Finger identification - Patent Review 4544267

What is claimed is:

1. Fingerprint processing apparatus comprising:

a source of an interrogating light beam,

means to provide a fingerprint object having ridge zones and valley zones in the path of said interrogating light beam,

said object modulating said interrogating light beam to provide a reflected light beam modulated with fingerprint information,

linear displacement optical scanning means for causing said interrogating light beam to scan the fingerprint object and for maintaining the angular relationship between said interrogating beam and the 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 a fingerprint image at said array,

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 object, and

electrical scanning means to electronically scan the information signal outputs of said array and to synchronize said electric 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, said slower optical scan being effected by the linear displacement between the light beam and the fingerprint object.

2. The apparatus of claim 1 further comprising:

a platen in the path of said interrogating light beam and adapted to receive a finger thereon to provide a fingerprint object at a predetermined surface to cause differential scattering of said reflected light beam, the reflected light from zones corresponding to the finger ridges scattering substantially differently than the reflected light from zones corresponding to the valleys of said fingerprint object,

said fingerprint image at said array having differential light intensity representing valley and ridge zones, the difference in light intensity at said fingerprint image deriving substantially from said differential scattering of said reflected light from said fingerprint object.

3. The apparatus of claim 1 wherein:

said interrogating light beam is a partially diffused, substantially collimated light beam.

4. The apparatus of claim 2 wherein:

said interrogating light beam is a partially diffused, substantially collimated light beam.

5. The apparatus of claim 2 wherein:

said platen comprises a transparent substrate and a di-electric anti-reflective coating on the finger receiving surface thereof,

said reflected light from the zones corresponding to the ridges of said fingerprint object scatters substantially less than said reflected light from the zones corresponding to the valleys of said fingerprint object.

6. The apparatus of claim 1 wherein: the reflected light from one of said valley or ridge zones of said fingerprint object falls substantially within the optical aperture of said lens means and the reflected light from the other of said zones falls substantially outside of said optical aperture.

7. The apparatus of claim 2 wherein: the reflected light from one of said valley or ridge zones of said fingerprint object falls substantially within the optical aperture of said lens means and the reflected light from the other of said zones falls substantially outside of said optical aperture.

8. The apparatus of claim 5 wherein: the reflected light from said ridge zones of said fingerprint object falls substantially within the optical aperture of said lens and the reflected light from the valley zones of said fingerprint object falls substantially outside of said optical aperture.

9. Fingerprint processing apparatus comprising:

a source of a substantially collimated interrogating light beam,

a platen in the path of said interrogating light beam, said platen having a finger receiving surface to receive a finger there on to provide a fingerprint object having ridge zones and valley zones,

said finger receiving surface of said platen, when a finger is applied thereto, modulating said interrogating light beam by scattering the reflected light from said ridge zones substantially differently than the light reflected from said valley zones to provide a reflected light beam modulated with finger information,

linear displacement optical scanning means for causing said interrogating light beam to scan said fingerprint object on said platen and for maintaining constant the angular relationship between said interrogating beam and said fingerprint object being scanned throughout the optical scan,

an array of photoelectric transducers optically downstream from said platen, and

a lens positioned in said reflected light beam to provide at said array a fingerprint image having differential light 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 said electronic scan to incremental position 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.

10. The apparatus of claim 9 wherein:

said platen comprises a transparent substrate and a di-electric anti-reflective coating on the finger receiving surface thereof.

11. The method of examining a fingerprint in which a finger is placed on a platen to provide a fingerprint object having ridge zones and valley zones comprising the steps of:

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 interrogating light beam and said fingerprint object being scanned to remain constant throughout the optical scan,

modulating said interrogating light beam with said fingerprint object to provide a reflected light beam modulated with fingerprint information, said step of modulating being effected by scattering the reflected light from the ridge zones substantially differently than the reflected light from the valley zones,

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 valley zones thus providing a fingerprint image,

detecting said fingerprint 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

electronically scanning each of said detected lines, and providing a plurality of relatively faster electronic scans simultaneous with the relatively slower optical scan.

12. The method of claim 11 wherein said reflected light from the zones corresponding to the ridges of said fingerprint object scatters substantially less than does said reflected light from the zones corresponding to the valleys of said fingerprint object, and said step of detecting occurs substantially at the image plane of a lens used in said step of imaging.

13. The method of claim 11 wherein said reflected light from the zones corresponding to the ridges of said fingerprint object scatters substantially more than does said reflected light from the zones corresponding to the valleys of said fingerprint object, and said step of detecting occurs at a plane displaced from the image plane of a lens used in said step of imaging.

Description Submit all comments and votes

BACKGROUND 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 signals 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. The more sophisticated techniques that employ optical techniques tend to 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 unique ridge and valley finger characteristics, it should be understood that the term fingerprint is used by applicant generically. The term fingerprint object is used to refer to the actual configuration of the ridges and valleys of the finger when pressed against a surface. The term fingerprint image refers to the image of the fingerprint object that is obtained by using an optical system. In these optical systems, the finger of the subject individual is placed against the back of a transparent platen and the normally flat fingerprint object on the back surface of the platen is imaged through the front of the platen and projected as a fingerprint image onto receiving or processing equipment. This receiving equipment may take the form of a screen, a camera or an array of photocells.

For example, U.S. Pat. No. 3,138,059 discloses such a system. As 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. U.S. Pat. No. 4,053,228, issued on Oct. 11, 1977 discloses a holographic identification system the disclosure of which is incorporated herein by reference. As described in 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 subject's finger is pressed. The reflected light beam is modulated with the finger image and is correlated against a hologram of the same fingerpress to provide indentification.

However, systems of this type suffer from a number of disadvantages, foremost 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, focusing 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 or a mirror out of position.

Accordingly, it is a major purpose of this invention to provide a technique for processing a fingerprint or fingerprint object in a fashion that is simple and unambiguous, that avoids undue messiness, provides a high degree of reliability in operation 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 system to provide an accurate and unambiguous fingerprint image which in turn is susceptible to being encoded into machine readable signals.

In the holographic systems, stringent requirements 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 eliminate this problem, such a solution is impractical.

Accordingly, a further object of this invention is to provide a highly accurate and reliable finger identification and processing apparatus that includes a relatively inexpensive platen.

Another object of the invention is to provide a finger receiving platen, for such an apparatus, that is insensitive to the amount of oil on a finger.

A further object of the invention is to provide optical fingerprint processing apparatus with greatly enhanced optical contrast between the valley zones and the ridge zones of the finger image.

Another object of the invention is to provide an optical scanning system which minimizes optical distortion, requires a minimum of optical components, and can be used for scanning of a fingerprint object on a platen as well as the scanning of a fingerprint image on a card.

A further object of this invention is to provide a system which is adapted to be used for both verification and identification. Verification is required in access control. Identification is required in police work.

BRIEF DESCRIPTION

In brief, a light source provides a light beam which is shaped by lenses into an interrogating beam of light. This beam is collimated and scanned across a finger placed against a platen. A finger pressed against the back surface of the platen 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 modulated 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 beam is modulated by the object being scanned to produce light and dark spots corresponding to finger valley and ridge zones. This reflected modulated signal is projected on a linear array of photo-responsive devices; 512 charge coupled devices (CCDs) being employed in one embodiment. An imaging lens between the platen and the CCD array projects an image of the fingerprint. The CCD array is located at a plane in space that, depending on the choice of platen, is either at or displaced from the focal plane of the lens. The signals generated by the CCD array are serially interrogated to provide a digital output to represent the fingerprint information. An encoder is coupled to this scanning circuit and to the circuit that interrogates the array to make sure that the modulated beam of light is interrogated at regular predetermined intervals during the scan across the finger involved. In various embodiments, between 256 and 1024 such intervals are employed. By providing a mechanical optical scan in one direction and an electronic scan (of the CCDs) in the orthogonal direction, a simple implementation is provided to obtain a two dimensional scan with only a one dimensional displacement motion.

One of the platens employed is composed of multiple 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 deformable, resilient epoxy layer that provides a flat back surface. On top of the epoxy layer, a thin (3,000 .ANG. 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 surface to reflect the incident collimated light beam. The fourth layer is a fairly thin (for example, 0.05 mm. thick) layer of the same epoxy material. The subject's finger is applied to the back surface. When so applied, the epoxy deforms sufficiently to conform to at least the ridges. The result is a topographic map of the ridge and valley structure at the reflective silver surface. The ridges scatter the reflected light substantially more than do the valleys. This differential scatter provides a modulated reflected 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. These various layers also smooth or filter the ridge breaks in the input finger to provide a more usable and useful topographic map of the finger in the optically active silver layer surface.

Another embodiment disclosed herein employs a glass platen with a di-electric, anti-reflective coating on its back surface. A further embodiment employs a fingerprint card instead of the platen. There is also disclosed the use of an element to partially diffuse the collimated interrogating light beam to eliminate certain fine details and perform some of the filtering otherwise provided by the multiple layer resilient platen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical and mechanical schematic drawing of a first embodiment of a fingerprint processing apparatus constructed 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.

FIGS. 5 and 5A are a detailed sectional view, in somewhat schematic format, of one platen that may be used in the FIG. 1 and FIG. 3 apparatus with a finger impressed thereon. FIG. 5A is a very much enlarged view of the indicated portion of FIG. 5. It should be understood that FIG. 5A is even more schematic 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, illustrating 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 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 that is a di-electric.

FIG. 9 is an optical and mechanical schematic showing a third and presently preferred embodiment of the apparatus adapted for use with an input finger on a platen.

FIG. 9A is an enlarged illustration of a typical glass diffuser element having a ground surface used in the FIG. 9 embodiment.

FIGS. 9B and 9C schematically illustrate and contrast the finger image obtained with and without the FIG. 9A diffuser element. FIG. 9A illustrates a finger image without the diffuser. FIG. 9C illustrates the sme individual's finger image with the diffuser.

FIG. 10 is an optical and mechanical schematic of a presently preferred embodiment illustrating an application of the apparatus to reading or encoding of an input fingerprint card.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The various embodiments described below differ from one another in certain major respects. One distinction has to do with what it is that is moved to effect the light beam scan. A second distinction has to do with the type of platen employed to provide the fingerprint or fingerprint object that is interrogated. A third distinction has to do with whether a platen or a fingerprint card is being read.

In the FIGS. 1, 9 and 10 embodiments, the interrogating light beam 30 is moved so that it scans across a stationary object. By contrast, in the FIG. 3 embodiment, a platen 64 and the finger F on the platen move 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. The platen of FIG. 8 is a rigid glass platen with an anti-reflective di-electric layer 101.

The FIGS. 1, 3 and 9 embodiments are adapted for the reading or encoding of an input finger on a platen while the FIG. 10 embodiment is adapted for the reading or encoding of an input card having a fingerprint on the card.

The term collimate is used herein to refer to a light beam in which the light rays do not scatter. It is not essential that the light rays all be parallel to one another except at the focal point. It is only essential that they not cross over one another. Thus, a collimated light beam as used herein might be diverging, or parallel, or converging. In one embodiment of this invention the collimated 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 to be coherent. What is important is that the light beam be collimated in the broad sense of collimation as defined above. Accordingly, a point source has been found to be a useful light source. In one presently preferred embodiment the light source used is a high power solid state gallium arsenide infrared emitting diode, which is close to a point source. That light source provides a narrow frequency band of .+-.100 Angstroms. However, as is described in connection with the embodiment using the diffuser element 105, a small degree of diffusion or dispersion of the collimated light beam has been found desirable to filter very fine detail so as to provide a better image for electronic processing. Such partially diffused light is still substantially collimated. To put it another way, the collimation is still effective collimation except for very high spatial frequency features.

It should be understood in the following descriptions that the laser 16 is presently preferably replaced with a point source and the photodiode array is preferably a charge coupled device (CCD) array.

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 array 12 is conventional in construction and may, for example, comprise photoresponsive array Model No. CCD 131 manufactured by the Fairchild Semiconductor Division of Fairchild Camera & Instrument Co., of Mountainview, Calif. This particular array comprises 1,024 photoresponsive elements that extend in the longitudinal direction (i.e., along a line going into the paper in the configuration of FIG. 1). The elements are aligned in contact with one another and each diode is about 0.02 mm. on a side. Accordingly, the shape of the light receiving opening of the array is in the form of a slit wherein the long dimension of the slit corresponds to the longitudinal direction of the array. Each element of the particular array 12 used is a charge coupled device (CCD).

The source of the light beam is a laser 16. The laser 16 normally produces a circular light beam of relatively small diameter in the order of approximately one millimeter. Cylindrical lenses 18 and 20 are positioned in the path of the light beam into a slit. As 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 20 mm. along one axis. The transverse axis of the beam remains one millimeter so that the shape of the interrogating beam 30 has a slit format roughly conforming to the format of the linear photodiode array 12.

To aid in visualization, the laser 16, the cylindrical lenses 18, 20 and the mirror 22 in FIG. 1 are rotated 90.degree. to show the elongated light beam in the plane of FIG. 1. These elements must have an orientation that is rotated 90.degree. from that shown to be consistent with the orientation of the rest of the apparatus as shown in FIG. 1. In FIG. 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 FIG. 1.

Although a laser beam is shown, the light source need not be a laser nor even a source of coherent light. It is important however that the interrogating light beam 30 be collimated to maximize the differential scatter in the reflected light beam 54.

The modulated light beam 54 carries relatively light zones and relatively dark zones representing the ridges and valleys of the finger. By forming the light beam into a slit and then using that slit light beam to scan across the fingerprint, the level of light intensity available is much greater than in the arrangement shown, for example, in the U.S. Pat. No. 4,053,228 wherein the laser light beam is expanded so that it covers the entire area of interest of the finger involved. Shaping the light beam into the slit and scanning of the fingerprint with that slit light beam provides increased intensity of light at the array 12 to provide a signal level from the array 12 which facilitates a rapid electronic interrogation at each predetermined increment of the optical scan. This beam shaping has not been found necessary when using the infrared emitting diode mentioned above.

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 receives a finger F thereon in contact with the upper surface of the platen.

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 fo the support is a member 38 having side walls 40 and 42 which terminate in shelves 41. In one embodiment, the sidewalls 40 and 42 form an angle of 42.5.degree. with respect to the horizontal, for reasons noted in greater detail hereinbelow. The mirror 26 is affixed to the wall 40 and a mirror 44 is affixed to the wall 42.

A spring 46 between the support 28 and a wall 48 biases the support 28 toward the right, as shown in FIG. 1. An idler roller 50 is rotatably mounted adjacent the right-hand end of the support 28, as shown in FIG. 1. A motor drive cam 52 is 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 left. 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 path of a reflected modulated beam 54 from the platen 32 and reflects the light beam 54 to the CCD array 12. The beam 54, 56 is modulated with fingerprint information from the platen 32 as noted in detail below.

The angle of the mirror 26 causes the interrogating light beam 30 to strike the platen 32 at an angle 5.degree. off normal. that is, the angle of incidence of the beam 30 with respect to a line perpendicular to the platen 32 is 5.degree.. Similarly, the angle that the reflected beam 54 makes with the normal is also 5.degree.. This insures that the reflected beam 54 will diverge from the interrogating beam 30.

The fingerprint information is modulated onto the slit light beam 30 when the finger F is pressed against the platen 32. The mode of modulation contemplated involves differential scattering of the light incident at the valley zones and ridge zones. Where the platen used has an anti-reflective coating on the back surface thereof this mode of modulation may also incorporate differential absorption and reflection from the ridge and valley zones. This differential scattering mode of modulation is explained in connection with the discussion of the platen structures shown in FIGS. 5, 6, 7 and 8.

More specifically, when a finger F is pressed against the back surface of the platen 32, a fingerprint object is created. That portion of the incident light which is reflected from the fingerprint object is modulated by the ridges and valleys of the finger to provide a reflected light beam 54 that carries the fingerprint information. A lens 65 serves to project an image of the fingerprint object to an image plane downstream. Depending on the platen used, the array 12 may be at the image plane or displaced from the image plane. The modulated slit light beam 56 striking the 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 fingerprint information which can be retrieved or otherwise processed.

The incident light beam 30 is scanned across the finger (from right to left as taken in FIG. 1). The light beam 54 information 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 55 is conventional and produces a signal each time the support 28 moves an incremental distance. In one array 12 each of the diodes are about 0.02 mm. (about one mil) on a side. The encoder 54 produces 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 to generate synchronizing signals.

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 modulated and reflected as light beam 54, 56 to the array 12. The encoder 55 produces a synchronizing signal which is applied to scanning circuit 58 (FIG. 2) via the lead 57. The scanning circuit 58 is conventional in construction and serially interrogates each element of the CCD array 12 in response to the synchronizing signal. The output of the scanning circuit 58, comprises a train of pulses for each scan line and is connected to the store or computer via a lead 60 so that the fingerprint information can be processed.

The cam 52 is energized with the laser 16 so that as the cam 52 rotates, the support 28 moves toward the left as taken in FIG. 1.

As the support 28 moves 0.02 mm towards the left, the light beam 30 similarly moves 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 58 to again interrogate each one of the CCD elements comprising the array 12 to produce a second train of pulses representative of the fingerprint information in the second scan line.

This operation continues until the entire fingerprint object has been scanned by the light beam 30. The interrogation of the array 12 is accomplished electronically at a rate much faster than the rate of movement of the support 28 so that all CCD elements will have been interrogated before the light beam 30 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 light beam 30 to be displaced without changing the angular relationship between the interrogating beam 30 and the fingerprint object being scanned and thus without changing the angular relationship between the reflected modulated light beam 54 and the fingerprint object being scanned.

The encoder 55 employed for the electronic scan is a position device. That is, it signals or requires an electronic scan at each predetermined incremental advancement of the optical scan. Accordingly, the system can tolerate small pertubations in the velocity of the optical scan. Thus there is no need to provide a highly sensitive and expensive constant velocity optical scan as long as the optical scan has a constant angle. By using position to guage the timing of the electronic scan a more stable result is achieved.

The FIG. 3 Embodiment

FIGS. 3 and 4 illustrate an arrangement in which the finger F is moved relative to the interrogating light beam 61 to effect the mechanical 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 constant angle displacement scan between an interrogating light beam and a platen and (b) a synchronized electronic scan of a linear photoresponsive array 12 to provide an electronic scan orthogonal to the optical scan.

The arrangement shown in FIG. 3 includes a fixed support 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 construction to the encoder 54. Additionally, the carriage 68 is maintained 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 support 62 has a recess 74, the length of which is substantially longer than the carraige 68. A rod 76 extends between one end of the recess 74 and the carriage 68 and is adapted to be slidingly received in an opening 78 within the carraige. A speed reducing device 80 is connected to the carriage 68 and receives the rod 76 therethrough and is adapted to increase the coefficient of friction between the rod and the carriage to limit the speed 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 finger image is scanned in the manner noted above. The speed limiting device 80 prevents generation of a synchronizing signal while the array is still being interrogated from the preceding scan.

The Platen-In General

The linear displacement scanning technique of this 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 provide an improved and more usable image than hitherto has been available. Four different platen arrangements are described in connection with FIGS. 5, 6, 7 and 8 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 scattered from under the ridge zones significantly differs from the degree to which reflected light is scattered from under the valley zones. This differential scattering results in differential processing 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. Its ability to focus a reflected light ray as part of the fingerprint image downstream from the lens 65 depends upon the angle at which the reflected light ray is incident on the imaging lens 65. Reflected light which remains collimated is received by the lens at an angel 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 lens, the lens 65 will for a number of additional degrees be able to refocus that light as part of the image at 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 substantially 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.