The use of holography in data processing for optical computing, pattern recognition, head up display, etc. induce a development of the methods for conceiving, realizing, and testing the holographic systems. The limitations of the mathematical models used up to now prevent any innovation in this field. As a first step to go beyond these limitations we propose : a) an alternative theory of diffraction by holograms, providing new methods for computing the grating parameters; b) applications to the conception of new devices for analysing or testing holograms, and of new holographic processors. The theory will be presented here only in the particular case of unslanted phase gratings without loss, but of course its domain of validity is larger. For the complete theory, see  (not yet published). Nevertheless, we do not restrict our study to the case of Bragg diffraction, as the classical coupled waves theories do, since our theory works independently of such an hypothesis.