DEVICE FOR CAPTURING IMPRESSIONS OF AUTOPODIA AND USE OF SAME

Abstract

A device for displaying information and for the contact-based, simultaneous capture of impressions of a plurality of blood-perfused skin regions of human autopodia, including: a placement surface for placing the autopodia, a display unit which is arranged below the placement surface and which has at least partially transparent display elements which are arranged in grid form and are individually controllable by a control unit, the display elements having display pixels which emit light in a direction of the placement surface. The device also includes optical sensor pixels which detect light incident on the sensor pixels. The sensor pixels may be arranged in a sensor layer arranged under the display unit viewed from direction of the contacting skin region or may be arranged in the display unit between the display elements. A cavity filter for angle selection is arranged in front of the optical sensor pixels.

Claims

1. A device for displaying information and for contact-based, simultaneous capture of impressions of a plurality of blood-perfused skin regions of human autopodia, comprising a placement surface for placing the autopodia, a display unit which is arranged below the placement surface viewed from a direction of a contacting skin region and which has at least partially transparent display elements which are arranged in grid form and are individually controllable by a control unit, the display elements having display pixels which emit light in a direction of the placement surface, optical sensor pixels which detect light incident on the sensor pixels from a direction of the placement surface, and i. the sensor pixels are arranged in a sensor layer arranged under the display unit as viewed from the direction of the contacting skin region, or ii. the sensor pixels are arranged in the display unit between the display elements, at least one cavity filter for angle selection, arranged in front of the optical sensor pixels in a direction of the placement surface.

2. The device according to claim 1, in which the optical sensor pixels are arranged in the display unit between the display elements, wherein one or more pixelated or pixel-shaped cavity filters are associated with each sensor pixel and arranged above the respective sensor pixel.

3. The device according to claim 1, in which the optical sensor pixels are arranged in a sensor layer below the display unit, wherein the at least one cavity filter is formed as a cavity filter layer which is arranged between the display unit and the sensor layer.

4. The device according to claim 1, wherein the display unit comprises actively luminous display pixels, wherein the display unit is configured as an LED unit with display pixels which are formed as LEDs, OLEDs, QLEDs or microLEDs.

5. The device according to claim 1, wherein the display unit comprises passively illuminated display pixels, and a first illumination unit with a transparent light guide layer body and first illuminants is arranged below the display unit and above the at least one cavity filter viewed from the placement surface, and the display pixels are diffusely illuminated by the light guide layer body in a first wavelength range.

6. The device according to claim 5, wherein the display unit is configured as an LC unit with display pixels formed as LC elements.

7. The device according to claim 4, comprising a second illumination unit with second illuminants which are formed for emitting directed light of a predetermined wavelength in a limited angular range of not more than around a predetermined central direction, wherein the central direction is determined depending on the predetermined wavelength of the directed light and on an angle selectivity of the at least one cavity filter for the predetermined wavelength, parallel to a normal of the placement surface.

8. The device according to claim 1, wherein the at least one cavity filter is formed as one or more individual cavity filters, one or more dual-cavity filters or one or more multi-cavity filters and as layer stacks with plane reflector layers and plane cavity layers separating the reflector layers, wherein the layers are arranged parallel to the placement surface, and a thickness of the cavity layers is determined depending on a wavelength range to be transmitted for capturing impressions of the plurality of blood-perfused skin regions of human autopodia and on an angular range to be selected by the cavity layers for this wavelength range.

9. The device according to claim 8, wherein the thickness of the at least one cavity layer is variably adjustable by piezoelectric materials.

10. The device according to claim 8, wherein the reflector layers are configured as Bragg mirrors comprising alternating layers of two dielectrics or a metal.

11. The device according to claim 8, wherein the at least one cavity layer between the reflector layers is metallic or dielectric, wherein, when there is more than one cavity layer, metallic and dielectric layers are combinable for an improved selection of the angular range to be selected.

12. The device according to claim 1, wherein the cavity filter or the cavity filters predominantly reflect light which, in case of active display pixels, is emitted by the latter or which, in case of passive display pixels, is emitted by an illumination unit in a direction of the sensor pixels in order to increase the luminous efficiency.

13. The device according to claim 1, wherein each of the sensor pixels is provided with an additional shutter, which reduces the selected angular range, wherein the additional shutters are alternately arranged along rows and columns in which the sensor pixels are arranged.

14. The device according to claim 1, wherein the optical sensor pixels comprise different color sensor pixels which are alternately arranged in columns and rows in which the sensor pixels are arranged.

15. A method of using the device according to claim 1 for distinguishing whether an object placed on the placement surface is an autopodium or a forgery of an impression of an autopodium, wherein the at least one cavity filter is constructed as a layer stack comprising planar reflector layers and planar cavity layers separating the reflector layers, which reflector layers and cavity layers are arranged parallel to the placement surface, comprising: placing an object is placed on the placement surface switching the optical sensor pixels on to detect light, switching the display pixels in such a way that the display pixels emit light of at least a first wavelength λ.sub.R which corresponds to a central wavelength of the at least one cavity filter, wherein the cavity filter is resonant with light of this first wavelength λ.sub.R and is transparent in a first angular range around a direction perpendicular to the placement surface, capturing a first overall image using the sensor pixels, switching the display pixels such that the display pixels emit light of a second wavelength λ.sub.F which is shorter than the first wavelength λ.sub.R, and wherein the cavity filter is transparent to light of the second wavelength λ.sub.F in a second angular range, wherein the second angular range cuts out a cone or conical ring around the direction perpendicular to the placement surface, capturing a second overall image using the sensor pixels, analyzing the first overall image and second overall image with respect to differences in their image characteristics.

16. A method of using the device according to claim 1 for distinguishing whether an object placed on the placement surface is an autopodium or a forgery of an impression of an autopodium, wherein the at least one cavity filter is constructed as a layer stack comprising planar reflector layers and planar cavity layers separating the reflector layers, which reflector layers and cavity layers are arranged parallel to the placement surface, comprising: placing an object on the placement surface switching the optical sensor pixels on to detect light, switching a first set of display pixels in such a way that the first set of display pixels emits light of at least a first wavelength λ.sub.R which corresponds to a central wavelength of the at least one cavity filter, wherein the cavity filter is resonant with light of this first wavelength λ.sub.R and is selective in a first angular range around a direction perpendicular to the placement surface, switching a second set of display pixels in such a way that the second set of display pixels emits light of a second wavelength λ.sub.F which is shorter than the first wavelength, and wherein the cavity filter is selective for light of the second wavelength λ.sub.F in a second angular range, wherein the second angular range cuts out a cone or cone ring around the direction perpendicular to the placement surface, wherein positions of the first set of display pixels and of the second set of display pixels in the display unit are so determined depending on the first wavelength λ.sub.R and second wavelength λ.sub.F and the respective selective angular ranges that, on the one hand, the light emitted by the first set of display pixels and by the second set of display pixels illuminates the same area of the placement surface from below and, on the other hand, light of the first wavelength λ.sub.R which is incident on the sensor pixels from a direction of the placement surface is detected by a first set of sensor pixels and is captured as a first image, and light of the second wavelength λ.sub.F which is incident on the sensor pixels from a direction of the placement surface is detected by a second set of sensor pixels and is simultaneously recorded as a second image, analyzing the first image and second image with respect to differences in their image characteristics.

17. The method according to claim 16, in which, in order to scan the largest possible portion of the placement surface, the area illuminated from below is successively displaced after the first image and second image have been captured so that the first set of display pixels and the second set of display pixels are also displaced and further first and second images are captured and are combined to form a first aggregate image and a second aggregate image.

18. The method according to claim 16, wherein display pixels of the first set of display pixels and display pixels of the second set of display pixels are distributed in a checkerboard manner, and an allocation to the first set or second set of display pixels is carried out based on color filters.

19. The method according to claim 1, wherein the first angular range and the second angular range have an empty intersection.

20. The method according to claim 15, wherein the second angular range includes angles around and close to the angle of total internal reflection at the placement surface.

21. The method according to claim 15, wherein the first wavelength λ.sub.R is selected from a first wavelength range between 500 nm and 550 nm, while the second wavelength λ.sub.F is selected from a second wavelength range between 450 nm and 480 nm, or the first wavelength λ.sub.R is selected from a first wavelength range between 600 nm and 650 nm, and the second wavelength λ.sub.F is selected from a second wavelength range between 500 nm and 550 nm.

22. The method according to claim 15, wherein the first angular range is selected between 0° and 15° and the second angular range is selected between 30° and an angle which is at most 5° greater than a limiting angle of total internal reflection at the placement surface.

23. The method according to claim 15, wherein more than two overall images are captured and compared for each angular range, wherein all of the overall images are captured with different colors.

24. The method according to claim 15, wherein the intensity or characteristics calculated from intensity, preferably a spatial structure or a latent impression of an autopodium, are used as image characteristics.

25. The method according to claim 15, wherein, as further steps, a comparison of the intensity ratios of the at least two images or overall images with a calibrated threshold range is carried out for detecting forged prints, and/or all of the images or overall images are superposed in order to achieve increased resolution or compensate for sensor defects and other errors in the image recording, and/or when an authorized fingerprint is detected one or more functions are enabled in an appliance in which the device is implemented, and/or the appliance is locked when a forged fingerprint is detected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The invention will be described in the following referring to exemplary embodiments in the appended drawings which likewise disclose features essential to the invention. These exemplary embodiments are to be considered as merely illustrative and not restrictive. For example, it is not to be construed from a description of an embodiment example having a plurality of elements or components that all of these elements or components are necessary for its implementation. On the contrary, other embodiment examples may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of various embodiment examples may be combined with one another unless otherwise indicated. Modifications and alterations which are described for one of the embodiment examples may also be applicable to other embodiment examples. To avoid repetition, like or similar elements are designated by like reference numerals in various figures and are not described multiple times. The drawings show:

[0055] FIGS. 1a-1d various basic possibilities for the construction of a device for displaying information and for the simultaneous contact-based capture of impressions of a plurality of blood-perfused autopodia;

[0056] FIG. 2 a schematic diagram of a device with additional illumination;

[0057] FIGS. 3a-3c the basic construction of cavity filters;

[0058] FIGS. 4a-c the angle selectivity of a cavity filter;

[0059] FIG. 5 the modulation transfer function of the device for 2.5 line pairs per millimeter depending on a cover layer thickness for using without a cavity filter and without a shutter (dotted line), with a cavity filter (dashed line) and with a cavity filter and additional shutter (solid line);

[0060] FIG. 6 an array with optical sensors and additional shutters;

[0061] FIG. 7 the effect of the additional shutters using two colors for increasing the spatial resolution;

[0062] FIG. 8 a flowchart for a first method for using the device;

[0063] FIG. 9 a flowchart for a second method for using the device;

[0064] FIG. 10 a detail of the beam path for two different wavelengths; and

[0065] FIG. 11 an overview of the working principle of the second method.

DETAILED DESCRIPTION OF THE DRAWINGS

[0066] FIGS. 1a to 1c show various basic constructions of a device for displaying information on the one hand and for the simultaneous contact-based capture of impressions of a plurality of blood-perfused skin regions of human autopodia on the other hand. All three constructions have a few key components in common: from the point of view of an observer or from direction of a finger to be placed thereon, this device comprises a placement surface 1 on which the autopodia are placed and under which a protective layer 2 is arranged. The placement surface 1 can also form the surface of the protective layer 2. The thicknesses and thickness ratios shown here are not actual and are only illustrative of the individual components. A touchpad 3 is arranged under the protective layer 2. In mobile devices such as smartphones, the touchpad 3 is used for entering control commands with one or more fingers. Further, it detects whether or not a finger is placed on the placement surface 1. However, the touchpad 3 is not compulsorily necessary for the functioning of the device. It can also be determined in some other way, for example, by means of the sensor pixels described in the following, whether or not a finger is placed on the placement surface 1. A display unit 4 is arranged under the touchpad 3 viewed from direction of the contacting skin region. The display unit 4 has at least partially transparent display elements with display pixels emitting light in direction of the placement surface 1, these display elements being arranged in grid form and individually controllable by means of a control unit, not shown here.

[0067] Further, all three configurations comprise optical sensor pixels which detect light that is incident on the sensor pixels from direction of the placement surface 1. In a first alternative, the sensor pixels are arranged in a sensor layer 5 which is arranged under the display unit 4 viewed from the direction of the contacting skin region. This is the case in the devices shown in FIG. 1a and FIG. 1b. In a second alternative, the sensor pixels are arranged in the display unit 4 between the display elements. This configuration is shown in FIG. 1c. Means for angle selection which comprise at least one cavity filter are arranged upstream of the optical sensor pixels in direction of the placement surface 1. In the configuration shown in FIG. 1a and FIG. 1b, the optical sensor pixels are arranged in a sensor layer 5 below the display unit 4. The at least one cavity filter is formed as a cavity filter layer 6 arranged between the display unit 4 and the sensor layer 5. In the configuration shown in FIG. 1c, in which the optical sensor pixels are arranged between the display elements in the display unit 4, one or more pixel-shaped cavity filters are associated with each sensor pixel and arranged above the respective sensor pixel. With thicker cover glasses, it is recommended that a pixel-based shutter structure 7 be inserted between the cavity filter layer 6 and the sensor layer 5, as a result of which the angular range transmitted by the cavity filter can be further limited.

[0068] The display unit 4 can essentially be configured in two different ways. In a first configuration, the display unit 4 comprises actively luminous display pixels and is preferably configured as an LED unit. The display pixels can be formed as LEDs, OLEDs, QLEDs or microLEDs. As has already been mentioned, the display unit 4 is partially transparent to light coming from the placement surface 1. In a second configuration, the display unit 4 comprises passively illuminated display pixels and is configured, for example, as an LC unit with display pixels formed as LC elements. This is shown in FIG. 1d. A first illumination unit with a transparent light guide layer body 8 and first illuminants is provided for illumination. The light guide layer body 8 is arranged below the display unit 4 and above the cavity filter layer 6 viewed from the placement surface 1. The first illuminants can be arranged laterally, for example, and the light thereof can be coupled into the light guide layer body 8 from where it diffusely illuminates the display pixels in a first wavelength region with corresponding out-coupling structures or scattering elements.

[0069] Regardless of whether or not a first illumination unit is provided for illuminating the passively illuminated display pixels, the device can comprise a second illumination unit 9 with second illuminants as is shown schematically in FIG. 2. As indicated by the arrows in FIG. 2, the second illumination unit 9 which can likewise comprise a light guide layer body can be integrated at different locations in the layer stack of the device. The second illuminants are formed for emitting directed light of a determined wavelength in a limited angular range of no more than 20° around a predetermined central direction. The central direction is determined depending on the predetermined wavelength of the directed light and on an angle selectivity of the at least one cavity filter for the predetermined wavelength. The central direction preferably lies parallel to a normal of the placement surface 1. In this way, an improved image quality can be achieved for capturing fingerprints because the intensity arriving at the optical sensors is increased. A further advantage of the second illumination unit is that it can be switched independently from the display screen. It can be variably adapted to different situations, for example, skin color, ambient light or moisture of the skin and/or of the placement surface 1 for capturing fingerprints. The normal output on the display screen is not influenced in this way. With a corresponding configuration of the cavity filter, invisible light—for example, near infrared or near ultraviolet—in particular can also be used for capturing fingerprints. In this way, negative visual effects on the display screen resulting from fingerprint recording are prevented.

[0070] The configuration and action of a cavity filter will be described in the following referring to FIGS. 3 to 5. FIG. 3a shows the basic construction of a cavity filter which is a special type of interference filter. A cavity layer 10 with the approximate optical thickness on the order of the light wavelength to be transmitted, that is, with which the cavity filter is resonant, is embedded between two reflector layers 11. By inserting the cavity layer 10, cavity modes are formed which allow light in this wavelength range or band to pass through the reflector layers 11. The thickness of the cavity layers 10 can be controlled during production and is determined depending on a wavelength range to be transmitted for capturing impressions of a plurality of blood-perfused skin regions of human autopodia and on an angular range to be selected by the cavity layers 10 for this wavelength range. Further, it is also possible to variably adjust the thickness of the cavity layer 10, for example, by means of piezoelectric materials. In particular, the cavity filters can also be configured as dual-cavity filters or multi-cavity filters. The more cavity layers 10 the filter contains, the sharper the delimitation of the transmitted wavelength range from the shape of a Lorentz curve to a box shape. A dual-cavity filter is shown, for example, in FIG. 3b. The dots on the bottom indicate that further cavity filter layers may follow. The cavity filter layer 10 can be formed as a metal or a dielectric. In the case of a metal layer, for example, of aluminum, it has a thickness of approximately 100 nm. If a plurality of cavity layers 10 are used, an improved selection of the angular range to be selected can be determined.

[0071] The construction of a reflector layer 11 is shown in detail in FIG. 3c. High-index layers 12 of a material with a high refractive index alternate with low-index layers 13. Each layer has the optical thickness of n×d=λ/4, where n is the refractive index, d is the actual thickness, and λ is the wavelength of the light which is to be transmitted and with which the cavity filter is accordingly resonant. The cavity layer 10 often has an optical thickness of λ/2. The reflector layers 11 can be formed as Bragg mirrors from alternating layers of two dielectrics or metals or combinations of dielectrics and metals. The advantage in using exclusively dielectric reflectors compared to metal reflectors consists in that the reflectivity is more than 99% higher and asymptomatically approaches 100% with an increasing quantity of layer pairs. Absorption is virtually nonexistent. When a metallic cavity layer 10 is used in addition, sidebands which occur in all-dielectric reflector layers and which can possibly lead to interference signals for the optical sensors can also be efficiently suppressed. Because of the high reflectivity, the cavity filters can be used to increase the luminous efficiency by predominantly reflecting light which, in case of active display pixels, is emitted by the latter or, in case of passive display pixels, is emitted by an illumination unit in direction of the sensor pixels.

[0072] For use in connection with fingerprint checking, an otherwise insignificant property of the cavity filter is utilized, namely, the capability of allowing the transmission of wavelengths only in particular angular ranges, i.e., an angle selection depending on wavelength. As will be explained in the following referring to FIG. 4 and FIG. 5, the use of cavity filters on the one hand allows the choice of a greater distance between the placement surface 1 and the optical sensor layer 5 so that the optical sensor layer 5 can easily be arranged as the bottommost layer of a display screen unit and, therefore, need not be transparent and does not negatively affect the imaging of the normal display. On the other hand, the detection of forgeries relating to fingerprints is facilitated.

[0073] A layer stack which is formed only of reflector layers 11 reflects any light substantially in an angular range from 0° to approximately 40° with respect to the normal of the surface. Inserting the cavity layer 10 between two reflector layers 11 at a resonant wavelength for which the cavity layer 10 is designed leads to an increased transmission through the reflector layers 11. Resonance exists when the light wave is in phase with the starting point after being reflected twice at the reflector layers 11 and after operation through the cavity layer 10. The spectral permittivity of the cavity mode shows a strong angular dependence and shifts to shorter wavelengths because of the altered phase length at oblique incident angles of the light. The displacement in dependence on the angle of incidence increases as the effective refractive index n* of the dielectric structure decreases. For a high-index cavity layer, the effective refractive index n* is given by

n*=√{square root over ((n.sub.Hn.sub.L))}  (1),

where n.sub.H is the refractive index of a reflector layer configured as high-index layer 12 and n.sub.L is the refractive index of a reflector layer configured as low-index layer 13, and for a low-index cavity layer is given by

[00001]$\begin{array}{cc}{n}^{*}=\frac{n_L}{\sqrt{1\frac{n_L}{n_H}+{\left(\frac{n_L}{n_H}\right)}^2}},& \left(2\right)\end{array}$

where the angular dependence Δλ(θ.sub.0) of the transmission can be described approximately by the term:

[00002]$\begin{array}{cc}\frac{\Delta \lambda }{{\lambda }_0}=\frac{n_0{\theta }_0^2}{2n^{*2}},& \left(3\right)\end{array}$

where θ.sub.0 is selected in radiants and no corresponds to the refractive index of the environment of the filter by which is meant the adjoining materials, typically glass in the present application. The same dielectric material as that used for the high-index or low-index reflector layers can be used for the high-index and low-index cavity layers so that at least two materials with different refractive indices are needed for the construction of a dielectric cavity filter.

[0074] The angular dependence Δλ(θ.sub.0) of the transmission of a cavity filter is shown by way of example in FIG. 4a. In the example shown here, the cavity filter is transmissive in the shaded area and has a resonant wavelength range Δλ.sub.R at approximately 550 nm. For this wavelength, it is transmissive in a narrow conical angular range of 0° to approximately 15° as is shown in FIG. 4b. The shaded region corresponds to the transmission range for the resonant wavelength range. Since the cavity filter layer 6 is arranged directly above the sensor layer 5, the angular range for the light which is detected in the optical sensor layer 5 can be sharply limited. For light sources which only emit in a narrow spectral band or even only emit monochromatic light the angular range which is detected by the optical sensors can be limited to approximately 10° around the surface normal of the placement surface 1. The point spread function PSF is also correspondingly reduced. This in turn has an effect on the modulation transfer function MTF which corresponds to the amount of the Fourier transform of the PSF. The MTF is a measurement of the dependence of normalized modulation on spatial frequency and can be utilized to determine the spatial resolution of a predetermined configuration. In particular, a maximum distance of the optical sensor layer 5 from the placement surface 1 beyond which fingerprints can no longer be resolved can be determined in this way. This matters in the conception of display screens for mobile devices which are to be capable of identifying fingerprints and, therefore, so as not to impair the image quality of contents to be displayed on the image screen because the sensor layer is to be arranged as the bottommost layer. Because of their position as bottommost layer, the sensors need not be transparent or semitransparent and do not pass any display illumination. Accordingly, the requirements for the optical sensor are looser, and less expensive sensors such as, for example, CMOS-based sensors or organic photodiodes, can be used instead of, e.g., TFT-based sensors on glass substrates.

[0075] For the human fingerprint, a period length of approximately 400 μm is measured from ridge to ridge, which corresponds to a spatial frequency of k=2.5 line pairs per millimeter. The point spread function which is given by the reflection of a typical display illumination—i.e., without the use of a cavity filter—on the placement surface 1 yields the curve—shown by a dotted line in FIG. 5—of the normalized modulation depending on the cover layer thickness, i.e., the thickness of all of the material layers located between the placement surface 1 and the sensor layer 5. It has been shown that the modulation approximates zero above 300 μm, which makes fingerprints no longer clearly detectable. If a resonant cavity filter is introduced which realizes an angle selection in the range of the resonant wavelength to angles of less than 10° relative to the surface normal, the curve shown in a dashed line results for the normalized modulation. Distinct contrasts are also still measured at cover layer thicknesses above 800 μm. When pixel shutters are additionally arranged for the optical sensor pixels as will be described below referring to FIG. 6, the curve of the normalized modulation shown by the solid line results, i.e., the possible cover layer thickness can be further increased by a factor of 2 so that distinct contrasts can still be measured with the additional shutter and the cavity filter even with cover layer thicknesses greater than 1 mm.

[0076] By using the additional shutter structure as shown in FIG. 6, the angular range can be limited even more and the spatial resolution can therefore be increased. The limiting is carried out in that the shutter structure—referring to the spherical angular space—only passes angles from one half of the upper hemisphere; the lower hemisphere would correspond to illumination from below. The array of sensor pixels 14 is shown. A detection area 15 is arranged in each sensor pixel. Each sensor pixel 14 is provided with a shutter 16 so that, for each pixel, light can impinge on the pixel only from the direction of the respective arrow shown in the drawing so that the spread of the angular spectrum can be halved, which makes it possible to double the cover layer thickness. Further, the shutters 16 cover the detection areas 15 almost completely in the direction of the arrows. In order to compensate for the generation of an asymmetrical MTF, the shutters 16 are arranged in an alternating manner as is shown in FIG. 6. However, the maximum possible measureable spatial frequency, or Nyquist frequency as it is called, is at least halved in this way. In order to mitigate or circumvent this limitation, the cavity filter or the angle selectivity of the cavity filter must be configured in such a way that recordings with at least two colors λ.sub.1, λ.sub.2 can be detected, where a zone can be scanned on the placement surface for one color λ.sub.1 that is not scanned for the other color λ.sub.2. This relationship is shown in FIG. 7. In this instance, d designates the total cover layer thickness between the sensor layer and the placement surface 1 on which—in this instance—one finger 17 is placed. In order to make this increased scanning possible, illumination must be carried out sequentially in case of a monochromatic optical sensor and two images must be captured. This restriction is omitted for color sensors, but the color sensor pixels are usually arranged in an alternating manner in this instance or correspondingly determined patterns so that the Nyquist frequency is lower from the outset.

[0077] Because of the angle selectivity of the cavity filter, it is possible to configure the cavity filter in such a way that recordings can be made in two colors. However, it must be taken into account that the cavity filter is transparent to light of this second color in a different angular range, which is also a precondition for scanning two different zones on the placement surface with light of two colors. The different angle selectivity for different colors is also explained referring to FIG. 4a. A further wavelength range Δλ.sub.F around a second wavelength λ.sub.F is labelled above the wavelength range Δλ.sub.R around the resonant wavelength. This second wavelength λ.sub.F is shorter than the resonant wavelength and, in the example, resides in the blue region at about 460 nm, whereas the resonant wavelength resides in the green region at about 550 nm. For this further wavelength range, the cavity filter is only transparent in an angular range between approximately 25° and 40°, i.e., does not overlap with the angular range for which the cavity filter is transparent in the resonant wavelength. This can be utilized for detecting fake fingerprints as will be explained below referring to FIGS. 8 to 11.

[0078] The device described above can be used to differentiate between an object (finger) placed on the placement surface 1 and a forgery of a fingerprint. A first method for detecting fakes is described referring to FIG. 8. A precondition for the flow of the process is the placing of an object—either the finger or the forgery—in a step 210. In the next step 220, the device detects—for example, by means of a touchpad—that an object is placed thereon. In a step 230, the optical sensor pixels in the sensor layer 5 are switched to detect light. In step 240, the display pixels of the display unit 4 are switched such that they emit light of at least a first wavelength λ.sub.R which corresponds to a central wavelength of the at least one cavity filter. The cavity filter is resonant with light of this first wavelength λ.sub.R and is transparent in a first angular range around a direction perpendicular to the placement surface 1. In a subsequent step 250, a first overall image is captured by means of the sensor pixels. In the next step 260, the display pixels are switched such that they emit light of a second wavelength λ.sub.F which is shorter than the first wavelength λ.sub.R. The cavity filter is transparent to light of the second wavelength λ.sub.F in a second angular range, which second angular range cuts out a cone around the direction perpendicular to the placement surface 1. In particular, the first angular range and the second angular range can have an empty intersection. In the subsequent step 270, a second overall image is captured by means of the sensor pixels. There follows a step 280 in which the first overall image and second overall image are compared with respect to differences in the image characteristics thereof. In particular, intensity or characteristics calculated from intensity, for example, a spatial structure or a latent print of a finger, can be used as image characteristics. The result is queried in a step 290. If the authenticity of the fingerprint is confirmed, the display screen can be cleared for further input, for example, in step 291. If a fake is detected, this can be indicated, e.g., in step 292, and the mobile device can be locked against further input. Steps 291 and 292 serve only as examples; further functions—in particular those connected to the enabling of actions—can be associated with step 291 and subsequent steps.

[0079] For the detection of forgeries, it is advantageous when the second angular range for the second wavelength λ.sub.F includes angles which are smaller than the angle of total internal reflection at the placement surface 1, since no light can be transmitted at angles >42° by many displays because of an integrated air layer due to total internal reflection at the interface. When an actual finger is placed on the placement surface 1, the differences between the first overall image and the second overall image are clear. While the first wavelength λ.sub.R can be used for imaging the fingerprint in the first overall image, the detected signal decreases significantly during illumination with the second wavelength λ.sub.F because the total internal reflection is frustrated or significant portions of light are also coupled out at angles close to total internal reflection. In contrast, there is no drop in intensity detectable in a forgery when illuminated with the second wavelength λF.

[0080] Of course, it is also possible to capture and compare more than two overall images per angular range, and all of the overall images are captured with different colors. This increases the accuracy in determining whether or not a forgery is present.

[0081] Since a plurality of images must be captured sequentially in the method described above, this could possibly be perceived as a disadvantage by users of a mobile device. In order to accelerate the detection of forgeries, the method described above can be somewhat modified and a plurality of images can then be captured simultaneously. This will be explained in the following referring to FIG. 9. Method steps which are identical to the method described above and in FIG. 8 have the same reference numerals and will not be explained again. In contrast to the method described above, not all of the display pixels are switched in the same way after the optical sensor pixels for detecting light have been switched on in step 230; rather, in step 340 a first set of display pixels is switched in such a way that the first set of display pixels emits light of the first wavelength λ.sub.R, and in step 350, which can be executed simultaneous with step 340, a second set of display pixels is switched in such a way that the second set of display pixels emits light of the second wavelength λ.sub.F. The conditions for the first wavelength λ.sub.R and the second wavelength λ.sub.F are the same as those described above as relates to their tuning with respect to the cavity filter and the first and second angular ranges. However, the positions of the first set of display pixels and of the second set of display pixels in the display unit 4 are so determined, depending on the first wavelength λ.sub.R, the second wavelength λ.sub.F and the respective selective angular range, that, on the one hand, the light emitted by the first set of display pixels and second set of display pixels illuminates the same area of the placement surface 1 from below and, on the other hand, light of the first wavelength λ.sub.R which is incident on the sensor pixels from the direction of the placement surface 1 is detected by means of a first set of sensor pixels and is captured as first image in step 360, and light of the second wavelength λ.sub.F which is incident on the sensor pixels from direction of the placement surface 1 is detected by a second set of sensor pixels and is captured simultaneously in step 360 as second image. In step 83, the first image and second image are analyzed with respect to differences in their image characteristics before executing further steps which depend on a result of the analysis. These further steps correspond to those which were described in connection with FIG. 8.

[0082] In contrast to the method shown in FIG. 8, no overall images are captured in the method shown in FIG. 9 but rather only images showing a section of the total placement surface 1. This has to do with the fact that, while the same area of the placement surface 1 is illuminated by both wavelengths, this area is detected by means of different sensor pixels so that twice as many sensor pixels are needed for capturing a section compared to the first method described with reference to FIG. 8, since different sensor pixels as well as different display pixels are required for illumination and detection with the two wavelengths. This will be described in the following referring to FIG. 10 and FIG. 11. FIG. 10 shows the basic illumination conditions. Light of the resonant wavelength λ.sub.R is emitted by a first pixel in the display unit 4 as shown by the dotted line. The emission is effected in all possible angles, but only light which impinges on the cavity filter in the narrow angular range around the surface normal of the placement surface 1 as symbolized by the dotted arrows is passed through to the sensor pixels in the sensor layer 5 because of the cavity filter layer 6. A further pixel in the display unit 4 emits light of the second wavelength represented here by the dashed line. This light is also emitted in all possible angles, but only light in the second angular range is allowed by the cavity filter layer 6 to pass through to the sensor layer 5 as symbolized by the dashed-line arrows. Accordingly, one and the same point on the placement surface 1 can be simultaneously scanned with different colors and detected at different places on the sensor. The regions of the sensor layer 5 which correlate with a location on the placement surface 1 depend on the cover layer thickness between sensor layer 5 and placement surface 1 and on the distance between the display pixels and the placement surface 1. Small groups of adjacent display pixels can also be switched together and used together for illumination so that the amount of light detectable on the sensor is increased. The signal can then be averaged, for example, provided there is no overlapping of the reflections of the different light colors on the sensor layer or sensor pixels.

[0083] FIG. 11 illustrates this procedure once again for a larger area of the device. In this instance, the display unit 4 is symbolized by its display pixels and the sensor layer 5 is symbolized by its sensor pixels. Relatively large surface areas of the placement surface 1 can be captured in this way, the display unit being controlled in such a way that every sensor pixel can detect, at most, only light of a single color. This rules out an overlapping of colors, and more information can be received from one and the same object point on different sensor pixels. The size x.sub.2 of the area depends on the distance d.sub.1 between the display unit 4 and the placement surface 1, the distance d.sub.2 between the sensor layer 5 and the display unit 4 and the angular range for which the cavity filter or cavity filter layer 6 is transparent to the second wavelength λ.sub.F. In order to scan the largest possible portion of the placement surface 1, the area illuminated from below can also be successively displaced after the first image and second image have been captured, as a result of which the first set of display pixels and second set of display pixels are also displaced. Further images are then captured and are combined to form a first aggregate image and a second aggregate image. This is indicated by the dashed arrow in FIG. 9. Alternatively, the display pixels of the first set of display pixels and the display pixels of the second set of display pixels can be distributed in a checkboard manner, and allocation to the first set of display pixels or second set of display pixels is carried out based on color filters or by means of a corresponding control.

[0084] Another possibility for circumventing sequential recordings is to use color sensors which only detect light for determined spectral ranges instead of expensive monochromatic optical sensors. The optical sensor pixels then comprise different color sensor pixels which are alternately arranged in columns and rows in which the sensor pixels are arranged. If the color to be detected is adapted to the transparent region of the cavity filter, a plurality of overall images can be imaged in a recording and evaluated.

[0085] The comparison of image characteristics after the images have been captured can be used not only to detect forged fingerprints but also, for example, to achieve an increased resolution and compensate for sensor defects or other errors in the image recording by means of superimposing the images or overall images.

[0086] As has already been mentioned, the cavity filter can be designed for a plurality of different angular ranges and wavelength ranges, which should preferably be selected in such a way that the wavelengths are far enough apart to exclude an overlapping of angular ranges.

[0087] By means of the device described above and the method described above, an authentication of multiple fingers, for example, can be integrated in mobile telephones without impairing the quality of the display of information on the display screen. Also, almost the entire surface of the device is available for displaying information because the entire surface of the display screen can be used for detecting the fingerprint and no area need be reserved exclusively for detecting prints. The detection of fingerprints is carried out inconspicuously and is hardly noticed by the user. Integration in thicker display screens is also possible through the use of cavity filters.

REFERENCE NUMERALS

[0088] 1 placement surface [0089] 2 protective layer [0090] 3 touchpad [0091] 4 display unit [0092] 5 sensor layer [0093] 6 cavity filter layer [0094] 7 shutter structure [0095] 8 light guide layer body [0096] 9 second illumination unit [0097] 10 cavity layer [0098] 11 reflector layers [0099] 12 high-index layer [0100] 13 low-index layer [0101] 14 sensor pixel [0102] 15 detection area [0103] 16 shutter/additional shutter [0104] 17 finger