Speckle characterization in a cinematography projection configuration

Abstract

Due to high exploitation costs and other environmental issues, it would be desirable to phase out large cinema projection systems based on standard xenon lamps in favor of laser based projection devices. Lasers provide longer lifetime and wider color gamut of light output. But the high degree of coherence of these sources also lead to the formation of granular structures, usually known as speckle. When an imaging system is involved, as in the cinema projection case because of the capacity of the human eye to form an image of the screen, we speak about subjective speckle. In order to remove this spatial random pattern, different methods have been studied as temporal and/or spatial coherence reduction. But most of them can't be used in the context of cinema projection because they don't respect the cinematography projection standard. In our work, we have studied the possibility to reduce the subjective speckle either by changing the coherence of the light source or by studying the influence of the different elements constituting the projection display in the conditions imposed by cinematography industry. Thanks to a lasers array formed by N independant semiconductor lasers, we have measured the evolution of the subjective speckle contrast in function of the number of sources. The resulting contrast discreases as a square root function and reach a saturation level when a light pipe is used. This behavior is directly due to the light pipe which limit at its output the spatial coherence of the source. Futhermore, in a different configuration, we have studied the influence of diffusers and the magnification of the projector zoom. It has been demonstrated that magnification plays an important role on the speckle formation because it increases the coherence length determined by the light pipe. On the contrary, the diffusers placed before the light pipe doesn't change the subjective speckle.