Abstract: A method for producing desired changes in a beam of coherent light radiating at .lambda..sub.1, comprising the steps of (a) splitting a beam of coherent light radiating at .lambda..sub.1, correcting one of the beams to achieve a desired wavefront and recombining this beam with the first, where a record medium (I) is placed such that the beams are separated by an angle determined by Bragg's law and the line spacing of the interference pattern formed by the two beams; (b) developing the first medium such that a diffraction grating of spacing d.sub.1 is recorded in a material, specifically sensitive to .lambda..sub.1, which will become the master; (c) illuminating the master with a second wavelength, .lambda..sub.2, selected by choice of a second medium which possesses very high diffraction efficiency. The second beam is split into two parts. The first is directed to the master. This diffracted order is recombined with the second beam.