstream and downstream from the
image plane.
As the angle of the reflected light that is incident
on the lens 65 deviates further from normal to the plane of the
1161250
lens, the lens 65 will for a number of additional degrees be able
to refocus that light as part of the image o~ the image plane.
However such light will be rapidly defocused above and below the
image plane.
Reflected light which is substantially scattered so
that it is incident to the lens 65 at angles that deviate sub-
stantially from normal will not be refocused at the image plane
or at any other plane upstream or downstream from the imaging
lens.
The term "angular pass band" will be used herein in
connection with the lens 65 to refer to that angular range within
which the lens 65 will substantially refocus incident light at
the image plane. Light that is scattered outside the angular
pass band will simply be lost to the system in that the lens 65
will not be able to use such light to provide an image of that
portion of the finger ob~ect which has so scattered the ligh~.
It should be understood that within the angular pass band signi-
ficant collimation is lost for light rays which are scattered at
the grea~er angles wîthin the angular pass band and that colli-
mation or substan~ial collimation is maintained only for light
rays which are reflected at angles well within the center of the
angular pass band.
Thus, depending on the relative differential scattering
of the reflected light from under the ridge zones and the valley
zones, the lens 65 will provide more or less focusing or defocus-
ing of the two zones either at the image plane or removed from
the image plane as is explained in greater detail in connection
with the following description of each of the four different
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1; 61250
platens that have been developed and tested. What may be useful
is to keep in mind that the basic notion behind all of the
viable platen arrangements is the generation of differential
scattering in the reflected ligh~ beam which is converted to
differential intensity for detection and reading at the arrayl2
by the use of an imaging lens 65.
-21-
~;~61~
The FIG 5 Platen.
The FIG. 5 platen 84 has two layers. The two layers
are a five millime~er (mm) thick transparent glass substrate
92 and a 0.25 millimeter thick, transparent, resilient, deform-
able epoxy layer 94. The readily deformable epoxy layer 94,
deforms in response to the pressure of finger ridges Fr. The
result is to provide a back surface for the platen 84 which pro-
vides a high degree of light scattering of whatever light is
reflected under the ridge zones and a minimal amount of light
- scattering of whatever light is reflected under the valley zones
Fv. As schematically shown in FIGS. 5 and 5A, when the colli-
mated light beam 61 impinges on the relatively flat surface under
the valley zone Fv, the interface between the epoxy layer 94 and
the air under the valleys Fv causes a small portion of the inci-
dent light 61 to be reflected, as part of the ligh~ beam 63,
because of the dif~erence in the index of refraction of the
epoxy material ~4 and air. Although only about 5% of the inci-
dent light is reflected under the valley zones Fv, this is suffi-
cient to provide the valley images downstream at the photodiode
array 12. ~nder the ~inger ridges Fr, the highly deformed areas
in,the highly compliant epoxy material 94 cause a mu~h greater
degree of light scatter in the reflected light. The reflected
light scatters at angles substantially outside of the angular
pass band of the simple imaging lens 65. As a consequence, at
the image plane, the valley zones are recreated while the ridge
~ones are essentially dark.
The result is a clear-cut pattern, received at the
array 12 as a slit having alternating light and dark spots. The
-22-
11~6~2~
spots corresponding to the finger valleys are relatively light
while the spots corresponding to the ridges are essentially
entirely dark.
The scattering depicted by the arrows in FI~S~ 5, 5A
and 6 is highly schematic. The arrows are not intended to sug-
gest actual,angles of scatter buk only that the reflected light
is scattered.
It is believed that there is some scattering of light
under the valley zones ~v~ but that this scattering is well with-
in the angular pass band of the imaging lens 65 so that substan-
tially all of the reflected light at the ~alley zone is reflected
as collirnated light and can be detected by the array 12 at the
image plane and at positions displaced from the image plane.
In addition, because of the fact that most fingers
carry substantial amounts of finger oil which has an index of
refraction very similar to that of the epoxy material 94, there
will be substantial absorpt;on of the incident light 61 at the
ridge zones Fr. Thus, in the FIG. 5 platen, the absorption of
light due to index of refraction match will complement the effect
of the scàttering of light under the ridge zones. However, one
advantage of the FIG, 5 platen is that even with a very dry
finger, in which there is little or no light absorption at the
ridge zones, the substantial scattering of light results in an
image downstream that has sufficient differential intensity
between the ridge zones and valley zones to provide a usable in-
put for the detecting array 12,
In order for the FIG. 5 platen to operate effectively,
it is important that khe index of refraction of the epoxy layer
il61250
94, or other readily deformable resilient material, be as close
as possible to that of the glass 92 or other transparent sub-
strate material that may be used. Matching the index of refrac-
tion of the layers 92 and 94 will minimize the amount of light
lost by reflection from the interface between the layers 92 and
94. In one embodiment, a transparent optical epoxy is used which
is manufactured by Epoxy Technology Inc., 65 Grove Street, Water-
town, Massachusetts 02172 and is designated by the Model No.
EPO-TEK No. 305. The index of refraction of this epoxy is 1.511.
The epoxy used should be specified as one which when cured will
readily de.form when pressure in the range of 5 ki~ograms per
square centimeter (about 2 p.s.i.) is applied.
It is also important that the epoxy material 94 be
resil.ient as well as readily deformable so that essentially no
latent image will remain in the epoxy once the finger F has been
removed.
Furthermore, because the FIG. 5 platen 84 generates
finger information by a different physical effect than by opti-
cal matching or mis-matching by finger oil causing different
reflectivity~ the problem of finger oil deposition is of little
concern. This eliminates the need to continuously clean the
working surface of the platen 84. Accordingly, the cost of the
platen 84 as compared with platens previously used is minimal
and the cost o:E operating with the platen 84 is also minimized.
The array 12 is preferably at the image plane. But if
it is displaced from the image plane by a small amount it will
receive an image. Light spots representing the valleys will ap-
pear with an intensity that is a function of a scale factor due
to light beam divergence or convergence and that may be affected
by a small degree of light scatter that occurs at the valley zones
-24-
ll
1 ~ 6~2~
The FIG. 6 Platen.
The platen of FIG. 6 has four layers. One of the four
layers is the five mm. thick transparent glass substrate 92 shown
in FIG. 5. The other layers are the 0.25 mm. thick transparent,
/Angstr~m
resilient, compressible epoxy layer 94, a three thousand/(3,000
thick reflective silver layer 96 and a O~Q25 mm. thick protec-
tive lacquer layer 9g. The lacquer layer 99 is the surface layer
against which the subject's finger F is pressed. The lacquer
layer 99 i~ quite compliant and the pressure of the finger ridges
Fr is transmitted through to the silver layer 96 and epoxy layer
94. From the point of view of the reflected light, it is the
reflective silver layer 96 which can be considered as the back
surface of the platen. The readily deformable epoxy layer 94
deforms in response to the pressure o the finger ridges Fr. The
combined operation of the deformable layer 94 and the reflective
layer 96 is to provide enhanced c:ontrast between the ridges Fr
and the valley Fv of the image of the finger that is reflected
downstream from the finger F.
As schematically shown in FIG. 6, when the interrogat-
ing light beam 61 impinges on the flat surface of the silver
reflective layer 96, it bounces off with an angle of reflection
that is equal to the angle of incidence. Thus the light beam
portions 63 reflected from the flat surface portions of the
silver surface 96 will remain collimated. However, these 1at
portions of the silver surface 96 are only the portions under
the valleys Fv of the applied finger F. As also schematically
shown in FIG. 6, under the ridges Fr of the applied finger F,
the silve surface 9o is deformed in response to deformation of
-25-
I l 61250
the deformable, resilient epoxy 94. The rounded reflective sur-
face under these ridges Fr will cause the incident light beam 61
to scatter.
Thus the operation of the FIG. 6 embodiment is similar
to that of the FIG. 5 embodiment. One major difference is that
in the FIG. 6 platen, the reflective layer 96 assures that the
level of light intensity downstream will be very much greater
than that provided by the FIG. 5 platen. In particular~ the
pattern received at the array 12 wil] have a light intensity
determined by the fact that l00U/o of the incident light is reflec-
ted from the FIG. 6 platen as contrasted with the approximately
5% of light reflected in the FIG. 5 platen. A second difference
is that because of the moderating effect of the lacquer layer 99,
the amount of deformation due to the ridges Fr at the reflective
layer 96 is somewhat less than the amount of deformation in the
FIG, S platen. Accordingly, in t.he FIG. 6 embodiment, the degree
of scatter under the ridges is not quite as great as the degree
of scatter under the-ridges of the FIG. 5 embodiment. This may
l be one of the reasons why, in the FIG. 6 embodiment, the array
¦ 12 cannot, as a practical matter, be placed at the image plane
of the lens 65. The image at the image plane is a fairly uniform
bright plane in which whatever distinction there may be between
the valleys and ridges cannot be particularly noticed or detected
l by the array 12. In large part, this is because much of the
¦ light scatter under the ridges in the FIG. 6 platen is suffi-
ciently within the angular pass band of the lens 65 so that the
reflected light is essentially imaged at the image plane. However
one or two millimeters from the image plane, the scattered light
1 ~6~5~
from the rounded areas under the ridges is not properly focused
thereby providing a contrast between ridge and valley zones.
What happens is actually not entirely understood by
applicant at present. Downstream from the image plane, the
defocus~d ridges appear dark while the substantially collimated
light rom the valley zones remains quite bright. The contrast
between the dark spots and the light spots represent~ng ri~dges
and valleys respectively is a substantial contrast and certainly
very much greater than the magnitude of any contrast available
from the FIG. S platen. Upstream a millimeter or two from the
image plane, the valleys, as expected, remain substantially as
bright as at the image plane, However, somewhat surprisingly,
the ridges achieve a light intensity substantially brighter than
that o the valleys.
A flat reflective layer 96 can be achieved when apply-
ing and bonding the epoxy layer 94 to the glass substrate g2,
When so applying the epoxy, a highly precise, highly flat master
can be employed as the means for determining the epoxy surface.
In practive, silver is vacuum deposited on a highly flat master
glass element. The epoxy (10 to 15 mg.) in liquid form is placed
on top of the silver and then the glass substrate 92 is placed
on top of the epoxy and held in position 0,23 to 0~25 mm. spaced
from the master. When the epoxy is cured, the silver layer 96
lifts off the master glass together with the epoxy layer 94 and
glass substrate 92. Lacquer is then sprayed on the silver to
provide the layer 99.
-27-
~612SO
The reflective surface 96 of the FIG. 6 platen is
held to a high degree of flatness in order to minimize inter-
fering background noice.
Because the reflective layer blocks light from
behind it, finger oil has no effect on the operation of the
platen and thus noise due to a latent image or other inter-
f erce i~ e'.~ tod
-~8-
~ 5
I
I
The FIG. 7 Platen.
The FIG. 7 platen is the presently preferred platen.
It incorporates two layers in addîtion to those described in
connection with the FIG. 6 platen. These two layers are a second
deformable resilient epoxy layer 97 and a second silver layer 98.
The silver layer 96 is still the optically active reflective
surface.
The layers 94, 96, 97 and 98 in combination form a
sandwich which isolates the optically active surface at the layer
96 from the touching surface at the layer 99, This shields the
optically active surEace, specifically the silver layer 96, from
the effects of minute shrinking or expansion caused by tempera-
ture changes. Essentially, because the optically active reflec-
tive layer 96 is in suspension between two of the same materials,
the epoxy of layers 94 and 97, it exhibits no external surface
effects.
The lacquer layer 99 conforms very readily to the
incident finger and provides the durability required to protect
the layers beneath it.
A very important factor is that the layers 97, 98 and
99 together perform an important function of smoothing or fil-
tering ridge breaks in the input finger. Thus the topographic
map formed on the optically active surface of the layer 96 will
be a mechanically filtered, and thus a somewhat idealized, ver
sion of the actual finger image. This elimination of certain
discontinuities provides for a much simpler signal processing,
feature extraction and matching in the downstream electronics.
-29-
1~6i25~
Fabrication of the FIG. 7 platen is preferably one in
which the epoxy layer 94 and the silver layer 96 are initially
fabricated against an ultra precise flat master. The silver
layer 96 is the release agent for the epoxy layer 94. The silver
is initially deposited on this master and is removed with the
epoxy layer 94 when the epoxy layer 94 is molded against the
master. After the layers 94 and 96 are cured, the layers 97 and
98 are molded again against a precise master with the silver
layer 98 acting as a release agent. This second silver layer 98
would not be required except for its function as a release agent
in the molding of the second epoxy layer 97. The lacquer layer
99, being very compliant, does not require being placed on with
extreme flatness. The epoxy layer is useful not only for its
durability in protecting the other layers but also because it
operates as part of the mechanical filtering function mentioned
above.
In one embodiment, the thickness dimensions of the
layers in the FIG. 7 platen are a glass substrate 92 that is
nominally 6 mm. thick, a first epoxy layer 94 which is 0.25 mm.
thick, a second epoxy layer 97 which is 0.050 mm. thick and a
lacquer layer ~9 which is 0.025 mm. thick. The two silver layers
96 and 98 are approximately 3,000 Angstroms thick.
The FIG. 7 platen operates ~rery much as does the FIG. 6
l platen and the discussion of the FIG. 6 platen applies to the
¦ FIG. 7 platen. The FIG. 7 platen is preferred over the FIG. 6
¦ platen primarily because of the improved filtering and better
shielding of the reflective layer 96.
-30-
I~ 125~
The displacement of the array 12 from the image plane
is only a relatively slight displacement. In one embodiment
employing the FIG. 7 platen, the array 12 has an optimum and pre-
ferred position upstream from the image plane. The array 12
position is a f~mction of the quality of the finger image or
fingerprint being interrogated. In that embodiment, where a
good fingerprint is provided in which the distinctions between
the ridges and valleys are quite clear cut, the optimum position
of the array 24 is between 0.5 mm. and 0.7 mm. upstream from the
image plane where the imaging lens 65 has a 20 mm. focal length.
For degraded fingerprints, the optimum position of the array 12
is 2.5 mm. upstream from the image plane. A preferred embodiment
of the invention incorporates a mechanism for permitting operator
selection of the lesser or greater distance from the image plane
¦ by providing two different lenses 65 having slightly different
focal lengths. The two lenses 65 a~e mounted on a movable mount
that permits the operator to selectively position one or the
other of the two lenses 65 in the path of the modulated reflected
light beam 63.
The light intensity at the array 12 upstream from the
image plane is three to five times greater at the ridge zones
than at the valley zones. It is believed that this observed
brilliant intensification which occurs slightly upstream from
the image plane comes about because of the combined effects of
the non-planarity of the object being imaged and the smooth con-
tinuous ridge contours provided by the mechanical filtering due
to the layers above the reflective layer 96.
It is presently preferred to deploy the array 12
slightly upstream from the image ;lane rather than s].ightly
~ 5~
downstream from the image plane because of the noise problem in
the relatively dark areas representing the ridge zones downstream
The relatively constant valley zone illumination due to the essen
tially collimated light provides a minimum noise problem in the
valley zones either upstream or downstream from the image plane.
However, the distinctive intensification of intenslty represent-
ing ridge zones upstream of the image plane contrasts with the
somewhat noisy relatively dark zones representing the ridges
downstream.
It has been observed that the substantial light inten-
sification representing the ridge zones upstream is coupled with
a decrease in the width of the ridge zones image. Thus what is
observed is a sharp bright line representing the ridge zones at
the array 12. The electric ~ield reinforcement which provides
this substantially greater intensity at the ridge zones occurs
where ~he collimated light is also coherent. However, where
incoherent collimated light is employed as the interrogat~ng
light beam 61, the light intensification is also observed.~owever
the extent of the intensification is somewhat less than when
coherent light is employed and the significantly distinctive
narrowing of the ridge image zones does not occur.
~lthough some mechanical ~iltering does occur in con-
nection with the FIG. 6 design, the significant and dramatic
improvement in the providing of a stylized image that can be
readily processed in provided by the entire multi-layer arrange-
ment shown in FIG. 7. In particular, the second epoxy layer 97
appears to be the element ~hich is the most significant to pro-
vide the improved filtering effect,
-32-
I 1 612~
In a presently preferred embodiment of this invention,
the apparatus involved employs the FIG. 1 system with the FIG. 7
platen, in which the platen is stationery and the interrogating
beam moves across the finger. That embodimen~ incorporates the
following dimensional relationships. The approximately 1 mm.
laser beam is shaped by the lenses 18 and 20 to form a slit beam
that is 15 ~m. long a~ the second cylindrical lens 20. The two
lenses 18 and 20 are positioned so that their focal points are
not coincident. The front focal point of ~he lens 18 and back
focal poin~ o~ the lens 20 are slightly spaced from one another
so that the slit beam converges slightly as it travels downstream
from the second cylindrical lens 20. This convergence is only
a few degrees so that the slit beam 30 at the platen 23 has a
length o~ about 12 ~m. The imaging lens 65 has a 20 mm. focal
length and is about 110 mm. downstream from the finger object.
The converging slit beam is ~ocused at an ima~e plane.
which is about 24 mm. downstream from the lens. The photodiode
array 12 plane is one millimeter upstream from the image plane
and receives the modulated slit light beam having a length of
abou~ 2.3 mm. In tha~ embodiment an array of 256 photocells is
employed which has a length of about 3~3 millimeters so that not
all of the cells are employed. For degraded fingers, the 20 mm.
lens is replaced with one having a 25 mm. focal length. In that
case, the distance from the finger object to the lens 65 is about
115 mm., the image plane is 32 mm. from the lens 65 and the array
12 is 4 mm. upstream from the image plane.
_31_
7161250
The FI&. 8 Platen.
A fourth platen 100 which has been tried with success
is illustrated in FIG. 8. In this platen, the glass substra~e 92
has, on its back surface, an anti-reflective coating lOl. The
platen has no other layers. This anti-reflective coating 101 is
termed such because it reduces the magnitude of reflection at the
interface between air and the back surface of the platen 100.
As a consequence, at the interface, all of the incident light,
at least ideally? passes through the back surface of the platen
.and, as shown in FIG. 8, is incident against the valleys of any
finger F applied thereon. This coating further calls for the
magnitude of reflection at the discontinuity with a further layer
of glass (or with a further layer of any substance, such as
finger oil, having substantially the index or refraction of glass
¦ to be substantially greater than the magnitude of reflection with
the discontinuity of air. The important thing is to have as
great a difference between the two magnitudes of reflection as
possible, with the reflection at the discontinuity with air being
relatively minimal and the reflection at the discontinuity with
finger oil heing relatively maximal.
- With such a platen, it is possible to obtain a usable
image at the image plane downstream from the lens 65 regardless
of how oily or dry is the finger F. What happens is that if
there is appreciable finger oil, then the reflection of the inci-
dent light at the ridges is maximum. In one embodiment, about
10% of the incident light is reflected. At the valley zones,
essentially no light is reflected from the interface between
-3~-
IIG1250
platen and air. The light reflected from the valleys is so
highly scattered that it falls outside of the angular pass band
of the lens 65 and thus is not focused.
However, where the finger is extremely dry, there will
be a refraction index mismatch under the ridges. But, the ridges
themselves reflect the incident light. Because the ridges are
pressed against the back surface of the FIG, 8 platen~ they are
sufficiently flat so that the degree of scatter of the reflected
light is within the angular pass band of the lens 65. Thus an
L0 image will be formed at the image plane although the brightness
of the zones representing the image will be less than when an
oily finger is applied to the platen. In this fashion, by use
of an anti-reflectant roating 101, the range of fingers which can
be processed by the system of this invention is greatly extended
because the finger oil is not a prerequisite for the system to
interrogate the finger being presented,
Because there is scattering of the reflected light from
the ridge zones when a dry finger is applied, there is a great
deal of defocusing of the rldge zone light at positions o~f the
image plane. Accordingly, when using a FIG, 8 platen, it is
important that the detection array 12 be positioned essentailly
at the image planP.
FIG. 8 represents a scattering under the ridges although
the degree of scatter, like the degree of scatter under the
~5 ridges in FIG. 6, is within the angular pass band of the lens 65.
By contrast the degree of s~atter under the valleys in FIG. 8,
like the sca~ter under the ridges of FIG, 5A, is substantially
outside the angular pass band of the lens 65.
~ 3 ~1~50
One problem with th~ FIG. 8 type of platen is that the
finger oil left by an applied finger can be fixed in place by a
charge effect thereby leaving a latent image which will tend to
interfere with the next subsequent applied fingerprint. This
problem is substantially eliminated by use of an electrically
conductive anti-reflective coating 101. Because, the coating 101
is selected to be elecrically conductive, a capacitive charge
will not build up across it and thus a latent finger image will
not be fixed on the back surfa~e of the platen.
A product designated as No. 7156 from Metavac, Inc.
of 44-68 162nd Street, Flushing, New York 11358 is a useful
anti-reflectant coating with adequate electrical conductivity.
When used as the coating 101 9 it provides a reflection of sub-
stantially under 1% of the incident light at a discontinuity with
air; that is, under the valley zones, and a reflection of about
10% at discontinuity with finger oil; that is, under the ridge
zones. With this coating, where a dry finger is applied, the
/at the ridges
amount of reflected light/may drop to as little as 5% of the
incident interrogating light but that has been found adequate
to provide a distinctive image at the image plane.
In general most light sources, whether coherent or
incoherent, tend to have a frequency range within which the light
is concentrated. The freqtlency range generally has to be speci-
fied as part of the specification for the developemnt of the
anti-reflectant coating. The particular Metavac product No. 7156
is employed with a laser having a frequency of 6,328 Angstrom
(A).
.
-36- ~
I 1 61250
The Pla~ens Compared.
An analysis of how the system of this invention oper-
ates with the different types of platen arrangements sho~m
requires, in general, consideration of the effects of (a) the
extent to which the ridge zones cause scattering of reflected
light and (b) the extent to which the finger oil from the ridge
zones affects optical matching or mismatching to absorb or re-
flect indicent light.
Where the reflective layer 96, such as is shown in
FIGS. 6 and 7 is employed, the finger oil has no effect. That
is one of the advantages of those embodiments.
Other embodiments can be selected so that the scatter-
ing cooperates with the oil matching or mismatching effect to
provide an enhanced result. Thus in the FIG. 5 embodiment, the
optical matching between whatever oil is on the finger and the
epoxy layer 94 results in a reduction of reflected light in the
ridge zones. Whatever light is re~lected is scattered to such
a large degree that much of it is outside of the angular pass
band of the lens 65 and thus is not reformed at the image plane.
Accordingly in the FIG. 5 platen embodiment, light absorption
due to the oil and light scattering due to the ridges are rein-
forcing effects to provide a dark ridge zone at the image plane.
Where a very dry finger is involved, so that there is less than
the usual amount of absorption due to optical match between oil
and epoxy, the high degree of scattering under the ridges tends
to assure a usable image in the image plane.
Similarly, in the embodiment shown in FIG. 8, where an
oily finger is applied, the anti-reflectant coating 101 causes
i~6~25~
essentially collimated light reflection under the ridge zones and
highly scattered reflection under the valley zones. Where a very
dry finger is applied, the light reflected from the relatively
flattened ridge zones is refocused to the image plane. Thus
whether an oily inger is applied or a very dry finger is applied
there will be a usable and significant ridge zone image at the
image plane. In this fashion, light reflection due to the oil
and light reflection due to the ridges are reinforcing effects
to provide a light ridge zone at the image plane. When contrast-
ing the FIG. 8 platen with the FIG. 5 platen, it must be kept in
mind that because of the flattenlng of the ridge zones the exten
of the light scattering from the ridge zones in the FIG. 8
arrangement is substantially less that in the FIG. 5 arrangement
so that most of the light scattered from the ridge zones in the
FIG 5 arrangement is outside of the angular pass band of the
lens 65 and much of the light reflected from the ridge zones in
the FIG. 8 arrangement is within the angular pass band of the
lens 65.
-38-
