We investigate the level of speckle that can be tolerated in a laser cinema projector. For this purpose, we equipped a movie theatre room with a prototype laser projector. A group of 186 participants was gathered to evaluate the speckle perception of several, short movie trailers in a subjective ‘Quality of Experience’ experiment. This study is important as the introduction of lasers in projection systems has been hampered by the presence of speckle in projected images. Laser display technology is widely considered as the most promising technology for high-lumen future projection applications. These sources can combine a high lumen output with a low étendue, which leads to a high optical efficiency in the projector. Lasers also offer a long lifetime and their output power does not degrade strongly. Laser projectors can therefore potentially outperform current arc-lamp projectors in many different aspects. But the speckle pattern originating from the (partly) coherent laser beam is displeasing for a human observer and therefore it needs to be reduced to an acceptable level. We identify a speckle disturbance threshold by statistically analyzing the observers’ responses for different values of the amount of speckle, which was monitored using a well-defined speckle measurement method. The analysis shows that the speckle perception of a human observer is not only dependent on the objectively measured amount of speckle, but it is also strongly influenced by the image content. We find that, for moving images, the speckle becomes disturbing if the speckle contrast becomes larger than 6.9% for the red, 6.0% for the green, and 4.8% for the blue primary colors of the projector, whereas for still images the speckle detection threshold is about 3%. The speckle disturbance limit for movies thus turns out to be substantially larger than that for still images, and hence is easier to attain.
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