JP2005228191A - Fingerprint image input device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fingerprint image input device by an optical system which has high contrast ratio and is not affected by dry fingers by solving problems of a reflected light system and a full-reflected light system in the optical system. <P>SOLUTION: This fingerprint image input device 3 for fingerprint authentication comprises a light guide body 3c of a transparent material having a fingerprint detection surface for placing the fingerprint of a finger of authentication object, and reflecting incident light from a light source by the fingerprint detection surface so as to be received by a light receiving element through a micro lens. In the light conductor 3c, a light incident surface 3f on which the light of the light source 3d is incident is laid in a state substantially orthogonal to the optical axis d of the light source 3d, and the incident angle to the fingerprint detection surface 3e of incident light a in the light guide body is set to an angle causing reflected light and irregularly reflected light. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fingerprint image input device used for fingerprint discrimination in personal authentication.

  Conventionally, collation by fingerprint authentication has been performed as a security measure at the entrance and exit. Such security measures will spread in highly confidential society in the future, and it will be necessary to deal with personal use. For this purpose, a fingerprint image input device is required to be light, thin, small, inexpensive, faithful to the original image, countermeasures against static electricity, and operability.

  Such a fingerprint image input apparatus is mainly a reduction optical system, but it requires a light path length of about 50 mm. Thinning was difficult. In addition, the use of prisms and mirrors to reduce the thickness results in distortion and enlargement / reduction of fingerprint images, and fingerprint images from different fingerprint input devices cannot be recognized as the same fingerprint. It was difficult.

  Therefore, development of non-optical methods such as a capacitance method and an electric field method in which a fingerprint image is directly detected on the surface of a semiconductor has become mainstream. However, the non-optical methods such as the electrostatic capacitance method and the electric field method have a problem that the sensor element is destroyed by static electricity because the human body directly contacts the semiconductor surface of the sensor portion.

  On the other hand, in the field test of fingerprint authentication algorithm, when a retry occurs due to a failure of fingerprint authentication, it is said that the optical system that reads optical information close to human vision has a smaller resistance to retry of the subject.

  As shown in FIG. 5, such an optical fingerprint input device includes a reflected light method in which input light is reflected by a fingerprint surface and bounced off by a light receiving element, and a light guide body as shown in FIG. There is a total reflection method in which diffused light generated when light is confined and a fingerprint and a light guide are in close contact with each other is received by a light receiving element.

  As shown in FIG. 5, when the light incident surface 1f in the light guide 1c is irradiated with light from the light source 1d, the reflected light method passes through the light guide 1c and the light in the light guide 1c. It goes to the fingerprint detection surface 1e. The light reaching the fingerprint detection surface 1e in the light guide 1c collides with the fingerprint surface to generate reflected light. The reflected light generated on the fingerprint detection surface 1e in the light guide 1c is condensed by the gradient index microlens 1b and formed as an erecting equal-magnification image on the light receiving element 1a. The reflected light received by the light receiving element 1a is converted into an electrical signal and output to an external device as an analog signal.

  Even when the fingerprint surface of the finger is not completely in close contact with the fingerprint detection surface 1e with a dry finger or the like, the light that has passed through the light guide 1c generates reflected light on the fingerprint surface. It is possible to detect the unevenness of.

However, since the light from the light source 1d does not pass through all the light incident surface 1f in the light guide 1c but also reflects light, the reflected light deteriorates the contrast ratio. Here, the contrast ratio is obtained by a contrast transfer function (MTF value), and is a value calculated by the following equation from the light amount level of a light-receiving element that receives an image of an uneven fingerprint surface.

i _max -i _min
MTF value = ――――――――― × 100 (%)
i _max + i _min
(Here, i _max and i _min indicate the maximum value and the minimum value of the light amount level.)
The MTF value is a Modulation Transfer Function, which is a value representing how much light of a certain part of the subject can be collected at a corresponding position in the image. The imaging performance of the lens is inspected by numerical values. The diaphragm is inspected for each aperture from the center to the periphery of the screen. The higher the MTF value, the better the contrast ratio and the difference in the uneven surface clearly appears.

  In the total reflection system, as shown in FIG. 6, when light is incident on the light incident surface 2f in the light guide 2c from the light source 2d, the light passes through (contains) the light guide 2c, and the light guide It goes to the fingerprint detection surface 2e in 2c. Since the critical angle of total reflection of light reaching the fingerprint detection surface 2e in the light guide 2c is broken on the fingerprint surface in close contact with the light guide 2c, irregularly reflected light is generated. The irregularly reflected light generated on the fingerprint detection surface 2e in the light guide 2c is collected by the gradient index microlens 2b and formed as an erecting equal-magnification image on the light receiving element 2a. The reflected light received by the light receiving element 2a is converted into an electrical signal and output to an external device as an analog signal.

When the finger surface of the finger is not completely in close contact with the fingerprint detection surface 2e with a dry finger or the like, the light that has passed through the light guide 2c cannot generate diffusely reflected light on the fingerprint surface. As shown in FIG. 7, it becomes impossible to detect the unevenness of the fingerprint.
Japanese Patent Laid-Open No. 2003-323605

The problem to be solved is that, in the optical fingerprint image input device, in the reflected light method, the reflected light generated on the outer surface of the light guide deteriorates the contrast ratio of the fingerprint irregularities, and lowers the quality of the fingerprint image. In the total reflection light method, in the case of a dry finger in which the light guide and the fingerprint are difficult to be in close contact with each other, irregular reflection light is hardly generated on the fingerprint detection surface, and the unevenness of the fingerprint cannot be detected.
As described above, there is a demand for development of a fingerprint input device using an optical method having a high contrast ratio and strong against dry fingers.

  A fingerprint image input apparatus according to the present invention is a fingerprint image input apparatus for fingerprint authentication, and has a fingerprint detection surface on which a fingerprint of a finger to be authenticated is placed and reflects incident light from a light source on the fingerprint detection surface. In the light guide of transparent material that is received by the light receiving element via the microlens, the light incident surface on which the light of the light source is incident is substantially orthogonal to the optical axis of the light source, The incident angle of the incident light with respect to the fingerprint detection surface is set to an angle at which reflected light and irregularly reflected light are generated.

Further, when the light guide is a transparent material, the refractive index thereof is n1, and the refractive index of air is n2, the light in the light guide obtained by the formula θ = Sin ⁻¹ (n2 / n1) °. The critical angle of
An incident angle of light from a light source with respect to the fingerprint detection surface is 0 ° ≦ α ≦ θ °;
And the light receiving angle of the light from the fingerprint detection surface with respect to the fingerprint detection surface is θ ° ≦ β ≦ 90 °;
A background surface is provided on the opposite side to the light receiving element on the light receiving side with respect to the orthogonal axis of the fingerprint detection surface to increase the contrast of the fingerprint image with a width equal to or larger than the light receiving area width, and the background color is a single black color. Be,
It includes that a cured film of a crosslinkable resin is provided on the surface of the light guide.

  In the fingerprint image input device of the present invention, since the light source is substantially orthogonal to the light incident surface of the light guide, a total reflection light system is adopted for the light source, and the light incident on the light guide is described above. Since the incident angle with respect to the fingerprint detection surface is set to an angle at which reflected light and irregularly reflected light are generated, the reflected light method is adopted. Thereby, it is strong to a dry finger and can raise the contrast ratio of fingerprint unevenness. The good point of the both light systems can be adopted. Furthermore, a fingerprint image becomes clearer by providing a background surface.

  The object of the present invention is realized by combining the reflected light system and the total reflected light system.

  FIG. 1 is a diagram showing a schematic configuration of a fingerprint image input device 3 according to the present invention. A light source 3d is disposed under a transparent light guide 3c, and a gradient index microlens is obliquely below. 3b and a light receiving element 3a are disposed.

  The light guide 3c is a material that transmits light such as transparent resin such as transparent acrylic resin or glass, and a light incident surface 3f is provided so as to be substantially orthogonal to the optical axis d of the light source 3d. ing. This is for preventing the influence of the reflected light from the outer surface of the light guide 3c like the total reflection light system. A cured film of a crosslinkable resin is provided on the surface of the light guide 3c. This is for improving hardness, weather resistance, heat resistance, wear resistance and the like.

Furthermore, the incident angle α of the incident light a and the fingerprint detection surface 3e on which the fingerprint 5a of the finger 5 to be authenticated is placed is 0 °. The incident angle α is an inclination angle of the incident light a with respect to the orthogonal axis of the fingerprint detection surface 3e. The allowable range is that the refractive index of the light guide is n1, the refractive index of air is n2, and the critical angle of light in the light guide determined by the formula θ = Sin ⁻¹ (n2 / n1) °. Is θ, and the incident angle α of light from the light source is 0 ° ≦ α ≦ θ °. This is set to an angle at which reflected light is generated even with a dry finger by the light reaching the fingerprint 5a of the finger 5 as in the reflected light method.

Also, the reflection angle β of the reflected light b of the fingerprint detection surface 3e is obtained by the formula θ = Sin ⁻¹ (n2 / n1) °, where n1 is the refractive index of the light guide and n2 is the refractive index of the air. The light emitting surface 3h is provided so that the critical angle of light in the light guide is θ and θ ° ≦ β ≦ 90 ° and is orthogonal to the reflected light b. The background surface 3g has a length equal to or larger than the light-receiving area width of the following gradient index microlens 3b, and is provided on the side symmetrical to the light emitting surface 3h with the orthogonal axis of the fingerprint detection surface 3e as the center. ing. The color of the background surface 3g is a single black color.

  A gradient index microlens 3b and a light receiving element 3a are disposed at the tip of the reflected light b.

  The light source 3d is, for example, a monochromatic light source such as laser light or LED light, or a light source such as a fluorescent lamp, an incandescent lamp, an HID lamp, ultraviolet light, or infrared light.

  By such a fingerprint image input device 3, the reflected light generated by the fingerprint 5a of the finger 5 or the fingerprint detection surface 3e in the light guide 3c is obtained by sweeping the finger 5 on the fingerprint detection surface 3e in the direction of the arrow. Then, the irregularly reflected light is collected by the gradient index microlens 3b and formed as an erecting equal-magnification image on the light receiving element 3a.

  The reflected light and irregularly reflected light received by the light receiving element 3a are converted into electrical signals and output to an external device as analog signals. As a result, as shown in FIG. 2 to FIG. 3, a good fingerprint image having a contrast ratio of 70% or more can be obtained with both the normal finger 5 and the dry finger 5.

  In this embodiment, as shown in FIG. 4, the shape of the light guide 3c is changed, and the light source 3d and the light receiving element 3a are on the same side with respect to the orthogonal axis c of the fingerprint detection surface 3e. The incident light a enters the light incident surface 3f substantially orthogonal to the optical axis d and then is refracted by the incident light refracting surface 3i, so that the incident angle α is approximately 0 ° with respect to the orthogonal axis c. The fingerprint detection surface 3e is reached.

The background surface 3g has a width (length) that is equal to or larger than the receivable region width of the gradient index microlens 3b, and the microlens 3b, the light receiving element 3a, and the light are centered on the orthogonal axis c of the fingerprint detection surface 3e. It is provided on the side symmetric to the exit surface 3h. In addition, by making the color of the background surface 3g a single black color, a light amount level difference is generated in the uneven portion of the fingerprint image to be acquired. As a result, the contrast ratio of the fingerprint image is improved, and a clearer image can be obtained.
The light exit surface 3h is also provided so as to be orthogonal to the reflected light b. Such a fingerprint image input device 6 can provide the same operation and effect as the first embodiment.

  It can also be applied to the field of optically processing surface irregularities.

It is a front sectional view showing a schematic configuration of a fingerprint image input apparatus 3 according to the present invention. It is a figure of the fingerprint image acquired by the fingerprint image input device 3 of the same invention. It is explanatory drawing of the fingerprint image in the case of the dry finger acquired by the fingerprint image input device 3 of the same invention. FIG. 6 is a front sectional view showing a schematic configuration of a fingerprint image input device 6 in Embodiment 2 of the present invention. It is a front sectional view showing a schematic configuration of a reflected light type fingerprint image input apparatus 1 according to a conventional example. It is a front sectional view showing a schematic configuration of a total reflection light type fingerprint image input apparatus 2 according to the conventional example. It is explanatory drawing (A), (B) of the fingerprint image by the total reflection light system based on the prior art example.

Explanation of symbols

1, 2, 3, 6 fingerprint image input device,
1a, 2a, 3a light receiving element,
1b, 2b, 3b gradient index microlens,
1c, 2c, 3c light guide,
1d, 2d, 3d light source,
1e, 2e, 3e fingerprint detection surface,
1f, 2f, 3f light incident surface,
3g background,
3h Light exit surface.

Claims (5)

A fingerprint image input device for fingerprint authentication, having a fingerprint detection surface on which a fingerprint of a finger to be authenticated is placed and reflecting incident light from a light source on the fingerprint detection surface and receiving it on a light receiving element through a microlens In the transparent material light guide
The light incident surface on which the light from the light source is incident is made substantially orthogonal to the optical axis of the light source, and the incident light with respect to the fingerprint detection surface in the incident light in the light guide is reflected light and irregularly reflected light. A fingerprint image input device characterized by being set to an angle. When the light guide is a transparent material, the refractive index is n1, the refractive index of air is n2, and the critical angle of light in the light guide is θ = Sin ⁻¹ (n2 / n1) °, the fingerprint detection surface The incident angle α of light from the light source with respect to
0 ° ≦ α ≦ θ °,
The fingerprint image input device according to claim 1. When the light guide is a transparent material, the refractive index is n1, the refractive index of air is n2, and the critical angle of light in the light guide is θ = Sin ⁻¹ (n2 / n1) °, the fingerprint The light receiving angle β of light from the fingerprint detection surface with respect to the detection surface is
θ ° ≦ β ≦ 90 °,
The fingerprint image input device according to claim 1, wherein: A background surface is provided on the opposite side of the light-receiving element on the light-receiving side with respect to the orthogonal axis of the fingerprint detection surface to increase the contrast of the fingerprint image with a width equal to or larger than the light-receiving area width. There is,
The fingerprint image input device according to any one of claims 1 to 3. A cured film of a crosslinkable resin is provided on the surface of the light guide;
The fingerprint image input device according to claim 1, wherein:

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