It is shown that the light intensity distribution in the image plane of an optical system producing a moire patterns the relevant information is the phase angle. The above described analysis forms the basis of a method for data retrieval. The intensity of light distribution on a pattern is recorded in the form of density changes in a photographic film negative. The changes of density are then retrieved by a light sensor. If one analyzes the density record, one can see that the trace is not only modulated by the relevant quantities but also by various other processes. Furthermore the effect of the nonlinear characteristic curve of the recording film is to generate for each narrow-band componet of the signal, an infinite number of narrow-band high order components of the fundamental argument. To eliminate the effect of unwanted componets, of the fundamental or the effect of unwanted comonents, the digitalized vision of the signal is filtered by numerical narrow-band pass filters in-quadrature. The computer program gives the strains in the case of moire patterns and relative retardations in the case of photoelasticity. The given examples of application show that the proposed data retrieving and data processing technique increases both the sensitivity and the accuracy of the moire and photoelasticity. This result is of great practical value particularly in the moire method since it extends its applicability to the ranges of interest of practically all engineering materials.