Method and apparatus for determining object characteristics
Embodiments of the invention provide a method of determining one or more characteristics of a target object, comprising recording one or more diffraction patterns at a detector, wherein each diffraction pattern is formed by a target object scattering incident radiation, determining a phase map for at least a region of the target object based on the one or more diffraction patterns, and determining a refractive property of the target object based on the phase map.
1. A method of determining one or more characteristics of a target object, comprising:
recording one or more diffraction patterns at a detector, wherein each diffraction pattern is formed by a target object scattering incident radiation;
determining a phase map for at least a region of the target object based on the one or more diffraction patterns, wherein the phase map is indicative of a phase shift introduced into the incident radiation by the target object at each of a plurality of spatial locations within the target object; and
determining a refractive property of the target object based on the phase map.
2. The method of claim 1 , comprising:
determining a wavefront at a plane of the object based upon the phase map;
wherein the refractive property of the object is based upon the wavefront.
3. The method of claim 1 , wherein the refractive property is one or both of a refractive power and a refractive index.
4. The method of claim 1 , wherein the refractive property is a refractive property map.
5. The method of claim 1 , comprising:
fitting a three-dimensional shape to the wavefront;
determining a focal axis of the object corresponding to an axis of the shape.
6. The method of claim 5 , wherein the three-dimensional shape is a sphere.
7. The method of claim 5 , wherein the three-dimensional shape is fitted to portion of the wavefront associated with a predetermined region of the object.
8. The method of claim 7 , wherein the predetermined region is a region of the object within a predetermined radius of a centre of the object.
9. The method of claim 1 , comprising determining a first phase gradient for at least a portion of the phase map.
10. The method of claim 9 , wherein the first phase gradient is determined in linear directions.
11. The method of claim 9 , wherein the first phase gradient is determined at each of a plurality of points based upon a phase of a plurality of adjoining points.
12. The method of claim 9 , comprising determining a radial phase gradient based on the first phase gradient.
13. The method of claim 12 , wherein the radial phase gradient is based upon a focal axis.
14. The method of claim 12 , wherein the refractive power map is based upon a radial phase gradient.
15. The method of claim 12 , wherein the refractive power map is a liquid-immersed focal power map for the object immersed in a liquid medium.
16. The method of claim 13 , comprising converting the liquid-immersed focal power map to an air-based focal power map.
17. The method of claim 14 , wherein the converting comprises determining a location of a virtual object in the liquid medium.
18. The method of claim 13 , wherein the liquid-immersed focal power map is determined based upon an angle of refraction at a liquid-air interface.
19. The method of claim 1 , comprising smoothing the phase map.
20. The method of claim 19 , wherein the smoothing comprises filtering the phase map.
21. The method of claim 20 , wherein the filtering is performed using a Hamming Fourier filter.
22. The method of claim 1 , wherein each diffraction pattern is recorded at a respective position of the target object relative to the incident radiation.
23. The method of claim 1 , wherein the phase map is determined based on a coherent diffractive imaging (CDI) process.
24. An apparatus arranged to perform the method of claim 1 .
25. A computer program tangibly stored on a computer readable medium which, when executed by a processor, is arranged to perform the method of claim 1 .