In December of 2015, the Laser Illuminated Projector Association (LIPA) released its Recommended Practice for the Measurement of Speckle. The good news is that there’s an agreed method for measuring speckle from laser illuminated projectors. The bad news is that there’s no longer an excuse for not publishing a number. Or is there? In September last year, a study published in Nature.com by researchers at Brussels-based Vrije Universiteit Brussel in September 2015 reveals a lot about selecting a performance figure for speckle.
Scientists are well aware that speckle is caused by laser light. Speckle from a laser-illuminated projector appears as a granular pattern of spots on the image, caused by the interference of coherent laser light on a not-so-optically-flat projection screen. The Vrije Universiteit Brussel researchers point to several studies concerning the threshold of perception of speckle in still images, but little effort made to study in motion images. Their study, performed with a prototype cinema laser projector in a cinema environment, may be the first of its kind.
The researchers projected six movie trailers and a full-length movie under various test conditions designed to increase or decrease speckle contrast. (An increase in contrast would make speckle behavior more visible.) 186 subjects participated in the study. The projector was a prototype 4K-laser illuminated projector. It wasn’t stated, but it would seem fair to assume the prototype projector was intended for the cinema market.
Overall, the results point to a strong variation of speckle perception dependent on the content. Two different trailers, shown with identical projector configurations, were perceived to be different in speckle by a factor of 4. An even larger variation, 15x over the best performing trailer, was observed with another two trailers. While a polarizer was used to increase speckle contrast with the high speckle trailers, the researchers report that the measured increase in speckle contrast did not account for the increased perception of speckle. Further, in a blind test conducted with a full-length movie, there was little correlation between perception of speckle and the use of a polarized filter. This removes the polarizer as the cause of the huge increase in speckle perception, and again points to the dependence of perception on content.
The researchers suggest that a reasonable level of speckle for a laser illuminator would be that of a xenon lamp. But they also point out that the perception of speckle with a xenon lamp was about half that of the best performing trailer using the prototype 4K laser projector. Considering the worst performing trailer, speckle would need to be reduced by a factor of 30 to achieve that xenon levels. Such improvement may not be realistic. Projector manufacturers will not be keen to advertise numbers that can be unfavorably compared to xenon illuminators. Until further improvements are achieved, LIPA’s speckle measurement method may be left to sit on the shelf awhile.
A few interesting observations were made. Speckle is more apparent when observed from the front of the auditorium than the back. But observers in the middle of the auditorium, generally preferred by audiences, found speckle to be only slightly less annoying than those in the front. No significant correlation of perception was found with regards to age or gender. Unlike wine, it won’t get better with age. Quoting the report, the style of imagery that appeared to trigger the highest perception was “large grey areas (e.g. from a wall or a cloudy sky) with a rather uniform intensity distribution and that change slowly in time “
If measurements encourage unfavorable comparisons with xenon-illuminated projectors, it’s unlikely that we’ll see LIPA’s numbers in product specifications. The surprising news from this study is that speckle perception is wildly dependent on photography. This could lead to all sorts of guidelines. One can see it now: the yellow-band trailer appears before the movie stating “Warning. The Following Movie May Have Disturbing Levels of Speckle.” Oh dear.