When using partially coherent transfer functions for Diffraction Image Analysis, there are two computation methods available, Mostly Coherent and Mostly Incoherent. The method that you should use depends upon how broad the Gamma function that you are using is.

Narrow Gamma functions (i.e. Gamma functions that comprise less than 20% of the overall image) are narrow in the spatial domain and, thus, broad in the spatial frequency domain. As such, for these types of Gamma functions, it is more efficient to compute the Diffraction Image Analysis in the spatial domain. The Mostly Incoherent computation algorithm is designed for these narrow Gamma functions and, as a result, computations performed using this method are done in the spatial domain.

On the other hand, broad Gamma functions (i.e. Gamma functions that comprise more than 20% of the overall image) are broad in the spatial domain and, hence, narrow in the spatial frequency domain. Thus, for these types of Gamma functions, it is more efficient to compute the analysis in the spatial frequency domain. The Mostly Coherent algorithm is designed for these broad Gamma functions. For this algorithm, computations are performed in the more efficient spatial frequency domain.

If you try to use the Mostly Incoherent method for a broad Gamma function (or, conversely, the Mostly Coherent method for a narrow Gamma function), the computation will take significantly longer at best. Even worse, the computation may not be carried through to completion due to insufficient sampling. So, it is always important to visualize the Gamma function that you are using, as described on the previous page, to determine which computation method you should use.

In summary:

There is one more related parameter of interest, the Fraction parameter. Both the Mostly Incoherent and Mostly Coherent methods speed up the Diffraction Image Analysis computations by making the assumption that the Gamma function has a finite width. (In reality, Gamma and Sinc functions have infinite extent.) You have control over this width via the Fraction setting. This parameter sets the fraction of total energy within Gamma that should be considered. In general, Fraction values larger than 0.96 will significantly slow down Diffraction Image Analysis computations given the asymptotic nature of Gaussian and Sinc functions.