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Edge-illumination
Edge-illumination (EI) was developed at the Italian synchrotron (Elettra) in the late ‘90s ,as an alternative to ABI. It is based on the observation that, by illuminating only the edge of detector pixels, high sensitivity to phase effects is obtained (see figure).

Also in this case,the relation between x-ray refraction angle and first derivative of the phase shift caused by the object is exploited:


 * $$\Delta \alpha=\frac{1}{k}\frac{\partial \phi(x)}{\partial x}$$

If the x-ray beam is vertically thin and impinges on the edge of the detector, x-ray refraction can change the status of the individual x-ray from “detected” to “undetected” and vice-versa, effectively playing the same role as the crystal rocking curve in ABI. This analogy with ABI, already observed when the method was initially developed, was more recently formally demonstrated. Effectively, the same effect is obtained – a fine angular selection on the photon direction; however, while in ABI the beam needs to be highly collimated and monochromatic, the absence of the crystal means that EI can be implemented with divergent and polychromatic beams, like those generated by a conventional rotating-anode x-ray tube. This is done by introducing two opportunely designed masks (sometimes referred to as “coded-aperture” masks ),one immediately before the sample, and one in contact with the detector (see figure).

The purpose of the latter mask is simply to create insensitive regions between adjacent pixels,and its use can be avoided if specialized detector technology is employed.In this way,the EI configuration is simultaneously realized for all pixel rows of an area detector. This plurality of individual beamlets means that no scanning is required – the sample is placed downstream of the sample mask and imaged in a single shot (two if phase retrieval is performed ).It should be noted that, although superficially, the set-up might resemble that of a grating interferometer, the underpinning physical mechanism is different: while GI is an intrinsically coherent method, in which an incoherent source can be used only provided it is made sufficiently coherent through collimation via the source grating, EI is an incoherent technique, and was in fact proven to work with both spatially and temporally incoherent sources, without any additional source aperturing or collimation. Quantitative phase retrieval was also demonstrated with (uncollimated) incoherent sources,showing that in some cases results analogous to the synchrotron gold standard can be obtained. The highly simplified set-up, which however does not lead to reduced phase sensitivity, leads to a series of positive features,which include reduced exposure time for the same source power,reduced radiation dose,robustness against environmental vibrations,and easier access to high x-ray energy. Moreover, since their aspect ratio is not particularly demanding,masks are cheap, easy to fabricate (e.g.do not require x-ray lithography) and can already be scaled to large areas. The method is easily extended to phase sensitivity in two directions, for example, through the realization of L-shaped apertures for the simultaneous illumination of two orthogonal edges in each detector pixel .More generally, while in its simplest implementation beamlets match individual pixel rows (or pixels), the method is highly flexible, and, for example, sparse detectors and asymmetric masks can be used .So far, the method has been successfully demonstrated in areas such as security scanning, biological imaging ,material science , paleontology and others; adaptation to 3D (computed tomography) was also demonstrated. Alongside simple translation for use with conventional x-ray sources,it should be noted that there are substantial benefits in the implementation of EI with coherent synchrotron radiation,among which high performance at very high x-ray energies and angular resolutions higher than in other approaches.