Patent US 3,945,825

Pulse width modulated surface relief phase holograms are produced on a substrate by developing a holographic interference pattern recorded on a photoresist deposited on the surface of said substrate as an amplitude modulated sinusoidal surface relief pattern so as to expose the surface of said substrate as a function of the intensity of said interference pattern, uniformly etching said substrate and removing said photoresist leaving a pulse width modulated, substantially two-level rectangular wave diffraction grating. Focused image holograms produced by this method on hard durable substrates may be used as masters for replicating said holograms in a suitable recording medium.