Speckle reduction using a sinusoidal rotating grating

The coherent nature of laser light introduces the problem of speckle in laser display projectors. Speckle are random spatial intensity fluctuations which arise when coherent light is reflected off a rough surface. A novel idea for speckle reduction in laser display projectors uses the rotation of a diffraction pattern to cover different areas on a diffuse surface, producing independent speckle patterns. To test the principle a test configuration was implemented. The setup consisted of sinusoidal phase gratings patterned on a transmissive spatial light modulator (SLM), with each phase grating rotated with respect to the previous one. A reduction of speckle contrast to 0.27 was achieved, with further reduction possible.