Fingerprints are texture formed by uneven skin on surfaces of fingers. The fingerprints have texture characteristics of uniqueness and stability, and therefore are often used as a basis for identity recognition. A fingerprint sensor is a sensor for recognizing an identity by means of fingerprints.
As shown in FIG. 1, a fingerprint sensor includes a fingerprint sensing chip 10 and a cover 20 that is secured to a surface of the fingerprint sensing chip 10 by means of an adhesive 30. The cover 20 is configured to protect the fingerprint sensing chip 10. A plurality of sensing units 11 (pixels) is provided on the fingerprint sensing chip 10. As shown in FIG. 2, the plurality of sensing units 11 is arranged in a matrix. The sensing unit 11 is configured to sense a fingerprint signal and output data. The fingerprint signal is information about a depth of fingerprint texture. A surface of the cover 20 is used as a sensing part of the fingerprint sensor. When the sensing part is pressed by a finger, a matrix constituted by information about depths of fingerprint texture output by all the sensing units 11 is texture information of the finger.
In an actual environment, differences exist among sensing units 11 in the same fingerprint sensor, in addition, unevenness of the adhesive 30 and impurities therein, as well as a flatness matter of the cover 20 or the like, also exist in the fingerprint sensor, and therefore responses from all the sensing units 11 are uneven. Thus, it may be improbable to obtain accurate fingerprint texture characteristics directly from data output by the fingerprint sensor. Instead, the output data needs to be corrected to eliminate such unevenness.
For this purpose, in practice, a correction operation is performed on the fingerprint sensor before delivery. A two-point correction method is used in the correction operation. In the method, on the basis of a linear relationship between output data of a sensing unit 11 and an input signal of the sensing unit 11 (that is, information about a depth of fingerprint texture), the output data of the sensing unit 11 is represented by y and the input signal of the sensing unit 11 is represented by x, coefficients k and b satisfy a linear equation y=kx+b. Therefore, a linear equation shown in FIG. 3 can be obtained by means of two points, namely, P1(x1,y1) and P2(x2,y2), on a straight line.
The specific process of the two-point correction method includes: firstly, a frame of data output by a sensing unit 11 is obtained upon a condition that the fingerprint sensor is non-loaded (that is, no object presses the sensing part of the sensor), and then, another frame of data output by the sensing unit 11 is obtained when a flat metallic block 50 is placed on the fingerprint sensor (as shown in FIG. 4). An equation group as follows is established for each sensing unit 11 separately by using the two frames of data:
                    {                                                                                                                        kx                      1                                        +                    b                                    =                                      y                    1                                                                                                                                                                  kx                      2                                        +                    b                                    =                                      y                    2                                                                                ,                                    (        1        )                            where P1(x1,y1) represents data output in the non-loaded condition and P2(x2,y2) represents data output when the metallic block is placed, and it can be obtained that:        
                    {                                                                              k                  =                                                                                    y                        2                                            -                                              y                        1                                                                                                            x                        2                                            -                                              x                        1                                                                                                                                                                  b                  =                                                                                                              x                          2                                                ⁢                                                  y                          1                                                                    -                                                                        x                          1                                                ⁢                                                  y                          2                                                                                                                                    x                        2                                            -                                              x                        1                                                                                                                          ,                                    (        2        )            
In addition, as shown in FIG. 3, x1 represents a value of an input signal in the non-loaded condition, namely, 0; and x2 represents a value of an input signal when the metallic block is placed, which may be defined as 1. Substitutions of x1 and x2 into equation (2) yield:
                    {                                                                              k                  =                                                            y                      2                                        -                                          y                      1                                                                                                                                            b                  =                                      y                    1                                                                                ,                                    (        3        )            
After values of k and b are obtained, the output data can be corrected by using the following formula:
      [                                                      x                                                1                  ,                  1                                ⁢                                                                                        ⁢            …            ⁢                                                  ⁢                          x                              1                ,                M                                                                                      …            ⁢                                                  ⁢            …            ⁢                                                  ⁢            …                                                                          x                              N                ,                1                                      ⁢                                                  ⁢            …            ⁢                                                  ⁢                          x                              N                ,                M                                                          ]    =      [                                                                                        y                                      1                    ,                    1                                                  -                                  b                                      1                    ,                    1                                                                              k                                  1                  ,                  1                                                      ⁢            …            ⁢                                                            y                                      1                    ,                    M                                                  -                                  b                                      1                    ,                    M                                                                              k                                  1                  ,                  M                                                                                                      …            ⁢                                                  ⁢            …            ⁢                                                  ⁢            …                                                                                                            y                                      N                    ,                    1                                                  -                                  b                                      N                    ,                    1                                                                              k                                  N                  ,                  1                                                      ⁢            …            ⁢                                                            y                                      N                    ,                    M                                                  -                                  b                                      N                    ,                    M                                                                              k                                  N                  ,                  M                                                                          ]                  where N and M represent rows and columns of the sensing units 11 in the fingerprint sensor, respectively, y represents data output by the sensing units 11, and x represents data after correction (that is, the input signals of the sensing units 11).        
However, the above-described correction method for a fingerprint sensor in the prior art has the following defects:                1) correction needs to be performed before delivery and the correction process increases product costs.        2) a correction environment requirement is high: a quite flat metallic block is required for correction, and it also requires no impurity such as ash exists on a sensing part of a fingerprint sensor during correction.        3) the correction is only performed for once before delivery in the prior art, while physical characteristics of a fingerprint sensor changes over time, and therefore correction coefficients (that is, the values of k and b) obtained in the previous correction becomes invalid when the physical characteristics of the fingerprint sensor change, such that the fingerprint sensor cannot be normally used and the service life of the product is shortened.        