The name of the first practical mathematical inversion technique for turning ptychographical diffraction intensity data into an image was called PIE, for the Ptychographical Iterative Engine: hence the abbreviation ‘pi’.  Phi is the conventional symbol for phase: it is the phase of the scattered wave which we recover via PIE.  Both pi and phi allude to the complex plane, as illustrated in our logo, and rhymes with ‘hi-fi’: the quality of the images we hope to reconstruct. 

'Ptychography' comes from the Greek ptycho meaning to fold (as in to fold a garment). This is because the method uses the convolution theorem (or the mathematical process of 'folding' two functions together) to solve for phase. The method was first postulated by Walther Hoppe in 1969, and was developed and named by Hegerl and Hoppe in the early 1970s. However although at that time it was clear that ptychography should solve the phase problem, there was no easy method for performing the necessary inverse calculation.