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Indirect Fourier transform : ウィキペディア英語版
Indirect Fourier transform

Indirect Fourier transform (IFT) is a solution of ill-posed given by Fourier transform of noisy data (as from biological small-angle scattering) proposed by Glatter. IFT is used instead of direct Fourier transform of noisy data, since a direct FT would give large systematic errors.
Transform is computed by linear fit to a subfamily of functions corresponding to constraints on a reasonable solution.
If a result of the transform is distance distribution function, it is common to assume that the function is non-negative, and is zero at ''P''(0) =  0  and ''P''(''D''max)≥;0, where Dmax is a maximum diameter of the particle. It is approximately true, although it disregards inter-particle effects.
IFT is also performed in order to regularize noisy data.
== Fourier transformation in small angle scattering ==
see Lindner et al. for a thorough introduction 〔Neutrons, X-rays and Light: Scattering Methds Applied to Soft Condensed Matter by P. Lindner and Th.Zemb (chapter 3 by Olivier Spalla)〕
The intensity ''I'' per unit volume ''V'' is expressed as:
: I(\mathbf) = \frac\int_V\int_V\rho(\mathbf)\rho(\mathbf')e^-\mathbf')}\text\mathbf\text\mathbf',
where \rho(\mathbf) is the scattering length density. We introduce the correlation function \gamma(\mathbf) by:
: I(\mathbf) = \int_V\gamma(\mathbf)e^}\text\mathbf
That is, taking the fourier transformation of the correlation function gives the intensity.
The probability of finding, within a particle, a point i at a distance r from a given point j is given by the distance probability function \gamma_0(r). And the connection between the correlation function \gamma(r) and the distance probability function \gamma_0(r) is given by:
:\gamma(r) = b_i\cdot bj\gamma_0(r)V,
where b_k is the scattering length of the point k. That is, the correlation function is weighted by the scattering length. For X-ray scattering, the scattering length b is directly proportional to the electron density \rho_e.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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