Patent Publication Number: US-9898646-B2

Title: Method of validation intended to validate that an element is covered by a true skin

Description:
The present invention relates to the field of the detection of fraud in the field of identification/authentication by biometry. The invention relates more particularly to a validation method intended to validate the fact that an element is covered with a true skin, in particular in the context of biometric analysis, as well as a validation device implementing such a method. 
     A device for identifying an individual by his finger/palm print consists of a sensor, a comparison means and a decision-taking means. 
     The sensor has a lighting device and an acquisition device for the capture of images of one or more finger or palm prints. A template is next extracted from each finger or palm print revealing its discriminating features. The comparison means compares the captured image or the biometric templates that issue from the image with the biometric images or templates in a database that contains the images or templates of persons previously recorded in the identification device. The decision-taking means is intended to take a decision as to the identification of the individual from the result of the comparisons. 
     Some ill-intentioned individuals attempt to be identified fraudulently by using decoys in order to lead the identification device into error. 
     Various validation methods are known for validating the fact that the skin present in front of the sensor is true and therefore that the finger bearing the fingerprint is a true finger. 
     Some known methods are based entirely on the analysis of images, in particular by identifying artefacts making frauds. These methods are however not robust against careful frauds. 
     Other methods are also known for capturing a series of images of the finger and for measuring for example sweating, pulse, oximetry or whitening of the finger during pressing on the capture surface. 
     Such methods require in particular a non-compressible acquisition time since it is related to the rate of change of the phenomenon being observed, which degrades the ergonomics of the sensor. 
     The document US-A-2012/218397 describes a device that is provided for detecting whether a finger is true or false. The finger comprises a light source that illuminates the finger and an image sensor that captures an image of the finger. The light flow that illuminates the finger penetrates the finger and is diffused therein. The light flow thus diffused is extracted from the finger at the level of a diffusion zone that is away from the area of the finger that is illuminated by the light source. To check whether the finger is true or false, the image sensor is disposed so as to capture an image only of the diffusion zone. The characteristics of this image are then analysed in order to check whether the finger is true or false. 
     Such a device is therefore based only on an image of the diffusion zone and, in some cases of fraud, this may not be sufficient. 
     One object of the present invention is to propose a validation method intended to validate the fact that an element is covered with a true skin and which does not have the drawbacks of the prior art. 
     For this purpose, a validation method is proposed, intended to validate the fact that an element is covered with a true skin and implemented by a validation device comprising a light source at at least one wavelength, a sensor, an analysis module and a decision-taking module, said validation method comprising:
         a positioning step during which a surface of said element is placed in front of the light source and the sensor,   an illumination step during which the light source illuminates said element,   a capture step for capturing, by means of said sensor, for the or each wavelength, an image of said element thus positioned that encompasses an illuminated zone of said element directly illuminated by the light beam emitted by the light source and a peripheral zone, referred to as the diffusion zone of said element, which is peripheral to said illuminated zone,   an analysis step during which the or each image thus captured is analysed, and   a decision-taking step during which the decision-taking module takes a decision as to whether said element is covered with a true skin according to the results of the analysis step.       

     Advantageously, the analysis consists, for the or each image, in dividing an analysis zone covering the illuminated zone and diffusion zone into a plurality of calculation zones, establishing an average intensity for each calculation zone, an intensity curve and an intensity gradient curve according to the distance from the calculation zone to the boundary of the illuminated zone and comparing characteristics of these curves with those extracted from reference curves. 
     Advantageously, the light source emits in at least two distinct wavelengths, and the analysis step further consists of establishing, for each calculation zone of said analysis zone, the curve of the intensity ratio for two distinct wavelengths according to the distance from said calculation zone to the boundary of the illuminated zone, and the curve of the ratio of the intensity gradient for two distinct wavelengths according to the distance from said calculation zone to the boundary of the illuminated zone and comparing characteristics of these curves with those extracted from reference curves. 
     According to a particular embodiment, the light source is rectangular, and the division of the analysis zone consists, for the or each image, in dividing said image into a plurality of rectangular bands, the edges of which are equidistant from the edges of the illuminated zone. 
     According to a particular embodiment, the light source is circular, and the division of the analysis zone consists, for the or each image, of dividing said image into a plurality of concentric rings centred on the centre of the illuminated zone. 
     Advantageously, the light source emits in a wavelength of between 350 and 550 nm and a wavelength higher than 600 nm. 
     The invention also proposes a validation device intended to validate the fact that an element is covered with a true skin, said validation device comprising:
         a light source emitting at least one wavelength, and intended to illuminate said element,   a sensor configured to capture, for the or each wavelength, an image of said element positioned in front of the light source and the sensor that encompasses an illuminated zone of said element directly illuminated by the light beam emitted by the light source and a peripheral zone, referred to as the diffusion zone, of said element that is peripheral to said illuminated zone,   an analysis module intended to receive the or each image captured by the sensor and to analyse it, and   a decision-taking module intended to take a decision as to whether said element is covered with a true skin, from the information transmitted by the analysis module.       

     Advantageously, the analysis module comprises, for the or each image, means for dividing an analysis zone covering the illuminated zone and the diffusion zone into a plurality of calculation zones, means for establishing, for each calculation zone, the average intensity of said calculation zone, in order to establish the intensity curve and the curve of the intensity gradient according to the distance from the calculation zone to the boundary of the illuminated zone and means for comparing characteristics of these curves with those extracted from reference curves. 
     Advantageously, the light source emits in at least two distinct wavelengths, and the analysis module further comprises means for establishing, for each calculation zone of the analysis zone, the curve of the intensity ratio for two distinct wavelengths according to the distance from said calculation zone to the boundary of the illuminated zone, and the curve of the ratio of the intensity gradient for two distinct wavelengths according to the distance from said calculation zone to the boundary of the illuminated zone, and means for comparing characteristics of these curves with those extracted from reference curves. 
     According to a particular embodiment, the light source is rectangular, and the analysis module comprises, for the or each image, means for dividing said image into a plurality of rectangular bands, the edges of which are equidistant from the edges of the illuminated zone. 
     According to a particular embodiment, the light source is circular, and the analysis module comprises, for the or each image, means for dividing said image into a plurality of concentric rings centred on the centre of the illuminated zone. 
    
    
     
       The features of the invention mentioned above, as well as others, will emerge more clearly from a reading of the following description of an example embodiment, said description being given in relation to the accompanying drawings, among which: 
         FIG. 1  is a schematic representation of a validation device according to the invention, 
         FIG. 2 a    is an image of a fingerprint seen by the validation device according to one embodiment of the invention, 
         FIG. 2 b    is an image of a fingerprint seen by the validation device according to another embodiment of the invention, 
         FIG. 3  is an algorithm of a validation method according to the invention, 
         FIG. 4  shows the reflectance of a true finger as a function of the wavelength of the light flow that illuminates it, 
         FIG. 5  shows an example of an intensity curve, and 
         FIG. 6  shows a curve representing the penetration in cm of the light in the skin as a function of the wavelength. 
     
    
    
     The principle of the invention consists of validating the fact that an element, in particular the bottom surface of one or more fingers or of the palm, is covered with a true skin and is therefore part of a human body. The principle of the invention can also apply to other parts of the human body such as the face. The principle of the invention consists more precisely of illuminating the surface of the element by means of a light source making it possible to illuminate only a well defined zone of the element and keeping a zone without direct illumination, capturing an image of these two zones, namely the zone directly illuminated and the zone not directly illuminated, and analysing this image in order to deduce therefrom whether said element is covered with a true skin or a false skin. 
     The illumination can be achieved via a capture of images without contact or with contact through a contact surface, in particular a prism or an optical plate. 
     In the remainder of the description, the element is a finger, but the invention applies to all other parts of the body, such as a plurality of fingers, a palm or a face. 
     In particular, one advantageous embodiment allows a sharp transition between the directly illuminated zone and the zone without direct illumination. One example embodiment may be one or more point light sources with low spread or one or more lines. 
       FIG. 1  shows a validation device  100  that is intended to validate the fact that an element  10 , here a finger, is covered with a true skin and is therefore a true finger  10 . 
     The validation device  100  comprises:
         a light source  102  with at least one wavelength intended to illuminate a surface of the finger  10 , and   a sensor  104  intended to capture, for the or each wavelength, an image  200   a - b  which, as described below, comprises both the surface illuminated directly by the light source  102  and the non-illuminated surface immediately adjacent to the illuminated surface, that is to say an image  200   a - b  having a reflection zone  202   a - b  where the light beam emitted by the light source  102  is directly reflected by said element  10  and a diffusion zone  204   a - b  where part of the light beam is diffused through said element  10 .       

     In the remainder of the description, the reflection zone  202   a - b  is referred to as the illuminated zone  202   a - b  and is limited to the zone of the element  10  that is directly illuminated by the light beam, and the diffusion zone  204   a - b  is the zone of the element  10  that is not illuminated directly by the light beam but receives and emits light by diffusion of the light beam in the element  10 . 
     The validation device  100  also comprises:
         an analysis module  106  intended to receive the or each image  200   a - b  captured by the sensor  104  and to analyse it as described below, and   a decision-taking module  108  intended to take a decision as to whether the finger  10  is covered with a true skin, from the information transmitted by the analysis module  106 .       

       FIG. 2 a    shows an image  200   a  captured by the sensor  104  in the case of a circular light source  102 . 
       FIG. 2 b    shows an image  200   b  captured by the sensor  104  in the case of a rectangular light source  102 . 
     The light source  102  has an angular opening reduced in at least one direction, so that the illumination of the surface is not uniform over the entire image  200   a - b  and is limited to a restricted zone constituting the illuminated zone  202   a - b.    
     Thus the surface of the finger  10  is illuminated at the illuminated zone  202   a - b  where the light beam emitted by the light source  102  directly illuminates the surface (the backscattered light, comprising a diffuse reflection component, is therefore imaged) and the surface of the finger  10  is illuminated at a peripheral zone referred to as the diffusion zone  204   a - b  (not illuminated directly by the light source  102 ) because part of the light beam is transmitted into the finger  10 , and is then diffused at the diffusion zone  204   a - b  that extends over the periphery of the illuminated zone  202   a - b.    
     The image  200   a - b  that is captured by the sensor  104  shows the illuminated zone  202   a - b  and the associated diffusion zone  204   a - b , and simultaneous analysis of these zones  202   a - b  and  204   a - b  makes it possible to conclude whether the finger  10  is true or false. 
     The illumination of the surface is said to be non-uniform over the entire image  200   a - b  when the illuminated zone  202   a - b  and the diffusion zone  204   a - b  are visible simultaneously to the sensor  104  but the diffusion zone  204   a - b  does not experience direct illumination from the light source  102 , unlike the illuminated zone  202   a - b.    
     The analysis therefore takes account both of the reflection on the finger  10  and of the diffusion by the finger  10  rather than only one of these two phenomena. 
     Thus, even if a decoy is used instead of a true finger, it will be difficult to produce a decoy having the optical characteristics particular to a true finger for reflection and diffusion. 
     In the particular case of capture with contact and to facilitate the positioning and holding of the finger  10  during capture of the image  200   a - b , the validation device  200  may comprise a capture surface on which the finger  10  is placed in abutment and which guides the light between the light source  102 , the finger  10  and the sensor  104 . 
     In some embodiments with contact, a translucent plate is interposed between the finger  10  and the sensor  104  and the light source  102 . 
       FIG. 3  is an algorithm of a validation method  300  intended to validate whether the finger  10  is covered with a true skin and used by the validation device  100 . 
     The validation method  300  comprises:
         a positioning step  302  during which a surface of the finger  10  is placed in front of the validation device  100 , and more particularly in front of the light source  102  and the sensor  104 ,   an illumination step  304  during which the light source  102  illuminates the finger  10  so that the surface of the finger  10  presents the illuminated zone  202   a - b  and the diffusion zone,   a capture step  306  during which the sensor  104  captures, for the or each wavelength, an image  200   a - b  of the illuminated zone  202   a - b  and of the diffusion zone  204   a - b,      an analysis step  308  during which the illuminated zone  202   a - b  and the diffusion zone  204   a - b  of the or each image  200   a - b  thus captured are analysed, and   a decision-taking step  310  during which the decision-taking module  108  takes a decision as to whether the finger  10  is covered with a true skin according to the results of the analysis step  308 .       

     The analysis step  308  consists, for example, for the or each image  200   a - b , of dividing an analysis zone  206   a - b  covering both the illuminated zone  202   a - b  and the diffusion zone  204   a - b  into a plurality of calculation zones  208   a - b , establishing an average intensity for each calculation zone  208   a - b , an intensity curve and a curve of the intensity gradient according to the distance from said calculation zone  208   a - b  to an origin and comparing characteristics of these curves with those extracted from reference curves. 
     Simultaneous analysis of the illuminated zone  202   a - b  and the diffusion zone  204   a - b  makes it possible to have information on the reflection and on the diffusion of the finger  10  and therefore to have complementary information which, in a single capture and a single analysis, makes it possible to determine the veracity of the finger  10 . Such a method therefore allows time saving and better robustness against attempts at fraud. 
     The origin of the analysis zone  206   a - b  depends on the shape thereof and is determined by means of the use of shape-analysis methods known to persons skilled in the art, such as skeletisation and artefact-removal methods. These methods make it possible to ensure that each point of the origin is equidistant from the closest point of the illuminated zone  202   a - b.    
     The distance used is for example and conventionally the minimum distance between the contour of the calculation zone  208   a - b  and the contour of the illuminated zone  202   a - b.    
     In the particular case of circular illumination ( FIG. 2 a   ), the illuminated zone  202   a  and the diffusion zone  204   a  are concentric circles, and the origin is fixed on the boundary of the illuminated zone  202   a . The analysis zone  206   a  is divided into calculation zones  208   a  that are here rings  208   a  concentric with the illuminated zone  202   a.    
     In the particular case of rectangular illumination ( FIG. 2 b   ), the illuminated zone  202   b  is a rectangle and the diffusion zone  204   b  is a larger rectangle, the edges of which are equidistant from the edges of the illuminated zone  202   b , and the origin is fixed on the boundary of the illuminated zone  202   b . The analysis zone  206   b  is divided into calculation zones  208   b  that are here rectangular bands  208   b , the edges of which are equidistant from the edges of the illuminated zone  202   b  and the corners of which are rounded in an exaggerated manner in order to show the consistency of the distance between the contour of the calculation zone  208   b  and the contour of the illuminated zone  202   b.    
     The boundary of the illuminated zone  202   a - b  that serves as an origin is determined for example by a calibration process based on the acquisition of a non-diffusing white target pattern through the sensor under non-uniform illumination as defined previously. The image obtained by capture of this material reflects only the direct reflection effect and not the diffusion effect through the nature of the material. Any illumination defects that may be observed (for example non-uniformity in the direct reflection zone, that is to say the illuminated zone  202   a - b ) will be compensated during calculation of the intensity profiles in order to obtain an intensity plateau in the direct reflection zone. The boundary of the direct reflection zone is defined at each point on the contour of the source, that is to say at each boundary point of the intensity plateau measured on the non-diffusing material. 
     The analysis module  106  then comprises, for the or each image  200   a - b , means for dividing the analysis zone  206   a - b  into a plurality of calculation zones  208   a - b , means for establishing, for each calculation zone  208   a - b , the average intensity of said calculation zone  208   a - b , and then for establishing the intensity curve according to the distance from the calculation zone  208   a - b  to the origin and the curve of the intensity gradient according to the distance from the calculation zone  208   a - b  to the origin, and means for comparing characteristics of these curves with those extracted from reference curves. 
     The characteristics to be compared are extracted from each image  200   a - b  advantageously previously processed with a view to not taking into account the background pixels. One example of such a processing is the application of a simple frequency filter or the use of a mask for locating the peak pixels generated by a template extractor. 
     The characteristics to be compared are thus in particular the absolute value of the intensity of each calculation zone  208   a - b  and the intensity gradient at each calculation zone  208   a - b.    
       FIG. 5  shows an example of an intensity curve  502  in function of the distance with respect to the boundary of the illuminated zone  202   a - b  corresponding to the origin defined by the method described above. 
     For the finger  10  to be validated, it is necessary that the intensity curve and the gradient curve in function of the distance each remain between two reference bounds extracted from reference curves. 
     It is also possible to measure at each point the gap of how much the intensity value and the value of the gradient of said point depart with respect to the corresponding two reference bounds and to sum these differences in absolute value and to compare this sum with an acceptable limit threshold. 
     The reference curves are here the intensity curve and the intensity gradient curve that were established with a large panel of true fingers  10  and for the wavelength considered. 
     In the case of a circular light source  102 , the division of the analysis zone  206   a  consists, for the or each image  200   a , in dividing said image  200   a  into a plurality of concentric rings  208   a  centred on the centre of the illuminated zone  202   a , at least one ring  208   a  of which is situated in the illuminated zone  202   a  and at least one ring  208   a  of which is situated in the diffusion zone  204   a , calculating the average intensity on each ring  208   a  of the previously processed images, establishing a curve of the average intensity thus calculated as a function of the distance from the ring  208   a  to the boundary of the illuminated zone  202   a - b , and a curve of the intensity gradient as a function of the distance from the ring  208   a  to the boundary of the illuminated zone  202   a - b  and comparing these curves with reference curves, for example by difference and verification with respect to a threshold. 
     The reference curves are here a curve of the average intensity in function of the distance from the ring  208   a  to the boundary of the illuminated zone  202   a  and a curve of the intensity gradient in function of the distance from the ring  208   a  to the boundary of the illuminated zone  202   a - b  that were established with true fingers  10  for the wavelength in question. 
     The analysis module  106  then further comprises, for the or each image  200   a , means for dividing said image  200   a  into a plurality of concentric rings  208   a  centred on the centre of the illuminated zone  202   a.    
     In the case of a rectangular light source  102 , the principle is similar except that the calculation zones  208   b  are rectangular bands  208   b  rather than rings  208   a . The division of the analysis zone  206   b  then consists, for the or each image  200   b , in dividing said image  200   b  into a plurality of rectangular bands  208   b . The analysis module  106  then further comprises, for the or each image  200   b , means for dividing said image  200   b  into a plurality of rectangular bands  208   b , the edges of which are equidistant from the edges of the illuminated zone  202   b.    
     When the light source  102  emits in at least two distinct wavelengths, it is possible to combine the results obtained for each wavelength with the analysis results for each pair of distinct wavelengths. 
     The analysis step  308  then further consists of establishing, for each calculation zone  208   a - b  of said analysis zone  206   a - b , the curve of the intensity ratio for two distinct wavelengths in function of the distance from said calculation zone  208   a - b  to the boundary of the illuminated zone  202   a - b  and the curve of the ratio of the intensity gradient for two distinct wavelengths in function of the distance from said calculation zone  208   a - b  to the boundary of the illuminated zone  202   a - b  and comparing characteristics of these curves with those extracted from reference curves. 
     Thus the method performs a relative analysis of the quantities in function of the wavelengths, calculating the ratio between the quantity (here the intensity gradient and the intensity) measured for one wavelength with the same quantity for another wavelength. Since true skin diffuses and absorbs in a typical manner according to the wavelengths and is difficult to imitate by means of an artificial element, this relative analysis facilitates verification of the veracity of the finger  10 . 
     The reference curves are here the intensity ratio and intensity gradient ratio curves that were established with a large panel of true fingers  10  and for the two wavelengths considered. 
     The analysis module  106  then further comprises means for establishing, for each calculation zone  208   a - b  of the analysis zone  206   a - b , the curve of the intensity ratio for two distinct wavelengths in function of the distance from said calculation zone  208   a - b  to the boundary of the illuminated zone  202   a - b , and the curve of the ratio of the intensity gradient for two distinct wavelengths in function of the distance from said calculation zone  208   a - b  to the boundary of the illuminated zone  202   a - b  and means for comparing characteristics of these curves with those extracted from reference curves. 
     In the various embodiments presented, there are for example 5 to 50 rings and the difference in radii between two consecutive rings is around 0.1 to 0.3 mm depending on the wavelength used. 
     In the case of a light source  102  with a plurality of wavelengths, the sensor  204  preferably takes the form of a camera having a signal output for each red-green-blue wavelength. The camera is for example equipped with a Bayer filter, which makes it possible, in a single capture, to recover the image  200   a - b  corresponding to each wavelength on the appropriate signal output. 
     According to another particular embodiment, the light source  102  emits a white light. 
     In another embodiment with a plurality of wavelengths, it is possible to use a monochrome camera and to use a light source emitting the various wavelengths one after the other. One image for each wavelength is then obtained and this embodiment makes it possible in particular to use a wavelength in the near infrared. 
     It can also be envisaged, for each wavelength, for the zone of the illuminated finger  10  to be different. There may for example be a plurality of light sources  102 , each illuminating according to a particular wavelength and illuminating a different zone of the finger  10 . 
       FIG. 4  shows the curve of the reflectance  400  in terms of percentage of a true finger  10  in function of the wavelength in nm of the light flow that illuminates it. 
       FIG. 6  shows the curve  602  of penetration in cm of the light in the skin in function of the wavelength. This curve is related to the reflectance curve  400  through the fact that the wavelengths not absorbed by the epidermis reaching the dermis, which is a less absorbent tissue, will be able to be diffused over a greater distance than the wavelengths remaining in the epidermis. 
     To maximise the difference in behaviour between reflection and diffusion, it is preferable to choose wavelengths having very different behaviours on a finger  10 . 
     The reflectance  400  and penetration depth  602  curves show these very different behaviours. In the low wavelengths, that is to say for wavelengths ranging from 350 to 550 nm (UV, blue, green), the light remains in the superficial layers of the skin and is absorbed. For higher wavelengths, that is to say for wavelengths ranging from 600 to 900 nm and beyond (orange/red, near IR), the light penetrates the dermis and diffuses further therein. 
     Thus, in cases where two wavelengths will be used, the choice will be preferably for a wavelength of between 350 and 550 nm and a wavelength above 600 nm. 
     Naturally, the present invention is not limited to the examples and embodiments described and depicted but is capable of numerous variants accessible to a person skilled in the art. 
     For example, the invention has been particularly described in the case of a single light source  102 , but it is possible to have a plurality of light sources  102  each pointing to a different surface of the element  10  to be validated. 
     There will then be the same number of illuminated zones  202   a  and diffusion zones  204   a , which will undergo identical treatment and which will allow consolidation between a plurality of values and thus avoid complex frauds presenting parts of true skins and parts of false skins. 
     It is possible to have a plurality of illuminated zones that are separate, not connected and therefore discontinuous with each other, such as for example parallel bands, a matrix of dots . . . . 
     In this case, the distance separating two separate non-connected illuminated zones is greater than at least twice the minimum distance separating the illuminated zone from the analysis zone.