Patent Grant
10366273
The present application claims priority to German Patent Application No. 10 2017 119 983.7, filed on Aug. 31, 2017, which said application is incorporated by reference in its entirety herein.
The invention relates to a device for the contact-based simultaneous capture of prints of several areas of skin of human autopodia supplied with blood, by means of disturbed total internal reflection.
The identification and recording of people via biometric features is becoming increasingly important. Alongside other biometric features, fingerprints play an important role. On the one hand, there are systems which are used to verify biometric features in which there must therefore be a match with stored features, for example in order to enable entry or access control. Other systems are used for identification by searching and storing in reference databases, for example in the case of border controls at airports or in the case of identity-recording by the police. For the latter systems there is a large number of requirements in terms of the quality, the resolution and the faithfulness to the original of the captured images of the skin textures. Not least because of the high demands of organizations entrusted with identity-recording measures, such as for example the Federal Bureau of Investigation (FBI), there is a high degree of standardization with these systems in order, on the one hand, to ensure as definite an identification as possible and, on the other hand, to make data sets which were captured by different systems comparable. For example, such systems must have a resolution of at least 500 ppi (points per inch), which corresponds to a pitch of the sensor elements of 50.8 μm. In addition, particular demands are made on the contrast transfer function (CTF), the signal-to-noise ratio (SNR) and the distortion. Finally, the grey scale must comprise at least 200 greyscale values and the image field must be illuminated as homogeneously as possible both in the immediate vicinity of the pixel and in the image as a whole.
All of the demand criteria require a balanced and high-quality system design. In the case of an optical system, this means, for example, that not only the acquisition sensor or acquisition sensors must satisfy the demands but also the illumination and all of the other components necessary for the image generation.
For recording finger- and handprints which fulfil the named high quality demands, at the present time, optical arrangements are predominantly used which operate according to the principle of disturbed total internal reflection. For this, a prism is applied, the surface of which provided for capturing the print must be larger than the surface required for capturing the print because of mechanical and optical demands. The size of the prism resulting from this often as a larger component in the capturing channel has a decisive influence on the minimum overall size and the minimum weight of a device.
On the other hand, however, the high image quality permits a rapid and reliable recording and identification of people, in particular even in the case of applications where, in addition to forensic accuracy, a high throughput of people also plays a role, for example in the case of border controls. In addition to the overall size and the weight, it is also disadvantageous that the use of complex mechanical components is necessary; moreover calibration and assembly are highly technically complex and time-consuming.
In order to combine the advantage of high image quality which can be achieved with disturbed total internal reflection with small, in particular flat, overall size, approaches are described, for example, in U.S. Pat. No. 9,245,167 B2. The fingerprint sensor disclosed there, in which the finger is placed on a TFT display (thin-film transistor display), captures a fingerprint and transmits this via an electronic system to a computer system. The brightness profile corresponding to the fingerprint forms—as in the case of arrangements with prisms—in that the epidermal ridges, the so-called papillary ridges, lying on the surface of the TFT display disturb the internal reflection of the light from the light source, while in the valleys between the papillary ridges, i.e. the epidermal valleys or papillary valleys, no contact occurs between skin and TFT display surface and there the light from the light source is reflected internally at the surface of the TFT display. A negative image of the fingerprint forms in this way on the light-sensitive areas of the TFT display. On the one hand, this solution assumes that the distance between the light-sensitive areas in the TFT display and the contact surface has a minimum size so that the light can strike the light-sensitive areas of the TFT. On the other hand, the illumination must fulfil certain requirements with regard to the direction of incidence and aperture angle.
U.S. Pat. No. 9,245,167 B2 discloses various possibilities as to how an illumination can be realized. One possibility consists of a light guide, arranged below the sensor layer, into which light is coupled from the side, which is coupled into the sensor layer from there. Because the device described there is designed for the examination of a single finger, the light guide can be kept relatively compact, with the result that a fall-off in the illuminance as the distance from the light source increases does not become disruptively noticeable. In the case of larger contact surfaces which are provided for the placement of several autopodia or a whole hand, however, the illuminance decreases with the result that the autopodia located at the edge are no longer correctly illuminated or their images are no longer completely usable due to a reduced contrast. In order also to obtain a sufficient illuminance for the finger placed on furthest from the light source, a powerful light source is required which, however, leads to the fingers which are closest to the light source being illuminated with excessive intensity, which also has an adverse effect on the contrast or leads to overexposure of these areas. An illumination which is optimal for these autopodia means, in contrast, that the autopodia which are placed on furthest from the light source are underexposed and can thus also not be represented. However, this type of illumination is not used with disturbed total internal reflection (TIR) in U.S. Pat. No. 9,245,167 B2; for such cases the use of a microprism array arranged between light guide and sensor layer is proposed when the light source is arranged below the sensor layer.
In other arrangements, the light is also coupled directly into a prism structure arranged above the sensor layer and above a cover glass. Another embodiment described in U.S. Pat. No. 9,245,167 B2 uses light sources which are arranged between the light-sensitive sensor elements; here, however, total internal reflection is not used for the image capture. This embodiment can also be used in conjunction with the scanning of a document.
When a microprism array is used, double images can form due to reflections in the light guide itself, furthermore parasitic scattered light from the surroundings can impair the image quality. In the case of the methods described in U.S. Pat. No. 9,245,167 B2, the protective layer on the sensor layer has to be kept as thin as possible since otherwise diffuse light generates optical crosstalk, which reduces the resolution. By this is meant the case where light from different angles of incidence strikes the same area of the contact surface and in the process records the same or very similar items of information about the finger placed on or not placed on at this specific point, but is also emitted at different angles of reflection due to the different angles of incidence and thus detected at various points, with the result that the resolution is impaired.
Further fingerprint scanners which operate according to the principle of disturbed total internal reflection with prisms, objective lenses and sensors are known in the state of the art, for example from U.S. Pat. No. 6,647,133 B1, from WO 2011/059496 A1 or from WO 2007/115589 A1. An optical fingerprint scanner with a touch-sensitive surface and an illumination via a light guide is described in WO 2008/033265 A2. Capacitive TFT sensors with a touch-sensitive control system are likewise known, for example from WO 2001/36905 A1 or from the already named U.S. Pat. No. 9,245,167 B2.
As already mentioned, prism devices are significantly larger and heavier than direct scanners, in particular when they are dimensioned for the simultaneous capture of several fingers. The calibration has to be effected very accurately, which is technically complex. Also ruled out is user guidance directly in the capture area of the skin prints. The capture of documents is also not possible using this principle.
Direct optical fingerprint scanners only work with a small distance between sensor element and skin surface, namely with distances of less than 25 μm. Although a robust encapsulation of the sensor/pixel structures can be achieved by means of such thin cover glass, the robustness against external mechanical influences is however reduced compared with thicker cover glasses. If an at least partially transparent coating is used instead of glass or ceramic, the stability vis-à-vis electrostatic discharges (ESD stability) is reduced compared with glass, and in addition such a protective layer only has a lower hardness and abrasion resistance than glass or ceramic. Such devices are therefore less suitable for scanning documents. Although glass with a thickness of less than 30 μm is commercially available and suitable for series production, it is however complex to process on large contact surfaces; in particular bubble-free optical bonding is challenging. Transparent ceramics in this small thickness are not available in series production. In principle the use of a thicker glass cover layer would be desirable, however this is accompanied by the associated reduced resolution described at the beginning.
Capacitive devices on the other hand cannot be used for the capture of documents; in addition the sensors are usually not transparent with the result that to date neither user guidance nor user feedback has been realized in the capture surface by a display mounted below it.
An embodiment device comprises, from the direction of an area of skin that is to be placed on, for example from the direction of a person who places a finger or a hand on a fingerprint sensor, a protective body of predefined thickness with a contact surface for placing on at least one area of skin, wherein the contact surface has a size which makes possible the simultaneous placement of several autopodia, as a rule several fingers. The device also comprises a sensor layer which can itself be part of a layer system. The sensor layer comprises light-sensitive sensor elements, arranged in the form of an array, for detecting light of at least one predefined wavelength range. Passband areas which are transparent for light of at least one predefined illumination wavelength range are arranged between the sensor elements, which are also referred to as pixels. Finally, the device also comprises a light guide made of a material that is transparent for light of the predefined illumination wavelength range. The light guide is designed in the form of a plane-parallel plate and has a lower large face and an upper large face arranged parallel thereto; the upper large face faces the sensor layer. Corresponding to the shape of a plane-parallel plate, the light guide also has several edges connecting the large faces to each other; the light guide itself is as a rule constructed as a solid glass or plastic body. One of the edges of the light guide is formed as a coupling-in surface for illumination light. For this a light source emits illumination light along a longitudinal direction of the coupling-in surface, i.e. it extends along this longitudinal direction and emits the light onto the coupling-in surface. The longitudinal direction of the edge refers to a direction which lies parallel to the planes of the large faces.
An object of the invention is to further develop a device of the type described at the beginning for capturing fingerprints to the effect that on the one hand the prints of several autopodia can be captured simultaneously, on the other hand thicker protective bodies such as cover glasses can be used, which optionally also allow documents to be scanned. The contact surface should be flat; in particular the device must be capable of achieving an image quality according to the quality requirements specified by regulations for all autopodia placed on simultaneously. User guidance directly on the contact surface should in principle be possible.
This object is achieved for a device of the type described at the beginning in that the light source is designed so as to emit light in a limited angular range around a preferred direction, wherein the preferred direction makes an angle of incidence with the large faces, which is predefined such that a predominant proportion of the light which is emitted in the angular range would, after entering the light guide, be totally internally reflected at the large faces in the case of a medium with a refractive index of air in contact therewith. “Predominant” means a proportion of more than 50%. However, in order to utilize the light quantity as completely as possible the configuration is preferably chosen such that all of the light which is emitted in the angular range around the preferred angle would be totally internally reflected under the named condition.
In addition, between the sensor layer and the light guide a mirror layer is arranged which reflects one portion of the illumination light back into the light guide and transmits another portion of the illumination light. Finally, the device also comprises a homogenizer for homogenizing the illuminance of illumination light which exits the light guide through the upper large face thereof and strikes the contact surface. The device additionally or alternatively also comprises a homogenizer for homogenizing a light quantity to be detected depending on the illuminance of the illumination light from the protective body striking the contact surface. The homogenization is effected in relation to the distance to the light source or depending on the distance thereto.
In order to implement the principle of total internal reflection, a beam propagating in the light guide must strike the upper or lower large face at a corresponding angle, at which total internal reflection is possible. The critical angle for total internal reflection is determined by the refractive indices of the two media involved; a prerequisite for the occurrence of total internal reflection is also that the refractive index of the medium in which the beam propagates is higher than that of the medium on the other side of the large face. In the present case, the light guide, on its upper large face, is joined to a further material layer, for example the mirror layer, for example in that both layers are optically coupled with a transparent adhesive, or also only laid alongside each other and clamped together, which likewise leads to a sufficient connection and sufficient boundary surface contact in the case of a highly accurately polished surface. The medium in contact with the lower large face is air.
A light beam propagating along the preferred direction first strikes the lower or upper large face—depending on the coupling-in direction. It is totally internally reflected at the lower large face and directed at the same angle in the direction of the upper large face. The medium on the other side of the upper large face is in general not air, but rather, for example, the mirror layer or another material layer. The mirror layer does not reflect 100% of the beam, but allows a fraction of the illumination light to pass through, i.e. transmits it, while another, predominant portion is reflected, with the result that the light guide can emit light over the whole extent of the large faces through the upper large face as light can penetrate the light guide as far as an end edge, i.e. an edge which lies opposite the coupling-in surface.
The transparent materials of the layer stack, in particular consisting of the mirror layer, the sensor layer and the protective body, preferably have similar refractive indices, with the result that a beam entering the medium at an angle other than zero is refracted as little as possible. This is not absolutely necessary, however it must be ensured when combining the materials that at least the light beam propagating below the preferred direction in the light guide strikes the protective body after passing through all of the media as far as the underside of the contact surface at such an angle that the beam is also totally internally reflected here if the medium air is located on the other side of the contact surface. The angle which the preferred direction makes with the normal of the large faces of the light guide therefore on the one hand has to be chosen such that total internal reflection takes place at these large faces themselves if air as medium is on the other side; on the other hand the angle also has to be chosen such that total internal reflection likewise takes place at the contact surface of the protective body after passing through all of the media which are stacked in the form of layers if air is on the other side. When a finger is placed on, the beam is not totally internally reflected, but enters the finger and is scattered in many directions, with the result that the skin particles lying thereon appear as dark spots in the image.
The different layers are preferably optically coupled to one another, for example by optical bonding. The light source emits light only in a limited angular range. In order to keep the coupling into the light guide as effective as possible and the broadening of the angular range by refraction as small as possible, the angular range which also defines an acceptance cone in the case of a substantially punctiform light source is preferably oriented symmetrical about the preferred direction wherein, in an advantageous embodiment, the normal of the coupling-in surface corresponds to this preferred direction or lies parallel to it.
The light source should advantageously be aligned such that all angles contained in the limited angular range still correspond to beams which after entering the light guide make an angle with the normals of the large faces which makes total internal reflection possible with air as adjacent medium. The size of the aperture angle also limits the possible thickness of the protective body as the diameter of an acceptance cone—in the case of a symmetrical aperture angle—becomes larger with the light path covered and the resolution is thus reduced in the case of thicker cover glasses. The smaller the angular range, the thicker the protective body can be chosen to be. LEDs are suitable as light sources that are favourable with regard to price. Particularly small aperture angle ranges can be achieved when lasers are used as light sources.
When the angular range is limited to 20°, i.e. to a ±10° deviation from the preferred direction, the thickness of the protective body can be approximately up to 0.1 mm in order still to achieve a resolution of 500 dpi when, for example, thin glass with a refractive index of 1.5 is used as protective body; other angular ranges and maximum thicknesses can result depending on the material. The light emitted by the light source is guided to the finger lying on the contact surface, wherein the angles of incidence of the light which result from the limited angular range, relative to the normal of the contact surface, are preferably all greater than the critical angle of the total internal reflection between the protective body and air, with the result that the light is reflected back onto the sensor in the papillary valleys between the papillary ridges with fingers placed on. This total internal reflection is disturbed at the papillary ridges with the result that these appear as dark areas.
The sensor is preferably an optical TFT sensor, such that fingerprints can be captured directly and in a contact-based manner, for example. The sensor is advantageously semi-transparent in order to allow as much light as possible from the light guide arranged below the sensor layer to pass through.
An important component of the device is the homogenizer means for homogenizing the illuminance of illumination light, or the homogenizer means for homogenizing a light quantity to be detected, in each case depending on the distance to the light source. By means of the homogenization it is achieved that all autopodia lying on the contact surface, for example four fingers or a whole hand, are illuminated or detected simultaneously with the same quality. This can be achieved on the one hand in that illumination light which exits the light guide through the upper large face thereof and which strikes the contact surface is homogenized, i.e. anywhere on average has the same illuminance. Another possibility consists of homogenizing not the illumination light, but rather the light to be detected or the light quantity to be detected. This can also be effected depending on the illuminance of the illumination light from the protective body striking the contact surface as this ultimately also determines the light quantity to be detected. The homogenization is effected depending on the distance to the light source. Through scattering losses and coupling-out, the illuminance or the intensity of the light which is emitted from the light guide into the sensor layer decreases as the distance from the light source increases in the case of a homogeneous mirror layer which is transparent up to a certain degree of, for example, 10%; this decrease taking place continuously for layers that are homogeneous per se. Without a homogenization layer the illuminance is therefore greater in the vicinity of the light source. The means for homogenizing the illuminance now ensure that the illuminance of the illumination light which strikes the contact surface is substantially homogeneous all over the contact surface, i.e. is constant. For this, the illuminance in the vicinity of the light guide has to be reduced to a value which corresponds to the illuminance in the areas of the contact surface furthest from the light source. The closer an area of the contact surface is to the light source, the greater the extent to which the illuminance has to be reduced at this point, whereas in the area of the furthest points on the contact surface the reduction in the illuminance is to be as small as possible or negligible. Between these two extreme positions the illuminance is correspondingly reduced, wherein on average, i.e. over a representative surface area on the contact surface, for example the size of a phalanx, the variation preferably takes place continuously and not erratically.
The device can alternatively or additionally also comprise a homogenizer for homogenizing a light quantity to be detected depending on the illuminance of the illumination light from the protective body striking the contact surface. The illumination light is then first allowed to pass through completely, and therefore illuminates the contact surface inhomogeneously and the autopodia lying closer to the light source more strongly. The homogenizer or means for homogenizing the light quantity to be detected may comprise, for example, diaphragms which are arranged above the sensor elements or light-sensitive areas thereof. The apertures of these diaphragms increase as the distance from the light source increases, with the result that the light quantity striking the light-sensitive areas of the sensor elements is regulated. The homogenizer for homogenizing the light quantity to be detected can additionally or alternatively also comprise a means to control sensitivity of the sensor elements such as a controller for varying the sensitivity of the sensor elements. The light sensitivity of the sensor elements in the vicinity of the light source is reduced or that of those further from the light source is increased, wherein the two can be combined with each other in order to obtain the best possible signal-to-noise ratio. The higher the sensitivity, the greater the light quantity that is detected. A further possibility for homogenization is the introduction of a layer between the light-sensitive sensor elements and the contact surface, which has an absorbing effect, wherein the degree of absorption at various locations of the layer is different. In the vicinity of the light source the absorption is strong; further from the light source it is less.
In a particularly preferred embodiment of the invention, the homogenizer for homogenizing the illuminance are formed on the mirror layer, i.e. the mirror layer additionally also assumes the function of a homogenization layer, in that the degree of reflection of the mirror layer decreases and the degree of transmission of the mirror layer increases as the distance from the light source increases, which can be realized in different ways. The variation of the degree of reflection or of the degree of transmission need not be effected continuously as the distance increases, but rather can also be effected more or less erratically or not at all for individual, pixel-sized areas: the requirement to decrease the degree of reflection or to increase the degree of transmission applies to areas which have a representative size in relation to the overall size of the contact surface, for example approximately 10% of the contact surface or an area the size of a finger or phalanx. The size and shape of the areas also depends on the manner in which the homogenization in the mirror layer is achieved. For example, a homogenization can be achieved by a pattern of reflecting and transmitting strips like a barcode, wherein the strips run parallel to the light source and the width of the reflecting strips decreases as the distance from the light source increases, while the width of the transmitting strips increases. In this case, a representative area in the direction away from the light source comprises at least one area which corresponds to the widest strip with the highest reflectivity and the adjacent narrowest space between with the lowest reflectivity or the highest degree of transmission. If the representative area is then gradually moved away from the light source, perpendicular to the longitudinal direction of the coupling-in surface or of the strips, the reflectivity in the representative area decreases on average—i.e. averaged over the representative area—and the transmissivity increases on average.
In order to achieve the homogenization, the mirror layer therefore preferably comprises reflecting areas and transmitting areas, wherein the proportion of the reflecting areas per unit area—or relative to an area of representative size—decreases on average with the distance from the light source and the proportion of the transmitting areas per unit area increases on average with the distance from the light source. By the expressions “per unit area” and “on average” is meant here that as the distance from the light source increases, the change can possibly also be effected erratically or not at all in the smallest, pixel-sized areas, but rather only over larger representative surface areas. The size and shape of these representative surface areas depends on the size and shape of the transmitting or reflecting areas, and can be up to a quarter of these or even more relative to the whole contact surface.
Density—by this is meant the number of areas per surface or unit area—of the reflecting areas advantageously decreases as the distance from the light source increases, while the density of the transmitting areas increases. In this case, the transmitting areas—or in another embodiment the reflecting areas—can in each case be the same size. However, it is alternatively or additionally also conceivable that the size of the reflecting areas decreases as the distance from the light source increases, while the size of the transmitting areas increases. A reflecting area has a degree of reflection significantly greater than 50% of the incident light, for example between 70% and 90%. The transmitting areas in turn should have a degree of transmission significantly greater than 50%, for example between 70% and 90%.
The areas themselves can have any desired shapes, for example circular or rectangular; other polygonal shapes are also conceivable. The areas can themselves also be delimited by irregular polygons the shape of which is predefined, for example stochastically; in this way artefacts, which can form due to regular structures, can optionally be avoided during the illumination.
Another possibility for achieving a homogenization of the illuminance with the aid of the mirror layer consists of varying the thickness of the mirror layer, wherein the thickness of the mirror layer decreases as the distance from the light source increases, this decrease preferably taking place continuously. In this way the degree of transmission of the layer is gradually increased as the distance from the light source increases. The mirror layer can then be formed wedge-shaped, for example in a cross section through the layer structure—perpendicular to the contact surface and away from the light source; to preserve the parallelism of the contact planes of the different layers it can be combined with a transparent compensation layer, also wedge-shaped.
All the layers, i.e. light guide, mirror layer, optionally a compensation layer, sensor layer and protective body, are optically coupled to each other according to their sequence in the stack. This means that there is no air layer between two layers, but two layers are in each case joined directly or by an adhesive layer, which is preferably as thin as possible; this type of connection is also referred to as optical bonding.
A further possibility for achieving the homogenization consists of using a mirror layer in which the density of a material which is essentially responsible for the reflections in the mirror layer decreases as the distance from the light source increases. This embodiment is particularly advantageous when light of only specific wavelengths is used, for example laser light.
The mirror layer can be, for example, a metal mirror and it can be produced in various ways, for example in an inkjet printing method, photolithographically, by means of PVD (physical vapour deposition) or laser ablation. The homogenization means can, however, also be realized on the mirror layer, in that the latter is designed as an interference mirror, which is in turn advantageous when only individual selected wavelengths are used.
The homogenizer for homogenizing the illuminance can, however, also comprise an absorption layer which is arranged between the mirror layer and the sensor layer or between the sensor elements in the passband areas, wherein as the distance from the light source increases the degree of absorption of the absorption layer for the illumination light decreases and the degree of transmission increases. The mirror layer can then be designed with high reflectivity throughout, with the result that a high proportion of the light is reflected up to the end of the light guide, which ensures a comparatively higher brightness there.
In order to keep the angular distribution as symmetrical as possible and as limited as possible around the preferred direction, it is advantageous for the normal of the coupling-in surface to lie parallel to the preferred direction; the coupling-in surface therefore makes an acute angle with the upper or lower large face. The angular distribution should preferably be chosen such that all angles at which light is coupled in lead to total internal reflection in order to keep the losses as low as possible and to achieve as high an image quality as possible.
In order to prevent the occurrence of double or multiple images in the area of the sensor, a suppressor or means for suppressing the formation of returning beams are formed or arranged to reduce the reflection at an edge of the light guide formed as end edge, which lies opposite the coupling-in surface in the light propagation direction. The reduction in the reflection at the end of the light guide can, for example, be achieved in that an absorbing layer is applied to this end or the edges are roughened. A further possibility which can be combined with the one named previously also consists of the addition of a chamfer similar to the coupling-in surface, i.e. the end edge is then arranged substantially perpendicular to the propagation direction of light that has entered the light guide along the preferred direction, wherein the propagation direction of the light depends on the angle of entry and, in the area of the end edge, on the extent of the light guide in the propagation direction. An embodiment with several absorption surfaces which are preferably provided with absorption layers and, in pairs in each case, make angles other than 0° and 180° with each other is also conceivable. The angles are predefined depending on the propagation direction of the light such that the absorption is maximized. The actual position of the absorption surfaces likewise depends on the orientation of the preferred direction in relation to the large faces and on the longitudinal extent of the light guide between coupling-in surface and end edge; the maximization of the absorption is in this respect a design problem with a plurality of equivalent possible solutions.
The protective body advantageously has a thickness of between 50 μm and 1000 μm, as already stated above, and it is optically coupled to the sensor layer. The protective body can consist of different materials, for example glass, ceramic or plastic. The narrower the light cone around the preferred direction, i.e. the narrower the irradiated angular spectrum, the greater the thickness of the protective body that can be chosen, i.e. the greater the distance of the contact surface from the sensor layer can be. Since according to the invention a directed edge illumination by means of LEDs or lasers is used and the effect of disturbed total internal reflection is utilized, in contrast to the state of the art where an internal diffuse illumination is used, commercially available thin glass with a thickness of more than 50 μm can be used, which proves to be less complex in terms of the procurement and processability, i.e. the process stability during the optical bonding or during the coating. Furthermore, protective films or protective glasses which can be removed without leaving a residue, as are used in the display industry, can also be used as protective body, whereby renewal of the sensor protection is available through easier replaceability. Transparent ceramics can also be used. Due to the increased thickness of the protective body, the sensor layer is encapsulated extremely stably, mechanically and chemically. In addition, the entire module can be produced both with a low weight of less than 300 g and also thin with a thickness of less than 2.5 cm.
A diaphragm layer is advantageously arranged between the contact surface and the sensor layer to suppress the detection of ambient light, wherein a diaphragm is in each case arranged above a sensor element. This serves for protection against ambient light as according to some users' requirements the sensors must also function during irradiation in direct sunlight. The detectable angular spectrum is limited by the diaphragms to the relevant range which is defined by the light coupling-in conditions. A spectral filter layer can additionally or alternatively also be formed between the contact surface and the sensor layer, or the protective body itself can be formed as a spectral filter layer. The spectral filter layer can also be only part of the protective body. It is also possible to form the sensor elements themselves so as to be wavelength-selective only for light of the illumination wavelengths. All measures can also be combined with each other.
A restriction of the spectral range by integrating the spectral filter layer in the protective body can be achieved, for example, with absorbent organic or inorganic dyes, particles, plasmonic filters—resonant metal nanoparticles—or interference filters. If an adhesive is used to join the protective body to the sensor layer, the protection against ambient light can also be integrated therein; the spectral filter layer then corresponds to the adhesive layer. Protection against ambient light at a pixel level is possible through a structured spectral filter. Here, only the light which propagates from the area of skin to be captured through the contact surface to the light-sensitive area of the sensor element is spectrally filtered and allowed to pass through, but not the illumination light beams which propagate through the sensor component and the contact surface and illuminate the area of skin to be captured.
In a particularly preferred embodiment of the invention, an additional light source is arranged below the lower large face of the light guide, and is spaced apart from the lower large face of the light guide. A layer, the refractive index of which is lower than that of the light guide, is located between the light guide and the additional light source. The light guidance in the light guide is not meant to be prevented thereby; one possibility for achieving this is an air gap between light guide and additional light source. The additional light source emits light in planar form; it can be designed, for example, as background lighting, as used in the display industry, or also as a complete display. A display makes it possible to represent items of information on the contact surface; direct user guidance is made possible in this way. The additional light source emitting light in planar form is not directly connected to the lower large face of the light guide, i.e. is not optically coupled thereto; for example, a thin air layer is located between the two in order not to disturb the total internal reflection at the lower large face. It is also possible by means of the additional light source to capture documents, because the light from this light source is as a rule undirected and the principle of total internal reflection does not apply here. However, the illuminance of the additional light source is advantageously adapted to the degree of transmission of the homogenizer, with the result that the object to be captured is also illuminated as far as possible with the same illuminance all over during the capture of documents.
The additional light source can be formed such that it simultaneously or successively emits light—preferably via control—in several different wavelengths or in several different wavelength ranges—in particular not overlapping one another. This makes control images for authentication of security documents and banknotes possible. For this, it is advantageous that the light source can also emit light in the IR or in the UV range in addition to the wavelength ranges visible to the human eye. To capture documents in color, it is furthermore advantageous when the additional light source can emit light from the red, the blue and the green wavelength ranges one after the other. Light of these wavelength ranges can preferably be switched on and off individually by means of a controller, in particular if the sensor elements record only the intensity, but not the colors—a color image can then be evaluated by means of corresponding image processing algorithms.
The additional illumination can also consist of individual light sources which can then also be arranged between sensor layer and mirror layer; in this case an adaptation of the illuminance to the degree of transmission of the homogenizer can be dispensed with. However, an arrangement below the light guide is to be preferred as there is a greater flexibility with respect to the choice of additional light source here and the element which bears this additional light source need not be transparent. Moreover, no further layer need then be penetrated by the illumination light, which in every case is associated with losses. The light source which emits the illumination light for capturing the areas of skin can advantageously be switched off when the additional illumination is used.
An embodiment of the invention relates to a device for the contact-based simultaneous capture of prints of several areas of skin of human autopodia supplied with blood, by means of disturbed total internal reflection. Seen from the direction of an area of skin in contact, the device comprises a protective body of predefined thickness with a contact surface, wherein the contact surface has a size which makes possible the simultaneous placement of several autopodia. A sensor layer follows, which comprises light-sensitive sensor elements, arranged in the form of an array, for detecting light of at least one predefined wavelength range, wherein passband areas which are transparent for light of at least one predefined illumination wavelength range are arranged between the sensor elements. Finally, the device also comprises a light guide made of a material that is transparent for light of the predefined illumination wavelength range, wherein the light guide is designed in the form of a plane-parallel plate. The plane-parallel plate comprises a lower large face and an upper large face arranged parallel thereto which faces the sensor layer. The large faces are connected by several edges, wherein one of the edges is formed as a coupling-in surface for illumination light which is emitted from a light source along a longitudinal direction of the coupling-in surface.
According to the invention the light source emits light in a limited angular range around a preferred direction. The preferred direction makes an angle of incidence with the large faces, which is predefined such that light which is emitted in the angular range would, after entering the light guide, be totally internally reflected at the large faces in the case of a medium with a refractive index of air in contact therewith. Between the sensor layer and the light guide a mirror layer is arranged which reflects one portion of the illumination light back into the light guide and transmits another portion of the illumination light. The device comprises means for homogenizing the illuminance of illumination light which exits the light guide through the upper large face thereof and strikes the contact surface. The device additionally or alternatively also comprises means for homogenizing a light quantity to be detected depending on the illuminance of the illumination light from the protective body striking the contact surface, wherein the homogenization is effected depending on the distance to the light source.
It is understood that the features named above and those yet to be explained below can be used not only in the stated combinations but also in other combinations or alone, without departing from the scope of the present invention.
In the following, the invention is explained in yet more detail by way of example with reference to the attached drawings, which also disclose features essential to the invention. There are shown in:
The light guide 106 is designed in the form of a plane-parallel plate and comprises a lower large face 155 and an upper large face 156 which is arranged parallel to the lower large face 155 and faces the sensor layer 104. The two large faces 155, 156 are joined to each other by several edges which are not represented in
The light source 112 emits light in a limited angular range around a preferred direction 113, which preferred direction 113 makes an angle of incidence with the two large faces 155, 156, which is predefined such that light which is emitted in the angular range would, after entering the light guide 106, be totally internally reflected at the two large faces 155, 156 in the case of a medium with a refractive index of air in contact therewith. In fact air is in contact with the lower large face 155 as medium, with the result that the condition for total internal reflection is fulfilled there. However, a mirror layer 105 arranged between the sensor layer 104 and the light guide 106 which reflects one portion of the illumination light back into the light guide 106 and transmits another portion of the illumination light is located on the upper large face 156. Protective body 103, sensor layer 104, mirror layer 105 and light guide 106 are optically coupled to each other, i.e. for example bonded to each other by optical bonding, in each case according to their layer sequence. The adhesive and the materials of the layers are chosen such that they have refractive indices that are as similar as possible in order to minimize the extent to which the spectrum of angles of incidence fans out around the preferred direction 113.
The light emitted by the light source 112 is coupled into the light guide 106 along the preferred direction 113. A total internal reflection of the light takes place at the lower large face 155 of the light guide 106. With every reflection only a fraction of the light is transmitted as a transmitted beam 107 by the mirror layer 105 lying between the light guide 106 and the sensor layer 104 and is guided through the passband areas 202 in the sensor layer 104 and through the protective body 103 to the contact surface 102.
In this way it is ensured that the light guided in the light guide 106 is reduced only slightly over many reflections. Without a mirror layer 105, a large proportion of the light would be absorbed in the sensor layer 104 depending on the degree of transmission of the sensor layer 104, which would lead to a very uneven illumination, which is furthermore still dependent on the position of the fingers 101 placed on. Without a reflective layer, the illumination intensity would moreover fall off to a even greater extent as the distance to the light source 112 increases, this fall-off being minimized by the mirror layer 105.
If an object, such as the fingers 101 shown here by way of example, lying on the contact surface 102 is not in contact with the latter at the point of reflection of the transmitted beam 107, a total internal reflection takes place. A beam 108 reflected at the contact surface 102 strikes the sensor element 201 and is recorded on the basis of its light sensitivity. The corresponding sensor element 201 lies opposite the point of reflection on the contact surface 102 moved away from the light source by Δz=d·tan(α), wherein d denotes the thickness of the protective body 103 and a denotes the angle of the reflected beam 108 to the surface normal of the contact surface 102.
Areas of skin, in particular fingers, are not smooth surfaces. For example, fingers have papillary valleys and papillary ridges, such that the known, individual structures form. In the case of the fingers 101 shown here, the papillary valleys 110 are not in contact with the contact surface 102; the beam is totally internally reflected and recorded here. However, the fingers 101 lie on the contact surface 102 with the papillary ridges 111, and at this point the fingers 101 are in contact with the contact surface 102. The transmitted beam 107 penetrates the finger 101 there. The light is scattered within the finger at naturally present scattering centres 109, the predominant portion of the light remains in the finger, and only a fraction is coupled into the protective body 103 again via the contact surface 102 and has the possibility of striking the light-sensitive sensor element 201. Papillary ridges therefore become noticeable in the image due to a lower intensity; their image is darker than that of the papillary valleys 110. Overall, in this way a light-dark pattern is formed via all the sensor elements 201 of the sensor layer 104, which corresponds to the prints of the fingers 101 placed on. The protective body 103 serves as an optical medium in order to guide the transmitted beams 107 that have passed through the passband areas 202 with illumination light onto the light-sensitive sensor elements 201, where the light is converted to an electrical signal corresponding to the intensity. Furthermore, the protective body 103 is also used to protect the sensor layer 104 against damage or destruction by objects placed thereon.
A beam which enters the light guide 106 along the preferred direction 113 is split into a transmitted beam 107 and a reflected beam 114 when it strikes the upper large face 156; only a fraction of the light is guided further as a transmitted beam 107 due to the mirror layer 105. The reflected beam 114 is once again reflected at the lower large face 155 and again strikes the upper large face 156 as a reflected beam 115, where once again a fraction is coupled out as a transmitted beam and guided by the mirror layer 105 to the contact surface 102, while a predominant proportion of the beam 115 is once again reflected and is directed to the lower large face 155 as a reflected beam 116. With every reflection at the upper large face 156, a small portion of the intensity is coupled out and transmitted, with the result that the intensity of the beam propagating in the light guide 106 is increasingly reduced.
When a mirror layer 105 is used, although the illumination intensity drops to a lesser extent than without such a mirror layer 105 as the distance from the light source 112 increases, the degree of reflection of the mirror layer 105 must be chosen to be as high as possible in order to keep the illumination fall-off as low as possible. The system thereby becomes less efficient as a large portion of the light is guided unutilized through the light guide 106 and is coupled out or absorbed at the end. To correct the illumination fall-off in order to obtain a bright image which is illuminated as evenly as possible, the device therefore comprises a homogenizer or means for homogenizing the illuminance of illumination light which exits the light guide 106 through the upper large face 156 thereof and strikes the contact surface 102. The device additionally or alternatively comprises means for homogenizing a light quantity to be detected depending on the illuminance of the illumination light from the protective body 103 striking the contact surface 102. The homogenization corresponds to the fall-off in the illuminance as the distance from the light source 112 increases, and is therefore effected depending on the distance from the light source 112, wherein a non-linear progression of the fall-off in the illuminance is optionally taken into consideration. The means for homogenizing the illuminance or the light quantity to be detected can be realized in various ways, some of which are explained by way of example in the following.
A first possibility consists of forming the homogenizer for homogenizing the illuminance on the mirror layer 105; the mirror layer 105 is optically coupled to the sensor layer 104 and the light guide 106, as has already been explained previously. A formation of the homogenizer on the mirror layer 105 comprises an embodiment of the mirror layer 105 described above to the effect that the degree of reflection of the mirror layer 105 decreases as the distance from the light source 112 increases and the degree of transmission increases. Examples of such mirror layers which have a decreasing degree of reflection and an increasing degree of transmission as the distance to the light source 112 increases are represented in
In
Whereas in the previous examples the transparent areas were represented as light and the reflecting areas as dark, the mirror structures can also be prepared inversely, i.e. the dark areas which were previously described as reflecting are then transparent and vice versa. In these cases, the illumination is effected from the opposite side in order that the degree of transmission of the mirror layer 105 becomes greater as the distance from the light source increases and the degree of reflection smaller, i.e. the light source 112 and the coupling-in surface would then be arranged on the right-hand side of the mirror layers 105 shown in the case of the mirror layers 105 formed in the inverse manner to those in
As an alternative or in addition to the examples described previously, the density of a material which essentially brings about the reflection at the mirror layer 105, i.e. of a component of the mirror layer 105, can also decrease as the distance from the light source increases.
In the preferred embodiment represented in
A suppressor, or means, for suppressing the formation of returning beams are preferably formed on an edge of the light guide 106 formed as end edge 300, which lies opposite the coupling-in surface in the light propagation direction.
The returning beams are suppressed by the measures described in the following. A simple measure is to provide the end edge 300 with an absorption layer 302, represented in
Further embodiments are shown in
The beam propagation in the light guide 106 is now explained in more detail with reference to
The protective body 103 can be applied to the sensor layer 104, for example by means of deposition processes. In comparison with known arrangements, however, thicker protective bodies 103 can be used, which can also be optically coupled to the sensor layer 104, by so-called optical bonding, by means of an adhesive agent adapted with respect to the transparency for illumination light and the refractive index. Such an example is shown in
Even better, both aspects, the absorption of ambient light and the homogenization of the light quantity to be detected, can be realized when the diaphragm layer 124 with pixel aperture diaphragms described in conjunction with
In order to couple the light from the light source 112 into the light guide 106 as effectively as possible and with high yield, it is advantageous to form the coupling-in surface correspondingly, in that the normal of the coupling-in surface lies parallel to the preferred direction 113, for example. Examples of particularly suitable coupling-in surfaces are represented in
Furthermore, there is also the possibility of reducing the angular range 151 in that diaphragms in the form of wide slots are attached to the coupling-in surface 158, and the distance from the light source 112 to the coupling-in surface 158 can be increased, however these measures are associated with a loss in irradiation intensity, which causes an unnecessary energy loss and shorter battery life, in particular in the case of mobile devices.
The additional light source 320 can additionally or alternatively also emit light sequentially or simultaneously in the limited wavelength ranges for red, green and blue light. With sequential illumination, an image can be captured in the three colors one after the other and these can be assembled to form a color image by corresponding image processing algorithms, as are known in the state of the art, when the sensor elements 201 record only the intensity. The illumination with the different wavelength ranges can also be effected simultaneously, wherein it is then necessary for the sensor elements 201 to be designed with corresponding sub-pixels which in each case detect only this wavelength range, and/or to be designed with different color filters, with the result that the different sub-pixels can in each case detect different wavelength ranges.
The additional light source 320 can furthermore also be used to display items of information on the contact surface which show a user directions for the use of the arrangement in the manner of operating instructions or additional items of information. For this the additional light source 320 must, however, consist of individually controllable elements, e.g. realized as a display.
With the previously described device it is possible to capture the prints of several fingers simultaneously with the same capture quality for all areas of skin due to the homogenization, wherein the device is constructed flat compared with other arrangements based on total internal reflection for capturing autopodia. Additionally, items of information such as for example user guidance or an evaluation can be displayed on the contact surface, which can also be used as a screen, and documents can be captured by means of the additional light source and checked for authenticity, for example with the aid of a comparison by means of connected databases.
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