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 ( > 3 m), the speckle contrast can be eliminated well ( ) by reducing the spatial coherence and the observer do not observe the speckle phenomenon. When the viewing distance is smaller than 3 m, however, the observer will feel the speckle all of the time. To further reduce the speckle contract, a method of wavelength modulation is used to reduce the temporal coherence while the spatial coherence is well reduced. The speckle contrast is reduced to 4% at the viewing distance of 1 m by reducing both the temporal and spatial coherence of the laser.