Laser illumination is a significant recent advancement in high-end projector systems that have traditionally used high power lamps. Lasers offer improved brightness, lower etendue, extended color gamut, low operation cost and longer life. These advantages are however accompanied by the presence of speckle that can degrade the perceived quality of projected images. When coherent laser light scatters from rough objects (rough on wavelength scale) like display screens, a granular pattern is seen as a result of interference of waves from multiple scattering centers. This granular pattern is known as speckle. The statistical properties of speckle and its dependence on source spectrum is a well understood topic. The speckle is typically characterized by a contrast parameter C = σ / <I>
, which is the ratio of the standard deviation of the intensity fluctuations and the mean intensity in a speckle pattern and the value of C
typically lies between 0 and 1, with values close to 0 implying low influence of speckle. While the information contained in a speckle pattern can be interesting for a remote sensing application, the presence of speckle can completely negate the advantage of using lasers in a projection display system. It is therefore important to understand the acceptable limits on the presence of speckle for a typical human observer.
In their comprehensive study involving human observers, Roelandt et al. used a two projector system consisting of a laser projector and a lamp projector. The projection areas of the two systems overlapped considerably. Further, one half (left or right, selected randomly) of the projected image from the two projectors was blanked, so that the observers had no prior knowledge about the location of the image from the laser projector. The researchers consider two speckle contrast limits – contrast value C
at which (a) observers just barely perceive the presence of speckle, and (b) a higher contrast value when observers feel that the presence of speckle is disturbing for image viewing. These contrast limits are studied for different primary colors. While most studies of this type traditionally project a uniform intensity pattern over a screen, the researchers study the dependence of these limits on scene content, as this is a more practical situation. The analysis presented here can be a valuable guideline for designing laser projection systems that will have increasing presence in future. The authors also indicate that these limits may have to be studied further when video streams are to be projected instead of still images as in the present study.
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