Method and arrangement for touchless detection of data of uneven surfaces

[0001] The present invention relates to a method and an arrangement for detection of data of uneven surfaces, in particular for acquisition of biometric data at faces and fingers, using a light source for illumination of the uneven surface, an imaging optical system and an analyzing facility for electronic image processing.

[0002] The acquisition of biometric data is required for many applications. In particular, for the identification of persons and for control of authorization, the accurate and as forgery-proof as possible acquisition of this data is required. Because of their manifold advantages, touchless methods are often used thereby.

[0003] On this, a set of possibilities is described in the prior art.

[0004] According to European Patent Document EP 0 194 783 B1, a device for detection of data of uneven surfaces is known, where a transparent plate exhibits two opposing surfaces, wherein an uneven surface having recesses and protrusions, which is to be detected, is pressed against one of the even surfaces, comprising a light source for illumination of the uneven surface at the contact section through the transparent plate and an optical element for guidance of the light, which is scattered by the uneven surface, to the light detector.

[0005] The light source is arranged in such a way that it illuminates the contact section at an illumination angle which is smaller than a critical angle, in order to that the light incident from the source onto the contact section is not totally reflected, whereby a part of the light, which is scattered at the protrusions of the uneven surface, is undergoing an inner total reflection at least at the second surface of the plate, in order to propagate in a direction against the optical element, whereas the light which is scattered at the recesses is undergoing none such inner total reflection, whereby the two kinds of scattered light are separated spatially, wherein the optical element is arranged to guide only light scattered at the protrusions to the detector.

[0006] Furthermore, in European Patent Document EP 0 359 554 B1, an arrangement for the determination of fingerprints is described, with which zones of the finger are imaged to a light receiver. The arrangement possesses a light source, means for guiding the light rays which are running from the light source onto the surface of the finger to be detected, an optical imaging system which creates an image of an irradiated part of the sample, a light detector facility for detecting the image and a facility for the output of a detection signal. Using the arrangement, it shall be determined if the sample is a biological object or an imitation. Therefore, the light detector is provided with a light receiver area which is divided into zones so that an image of the irradiated part is created on the light receiver area. The light detector possesses separate optical outputs for the plurality of light rays received respectively. The discrimination if it is the matter of an authentic object, i.e. a vital finger, or an imitation is carried out by analyzing the course of the light, wherein the phenomenon that, in case of an authentic finger, the light partly enters the finger and, in case of an imitation, does not enter into it so that different courses of light result, is exploited.

[0007] In addition, according to European Patent Document EP 1 073 988 B1, a system for recognition of hand and finger lines is known, which serves for the identification of persons. Using this system, hand and/or finger lines, patterns of dermal ridges, patterns of the subcutis or the like are acquired optically for acquisition of an image by using a light source, a polarization filter and a camera without mechanical movements of the arrangement. The optical acquisition is carried out by means of polarization filters placed in the optical path of illumination and in the optical path of imaging and by means of a fixedly arranged camera.

[0008] In the case of the known arrangements, it is disadvantageous that the contrast for the imaging of the structures is low so that a secure analysis of the images is made very difficult.

[0009] An object of the present invention is to provide a method and an apparatus of the initially said type which enable touchiessly scanning of uneven surfaces, in particular of a relief made up of dermal ridges, and creating an image of the surfaces true to original with high contrast.

[0010] The present invention provides a method for recognition of biometric data, in particular for the recognition of characteristics of fingers and of faces of persons, comprising: illuminating at least one of stripes and a grid on the uneven surface using at least one light source; detecting, without a touching contact, light reflected from the uneven surface at a plurality of discrete locations so as to create a partial image of the uneven surface at each of the plurality of discrete locations; selectively analyzing each of the partial images; and combining at least portions of the partial images into an overall image of the uneven surface. The present invention also provides an arrangement for a touchless detection of data of an uneven surface of an object, comprising: an imaging optical system including an electronic camera oriented perpendicular to the uneven surface; a plurality of line-shaped light sources for illuminating the uneven surface disposed in row on each side of the electronic camera; an analyzing unit for electronic image processing including an electronic control unit couple to the electronic camera and configured to assign a partial image corresponding to each of the plurality of light sources and to process the partial images into an overall image.

[0011] Advantageous embodiments are described in the specification and in the claims.

[0012] The present invention preferably serves for the optical scanning of dermal ridges. The dermal ridges are embossed and make up a characteristic relief. They distinguish themselves by a higher gloss than the adjacent spacings. By the glossy surface, the light is reflected directionally to a large fraction, wherein the coloration of the reflected light is small. In contrast, the spacings reflect diffusely with a red coloration.

[0013] As glossy surfaces reflect directionally, i.e. the light is reverberated according to the law of reflection, this means that light sources mirror themselves in the reflecting area. The dermal ridges thus mirror the light coming from the light source. To achieve a true-to-original and high-contrast image of the ridges, it has to be ensured that only a top edge of the ridge is imaged. As the slope of the finger ridges varies up to about 50° along the finger, for the illumination of the outer ridges which are sloped to a higher extent, the light source is placed differently than the light source intended for the illumination of the object's center.

[0014] Conveniently, the camera is arranged perpendicularly to the object so that the light source serving for illumination should be arranged directly next to the camera. However, the camera placed in such a way does not illuminate the outer ridges at their top edges anymore, but rather laterally of those edges. This causes an apparent deformation of the ridges in the image. In order to illuminate the border areas of the object, the light sources are therefore arranged farther outwards.

[0015] An advantageous embodiment of the present invention uses the light's color with which the object is illuminated in order to increase the contrast of the image.

[0016] If the object is illuminated directionally using white light, the finger ridges appear glossy white, but the spacings diffusely red. By color filtering during the imaging and/or by colored illumination, the contrast can be raised.

[0017] In the method according to the present invention, an object to be detected is illuminated in such a way that, in each area of the object, the top edge of the dermal ridges reflects.

[0018] In order to identify a finger, the illumination is carried out in such a way that the surface normal in each area of the finger makes up the bisecting line of the angle between the illumination and the camera's objective with the finger's axis as the angular point. But the light source shall not be extensive thereby, but shall not exceed an extent of 15 mm transverse to the finger's axis.

[0019] The method can be realized using different arrangements. In particular, such arrangements are suitable which perform the following functions:

• • [0020] moving the illumination on a path arranged around the object,
  • [0021] moving the camera on a path arranged around the object,
  • [0022] arranging several cameras around the object,
  • [0023] arranging several switchable illuminations,
  • [0024] arranging several differently colored illuminations and several color-selective cameras,
  • [0025] arranging several differently colored illuminations and one color camera in an arrangement, wherein the above condition of angle is satisfied.

[0026] The spacings between the single dermal ridges are not glossy indeed, but also not dark. In the image, they appear colored red. However, in red light, the structure of the object is hardly recognizable. The contrast of the image can be further increased by creating an image congruent to the images of the dermal ridges in the red or infrared spectral range respectively and subtracting this image from the structure image as a uniform darkness value.

[0027] Using the arrangement according to the present invention, a preferably large part of the object is illuminated by the given arrangement in such a way that the light source mirrors itself always exactly in the top edge of the dermal ridge. Only in this case, a true-to-original image results.

[0028] If the illumination was carried out using a sufficiently large light source it would indeed be ensured everywhere that the top edge of the ridges mirror any part of the light source. But the side surfaces shine, too, as the remaining parts of the light source responsible for other areas illuminate them. In this case, the dermal ridges appear broad, the contrast between them can get lost completely and the borders of the ridges are imaged in a deformed manner.

[0029] It is possible to design the light source in a way such that only light rays are emitted which meet in an axis of the object. This can be achieved, for example, by using parallel light which is focussed by means of a cylindrical lens. Admittedly, this requires a certain effort. Furthermore, the object needs to be placed exactly centrically.

[0030] In order to prevent disturbing light from falling onto the side surfaces, the light source can be moved or the light sources are switched on one after the other. Then, for each situation, a separate image is taken.

[0031] An advantageous embodiment is designed to arrange several cameras of whose single images an overall image is composed which uses only the accurately illuminated image parts. The difficulty occurring thereby, that the boundaries do not fit exactly because of the unavoidable differences of the single perspectives, can be solved using an appropriate software.

[0032] Therefore, it is advantageous to create a separate image using only one camera for each light source and to concatenate the respective accurate areas of the single images to an overall image. Thereby, a large fraction of the images is not required.

[0033] This is achieved, for example, by using a CMOS camera, from which each arbitrary pixel can be read out selectively separately. Then, for each lighting situation, one can sequentially read out the well illuminated area of the matrix, as it is known where this area must lie for each light source. Therewith, creating the image is much faster than creating an overall image by joining together. Practically, this can be as fast as if one overall image is taken. During the imaging, the illumination is just switched synchronously through the lighting situations.

[0034] Another embodiment uses colored light sources so that the separation is carried out via the single color excerpts. So here, after taking the image, from each color excerpt the optimally illuminated area is taken out and an overall image is created in a frame buffer as black and white image. It is advantageous here that the procedure is performed simultaneously. The effort in software and the lower resolution of the color camera have to be regarded to be disadvantageous.

[0035] An especially advantageous embodiment for the recognition of fingers is constituted by an arrangement, wherein a successive switching of the lighting situation is carried out and a CMOS camera is read out synchronously and selectively. Advantageously, the camera is arranged or the reading out is designed, respectively, in such a way that areas parallel to the finger's axis are always read out simultaneously. Purposefully, the light source is also arranged longitudinally parallelly thereto. Thus, the finger is scanned synchronously and selectively line by line, wherein the lines are aligned parallelly to the finger's axis.

[0041] In case of the arrangement depicted in FIG. 1, the finger 1 to be scanned is illuminated using a light source consisting of eight LED rows 3 and imaged using a centrally arranged camera 2.

[0042] The LED rows 3.1 to 3.4 and 3.1′ to 3.4′ are arranged in an approximately circular path around the finger 1, wherein adjacent LED rows always exhibit different colors, respectively. The colors repeat after a distance in which no mutual interaction takes place. In the present embodiment, LED rows of four different colors 3.1 to 3.4 and 3.1′ to 3.4′ respectively, are used. The camera 2 is provided with a facility which creates a separate image for each color.

[0043] It is also possible that the light sources are arranged in an array of light emitting diodes.

[0044] In FIG. 2, an advantageous embodiment is depicted, where, between the LED rows 3.1 to 3.4 and 3.1′ to 3.4′ respectively, an additional LED row 3.R which emits red light is arranged between adjacent rows respectively. Therewith, in the camera 2, a fifth color excerpt image can be created which exhibits a weak structure only and serves as a reference of brightness for calculationally joining together the single color excerpts to an overall image. Using the camera 1, for each light source a separate image is created and the single images are concatenated to an overall image. This joining together is carried out using an electronic imaging unit not depicted here.

[0045] Preferably, for camera 1 a CMOS camera is used at which the pixels can be read out selectively and can be processed further. For each LED row 3, the matrix area which is assigned to the appropriate edge is read out successively. During the imaging, the lighting situations are synchronously switched through.

[0046] FIG. 3 shows an embodiment in which several cameras 2 are used. In this variant, an overall image is composed of the single images gained by all cameras 2, by means of the electronic imaging unit, wherein the overall image uses the accurately illuminated image parts only.

[0047] Thereby, illumination and/or receivers can be merged by means of beam splitters.