Method and apparatus for determining the shape, distance and orientation of an object

• US 7,841,719 B2
• Filed: 01/24/2008
• Issued: 11/30/2010
• Est. Priority Date: 02/15/2006
• Status: Active Grant

• Current Assignee: Optos Plc (Nikon Corporation)
• Original Assignee: OTI Ophthalmic Technologies Inc.
• Inventors: Podoleanu, Adrian G. H.
• Primary Examiner(s): DANG, HUNG XUAN
• Application Number:
  US12/019,121
• Publication Number:
  US 20080170204A1
• Time in Patent Office: 1,041 Days
• Field of Search: 351/206, 351/207, 351/211, 351/212, 351/221, 351/246, 351/247, 356/479, 356/354, 356/345, 356/351, 356/349, 600/476
• US Class Current: 351/212
• CPC Class Codes: A61B 3/102 for optical coherence tomography [OCT]
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 9/02028 Two or more reference or object arms in one interferometer
• CPC Class Codes: G01B 9/02077 of the object
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation
• CPC Class Codes: G01B 9/02091 Tomographic interferometers, e.g. based on optical coherence

Abstract

A method of determining the external three dimensional external contour of a curved object in one step, which produces and processes a single composite en-face OCT image (a C-scan), which involves generating and processing a set of two dimensional image contours of the object in different image planes using a plurality of optical path lengths in an OCT system with all such contours present in the composite en-face OCT image. The multiple path lengths are generated using a multiple delay element in bulk or in fiber inserted in the path from the source to the OCT system or in one of the OCT arms. The image planes have a known spatial relationship with each other. The three dimensional external contour of the curved object is computed from the two dimensional contours in the composite en-face OCT image and the known spatial relationship between the image planes. Axial position and tilt of a sample are evaluated using an optimum shape object attached to the sample in one step, by processing a s composite en-face OCT image generated by using a plurality of optical path delays.

Claims

1. A method of obtaining geometrical information about an object, comprising:

transversely scanning the object with an OCT instrument including a low coherence interferometer to obtain an en-face OCT image;

introducing differential delays with known values into a path of light, wherein said light passes through said interferometer to produce an output beam comprising multiple parallel beam portions having different path lengths associated therewith, and thereby generating in said en-face OCT image a set of two dimensional image contours at different depths, each said two dimensional image contour corresponding to a said respective beam portion, and said two dimensional image contours having a known depth relationship determined by said differential delays; and

computing the geometrical information about the object from at least two of said of said sets of two-dimensional contours in the said en-face OCT image.

2. A method as claimed in claim 1, wherein said path includes a source path extending from a source to the interferometer, an object path, and a reference path, and said differential delays are introduced into said source path.

3. A method as claimed in claim 1, wherein said image contours are located on surfaces that are located at fixed distances apart in the depth direction.

4. A method as claimed in claim 1, wherein a differential delay element is introduced into said path to incrementally vary the optical path length of said sub-beam portions.

5. A method as claimed in claim 1, further comprising temporarily removing said differential delay to create full OCT en-face image of said object.

6. A method as claimed in claim 1, wherein said differential delay is provided by a differential delay element in the form of a transparent block partially intercepting the path of light and having a graduated or stepped increase in thickness across its width, wherein the individual steps are larger than the coherence length of the light used to obtain the images.

7. A method as claimed in claim 6, wherein the differential delay element is adjusted to intercept the beam such that the portions of the beam having different optical path lengths have a uniform intensity.

8. A method as claimed in claim 1, wherein said geometrical information is the external shape of the object or axial position of the object.

9. A method as claimed in claim 1, wherein a delay element is inserted into a collimated portion of a beam of said light.

10. A method as claimed in claim 1, wherein a polar value for the curvature of the said object, r_p,m(θ

), is obtained from polar radial distances, X(θ

)_m and X(θ

)_p evaluated along a polar orientation θ

in the said composite C-scan OCT image;

11. A method for determining the three dimensional external contour of the cornea, comprising:

creating an OCT en-face two-dimensional composite image of the cornea in a common image plane with a multiple delay OCT instrument employing a reference beam and an object beam, wherein the optical path length difference between the object and reference beams has a plurality of values produced by a differential delay element, whereby said composite image lying in said common image plane created by said multiple delay OCT instrument contains a series of superimposed two dimensional image contours of the cornea taken from planes at different depths determined by the optical path length differences; and

computing the three dimensional external contour of said curved object from said two dimensional contours in said common image plane and the known optical path length differences at the corresponding planes at different depths.

12. A method as claimed in claim 11, wherein the plurality of optical path length difference values is obtained by varying the optical path delay across the object beam or reference beam or source beam by introducing a delay element therein.

13. An apparatus for obtaining geometrical information about a curved object, comprising:

an OCT imaging instrument for generating a composite en-face contour image of the curved object; and

said OCT imaging instrument employing an output beam having parallel sub-beam portions associated with different path lengths so as to generate a set of two dimensional image contours at different image depths within the composite en-face image, said image contours having a known depth relationship determined by the differences in the optical path lengths; and

a processor configured to compute the geometrical information from the image contours and the known depth relationship therebetween.

14. An apparatus as claimed in claim 13, further comprising a differential delay element creating said different path lengths, said delay element being in the form of a transparent block whose thickness varies in a stepwise manner across its width, the lateral size of the steps being less than the beam diameter divided by N+1, where N is the number of the plurality of optical path length differences.

15. An apparatus as claimed in claim 14, wherein the differential delay element is incorporated into the arm of the OCT interferometer leading to the object object.

16. An apparatus as claimed in claim 13, wherein further comprising a differential delay element creating said different path lengths in the form of multiple waveguides with different lengths.

17. An apparatus as claimed in claim 16, wherein said differential delay element comprises fiber patches of different length.

18. An apparatus as claimed in claim 13, further comprising a differential delay element creating said different path lengths, said differential delay element having portions intercepting the beam which vary according to the distribution of power within the transverse section of the beam.

19. An apparatus claimed in claim 13, further comprising an application module for approximating the contours in the en-face image with sharp contours.

20. An apparatus as claimed in claim 13, wherein said processor is configured to compute solutions for the curvature of the said object and for the axial distance away of said object from at least two distances between at least respectively three said contours and the lateral image size.

21. An apparatus as claimed in claim 13, further comprising an application module for computing an approximate axial distance by counting the number of contours in the composite en-face image and using the known optical path differences between the plurality of said beams.

22. A method of estimating the axial position of an object, comprising

generating simultaneously a series of two dimensional image contours from planes at different image depths to create a two-dimensional composite en-face image in a common image plane using an OCT imaging instrument employing a reference beam and an object beam, wherein the optical path length difference between the object and reference beams has a plurality of values produced by a differential delay element, said planes at different depths having a known depth relationship determined by the differences in the optical path lengths at the different planes;

and

estimating the axial position of the object from the number of contours present in said composite image and the differential optical path differences between adjacent paths.

23. A method of obtaining geometrical information about an object, comprising:

generating simultaneously a set of superimposed two dimensional image contours from planes at different depths to create a two-dimensional composite en-face image of the object in a common image plane using an OCT instrument employing a reference beam and an object beam, wherein the optical path length difference between the object and reference beams has a plurality of values produced by a differential delay element, said planes at different depths having a known depth relationship with each other; and

computing the geometrical information about the object from said dimensional contours in said composite image and the known depth relationship between said planes at different depths.

24. A method as claimed in claim 23, wherein the geometrical information about the object is tilt or axial position.

25. A method as claimed in claim 24, wherein the shape of the said object is optimized for best accuracy of axial length measurement, or best sensitivity or depth range.

26. A method as claimed in claim 23, wherein the object is the cornea, and Purkinje reflections are added to the composite en-face image.