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Coherent diffractive imaging (CDI) is a “lensless” technique for 2D or 3D reconstruction of the image of nanoscale structures such as nanotubes, nanocrystals, defects, potentially proteins, and more.〔 In CDI, a highly coherent beam of x-rays, electrons or other wavelike particle or photon is incident on an object. The beam scattered by the object produces a diffraction pattern downstream which is then collected by a detector. This recorded pattern is then used to reconstruct an image via an iterative feedback algorithm. Effectively, the objective lens in a typical microscope is replaced with software to convert from the reciprocal space diffraction pattern into a real space image. The advantage in using no lenses is that the final image is aberration–free and so resolution is only diffraction and dose limited (dependent on wavelength, aperture size and exposure). A simple Fourier transform retrieves only the intensity information and so is insufficient for creating an image from the diffraction pattern due to the phase problem. == The phase problem == (詳細はinverse problem as any phase could be assigned to the amplitudes prior to an inverse Fourier transform to real space. Three ideas developed that enabled the reconstruction of real space images from diffraction patterns.〔 The first idea was the realization by Sayre in 1952 that Bragg diffraction under-samples diffracted intensity relative to Shannon’s theorem. If the diffraction pattern is sampled at twice the Nyquist frequency (inverse of sample size) or lower it can yield a unique real space image.〔 The second was an increase in computing power in the 1980s which enabled iterative Hybrid input output (HIO) algorithm for phase retrieval to optimize and extract phase information using adequately sampled intensity data with feedback. This method was introduced〔 by Fienup in the 1980s. Finally, the development of “phase recovery” algorithms led to the first demonstration of CDI in 1999 by Miao using a secondary image to provide low resolution information . Reconstruction methods were later developed that could remove the need for a secondary image. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「coherent diffraction imaging」の詳細全文を読む スポンサード リンク
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