Method and arrangement for low-distortion recording of intensity patterns occurring on a contact surface through frustrated total reflection

• US 7,130,456 B2
• Filed: 12/13/2002
• Issued: 10/31/2006
• Est. Priority Date: 12/14/2001
• Status: Active Grant

• Current Assignee: Cross Match Technologies GmbH (Assa Abloy AB)
• Original Assignee: Smiths Heimann Biometrics Gnbh
• Inventors: Hillmann, Juergen
• Primary Examiner(s): COUSO, YON JUNG
• Application Number:
  US10/319,178
• Publication Number:
  US 20030111534A1
• Time in Patent Office: 1,418 Days
• Field of Search: 382/124, 382/127, 382/312, 382/324, 382/300, 235/454, 348/207.99, 348/147, 359/15, 359/700.71, 356/71
• US Class Current: 382/127
• CPC Class Codes: G06V 40/1347 Preprocessing; Feature extraction

Abstract

A method and an arrangement for recording intensity patterns occurring on a contact surface by frustrated total reflection, particularly for low-distortion recording of relatively large-area fingerprints, handprints or footprints or other parts of the skin. It is an object of the invention to find a novel possibility for recording intensity patterns occurring on a support surface according to the principle of frustrated total reflection which achieves a high-resolution, low-distortion image of the intensity pattern using simple optical arrangements. This object is met, according to the invention, in that a support surface for the measurement object is illuminated homogeneously by a light source at an angle of total reflection, an image of the support surface is imaged on the image sensor as an intensity pattern only by of an imaging optical system, so that the image is imaged on the image sensor so as to be distorted but sharp in all parts of the image, and light from the light source which is totally reflected at the support surface enters parallel to the axis of the optical system, the image of the support surface is scanned by the image sensor with substantially more image elements (pixels) than those required for the desired resolution in the output-side final image of the intensity pattern, and the optoelectronically converted, trapezoidally distorted image is rectified by displacement, interpolation and averaging operations based on the generation of combined pixel data from a data surplus occurring as a result of the increased quantity of recorded pixels compared to the necessary quantity of pixels in the final image.

Claims

1. An arrangement for low-distortion recording of intensity patterns occurring on a support surface through frustrated total reflection, comprising:

a light source;

a prism having a support surface; and

an image sensor;

said support surface illuminated by the light source being imaged on the sensor in such a way that light from the light source which is totally reflected at the support surface and whose total reflection is partially frustrated though contact with adjacent areas of a measurement object being imaged on the image sensor as an intensity pattern of the support surface;

only an imaging optical system being provided for imaging said support surface on the image sensor;

said image sensor being inclined opposite to the inclination of the support surface with respect to the axis of the optical system, so that an image of the support surface which is distorted but sharp in all parts of the image is present on the image sensor;

said image sensor being capable of recording a quantity of image elements (pixels) that is substantially greater than the quantity that would be necessary for a resolution required in an output-side final image of the intensity pattern so that there is a data surplus with respect to the final image; and

a processing unit following said image sensor for electronic or mathematical rectification of image data by changing pixel centroid data based on displacement processes and/or operations for interpolating and averaging from the data surplus, wherein a rectification is carried out with low losses as a result of the number of pixels which is increased over the necessary number of pixels of the final image of the intensity pattern.

2. The arrangement according to claim 1, wherein the support surface, insofar as it has different measurements in the two dimensions of the plane, has a longitudinal direction and a transverse direction and the longitudinal direction is the larger dimension, is oriented with the longitudinal direction parallel to a reflection plane defined by the incident and reflected light and extends perpendicular to the reflection plane, so that the image of the support surface fills an approximately square area of the image sensor, and the image of the support surface is imaged on the image sensor so as to be compressed in longitudinal direction and trapezoidally distorted in transverse direction.

3. The arrangement according to claim 1, wherein the scanned image of the support surface has a quantity of pixels in both dimensions that is greater by a factor of 1.1 than that quantity required for the desired resolution in the rectified final image.

4. The arrangement according to claim 1, wherein the density of pixels that can be scanned by the image sensor in one dimension of its receiver surface which is inclined relative to the optical axis of the optical system and associated with the longitudinal direction of the support surface is higher than the density of pixels that can be scanned in the other dimension.

5. The arrangement according to claim 4, wherein the image sensor has a light-sensitive receiver surface with different densities of the light-sensitive elements in the two dimensions, wherein the receiver surface is a matrix and has a higher density of light-sensitive elements in a dimension associated with the longitudinal direction of the support surface than in the other dimension.

6. The arrangement according to claim 4, wherein the image sensor has a receiver surface with uniform pixel spacing in both dimensions, and the receiver surface is a matrix and is displaceable by fractions of the matrix raster (dx;

dy) at least in one dimension associated with the longitudinal direction of the support surface in order to implement at least one subpixel scanning in this dimension.

7. The arrangement according to claim 3, wherein the quantity of scanned pixels of the image is adjustable in any desired manner with respect to the longitudinal direction and transverse direction of the support surface by a two-dimensional microscan process in the quantity of subpixel scans of the image.

8. The arrangement according to claim 4, wherein the quantity of scanned pixels of the image with respect to the longitudinal direction and transverse direction of the support surface is adjustable in any desired manner by a two-dimensional microscan process with a different quantity of subpixel scans of the image, wherein an increased quantity of scanning steps in the dimension of the image associated with the longitudinal direction of the support surface is adapted to the compression of the image in this dimension.

9. The arrangement according to claim 4, wherein the image sensor has a receiver surface with different densities of light-sensitive elements in both dimensions, wherein a receiver line is arranged in the dimension of the receiver surface associated with the transverse direction of the support surface and, in the dimension associated with the longitudinal direction of the support surface, this receiver line is moved by a scanning step size that is adapted to the reduction of the image in longitudinal direction of the support surface.

10. The arrangement according to claim 9, wherein the receiver line is moved by a successively decreasing scanning step size that is adapted to the actual compression of the image.

11. The arrangement according to claim 4, wherein the quantity of pixels in the image which are scanned by the image sensor in relation to the ratio of transverse direction to longitudinal direction of the support surface has a ratio of between 0.5 and 1.

12. The arrangement according to claim 1, wherein prisms with different refractive indexes are provided in order to adapt different ratios of longitudinal direction to transverse direction of the support surface to the dimensions of the receiver surface of the image sensor, wherein different angles of total reflection at the support surface and, therefore, different degrees of compression of the image of the support surface on the image sensor can be adjusted due to the different refractive indexes of selected prisms.

13. The arrangement according to claim 1, wherein beam deflecting elements are provided in the optical beam path between the support surface of the prism and the image sensor to fold the beam path so as to economize on space.

14. A method for recording intensity patterns occurring on a support surface through frustrated total reflection in which the total reflection at the support surface is partially frustrated through contact with adjacent areas of a measured object, comprising the following steps:

homogeneously illuminating a support surface by a light source at an angle of total reflection;

imaging the support surface on an image sensor in such a way that an image of the support surface is generated on the image sensor as an intensity pattern only by an imaging optical system, so that the image on the image sensor is distorted but is sharply imaged in all parts of the image and light from the light source which is totally reflected at the support surface enters the optical system;

recording the image of the support surface by the image sensor with substantially more image elements (pixels) than required for the desired resolution in the output-side final image of the intensity pattern; and

rectifying the optoelectronically converted, trapezoidally distorted image by interpolating and averaging operations based on the generation of combined pixel data from a data surplus occurring as a result of the increased quantity of recorded pixels compared to the necessary number of pixels of a final image to be outputted.

15. The method of claim 14, further comprises step of rectifying the optoelectronically converted, trapezoidally distorted image by electronic timed displacement and interlacing of pixel data streams in the course of reading out and storing an image, wherein the data surplus resulting from the increased quantity of recorded pixels compared with the necessary number of pixels of a final image to be outputted is used for generating corrected pixel correspondences.

16. The method according to claim 15, wherein the image is scanned by a receiver line in scanning steps of different sizes in a first dimension of the image sensor which is inclined relative to the axis of the optical system, wherein the size of the scanning step is changed successively during a scan of the complete image of the support surface in accordance with the actual compression of the image, and the optoelectronically converted image is electronically rectified by line-correct interlacing of pixel data streams and altered timing of the line data streams of lines of the image which have different lengths to condensed lines with uniform pixel numbers.

17. The method according to claim 16, wherein the scanning of the image in the second dimension oriented orthogonal to the axis of the optical system is carried out by a receiver line which is mechanically displaced in the first dimension that is inclined relative to the axis of the optical system.

18. The method according to claim 14, wherein the optoelectronically converted, trapezoidally distorted image from the data surplus resulting from the increased quantity of recorded pixels is rectified solely by calculating combined pixel data corresponding to the desired pixel raster of the final image on the output side based on interpolating and averaging operations.

19. The method according to claim 18, wherein the image of the support surface is scanned by a receiver matrix in at least one dimension of the matrix in subpixel steps, and the electronically converted image is rectified by electronic line-correct and column-correct interlacing and subsequent calculation of corrected pixel data which correspond to the metric pixel number of the desired final image and involve averaging and interpolation of signal percentages of adjacent pixels based on a pixel combination.

20. The method according to claim 19, wherein the image is more tightly scanned in a dimension of the matrix that is inclined relative to the axis of the optical system.

21. The method according to claim 19, wherein the image is scanned by a two-dimensionally uniform subpixel displacement of the matrix, wherein the scanning density is selected corresponding to the compression of the image occurring in the dimension of the matrix which is inclined relative to the axis of the optical system so as to be sufficiently great that it exceeds at least the number of pixels required in the final image on the output side when rectifying the compression, and the compression is rectified in the calculation of the combined pixels for the final image.

22. The method according to claim 14, wherein the optoelectronically converted, trapezoidally distorted image is rectified based on the data surplus resulting from the increased quantity of recorded pixels by a combination of electronic displacing processes and calculation of combined pixel data through mathematical interpolating and averaging operations.

23. The method according to claim 22, wherein the image in the first dimension which is inclined relative to the axis of the optical system is scanned with a higher density compared to the second dimension, wherein the image is stretched in said first dimension at least to a substantial extent electronically by suitable scanning and storage processes, and the complete rectification is carried out by calculating combined pixel data.

24. The method according to claim 23, wherein the image is scanned with a receiver matrix having a smaller spacing between the light-sensitive elements in the column direction than in the line direction.

25. The method according to claim 23, wherein the image is scanned with a symmetrical matrix, wherein the matrix executes a subpixel scan by a suitable displacing device in the first dimension which is inclined relative to the axis of the optical system with a greater quantity of scanning steps than in the second dimension.

26. The method according to claim 23, wherein the image is scanned by a receiver line in the second dimension which is oriented orthogonal to the axis of the optical system, and the receiver line is mechanically displaced in the first dimension which is inclined relative to the axis of the optical system, wherein the receiver line is moved by scanning steps which are smaller than the distances between the light-sensitive elements in the line direction.

27. The method according to claim 26, wherein the receiver line is moved in the first dimension by scanning steps which are smaller than the distances between the light-sensitive elements in the line direction and which are adapted successively in step size corresponding to the actual compression of the image.