# US 9,519,819 B2 · Method and electronic device for noise mitigation

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- Number: US 9,519,819 B2
- Document ID: US9519819B2
- Status: Granted
- Kind: B2
- Application number: 14/698,930
- Filed: 2015-04-29
- Published: 2016-01-14
- Granted: 2016-12-13
- Earliest claimed: 2014-07-14
- Assignees: Fingerprint Cards AB
- Applicants: Fingerprint Cards AB
- Inventors: Morten Rolle Hansen; Lars Puggaard Bøgild Christensen; Hans Thörnblom
- Web page: https://patentgrep.com/patent/US9519819B2

## CPC

- G06V 40/1347: {Preprocessing; Feature extraction}
- G06T 5/70: Denoising; Smoothing
- G06V 10/30: Noise filtering
- G06V 40/1306: {non-optical, e.g. ultrasonic or capacitive sensing}
- G06V 40/1365: {Matching; Classification}
- G06T 2207/10004: Still image; Photographic image
- G06T 2207/30196: Human being; Person
- G06V 40/15: {Biometric patterns based on physiological signals, e.g. heartbeat, blood flow}
- G06V 2201/00: Indexing scheme relating to image or video recognition or understanding
- G06V: IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
- G06T: IMAGE DATA PROCESSING OR GENERATION, IN GENERAL

## Abstract

The present invention generally relates to a method for removing disturbances in an image captured by a fingerprint sensor, and specifically to reduction of noise in an acquired fingerprint image by incorporating the sensing principle into the applied method for noise reduction, wherein the noise reduced image is used for determining a representation of a fingerprint pattern. Advantages with the invention include enhanced determination of fingerprint patterns from fingerprint images captured using a fingerprint sensor. The invention also relates to a corresponding electronic device and to a computer program product.

## Claims

### Claim 1

A method of determining a representation of a fingerprint pattern of a finger captured using a fingerprint sensor comprising a plurality of sensing elements, the method comprising:
selecting a sampling matrix, wherein the sampling matrix represents a sampling pattern for acquiring a fingerprint image using the fingerprint sensor;
acquiring the fingerprint image using the fingerprint sensor according to the sampling matrix;
applying a linear filter to the acquired fingerprint image, wherein setup of the linear filter depends on the sampling matrix and the linear filter is provided for noise reduction within the acquired fingerprint image;
determining the representation of said fingerprint pattern based on said filtered fingerprint image;
determining a log-likelihood ratio for noise being present within the fingerprint image,
comparing the log-likelihood ratio with a predetermined threshold; and
performing filtering of the fingerprint image only if the log-likelihood ratio is above the predetermined threshold,
wherein the fingerprint sensor is a two-dimensional fingerprint sensor and the sampling matrix corresponds to a selected portion of the plurality of sensing elements.

### Claim 2 (depends on 1)

The method according to claim 1, wherein the selected portion of the plurality of sensing elements are selected as an adjacently arranged group of sensing elements.

### Claim 3 (depends on 1)

The method according to claim 1, wherein the selected portion of the plurality of sensing elements are spatially separated over the fingerprint sensor.

### Claim 4 (depends on 1)

The method according to claim 1, wherein determining a log-likelihood ratio comprises evaluating a likelihood function for the assumption that noise is not present, and determining the likelihood function for the assumption that noise is present in accordance with the sampling matrix.

### Claim 5 (depends on 1)

The method according to claim 1, further comprising applying a non-linear mapping function to the acquired fingerprint image.

### Claim 6 (depends on 5)

The method according to claim 5, wherein the non-linear mapping function is a logarithmic function.

### Claim 7 (depends on 1)

The method according to claim 1, wherein the linear filter is a Wiener filter.

### Claim 8

A portable electronic device, comprising:
a fingerprint sensor having an array of pixels, and
a control unit electrically connected to the fingerprint sensor,
wherein the control unit is configured for:
selecting a sampling matrix, wherein the sampling matrix represents a sampling pattern for acquiring a fingerprint image using the fingerprint sensor;
applying a linear filter to the acquired fingerprint image, wherein setup of the linear filter depends on the sampling matrix and the linear filter is provided for noise reduction within the acquired fingerprint image; and
determining a representation of a fingerprint pattern based on said filtered fingerprint image;
determining a log-likelihood ratio for noise being present within the fingerprint image,
comparing the log-likelihood ratio with a predetermined threshold; and
performing filtering of the fingerprint image only if the log-likelihood ratio is above the predetermined threshold,
wherein the fingerprint sensor is a two-dimensional fingerprint sensor and the sampling matrix corresponds to a selected portion of the plurality of sensing elements.

### Claim 9 (depends on 8)

The portable electronic device according to claim 8, wherein the fingerprint sensor is employing capacitive sensing technology.

### Claim 10 (depends on 8)

The portable electronic device according to claim 8, wherein the fingerprint sensor comprises at least 160×160 pixels.

### Claim 11 (depends on 8)

The portable electronic device according to claim 8, wherein the portable electronic device is a mobile phone.

### Claim 12

A computer program product comprising a non-transitory computer readable medium having stored thereon a computer program for a control unit adapted for controlling a portable electronic device, the portable electronic device comprising a fingerprint sensor and a control unit, wherein the fingerprint sensor is a two-dimensional fingerprint sensor, and
wherein the computer program product comprises:
code for selecting a sampling matrix, wherein the sampling matrix represents a sampling pattern for acquiring a fingerprint image using the fingerprint sensor;
code for acquiring the fingerprint image using the fingerprint sensor and according to the sampling matrix;
code for applying a linear filter to the acquired fingerprint image, wherein setup of the linear filter depends on the sampling matrix and the linear filter is provided for noise reduction within the acquired fingerprint image;
code for determining a representation of a fingerprint pattern based on said filtered fingerprint image;
code for determining a log-likelihood ratio for noise being present within the fingerprint image,
code for comparing the log-likelihood ratio with a predetermined threshold; and
code for performing filtering of the fingerprint image only if the log-likelihood ratio is above the predetermined threshold,
wherein the sampling matrix corresponds to a selected portion of the plurality of sensing elements.

### Claim 13 (depends on 8)

The portable electronic device according to claim 8, wherein the fingerprint sensor comprises at least 192×192 pixels.

### Claim 14 (depends on 8)

The portable electronic device according to claim 8, wherein the fingerprint sensor comprises at least 208×80 pixels.