Speckle Suppression by Controlling the Coherence in Laser Based Projection Systems

In this paper, the relationship between the coherence and the speckle contrast in a laser-based projection display system is studied with the consideration of human visual percept. By using a varifocal liquid-crystal lens and a monochromatic charge-coupled device (CCD), a system which is used for simulating a human eye is set up to record the speckle pattern. An efficient method for controlling the spatial coherence by using a dielectric elastomeric actuator (DEA) is demonstrated. We find that there is a limitation for eliminating the speckle contrast in the laser projection system when only the method of reducing the spatial coherence is used. When the distance between the observer and the screen is sufficiently large (${>} \hbox{3 m}$), the speckle contrast can be eliminated well (${