Provision of image data
A method and apparatus are disclosed for providing image data. The method includes the steps of providing incident radiation from a radiation source at a target object and, via at least one detector, detecting an intensity of radiation scattered by the target object. Also via the at least one detector an intensity of radiation provided by the radiation source absent the target object is detected. Image data is provided via an iterative process responsive to the intensity of radiation detected absent the target object and the detected intensity of radiation scattered by the target object.
1. A method of providing image data for constructing an image of a region of a target object, comprising the steps of:
providing incident radiation from a radiation source at a target object and, via at least one detector, detecting an intensity of radiation scattered by the target object;
estimating an object function indicating at least one characteristic of a region of the target object;
estimating a probe function indicating at least one characteristic of the incident radiation; and
providing image data via an iterative process responsive to the detected intensity of radiation scattered by the target object, wherein in said iterative process each of the object function and probe function are iteratively re-estimated step by step with a running estimate of the probe function being utilized to determine a running estimate of the object function associated with the target object.
2. The method as claimed in claim 1 , wherein the iteratively re-estimating the object function and the probe function comprises the steps of:
multiplying the estimated object function by the estimated probe function to thereby provide an exit wave function;
propagating the exit wave function to provide an estimate of an expected scattering pattern; and
correcting at least one characteristic of said expected scattering pattern according to a detected intensity of radiation scattered by the target object.
3. The method as claimed in claim 2 , further comprising the steps of;
inverse propagating a corrected expected scattering pattern to thereby provide an updated exit wave function; and
updating the running estimate of the object function responsive to the updated exit wave function.
4. The method as claimed in claim 3 , wherein the object function is updated according to:
O
k
+
1
(
r
)
=
O
k
(
r
)
+
α
P
k
*
(
r
-
R
)
P
k
(
r
-
R
)
max
2
(
ψ
k
′
(
r
,
R
)
-
ψ
k
(
r
,
R
)
)
where O k (r) is the current (kth) estimate of the object function, ψ k (r, R) is the current exit wave, ψ′ k (r, R) is the updated exit wave whose Fourier Transform or Fresnel Transform has been corrected and P k (r−R) is the current (kth) estimate of the illumination function, α is a constant whose value can be adjusted to optimize the performance of the algorithm.
5. The method as claimed in claim 1 , further comprising the steps of:
propagating the estimated probe function to provide an estimate of an expected targetless scattering pattern; and
correcting at least one characteristic of said expected targetless scattering pattern according to an intensity of radiation detected absent said target object.
6. The method as claimed in claim 1 , further comprising the steps of:
inverse propagating a corrected expected targetless scattering pattern to thereby provide the running estimate for the probe function; and
updating the running estimate of the probe function responsive to the updated probe function.
7. The method as claimed in claim 6 , wherein the probe function is updated according to:
P
k
+
1
(
r
)
=
P
k
(
r
)
+
β
O
k
*
(
r
-
R
)
O
k
(
r
-
R
)
max
2
(
P
k
′
(
r
)
-
ψ
k
(
r
,
R
)
)
where O k (r) is the current (kth) estimate of the object function, ψ k (r, R) is the current exit wave, P′ k (r) is the updated illumination function whose Fourier Transform has been corrected using the measurement Ω P (u) and P k (r) is the current (kth) estimate of the illumination function, and β is a constant whose value can be adjusted to optimize the performance of the algorithm.
8. The method as claimed in claim 7 , wherein the estimated probe function is propagated to provide an estimate scattering pattern in a measurement plane of
Φ k ( u )= [ P k ( r )]
where the propagation operator suitably models the propagation between the plane of the object and the measurement plane, wherein comprises a Fourier Transform or a Fresnel Transform, Φ k (u) is the propagated kth illumination function guess, whose modulus must match the recording Ω P (u), and P k (r) is the current (kth) estimate of the illumination function.
9. The method as claimed in claim 8 , wherein the corrected targetless scattering pattern is inversely propagated back to an object plane as
P′ k ( r )= −1 [Φ′ k ( u )]
where the propagation operator −1 suitably models the propagation between the measurement plane and the plane of the object, wherein −1 comprises an Inverse Fourier Transform or an Inverse Fresnel Transform, P′ k (r) is the corrected illumination function in the plane of the object and Φ′ k (u) is the corrected diffraction pattern whose modulus matches the recorded modulus of the illumination function.
10. The method as claimed in claim 1 , further comprising the steps of:
updating the running estimate of the probe function and the running estimate of an object function simultaneously with each iteration.
11. The method as claimed in claim 1 , further comprising the steps of:
detecting the intensity of radiation scattered by the target object with the incident radiation or a post target aperture at a first position with respect to the target object;
re-positioning the incident radiation or post-target aperture at least one further position relative to the target object;
subsequently detecting the intensity of radiation scattered by the target object with the incident radiation or post-target aperture at the at least one further position; and
providing the image data responsive to the intensity of radiation detected as scattered at the first and at least one further position.
12. The method as claimed in claim 11 , further comprising the steps of:
estimating the object function indicating at least one characteristic of said region of the target object; and
estimating the probe function indicating at least one characteristic of incident radiation at the target object or the post-target aperture.
13. The method as claimed in claim 1 , further comprising the steps of:
detecting the intensity of radiation provided absent the target object before and/or after locating a target object between the radiation source and the detector.
14. The method as claimed in claim 1 , further comprising the steps of:
providing an initial estimate of the probe function as a prior modelled probe function or by providing a random approximation for the probe function.
15. The method as claimed in claim 1 , further comprising providing a diffuser arranged to diffuse radiation detected at the detector.
16. The method as claimed in claim 1 , wherein the target object is at least partially transparent to the incident radiation and detecting an intensity of radiation scattered by the target object comprises detecting an intensity of radiation transmitted by the target object.
17. The method as claimed in claim 1 , wherein the target object is at least partially reflective to the incident radiation and detecting an intensity of radiation scattered by the target object comprises detecting an intensity of radiation reflected by the target object.
18. Apparatus for providing image data for generating an image of a region of a target object, comprising:
a locating device for locating a target object at a predetermined location;
a radiation source for providing incident radiation at a target object located by the locating device;
at least one detector device for detecting an intensity of radiation scattered by the target object locating device for locating incident radiation or a post-target aperture at one or more locations with respect to the target object;
a memory storing an object function indicating at least one characteristic of a region of the target object and a probe function indicating at least one characteristic of the incident radiation; and
a processor for providing image data via an iterative process responsive to a detected intensity of radiation scattered by the target object, wherein in said iterative process the processor is configured to iteratively re-estimate each of the object function and the probe function step by step with a running estimate of the probe function being utilized to determine a running estimate of the object function associated with the target object.
19. The apparatus as claimed in claim 18 , wherein the incident radiation is substantially localized.
20. The apparatus as claimed in claim 18 , further comprising a diffuser arranged to diffuse radiation detected at the detector.
21. The apparatus as claimed in claim 20 , wherein the diffuser is located within a post target aperture.
22. A non-transitory computer-readable data storage medium having instructions stored thereon which, when executed by a computer, perform the method as claimed in claim 1 .