Abstract: In laser display system, speckle noise produced by laser coherence has a serious effect on image. The condition for a diffuser to produce the maximum speckle reduction with the minimum number of distinct phase patterns is derived. A binary realization of this optimum diffuser is obtained by mapping the rows or columns of a Hadamard matrix to the phase patterns. The method is experimentally verified by an example of green light. After using the diffuser，speckle contrast is reduced from 8.4% to 2.7%.