JPH0226540A - Finger print image former - Google Patents

Abstract

PURPOSE: To obtain a fingerprint image in which contrast between a protuberance zone and a dent zone is high, by providing the equipment with a contact plane of a dent cylinder type supporting a finger in a platen and a cylinder plane of a convex type and irradiating a finger with a questioning light beam from a decided direction. CONSTITUTION: A spherical lens assembly 12 and a plane and convex cylinder light convergence lens 14 form a light from a light source 10 into the questioning light beam irradiated to a finger F as a slit type light beam. The platen 18 has a contact surface 18b of a dent cylinder type and cylinder plane 18a of a convex type and the finger F is placed on a contact body 18b. An illuminated part of the finger F is converged on a linear photodiode line 22 by a convergence lens 20. All optical system excluding the platen 18 rotates by a motor 40 and surroundings of the finger F is scanned by the slit type questioning beam.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates generally to an apparatus for optically forming a fingerprint image, and more particularly to an apparatus for forming an image from optical scanning of a finger object supported on a platen. Regarding equipment.

[Prior art and problems to be solved by the invention] In conventional optical fingerprint image processing technology, the cost of the device is kept relatively low, the device is made as small as possible, and the reliability of the device is reliably maintained. In order to obtain a useful fingerprint image, the contrast between the ridge and depression signals should be as high as possible, and the background signal should be kept as low and stable as possible. Furthermore, the device must provide a high resolution image of the subject's fingerprint.

U.S. Pat. No. 4,322,163 describes an example of a fingerprint device that forms a fingerprint image to be encoded into a machine-readable signal. A slit-shaped interrogation light beam consisting of nearly parallel light is incident on a finger pressed against a flat platen. The interrogation light beam is at an angle to the normal and is displaced along the platen. A finger held on the platen modulates the interrogation light beam to produce a reflected light beam carrying fingerprint information.

The axis of the reflected light beam and the axis of the interrogation light beam are symmetrical with respect to the perpendicular.

To generate a series of output signals corresponding to the modulation information,
The modulated light beam is imaged onto an array of photoelectric converters. The output of the transducer array is interrogated successively at successive scan positions to form a series of signals containing fingerprint information.

Fingerprint devices capable of forming rolled fingerprint images are known in the art. An example of such a device having a curved finger rest platen is described in US Pat. No. 4,533,837, issued Nov. 19, 1985.

This device also generates information signals using a collimated slit-shaped interrogation light beam whose optical axis is at an angle to the perpendicular, and a modulated reflected light beam whose optical axis is symmetrical and whose optical axis is at an angle to the perpendicular. , this information signal is converted into an electrical signal in a photoelectric converter array.

In addition to the known flat platen configurations and curved platen configurations, there are devices that use triangular prisms as platens to support the finger to be examined. U.S. Patent No. 4.210.89
No. 9 is an example of such a device.

When a prism platen is employed, the interrogation light beam is at a 45 degree angle to the normal, and the axis of the reflected readout light beam is symmetrical to the axis of the interrogation light beam with respect to the normal; It forms a 45 degree angle.

In all three configurations, according to the law of reflection, five percent (5%) of the incident light is reflected from the interface between the lower part of the recess and the glass along the axis of symmetry. Approximately ninety-five percent (95%) of the light is transmitted through the interface and reflected and scattered from the recesses.

The raised part of the finger that is in contact with the platen effectively suppresses geometric reflections because the refractive index of the sebum of the raised part and the glass match. That is, nearly all of the light rays traveling towards the ridges of the finger actually make contact with the ridges and are reflected and scattered back into the glass. The fact that the ridges are substantially in contact with the contact surface, while the recesses are not, results in a significantly larger difference between the angular scattering from the ridges and the recesses.

One major difference is that the angle of recess scattering is spatially limited, whereas the angle of ridge scattering is unlimited. This will be discussed in detail below.

The second difference is that five percent (5%) of the recess-glass interface reflections are along the axis of symmetry, whereas the ridge reflections are scattered and are therefore exactly aligned with the same axis of symmetry. Don't do it. That is, the only light along the axis of symmetry is the recess light, and an optical system with an effective aperture close to zero diameter would receive only the recess signal. In such a configuration, the F-number of the optical system will necessarily be close to infinity and thus have virtually no resolution. A lens with an appropriately sized aperture will then receive five percent (5%) of the geometrically reflected light and sufficiently equal ridge scatter and depression scatter that the baseline of the signal will be symmetrical. The 5 percent concave zone light along the axis varies considerably more.

It can therefore be seen that in the optical fingerprint reader described above, the axis of the imaging segment is a mirror image of the axis of the interrogation segment. That is, the axis of the interrogation light beam is 2 from the perpendicular.
If off by 0 degrees, the optical aperture of the imaging segment is aligned with the center of the reflected light beam and the imaging system is aligned to extract as much as 5% of the light reflected along the line of symmetry. , that is, the axis of the reading system also forms an angle of 20 degrees with the perpendicular. Nevertheless, since almost 95% of the incident light is reflected by the recessed and raised zones in a scattered manner (and not along the line of symmetry), the undesired background becomes quite large and its It is difficult to extract the recess signal against the background.

Furthermore, the latent image interferes with the desired image. Due to the sebum left from the previous finger contact with the platen, the image will tend to scatter some of the 5% light reflected from the recess interface, making identification of the recess signal more difficult.

This latent image problem and other problems related to it are discussed in 198
U.S. Patent Application No. 630, filed July 12, 2016;
012, Name rDirect Finger Read
This problem has been solved by the method described in ing J. The optical fingerprint imaging device described in this US patent application does not use a platen. The subject finger is held in place and its surface is interrogated with a coherent collimated beam of light.

Preferably, the light beam is shaped in the form of a slit extending along the length of the finger surface to be examined. The slit beam is scanned around the finger by a rotational movement. The reflected modulated light beam is focused onto a linear array of photoresponsive transducers to generate an electrical signal image of the optical image focused on the array. A limitation of images formed in this manner is that they tend to lose focus during part of the scan, thereby losing image resolution and information. This defocus problem occurs for two reasons.

First, because the finger to be scanned is not a perfect cylinder, the position of the subject varies along the optical axis. Second, because there are large differences in the thickness of individual fingers, different people experience different degrees of defocus.

It is therefore an object of the present invention to provide a fingerprint image with high contrast between raised and recessed zones.

Another object of the invention is to provide a device in which the fingerprint signal is not interfered with by baseline or background signals.

Another object of the invention is to provide this high contrast signal with minimal baseline so that the image formed is in focus along the length and width of the finger being examined.

Yet another object of the invention is to provide an apparatus that is relatively inexpensive, provides reliable, reproducible images, and is relatively compact.

It is an object of the present invention to achieve the various objectives mentioned above in a device that is effective for a wide range of finger shapes present in a population.

[Means to solve the problem]

In one embodiment of the invention, a moderate thickness (e.g., 5
A curved platen of mm) is provided.

The platen has a concave cylindrical "contact" surface that supports the fingers and a convex cylindrical surface. These two surfaces are spaced apart from each other by the thickness of the platen.

The interrogation light beam in the form of a slit illuminates one slit zone of the finger that rests on the contact surface.

The finger on the contact surface provides reflected light that is modulated by the image of the finger surface being illuminated.

An optically downstream imaging lens focuses the illuminated finger zone onto a linear array of photoelectric transducers. The optical axis of the imaging lens intersects the platen at a point well off the axis of symmetry. The imaging optical axis intersects the platen within a relatively narrow zone. On one side of this narrow zone is a standard reading area, in which the emitted light is divided into five percent (5%) recess light along the axis of symmetry, plus reflected and scattered light from the recess;
This light is mixed with additional reflected and scattered light from the ridge.

On the other side of this narrow zone, all light reflected and scattered from the ridges is internally reflected, so there is no exit light. The imaging optical axis images the raised portions as bright images on the photoelectric converter array and the recessed portions as completely light-free areas.

Centered on a perpendicular to the platen contact surface, five percent (5%) of the light is geometrically reflected from the air-glass interface within the recesses, plus light reflected and scattered from the ridges and from the recesses. There is a cone in which the outgoing light, including all of the reflected and scattered light, is contained. The axis of the interrogation light beam is located within this cone, so that the light reflected along the axis of symmetry is inside the cone.

The light emerging from this cone is specifically excluded from the optical aperture of the imaging system by locating the axis of the imaging system of the invention outside the cone. Areas containing reflected scattered ridge light that are all internally reflected provide no exit light and therefore cannot contribute to the image of the photoelectric converter array. Between these two regions there is a limited region where the only emitted light is light reflected from the ridges. Between these regions is where the axis of the reading system extends.

This uses a conventional flat platen, or a curved platen, positioned such that the axis of the imaging system is located inside the cone and, in particular, is a mirror image of the axis of the interrogating light beam with respect to the normal. The equipment is very different.

To scan the fingerprint image along the photoelectric transducer array, the interrogation light beam, imaging lens, and photoelectric transducer array are all mounted on a bracket that rotates about the axis of the platen. During this rotational scanning, the optical axis of the interrogation light beam is kept perpendicular to the zone to be examined.

〔Definition of terms〕

Tsume. When light is refracted as it travels from one medium (air or glass) towards another medium (glass or air),
When the light goes from the glass to the air, it bends away from the normal, and when the light goes from the air to the glass, it bends towards the normal. Here, the optical axis of a light beam refers to a line that includes a bend. That is, since there is a difference in the refractive index of the medium through which the light beam passes, the optical axis is not a straight line. More importantly,
The modulated light beam, defined by the optical aperture of the focusing lens and the array of photodiodes, or the readout light beam, may extend along an axis that bends at the non-contacting surface of the platen. Therefore, here, the optical axis of the reading light beam refers to a dogleg-shaped line bent at the non-contact surface of the platen.

reading light beam. The read light beam is a beam of light that is focused onto an array of photodiodes. This light beam is not all of the light that is reflected from the finger being examined, nor is it all of the light that exits the platen. This is simply the portion of the light that is focused by the aperture of the focusing lens and projected onto the array of photodiodes. More specifically, the reading light beam does not include any internally reflected light. It also does not include light that is reflected along the axis of symmetry, ie along an axis that has an equal angle with the inspection light beam with respect to the normal. Furthermore, the reading light beam does not include any other reflected light that is not received inside the aperture of the focusing lens.

TIR, the acronym TIR, is used in various places in the specification and claims in place of the phrase "total internal reflection." The term "rTIR angle" refers to the angle at which no internal rays exit the platen. When the angle is greater than or equal to the TIR angle, light rays inside the platen that are incident on the surface of the platen are internally reflected. The angle is between the perpendicular to the surface and
It is measured between the optical path of the internal ray incident on the surface. In contrast, light rays within the platen that form an angle with the normal that is less than the TIR angle are partially reflected and partially transmitted. The TIR angle of glass when the surrounding environment is air is approximately 41 degrees.

MAR, the acronym MAR stands for maximum angle of refraction.

As the light incident on the glass platen passes through the glass platen, it is deflected due to the difference in refractive index between the air and the glass of the platen. The incident ray is bent toward the perpendicular line by refraction. The angle at which a light ray bends is determined by Snell's law. Incident light rays that are normal to the platen surface are not bent. The farther the incident ray is from the normal, the greater the bending. At the limit, the incident rays that are parallel to the platen surface will bend more than any other rays. The angle that such a limiting ray makes with the normal inside the glass is here called the "maximum refraction angle", or MA
It's called R. MAR of glass when the surrounding environment is air
is approximately 41 degrees. Light rays incident on the platen do not make an angle within the platen with respect to the normal that exceeds a MAR of 41 degrees.

critical angle. The MAR angle and TIR angle are equal angles. These angles are derived from the same basic relationship expressed by Snell's law. This angle is called the critical angle here. As shown in FIG. 5, because of this critical angle, all light incident on the contact surface and reflected and scattered from the recess falls within a cone centered on the normal. This cone has a half angle equal to the critical angle.

The light source producing spatially bounded recess scattering is reflected at the interface between the glass and the air below the recess.
It is not a percent (5%) light.

Rather, it penetrates the interface and hits the recessed surface,
This is light reflected from the surface of the recess. Due to this critical angle, all rays scattered from the recessed surface and incident again on the contact surface 18b (see Figure 3) cannot make an angle exceeding MAR/TIR with the normal, which is also the radius to the contact surface 18b. . Therefore, all concavity scattering boundaries will be confined to the inside of the cone where the half-angle maximum ray intersects the non-contact surface 18a at point C.

Thus, the critical angle at 18b is such that the exiting light is (a) geometrically reflected from the lower boundary surface of the recess 5
% (5%) of the light, (b) the scattered light reflected by the recesses, and (C) a portion of the scattered light reflected by the ridges. decide. The critical angle at the non-contact surface 18a further determines the area within which scattered radiation from the ridges is confined to exit the platen. For flint glass platens operating in air, this critical angle is 41.2 degrees.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

As shown in the drawings, one embodiment of the invention uses an incandescent white light source 10. The spherical lens assembly 12 and the plano-convex cylindrical condenser lens 14 form the light into an interrogating light beam 16 that is incident on the finger F as a slit-like light beam. Figure 2 shows
FIG. 2 is a view of a curved transparent glass platen 18 viewed from an end face. In FIG. 2, a slit-shaped interrogation light beam 16
runs perpendicular to the plane of the paper.

Concave surface 18b is a contact surface. That is, this surface 18b is a surface on which the finger F is placed. The illuminated portion of the finger F is focused onto a linear photodiode array 22 by an imaging lens 20 .

In one example actually tested, the light source 10 was 7.5
It is a small incandescent light bulb made by Watsuto, and has 22 photodiode rows.
is a charge-coupled diode (CCD) array of 2048 photodiode cells arranged along a line perpendicular to the plane of FIG.

In this example, lens assembly 12 is an aspherical condenser lens with a focal length of 40 mm located approximately 40 mm in front of the light source, and condenser lens 14 is approximately 6 +++ m downstream from lens assembly 12. apart and point A
This is a cylindrical condensing lens placed at a position approximately 25 mm away from the upstream side. The width of the slit light beam at the contact surface 18b is approximately 2+nm. In this example, the imaging lens 20 has a focal length of 50 m and the contact surface 18b
It is spaced about 135 mm downstream from the photodiode array 22 and about 80 ml 11 away from the photodiode row 22 on the upstream side.

Another example tested also included four infrared light emitting diodes (
Using a light source 10 consisting of a linear array of IREDs,
Implementable results were obtained. In this embodiment, four lenses with a focal length of 8 mm are used as the condenser lens assembly 12, one lens being located downstream of each of the four light sources. The resulting light pattern consisting of four mutually overlapping circles is condensed by the condenser lens 14 and is incident on the contact surface 18b as a slit-shaped interrogation light beam. Single incandescent light source 1
Using 0 is advantageous as it increases the light intensity compared to rRIED.

Downstream of the light source 10, to ensure that the interrogation light beam 16 does not carry an image, such as an image of a filament,
U.S. Patent No. 4,569.08, issued February 4, 1986.
A diffusing element 24 as described in No. 0 is arranged. Diffusing element 24 is not shown in FIG. 1 to simplify the drawing.

The entire optical system except platen 18 constitutes an optical interrogation and detection unit. This unit is mounted on a rotatable support 26 such that the interrogation light beam 16 can rotate about the axis X--X around the finger. As the slit-shaped interrogation light beam 16 rotates around the finger F,
The image on the non-contact surface 18a is scanned. This results in a complete rolled fingerprint image being scanned across the photodiode array 22.

As motor 40 rotates support 26, position encoder 25 generates a signal each time the optical interrogation and detection unit advances by a predetermined incremental angle, as shown in FIG. This signal is applied to the scanning circuit 37, and the scanning circuit 3
7 is a photodiode array 22 in response to each signal.
ask a question.

Scanning circuit 37 generates one output signal each time photodiode array 22 is interrogated. This output signal represents the fingerprint information at each location where photodiode array 22 is interrogated. The elements comprising the scanning device are conventional and described in issued U.S. Pat. No. 4,322.
It is similar to that described in No. 163.

The concave contact surface 18b of the platen 18 is part of a cylinder whose center lies on the axis x-X. Similarly, the non-contact surface 18a, which is an outer convex surface, is also a part of the cylinder. In one embodiment, the radius of the contact surface 18b is approximately 11.8 mm (
464 mils), and the radius of the convex non-contact surface 18a is approximately 16 mm when the thickness of the platen is 4.3 mm.
．． 1 mm (635 m i Is). In this embodiment, the length of the platen 18 is approximately 57 mm (2,2 mm).
5 inches).

FIG. 3 is an enlarged cross-sectional view of the platen 18 shown in FIG. 2, illustrating the nature of the relationship between the interrogation light beam 16 and the read light beam 30. As shown, the axis 28 of the read light beam 30 is not symmetrical with the axis 27 of the interrogation light beam 16.

The curved platen geometry is shown in FIG.

As shown, the interrogation light beam is along the glaze 27 forming the radius of the cylindrical non-contact surface 18a and the contact surface 18b, so that the axis 27 is perpendicular to the surface in the illumination zone A. The reading light beam has an off-normal axis 28. Axis 28 moves away from normal as it moves away from platen 18 because it exits the optically dense glass medium of platen 18 into the less dense medium of air. The portion of shaft 28 that is inside the platen is shown as 28a.

It is important that the axis 28 of the reading light beam intersects the non-contact surface 18a between points C and B, as shown in FIG. Between these points C and B, the exit ray carries only ridge information and no depression information. The light rays exiting between these two points C and B are those that are scattered only from the ridge surface. Therefore, the image focused by the imaging lens 20 onto the photodiode array 22 is an image in which the concave zone appears as an area completely devoid of light;
Maximum contrast is obtained between the recessed and raised zones. Briefly, point B is determined by the point at which all reflected and scattered light from the ridges is internally reflected inside the platen 18. Also, point C is determined by the limit of the zone on the non-contact surface 18a that includes all the light reflected from the recess.

Therefore, from FIG. 3, (i) from the non-contact surface 18a to the point B
It can be seen that (ii) there is no reflected light emitted to the right side of point C, and (ii) the light reflected from both the recessed portion and the raised portion is emitted to the left side of point C from the non-contact surface 18a.

Assume that platen 18 is crown glass with a refractive index of 1.517. Since the refractive index of air is assumed to be 1.0, the light reflected from the concave part of the finger enters the contact surface 18b, is bent toward the perpendicular line, and is reflected inside the platen 18. .

The amount by which an incident light beam reflected from a recess bends toward the normal is determined by Snell's law. In the limit case, if the incident ray is parallel to the contact surface 18b, then for a crown glass platen the ray will be directed towards the normal at an angle of about 41.2 degrees with the normal inside the platen 18. It is bent by bending.

Any incident light beam that makes an angle of less than 90 degrees with the normal will have an angle of 41.2 degrees with the normal inside the platen 18.
Refract so that it is smaller than the degree. Therefore, the angle of 41.2 degrees is the maximum refraction angle, and the angle MAR
It is shown as.

FIG. 5 illustrates this critical aspect of the basic optical system involved. FIG. 5 schematically shows a state in which the concave portion V and the raised portion R of the finger F are pressed against the flat surface 58b of the glass platen 58. The interrogation light beam axis 57 is perpendicular to the plane 58b. The part of the interrogation light beam that is not reflected along the axis of symmetry is transmitted intact and impinges on the recessed area of the finger F. When the light hits the irregular surface of the recesses V, it is scattered at all angles toward the platen face 58b.

In FIG. 5, three rays are shown. first ray X
is substantially parallel to plane 58b. That is, when the first light beam is incident on the surface 58b, it is refracted inside the glass platen 18 so that the light beam X forms an angle of 41.2 degrees with respect to the axis 57, which is a perpendicular line. The second ray Y and the third ray Z make a smaller angle with the normal in the air of the recessed region V than the limiting ray X, and therefore make a smaller angle with the normal inside the glass platen 58. . The limiting ray X corresponds to line 42 in FIG. In this way, all light reflected from below the recess is centered at 41.
It can be seen that it is contained inside a cone that has a half angle of 2 degrees.

Therefore, all recess zone information is included in the reflected rays that are within this MAR zone.

Unlike in the case of the recessed zone V, the ridge R is located at the contact surface 18
in close optical contact with b. Since finger sebum has approximately the same refractive index as glass, there will be a strength filling between the ridge surface and the platen contact surface 18b, and there will be no misalignment below the ridge, so the boundary Mirror reflections below the surface do not occur. What happens in the ridge zone R is that the irregular finger surface scatters the reflected light at all angles.
The situation is to form a focused ridge image onto the photodiode array 22 along the axis 28.

Because the axis 28 of the read light beam 30 in the embodiment of FIG. 2 is at an angle greater than the maximum angle of refraction with respect to the normal at the interrogation line A, the information contained in the read light beam 30 is not recess information. do not have.

As a result, the image of photodiode array 22 is very dark, forming a recessed zone image with zero light level. Therefore, the contrast between the image of the recessed zone and the image of the raised zone is quite high, so that the photodiode row 2
A much more useful image can be obtained downstream of 2.

Line 44 in FIG. 3 shows another limit of the axis 28 of the reading light beam. Light from interrogation line A representing ridge information is scattered by the ridge over a fairly large scattering angle. The light ray along the interrogation line A is directed to the non-contact surface 18, as shown at 44a.
It is emitted in the direction tangent to a. Question line 44 is TIR
defined by the angle. Any light that is reflected within platen 18 at non-contact surface 18H at an angle greater than the TIR angle relative to the normal does not exit from non-contact surface 18a.

All such light is internally reflected. Ray 46 is TIR
Represents a ray of light. Therefore, the portion 28a of the reading light beam axis
must intersect the non-contact surface 18a at an angle with the normal that is less than the TIR angle. Platen 18
If is a flint glass with a refractive index of 1.517 and the environment in which platen 18 functions is air, then this TIR critical angle is 41.2 degrees. Therefore, the portion 28a of the read light beam axis that is inside the platen is 41.
It must intersect the contact surface 18b at an angle greater than 2 degrees and must intersect the non-contact surface 18a at an angle less than 41.2 degrees with respect to the normal. The signals processed by imaging lens 20 and photodiode array 22 relate to the ridges and convey only ridge information. That is, all the light included in the read light beam 30 is from the raised zone and, most importantly, no light is included in the read light beam 30 from the depressed zone.

It should be noted that the orientation of the axis of the interrogation light beam is conveniently and preferably oriented perpendicular to the contact surface 18b, but even if it deviates from the perpendicular.
The relevant relationships will not change significantly. The maximum refraction angle MAR remains 41.2 degrees with respect to the normal. Furthermore, the angle of the interrogation light beam does not affect the line 44 that defines the minimum angle at which total internal reflection occurs. Naturally, the axis 27 of the interrogation light beam must be kept out of the zone between the two lines 42 and 44 of the third line, considering that these lines represent a conical surface.

The relationships depicted in FIG. 3 concern rays along a cross-section perpendicular to the axis x--x.

This is the marginal case in that the rays reflected or scattered from a point along zone A follow the same rules with respect to a plane parallel to FIG. 3, thus allowing for proper analysis. Additionally, line 42 defines a conical rotation zone about axis 27 for points along the interrogation line shown in FIG. A collection of lines 42 from each point along interrogation line A extends between interrogation line A and a line at C extending along non-contact surface 18H that is perpendicular to the plane of FIG. defines the plane in which the

The fact that there is a zone of emitted light containing only light reflected from the ridge between points B and C on the non-contact surface 18a means that
This is an important point that must not be forgotten.

As shown in Figure 3, the light exiting to the left of C includes both the light reflected from the concave zone and the light reflected from the ridge zone, and the area to the right of B contains no exit light. . Naturally, as the interrogation light beam in axis 27 rotates about the cylinder axis x-X, points A, C and B also rotate correspondingly, so that during rotational scanning The relationship remains unchanged.

During use, to accommodate fingers of various sizes,
Several different size platens 18 are utilized.

FIG. 6 shows why the invention cannot be applied in the case of flat platens. FIG. 6 is a schematic diagram of the optical relationship similar to FIG. 5, except that a non-contacting surface 58a of the platen is included. Since the non-contact surface 58a is parallel to the contact surface 58b, the limiting ray X inside the platen is directed to the non-contact surface 58H.
makes an angle with the normal at contact surface 58b equal to that made with the normal at contact surface 58b. This angle is the critical angle of 41
．． 2 degrees, the limiting ray X will be internally reflected. As a limit case, one could consider that the ray X emerges from the non-contacting surface 58a at an angle parallel to the non-contacting surface 58a. In any case, it is clear that all rays with angles greater than 41.2 degrees are all internally reflected. As a result, the flat platen non-contact surface 58a
The only images that can emerge from the plane are those contained in a cone centered on normal 57 and bounded by the MAR angle. The light inside the cone includes light reflected from the recess zone and light reflected from the ridge zone, and is thus a mixture of ridge and recess information.

It is this mixed information forming the back shadow baseline that makes it difficult to extract the 5 percent light reflected from the contact surface 58b below the recessed zone V.

[Brief explanation of the drawing]

1 is a perspective view of the curved platen-shaped embodiment of the present device with a finger in the predetermined reading position; FIG. 2 shows the optical and mechanical relationships of the device of FIG. 3 is an enlarged cross-sectional view of the cylindrical platen of the apparatus of FIG. 1 overlaid with the geometrical relationships associated with the embodiment of FIG. 1; and FIG. 4 is a diagram showing a fingerprint image. 57 is a block diagram illustrating an arrangement for electronically scanning an optical image to provide a two-dimensional set of pixel values representing a recessed surface; Figure 6 is an optical diagram similar to Figure 5 showing why there is no zone between the zone of mixed ridge and recess reflections and the zone of total internal reflection in a flat glass platen. This is a schematic diagram. DESCRIPTION OF SYMBOLS 10... Incandescent white light source, 12... Spherical lens assembly, 14... Plano-convex cylindrical condensing lens, 16... Interrogating light beam, 18... Glass platen, 18a... Non-contact surface, 18b... Contact surface, 20... Imaging lens, 22... Photodiode array, 26... Support body, 27... Axis of interrogating light beam, 28... Axis of reading light beam, 30...Reading light beam, 40...Motor, F...
・Finger, R...Protuberance, ■...Concavity.

Claims (1)

[Claims] 1. A curved platen having a concave finger contact surface is used to illuminate a finger in contact with the finger contact surface with an interrogating light beam to generate a modulated reflected light beam, and the reflected light beam is In an optical fingerprint imaging device having an imaging segment for imaging a light beam onto a photoelectric transducer and a platen having a convex, non-contact surface from which a reflected light beam exits, the imaging segment has a finger contact surface. an axis within a zone in the platen bounded by a first line making an angle equal to the critical angle with the normal at the axis and a second line making an angle equal to the critical angle with the normal at the non-contact surface; fingerprint imaging comprising: an interrogation light beam having an axis inside the platen such that the axis of symmetry is located outside of the zone; and the axis of the interrogation light beam intersects the non-contact surface. Device. 2. The fingerprint image according to claim 1, wherein the curved platen is a cylindrical platen having the finger contact surface as a concave cylindrical surface and the non-contact surface as a convex cylindrical surface. Forming device. 3. optical means for shaping the interrogation light beam to form an interrogation zone in the finger contact surface in the form of a slit with a long axis parallel to the axis of the cylindrical finger contact surface; further comprising means for rotating the interrogating light beam, the imaging segment, and the photoelectric transducer about the axis of the finger contact surface to perform optical scanning of a finger placed on the contact surface. A fingerprint image forming device according to claim 2. 4. The fingerprint image forming device according to claim 3, wherein the optical axis of the interrogation light beam is substantially perpendicular to the finger contact surface. 5. having an interrogation light beam having a raised zone and a recessed zone to form a reflected readout light beam toward the finger, and including a plurality of photoelectric transducers coupled to the readout light beam;
A fingerprint processing device having an image forming means for generating an electrical signal representative of a fingerprint on a photoelectric transducer, comprising: a first curved surface disposed in the interrogation light beam and serving as a contact surface; and a non-contact surface spaced apart from the contact surface. a curved platen having a second curved surface, a finger resting on the contact surface being illuminated by the interrogation light beam, and an optical axis of the interrogation light beam relative to a normal at the contact surface. , has an internal angle smaller than the maximum refraction angle of the platen, and the optical axis of the image forming means has a first internal angle formed with the normal at the contact surface and a first internal angle formed with the normal at the non-contact surface. 2, the first internal angle is greater than the maximum refraction angle of the platen, the second internal angle is less than the total internal reflection angle of the platen, and the axis of the interrogation light beam is Fingerprint processing device consisting of crossing a non-contact surface. 6. The fingerprint processing device according to claim 5, wherein the platen is a cylindrical platen, the contact surface is a concave cylindrical surface, and the non-contact surface is a convex cylindrical surface. 7. optical means for shaping the interrogation light beam to form an interrogation zone in the contact surface in the form of a slit with a long axis parallel to the axis of the cylindrical contact surface; 10. The method of claim 1, further comprising means for rotating an interrogating light beam, an imaging segment, and a photoelectric transducer about the axis of the contact surface to perform an optical scanning of a placed finger. 6. Fingerprint processing device according to item 6. 8. The fingerprint processing device according to claim 7, wherein the optical axis of the interrogation light beam is substantially perpendicular to the contact surface. 9. A method of optically imaging a finger in contact with a surface of a platen on a photoelectric converter, wherein the entire finger in contact with a concavely curved contact surface of a curved platen is imaged inside the platen. scanning the interrogating light beam at an angle less than a critical angle with respect to the normal at the curved contact surface to form a modulated reflected light beam exiting from the convexly curved non-contact surface of the platen; photoelectrically converting a modulated reflected light beam along an axis that is at an angle greater than a critical angle with the normal at the contact surface within the platen and greater than a critical angle with the normal at the non-contact surface within the platen; A method consisting of the process of forming an image on a device.