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. 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. As is also discussed in [Verschaffelt et al., Scientific Reports 5, art. nr. 14105, 2015] 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%. As we could not independently tune the speckle contrast of each of the primary colors, this speckle disturbance limit seems to be determined by the 6.9% speckle contrast of the red color as this primary color contains the largest amount of speckle. The speckle disturbance limit for movies thus turns out to be substantially larger than that for still images, and hence is easier to attain.
During the last years, a trend to replace commonly used short arc lamps in projection systems with alternative light
sources is seen. Next to LEDs for low light output products, lasers try to enter the projection area and have the ambition
to infiltrate from low (picoprojection) towards high light output systems (digital cinema).
One of the benefits of lasers is their narrow spectral bandwidth. As a consequence, the display can have a very large
colour gamut, if the lasers are carefully selected. Another benefit is the very low intrinsic étendue of the source. One can
imagine using less complex, more efficient, smaller but more powerful optical systems. This not only for scanning
projectors, but also for 2D light valve based projectors (LCOS, LCD, DLP). In addition, the limited lifetime of lamps has
serious impact on a system's cost of ownership, and puts light source reliability/lifetime high on the list of priorities for
future developments. For this reason, Barco entered the European FP6-project OSIRIS, in a subtask where a 300 lm laser
projector demonstrator has to be developed and evaluated.
So far, we found out that next to obvious challenges such as laser cost and laser power, the most critical issue regarding
image quality is speckle interference which counteracts the beneficial nature of the light source. This phenomenon is a
direct consequence of the coherent nature of a laser and cannot be solved as easily as is often claimed. We will describe
laser speckle in the context of laser projection and the theoretical limits of several reduction techniques. This leads to
guidelines which can make laser projection worth considering.