JPH03292578A - Fingerprint reading device - Google Patents

Abstract

PURPOSE:To obtain a fingerprint picture faithful to an actual fingerprint by providing an optical correcting member to correct a component parallel to the optical axis of the whole optical path length from a fingerprint detecting plane to the main point of a lens so as to be approximately equal to the optical path length on the optical axis between a fingerprint detecting member and the lens. CONSTITUTION:A correcting prism 8 makes all the light reflected at the any spot of a fingerprint detecting plane 2 trace equal distance and reach a main point K of the front side of a lens 11. All the light reflected at the fingerprint plane 2 reaches the main point K of the lens 11 with equal optical path length with passing through the correcting prism 8. Accordingly, actual fingerprints on the fingerprint detecting plane 2 are image-formed as a fingerprint picture on a CCD light receiving plane 12 at an equal magnification without relation with parts and the generation of magnification difference in the fingerprint picture is prevented. Thus, the fingerprint picture faithful to the actual fingerprints can be obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a fingerprint reading device, and more specifically, to a fingerprint reading device that reads a fingerprint as a fingerprint image in order to identify a registrant on a credit card or the like. .

[Conventional technology]

A conventional fingerprint reading device is described in Japanese Utility Model Application No. 63-99960. As shown in FIG. 13, this fingerprint reading device uses one plane of a triangular prism-shaped fingerprint detection prism 21 as a fingerprint detection surface 22, and allows light to be incident on the back surface of the fingerprint detection surface 22 at a predetermined angle from a light source (not shown). A lens 2 arranged to tilt the reflected light with respect to the fingerprint detection surface 22
3, the image is formed on the CCD light receiving surface 24.

Then, when a fingertip is brought into contact with the fingerprint detection surface 22 described above, only the convex portion of the fingerprint comes into close contact with the fingerprint detection surface 22, and air is interposed between the concave portion and the fingerprint detection surface 22. As a result, the light incident on the back surface of the fingerprint detection surface 22 is totally reflected only at the portion where air is present, and is imaged on the CCD light receiving surface 24 as a fingerprint image. Then, pattern matching is performed on the fingerprint image obtained in this manner with a fingerprint image stored in advance, and it is determined whether or not there is a match (whether or not the fingerprint image is the real person's identity).

[Problem to be solved by the invention]

However, in the fingerprint reading device described above, since the lens 23 is arranged at an angle with respect to the fingerprint detection surface 22, the optical path length to the lens 23 varies depending on the reflection location on the fingerprint detection surface. The magnification of the fingerprint image changes. That is, in FIG. 13, if the distance from point A of the fingerprint detection surface 22 to the front principal point K of the lens 23 is a, and the distance from the rear principal point ' of the lens 23 to the CCD light receiving surface 24 is b, then this The magnification m at point A is m = b /
It becomes a.

Further, if the focal length of the lens 23 is f, then from the general formula of the lens, it is expressed as 1/f-(1/a)+(1/b),', m=f/(a-f).

Also, a point shifted by ΔS from the above-mentioned point A to the side opposite to the lens 23 (for convenience of explanation, it is referred to as the negative side) is defined as a point B, and the distance from the point B to the front principal point K of the lens 23 is ao Lens 2
The distance from ' to the rear principal point of 3 to the CCD light receiving surface 24 is b.
', then the magnification m° at point B is m' = b' / a' = f/ (a' - f), where a' = a + ΔS cos θ, so m' = f/ (
a+Δ5cosθ−f).

Therefore, the magnification m at point B is smaller than the magnification m at point A. When such a difference in magnification occurs, if the detection positions at the time of fingerprint registration and fingerprint comparison are different, for example, when a fingerprint is registered at point A and compared at point B, pattern matching will be affected. In this case, the number of matching pixels decreases and the matching rate decreases. As a result, there is a possibility that even though the fingerprints are the same, they may be determined to be different fingerprints.

An object of the present invention is to provide a fingerprint reading device that can prevent magnification differences and obtain a fingerprint image that is faithful to an actual fingerprint.

[Means to solve the problem]

The present invention forms a fingerprint detection surface on a transparent fingerprint detection member, irradiates light from a light source to the back side of the fingerprint detection surface with the fingertip in contact with the fingerprint detection surface, and reflects light from the fingerprint detection surface. In a fingerprint reading device in which light is formed as a fingerprint image on an image sensor through a lens, the entire optical path length from the fingerprint detection surface to the principal point of the lens is provided between the fingerprint detection member and the lens. The gist of the fingerprint reading device is to provide an optical correction member that corrects the component parallel to the optical axis of the optical axis so that it is approximately equal to the optical path length on the same optical axis.

[Effect]

When a fingertip is brought into contact with the fingerprint detection surface and light is irradiated from a light source to the back side of the fingerprint detection surface, the light is reflected by the fingerprint detection surface, refracted by the optical correction member, and then directed onto the image sensor through the lens. image is formed. At this time, only the portions of the fingerprint detection surface where the fingerprint is not in close contact totally reflect the light, so that a fingerprint image corresponding to the unevenness of the fingerprint is formed on the image sensor.

As mentioned above, the light reflected on the fingerprint detection surface is refracted by the optical correction member, and the component of the optical path length parallel to the optical axis until reaching the principal point of the lens is approximately equal to the optical path length on the same optical axis. It will be corrected so that Therefore, all actual fingerprints on the fingerprint detection surface are imaged as fingerprint images on the image sensor at the same magnification regardless of their locations, and magnification differences in the fingerprint images are prevented from occurring.

〔Example〕

An embodiment embodying the present invention will be described below with reference to FIGS. 1 to 12.

FIG. 1 is a diagram showing a schematic configuration of a fingerprint reading device of this embodiment. As shown in this figure, in the fingerprint reading device of this embodiment, one plane of a fingerprint detection prism 1 as a triangular prism-shaped fingerprint detection member is used as a fingerprint detection surface 2. Further, two planes of the fingerprint detection prism 1 other than the fingerprint detection surface 2 are used as a light receiving surface 3 and a light projecting surface 4, and a translucent diffuser plate 5 is arranged so as to be in contact with the light receiving surface 3. Further, a light source 7 is arranged facing the diffuser plate 5 at a predetermined interval, and the light source 7 is constituted by a large number of LEDs 6 arranged in the vertical and horizontal directions. In the fingerprint reading device of this embodiment, the thickness of the diffusion plate 5 is set to be thicker on the side closer to the fingerprint detection surface 2.

Further, one vertex of a correction prism 8 serving as a triangular prism-shaped optical correction member is in contact with the light projection surface 4 side of the fingerprint detection prism 1. The correction prism 8 has a light-receiving surface 9 as a plane on one side of the apex with which it abuts, and a light-projecting surface 10 as a plane facing the apex. Light projection surface I of correction prism 8
A lens 11 is disposed at O so as to face each other at a predetermined interval, and on the opposite side of lens 11 is C as an image sensor.
A light receiving surface 12 of the CD element is provided.

The light emitted from the LED 6 of the light source 7 is transmitted to the diffuser plate 5.
After being diffused in the fingerprint detection prism 1, the light is introduced into the fingerprint detection surface 2.

Furthermore, the light is reflected on the back surface of the fingerprint detection surface 2 and guided to the outside of the fingerprint detection prism 1, and then introduced into the correction prism 8 and refracted, and then transmitted through the lens 11 to the CCD.
An image is formed on the light receiving surface 12.

The above-mentioned correction prism 8 is for making all the light reflected from any part of the fingerprint detection surface 2 follow the same distance (hereinafter referred to as optical path length) to reach the principal point K on the front side of the lens 11. . However, the optical path length mentioned here is lens 1.
1 refers to the sum of components parallel to the optical axis X passing through the center of the optical axis.

In other words, the actual optical path length from the fingerprint detection surface 2 to the front principal point K of the lens 11 is applied to the light passing on the optical axis The sum of the parallel components is applied as the optical path length. In other words, the shape, posture, etc. of the correction prism 8 are set so that all the light reflected by the fingerprint detection surface 2 reaches the principal point K of the lens 11 with the same optical path length.

Next, the operation of the fingerprint reading device configured as described above will be explained.

When light is emitted from the LED 6 of the light source 7, the light is diffused by the diffusion plate 5, enters the back side of the fingerprint detection surface 2 of the fingerprint detection prism l, and is reflected by the fingerprint detection surface 2.

Furthermore, after passing through the correction prism 8 and being refracted, the light is imaged on the CCD light receiving surface 12 via the lens 11.

Therefore, when the fingertip 13 is brought into contact with the fingerprint detection surface 2, only the convex portions of the fingerprint come into close contact with the fingerprint detection surface 2, and the concave portions and the fingerprint detection surface 2
There is air between them. As a result, the light incident on the back surface of the fingerprint detection surface 2 is totally reflected only at the portion where there is air, and the light is imaged on the CCD light receiving surface 12 via the lens 11 as a fingerprint image. And this CCD light receiving surface 1
The fingerprint image obtained by the processing device 14 connected to 2 can be pattern-matched with a fingerprint image registered in advance, and it can be determined whether there is a match (whether the fingerprint image is the real person's identity).

Further, as described above, all the light reflected by the fingerprint detection surface 2 passes through the correction prism 8 and reaches the principal point K of the lens 11 with the same optical path length. Therefore, the actual fingerprint on the fingerprint detection surface 2 is imaged as a fingerprint image on the CCD light-receiving surface 12 at the same magnification (all images related to its location), thereby preventing the occurrence of magnification differences in the fingerprint image.

On the other hand, as mentioned above, the light from the LED 6 is transmitted to the fingerprint detection surface 2.
Since the light is incident obliquely from the back surface side, the amount of light incident on the fingerprint detection surface 2 near the light source 7 is large. However, in the fingerprint detection device of this embodiment, since the thickness of the diffusion plate 5 is set to be thicker on the side closer to the fingerprint detection surface 2,
On the side of the fingerprint detection surface 2, the transmittance of the diffusion plate 5 becomes low, and the amount of light reaching the fingerprint detection surface 2 is limited. Therefore, the amount of light reaching the fingerprint detection surface 2 is made uniform, and the brightness of the fingerprint image reflected from the fingerprint detection surface 2 and imaged on the CCD light receiving surface 12 is also made uniform.

FIG. 11 shows a state in which a fingerprint image obtained by the fingerprint reading device of this embodiment is converted into a video signal of the NTSC system or the like for pattern matching.

Further, FIG. 12 shows a video signal from a fingerprint reading device when a flat diffuser plate is used or when no diffuser plate is used. In these figures, the horizontal axis indicates positional displacement on the fingerprint detection surface (for example, positional displacement in the left-right direction in FIG. 1).

In FIG. 12, since the brightness of the fingerprint image is uneven, the video signal obtained by converting the fingerprint image is tilted. Therefore, when the video signal is binarized using the binarization threshold, a part of the video signal that should originally represent the unevenness of the fingerprint (T part in FIG. 12) disappears, and a faithful signal is obtained. I can't. On the other hand, in the fingerprint reading device of this embodiment, since the brightness of the fingerprint image is uniform, the video signal converted from the fingerprint image is also flat, and the video signal is divided into two.
Even if it is converted into a value, the video signal representing the unevenness of the fingerprint is prevented from disappearing.

Next, a procedure for determining the specific specifications of the fingerprint reading device of this embodiment, such as the shapes and relative positions of the fingerprint detection prism 1 and the correction prism 8, will be described.

First, the magnification of the fingerprint image on the COD light-receiving surface 12 according to the part of the fingerprint detection surface 2 is calculated based on FIG. In FIG. 2, the vertices of the fingerprint detection prism 1 are designated as A, B, and C, and the vertices of the correction prism 8 are designated as E and F. Further, the center of the fingerprint detection surface 2 is set as point G (0, O), and this point G is set as the origin position of the coordinates (0, O). Furthermore, the light reflected at point G is defined as the optical axis X, and the lens 11 and the CCD light receiving surface 12
is arranged perpendicularly to this optical axis X.

Further, on the fingerprint detection surface 2, a point shifted by ΔS toward the light source from point G is defined as point Q, and points shifted by ΔS toward the lens 11 from point G are defined as 8 points. Also, AC=BC=L, DE=DF=R,
CD=X, m=center magnification, NU=detection length of CCD light receiving surface 12/2, refractive index of fingerprint detection prism 1=n1, and refractive index of correction prism 8=n2. Furthermore, the reflection angle of the ray at point G is 67, and the reflection angle of the ray at Q point is θ14+
Let the reflection angle of the ray at point S be θ1. ′. Under the above conditions, each point in FIG. 2 is expressed as follows.

First, the coordinates of point H, which is the position where the light reflected at point G of fingerprint detection prism 1 is emitted to the outside of fingerprint detection prism 1, are determined as follows: H(Hx, Hy) Hx=Lsinξo/[tanξ.

+tan(π/2-θ,)) HV --tan (π/2-θ,)Hx.

Also, the coordinates of the apex, E, and F of the correction prism are as follows.

D (Dx, Dy) D x = Xsin (π/ 2-ξ0) Dy = Xco
s(π/2-ξo)Dx -Lcos(π/2-ξO) E (Ex, Ey) Inclination of DE with respect to the y-axis: α α=jan(ξ0-01) Intersection of DE with the y-axis: β β =Dy-jan(ξ.-θ,)Dx The desired point E (Ex, Ey) is, from (Ex-Dx)"+ (EV-Dy) 2=R",
Ex= [-ν+r ding "-T7 ding stone/2 (l+α2) Schiff 2αβ-2Dx-2αDy δ=Dx2"+β"-2D Vβ+Dy2-R2EV=
αEx+β Here, point I which is the incident position of the light beam on the correction prism 8
And point J, which is the emission position, is as follows.

I (Ix, Iy) θ1-θ, -ξ0 θ2=sin-' (n 1 sinθ1) 1 point is DE
Since it is the intersection of and HI, Iy=jan(ξ0-θ,)Ix+Dy-jan(ξ,
-〇,)Dx and I V =-jan (π/2-θ, -θ2)IX+
The following two equations hold true: Hy+tan(π/2−θ, −θ2)HX. Therefore, I x = (Hy + tan (π/ 2-θ, -θ
2) HX+Ey+tan(ξ0-θ,) Ex) /
[tan (ξ0-θ,) +tan (π/2-θ, -
62)) I y = -tan (π/2-θ, -θ2
) IX+HV+tan(π/2-θ,-θt)Hx J (Jx, Jy) θ3=01+θ2 θ,=3i1-' (sinθa/n2) μm=π/2
-θ, -θ2+θ3-θ4J points are the intersections of IJ and EJ, so J V=-jan (μ+) J x+ I y+ja
n (μm) I x and 1, Jy=jan(ψ1+ξ.-θ,) Jx+Eytan
Two equations (ψ1+ξ0-θ,)Ex hold true. Therefore, J x= [1y+tan (μt) I x−Ey+
tan (ξ.-θ, +ψ+)Ex) / (tan (ξ0-θ, +ψl) +jan (μm)) J y=-tan (μ+) J x+ I y+ j
an (μ+) I
f(1+m)/m. Also, both prisms 1 and 8
The distance in the air is I/n (n is the refractive index of the prism), so JK = Il and the principal point is K (Kx, Ky) θ5 = π - ψ1 - ψ2 - θ4 θe=sin 1 (n 2 sin θ5)μ2=μm
+θ6−θ.

ε=-jan(μ2) ζ=JV+jan(μz)Jx η=2εζ-2Jx-2Jyε t=J x2+J y”-J K”10ζ2-2 J
yζKx= [−η+ η”−4(1+ε2)t)/
(2/ (1+ε2)) Ky=-jan (μt) Kx+J y+'jan
(μ = JX) In the above manner, the trajectory of the light ray (optical axis X) from point G on the fingerprint detection surface 2 can be simulated.

Next, under these conditions, the lowest magnification side (the ray that reaches point Q)
Simulate the trajectory of.

Assuming that the distance from the rear principal point of the lens 11 to the CCD light-receiving surface 12 is l, and the relative angle of the low-magnification side ray to the optical axis X is θ7, the size of the CCD light-receiving surface 12 and the magnification m are
Therefore, θ7 is θ7= jan (N U/ (! b)l b= f
It is expressed as (1+m).

This light beam is traced back from U on the CCD light receiving surface 12 to determine which position on the fingerprint detection surface 2 of the fingerprint detection prism 1 it reaches. First, the coordinates of point M, which is the position of incidence on the correction prism 8, are found: M (Mx, My) μ3; μ2−θ7 Since point M is the intersection of two straight lines ME and MK, My=j
an(ξ.-θ, +ψl)MX+EV-jan(ξ.-
θ, +ψ+)Ex and My=-jan (μ3) Mx+Ky+tan(μ3
) KX holds true. Therefore, Mx= fKy jan (μ3) Kx−Ey+ta
n (ξ0-θ, +ψ+)Exl/ [tan (ξ.-θ1+ψ,) +tan (μ3
)) My=jan(ξ0-θ,+ψ+)Mx+Eyta
n (ξ0-θ, +ψ,)Ex.

Next, when finding the point 0 which is the exit position from the correction prism 8, 0 (Ox, Oy) θ8=06-θ7 θ5-sin' ((sinθg)/n2]μ4=
μ3-θ9+θB Since the 0 point is the intersection of OE and OM, 0y=jan(ξ.-θ,)Ox+Ey-jan(ξ0
-θ, )Ex Oy=-jan (μ4) Ox+My+tan(μ4
)MX, so Ox= (My+jan (μ4) Mx-Ex+ja
n(ξ.-θ,)Exl/ (tan (ξ.-θ,) +tan (μ4))Oy
=jan(ξ0-θ,)Ox+Ey-tan(ξ.-θ
, )Ex, and the point P, which is the position of incidence on the fingerprint detection prism l, is P (Px, Py) θ10"π−ψ1−ψ2−θ9 θ+x=sin −' (n 2 sinθto)P point is the intersection of BP and PO, so p y ” tan
ξoPx−Lsinξ.

and py=-tan(μ<-θ1□+θ1o)Px+Q y
The following two equations hold true: +tan (μ4-θ1□+θto)Px. Therefore, Px= (Oy+jan(μ*-θ11+θto)Ox
+Lsin(ξo) ) /(tanξ.+tan (μ4-θ11+θ1°))
P y = tanξoPx−Lsinξ0. Then, the point Q, which is the position reaching the fingerprint detection surface l, is Q (Qx, O) θ12=θ1, −θ.

θ+3=Sjn-' [Sjn (θ12)/n
11μ5=μ4-θ11+θlO+θ12-θ13Q
x= (Py+jan (μs) Px) /ja
n (μs) θ14=π/2−μ5 (This θ14 must satisfy the critical angle condition) From the above, the distance of the ray from point Q to the center of the lens is as follows.

At this time, the required magnification is the distance of a ray of light parallel to the optical axis X from point Q to point Q, so first, the component parallel to the optical axis becomes. Next, the component parallel to the optical axis X in the line segment PO is POcos(
θ1□−θ. Also, the component parallel to the optical axis X in the line segment OM is OMcos(θ10-θ*)/n2, and the line segment M
The component of K parallel to the optical axis X is JK=MJsinθ. The magnification m' is the sum of all these values divided by lb.

, ', m' = 1 b/ IQ Pcos (θ1
4-θ, )/nl+POcos(θ1□-θ2) →-OMcos(θ1o-04)/n 2+ (J
K-MJsin θ6)) On the other hand, point S located on the opposite side of point Q with point G as the center on the fingerprint detection surface 2 can be found by the same procedure by replacing μ3 with μ3:μ2+67.

The magnification m'' at point S obtained from this is m = fb/
(QP'cos(θth' - θ,)/nl+PO
'cos (θ1□-θ2) +OM' cos (θ1o-θ4)/n2+ (JK
10MJ' + sin θ6)).

By making mr m' t m'' obtained in the above manner equal, the occurrence of a difference in magnification can be prevented.The magnifications m' and m at points Q and S are determined by the factors shown below. It changes with

■, Angle ξ0 of fingerprint detection prism 1 2, Angle θ between fingerprint detection prism 1 and correction prism 8
.

3. Angle ψ1 of the correction prism 8 4. Material of the fingerprint detection prism l and the correction prism 8 5. Focal length f of the lens 11 6. Angle of reflection θ of the reflected light ray and a wavelength of the reflected light λ 7. Image center magnification m 8, the length of the CCD light-receiving surface 12 Therefore, by setting these factors appropriately, each point G,
By making the magnifications m, m', and m'' at Q and S equal, it is possible to prevent a difference in magnification from occurring.

Next, specific specifications of the fingerprint reading device of this embodiment are selected based on a simulation program.

The multipliers at points G, Q, and S are determined by considering the factors of change in each of the above terms, but since there are many factors of change from term 1 to term 8, the following conditions are set in advance.

First, the angle ξ of one item. For fingerprint detection prism l
Let ξ be a right-angled prism with the length of the right-angled side being 25 mm. =45°. In addition, the material of the fingerprint detection prism l of the 4th item and the correction prism 8 is BK7, the wavelength of the reflected light λ of the 6th item is 660 nm, the center magnification of the 7th item is m = 0°, and the COD light receiving surface of the 5.8th item is 660 nm. 12 is selected to be 6.4 mm.

Also, point D, where the fingerprint detection prism l and correction prism 8 come into contact, needs to be selected at a position that does not block the light from point Q of the fingerprint detection prism l, so it is marked CD=5. Furthermore, the correction prism 8 is a right-angled prism, and its side DE
Since it is necessary to have a length that allows light from 8 points of the fingerprint detection prism to enter, DE=25 mm.

Under the above conditions, the angle θ formed by both prisms l and 8 in the two items is
1.3 items of the angle ψ of the prism 8; 1.5 items of the focal length f of the lens 11; and 6 items of the reflection angle θ of the reflected light ray are determined.

First, the angle ψl of the correction prism 8 is the reflection angle θ of the reflected light beam.
It is determined from the critical angle condition of . At this time, the focal length f of the lens 11 is 25 mm, and θ is 40°. FIG. 3 is a diagram showing the case where the angle ψ1 of the correction prism 8, the reflection angle θr at point G, the reflection angle θ14° at points Q and 8, and the θth degree are changed. ψl is a common angle 30° 45°
, 60°.

From FIG. 3, it can be seen that when ψ1 is 30°, 014' does not satisfy the critical angle condition unless θ is set to 46° or more. Similarly, when ψl is 45°, θ must be 45° or more, and when ψ1 is 60°, θ must be 44° or more. In this way, θ7 is restricted by the angle of ψ1.

Moreover, FIG. 4 is a diagram showing how the magnification difference of the entire image changes when the reflection angle θ at point G and the angle ψ1 of the correction prism 8 are changed, and when ψl is 30° , 45°,
Among 600, 45° has the smallest magnification difference, ψ1=4
It can be seen that the smaller θ is at 5°, the smaller the magnification difference is. Also, if ψl is 60°, there will be overcorrection (the difference in magnification will be negative) and the magnification on the side closer to the lens 11 will be lower than the magnification at the center of the prism, so from 45° to 6
It can be predicted that the optimum value exists between 0°.

Therefore, when ψ1 is increased by 1° from 45°, the magnification difference when θ is set to 45°, 46°, and 47° is calculated, and the set value with the smallest magnification difference is determined. Figure 5 is a diagram showing the change in the magnification difference, and from this figure, θ is 4
When the angle is 5°, the difference in magnification can be eliminated by setting ψ1 to 47°, and θ.

When is 46°, ψl is 49°, and when θ is 47°, ψ
It can be seen that if l is set to 51°, the difference in magnification can be eliminated. In a general 45° triangular prism, the magnification difference cannot be completely corrected because θ cannot satisfy the critical angle condition, but it is possible to suppress the magnification difference to 0.01 or less.

Therefore, the angle ψ1 of the correction prism 8 is 45°, and the lens 11
The focal length f is set to 25 mm in consideration of the size of the optical system. Also, the reflection angle θ of the reflected ray is 45 from Figures 3 and 5.
The critical angle condition is satisfied if the angle is greater than or equal to .degree., but in consideration of manufacturing and assembly errors, .theta.7 is set to 47.degree.

On the other hand, FIG. 6 is a diagram showing the magnification difference when the angle θ is set to 30°, and FIG. 7 is a diagram showing the magnification difference when the angle θ is set to 500°. As shown in these figures,
It can be seen that the angle θ1 has almost no effect on correcting the magnification difference.

However, this angle θ affects the fingerprint detection range. FIGS. 8 and 9 are diagrams showing increases and decreases in the fingerprint detection range when the angle θ1 is changed, and it can be seen that the larger θ is, the narrower the detection range is. In the current optical system layout, the image is input diagonally to the fingerprint, so the image is distorted in the vertical and horizontal directions of the fingerprint, in addition to the difference in magnification. Therefore, distortion can be eliminated by making the detection range in the vertical direction as short as possible. However, since the fingerprint detection length needs to be about 17 mm, θ is set to 40° in this case.

In addition, FIG. 1O is a diagram showing the change in the magnification difference when the length of the side DE of the correction prism 8 (size of the prism 8) is increased or decreased. As shown in this figure, the length of the side DE It can be seen that the magnification difference remains almost unchanged even if the value changes.

The above optimal values are summarized as follows.

1. The angle ξ0 of the fingerprint detection prism 1 is set to ξ by using a 45° right-angled prism (the length of the right-angled side is 25-). =45
°.

2. Angle θ between the fingerprint detection prism 1 and the correction prism 8
, is θ,=40°.

3. The angle ψ1 of the correction prism 8 is set to 45°.

4. The materials of the fingerprint detection prism 1 and the correction prism 8 are:
General resin K7 is used as the optical component.

5. The reflection angle θ of the reflected light beam is θ=47°, and the wavelength of the reflected light λ is 660 nm of the red LED.

6. The focal length f of the lens 11 is f=25 nm.

(In this case, the overall magnification difference is 0.008) 7. The center magnification m of the image is set to m=0.5.

8. The length of the CCD light receiving surface 12 is 6.4 mm because a 2/3 inch CCD element is used.

Other setting values were determined as follows: AC=BD=DE=DF=25mm, CD=5mm, ψ2=90°.

The present inventor compared the fingerprint reading device of this embodiment set as described above with a conventional fingerprint reading device that does not include the correction prism 8, and determined the magnification of point Q and point 8 in the actually obtained image. It was confirmed that while the difference was 0.064 times in the conventional fingerprint reader, it was reduced to 0.007 times in the fingerprint reader of this embodiment.

In this way, in the fingerprint reading device of this embodiment, there is an optical path parallel to the optical axis X of the entire optical path length from the fingerprint detection surface 2 to the front principal point K of the lens 11. A correction prism 8 is provided to correct the component so that it becomes approximately equal to the optical path length on the optical axis X.

Therefore, the actual fingerprint on the fingerprint detection surface 2 is imaged as a fingerprint image on the COD light-receiving surface 12 at the same magnification regardless of its location, and magnification differences in the fingerprint images are prevented from occurring. As a result, even if the position of the fingertip deviates between fingerprint registration and fingerprint verification, it is possible to match a predetermined number of pixels and prevent a decrease in the verification rate.

In addition, in the fingerprint reading device of this embodiment, the thickness of the diffusion plate 5 is set to be thicker on the side closer to the fingerprint detection surface 2, so that the amount of light reaching the fingerprint detection surface 2 is made uniform and the CCD
The brightness of the fingerprint image formed on the light receiving surface 12 is also made uniform. Therefore, even if the video signal is binarized for pattern matching, the video signal representing the unevenness of the fingerprint is prevented from disappearing, and a signal faithful to the actual fingerprint can be obtained.

Although the specific specifications of the fingerprint reading device of this embodiment were determined as described above, it is possible to further reduce the different specifications, for example, the magnification difference, by appropriately changing the items 1 to 8 above. It is also possible to reduce the size of the reading device by shortening the focal length f of the lens 11.

In addition, in the above embodiment, the thickness of the diffuser plate 5 was changed to equalize the amount of light reaching the fingerprint detection surface 2, but as described above, the arrangement density of the EDs 6 arranged in the vertical and horizontal directions was adjusted to detect fingerprints. The surface 2 side may be set to be sparse.

〔Effect of the invention〕

As described in detail above, the fingerprint reading device of the present invention exhibits the excellent effect of preventing the occurrence of magnification differences and obtaining a fingerprint image that is faithful to an actual fingerprint.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the schematic configuration of the fingerprint reading device of the embodiment, Fig. 2 is a diagram showing the path of light passing through each point on the fingerprint detection surface, and Fig. 3 is a diagram showing the reflection angle θ and the reflection angle. Angle θ14,014'
Figure 4 is a diagram showing the relationship between reflection angle θ and magnification difference, Figure 5 is a diagram showing the relationship between reflection angle θ and magnification difference, and Figure 6 is a diagram showing the relationship between reflection angle θ and magnification difference. Figure 7 shows the magnification difference when θ1 is set to 30°, Figure 7 shows the magnification difference when the angle θ is set to 50°, Figures 8 and 9 show the angle θ and fingerprint detection. A diagram showing the relationship with the range, FIG. 1O is a diagram showing the change in magnification difference when the length of the side DE of the correction prism is increased or decreased, and FIG. 11 is a fingerprint image obtained by the fingerprint reading device of the example. FIG. 12 is a diagram showing a video signal obtained by a reader using a flat diffuser plate or without a diffuser plate, and FIG. 13 is a schematic diagram of a conventional fingerprint reader. FIG. 1 is a fingerprint detection prism as a fingerprint detection member, 2 is a fingerprint detection surface, 7 is a light source, 8 is a correction prism as an optical correction member, 11 is a lens, 12 is a CCD light receiving surface as an image sensor, 13 is a fingertip, is the optical axis and K is the principal point.

Claims (1)

[Claims] 1. A fingerprint detection surface is formed on a transparent fingerprint detection member, and light is irradiated from a light source to the back side of the fingerprint detection surface while the fingertip is in contact with the fingerprint detection surface to detect the same fingerprint. In a fingerprint reading device in which light reflected from a surface is imaged as a fingerprint image on an image sensor through a lens, there is provided a space between the fingerprint detection member and the lens from the fingerprint detection surface to the principal point of the lens. 1. A fingerprint reading device comprising an optical correction member that corrects components of all optical path lengths parallel to an optical axis to be approximately equal to optical path lengths on the same optical axis.