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Author Topic: Laser Projectors Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 03-24-2019 07:38 AM                     Screen size has no impact. The speckls is created in the eye, it is an effect of interference in the coherent light from a laser. The speckle visible changes with color (wavelength), most visible with green in a 3P projector with rdd anx blue not so annoying. LP projectors filter green and red from a yellow phosphor that gets excitdd by blue laser light, the yellow is not laser light, not coherent, and has no speckle. Screen shakers use accoustic pressure to vibrate the screen surface very slightly, less than 1mm, using band limited sub audible LF noise through a bunch of speakers close behind the screen. Although the reflected laser light is still coherent, the shaking screen makes the speckle created inside your eye move around faster than you can see it, similar to a movie looking smooth rather than the series of still images because they change faster than we can notice. Odd: looking at reflected laser light with visible speckle, move your head sideways. The speckle moves with or opposite to the head movement... one way for short sighted people, opposite for far sighted.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 03-24-2019 08:45 AM                     There are hundreds of papers on speckle measurement and reduction for projection systems to be found by google. Barco had an interesting report on their early prototypes. You may decide to read through them, or simply accept that speckle is currently one remaining issue with 3P and 6P projection systems. It annoys the hell out of me in laser IMAX installations I have visited so far, and is exaggerated there because IMAX seems to prefer leaving metallized screens in place, as conventional gain and metallized screens make speckle worse. Why they do that, with the speckle becoming extremely annoying that way, is beyond my understanding. The laser IMAX Berlin even received a NEW metallized screen a few months after their laser IMAX update, so they are definitely doing it deliberately. They don't need silver screens for their 3D, so I can only assume that even with laser projectors they still need the screen gain to yield their brightness spec. For now, I would hesitate to buy one of those very costly 3P or 6P laser projectors - then being notified that the 'speckle-free' successor is just around the corner. Although it's not a yes/no issue, there are many speckle reduction techniques, and every single one only reduces speckle by a certain amount, overdoing it is an economic issue. So, the question is not wether a new projector has speckle or not, but wether it is able to reduce speckle on a given screen surface below most spectators perception threshold. - Carsten  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 03-24-2019 11:07 AM                     You, generally, don't see speckle on LP projectors. Many LP projectors use two Blue lasers, one to generate blue (least noticeable for speckle) and the other laser, via phosphor wheel to get the remaining spectrum. Even the Blue laser often goes through a diffusion wheel. Others use Red and Blue lasers with a second blue laser to get green. Green is the expensive laser to make. On those I'd think that red is the most visible (it is the one I often key off of on speckle). I've seen some shake the light source itself to minimize speckle too. RGB laser systems are going to be the most susceptible. Using a matte white screen is the best defense against speckle in the theatre. Matte-white is the best for light distribution too.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 03-24-2019 02:47 PM                     IMAX seemingly uses ultrasonic speakers to vibrate the screen, like Dave mentioned. In my opinion, this system is pretty worthless. Like Carsten already mentioned, the silver screen increases speckle and it doesn't look like the screen shaking really solves the problem. Since the speckle is present in all three primaries, you'll get a rainbow-pattern of pearls on the screen, which annoys the heck out of me. Dolby isn't entirely clear in what they're using for their Dolby Vision / Cinema setup, but it seems to be a system using physical actuators. It is very effective, as you won't really notice any speckle in a Dolby Cinema presentation, using 2 x 3P projectors. They're using a sort of double-stepped light engine though, so that might also help to reduce speckle even more. The amount of speckle in LP projectors depends on make and model, but most of them either have barely noticeable speckle in one of the colors or no noticeable speckle at all, even without screen shakers. quote: Carsten Kurz They already use that approach (and others) for spectrum spreading, but it is a limited option, and especially for green, they usually need to use multiple schemes to get speckle below perception threshold - using screen shakers is the final defense line. I was aware that they use like 3 or 4 different wavelengths, but I was told that varying every module separately wasn't possible yet, since the output wavelength is design dependent and not something you can easily adjust with e.g. a POT-meter. Quite some research has been done on modulating laser light spectra, maybe the solution would be to broaden the spectrum of each and every module. Those solutions are still pretty expensive though. While the coherence of laser light is often one of the key features, in projection, it's more of a hindrance. That's also why I don't think current laser technology will be around for very long. There are other avenues that don't need complicated methods to reduce speckle, think about giant LED arrays and LED-module based screens.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 03-24-2019 03:09 PM                     Varying wavelength certainly has limits (after all, their distinctive and precise wavelengths are a key aspect of laser sources), but it's not the specific wavelength that is immediately responsible for speckle. They can also 'just' alter polarization, and some of that is possible by power modulation. The trouble probably is, you want many laser sources to help in speckle reduction, and if you want to control every one to decrease speckle further, each one would need it's own modulating power supply, which is probably a major cost factor. We are probably talking hundreds of laser diodes here. Dolby Cinema may benefit from the outsourced laser banks of the Christie projectors, they stack up many of them, and each probably contains already many individual laser sources. Then maybe the fibre transmission offers additional ways to reduce speckle. That said - I saw some press reports for the Christie 6p system mentioning 'enhanced wavelength diversity'. https://www.christiedigital.com/en-us/about-christie/news-room/press-releases/cgv-using-christies-revolutionary-rgb-laser Don't know, though, wether that is a standard feature for the Christie 6P system, or a special option. There is probably a reason why a Christie 6p system costs much more than a Barco 3P? I don't find much about speckle reduction in the smaller RGB laser projectors now entering the market. Don't know if they compromise on it, or found better ways to achieve the same level in the smaller and lower cost devices. - Carsten  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 03-24-2019 03:31 PM                     That article is already a few years old, I guess it's more or less a standard option right now, as I cannot find anything particular in their product offering. The Christie 6P setup always uses a dual-headed setup (2x3P), whereas the Barco setup is a 6P single-projector setup. There are some obvious cost-savings in a single-headed setup. I don't think Barco has any 3P cinema offerings right now. All the native Barco 6P setups I've seen are using a matte white screen, most of them without screen shakers and although you can see a small amount of speckle in some scenes, it is generally not really distracting.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 03-24-2019 03:41 PM                     Well, in a 6p system you can use both 3p heads for 2D as well, and then benefit from twice as many uncorrelated sources. The Barco 60L offers the same operation mode, but the compact form factor maybe doesn't allow for as much source diversity as the discrete Christie laser banks allow. For the average cinemagoer, speckle is probably not such an issue (given that even the extreme amount of speckle in IMAX laser installations doesn't seem to bother the IMAX people and most visitors much). The question is, do you want to pay a premium for a premium projection system that still has some early adopter quirks, with a lower cost successor already around the corner? Even with few patrons noticing it, it is still not the premium experience as the 'laser projection' buzzword suggests. - Carsten  |  IP: Logged Bruce Cloutier Expert Film Handler Posts: 161 From: Gibsonia, PA, USA Registered: Aug 2016 posted 03-24-2019 04:53 PM                        This is interesting. I don't think that I have yet to see laser projected movie. I wonder if a patron would know of that detail before the presentation. I mention it because years ago (uh... many many years ago) when planetariums first augmented their revenue with laser programs the speckle in those was severe. In my case it almost immediately triggered a migraine and I recall having to advert my eyes for the balance of the show praying for an exit. I had since avoided those. I am sensitive to some of those cheap Christmas laser displays. Am I safe to assume that this isn't a concern with these LP and 3P projectors or should I worry?  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 03-24-2019 05:54 PM                     I think those planetariums use laser scanning projectors, where they rapidly scan a modulated and combined laser beam across the projection surface. So, here you have essentially a single beam tracking the projection. Many planetariums use aluminum sheeting as a projection surface, which only increases speckle. RGB cinema laser projectors work differently. They combine multiple laser sources to a powerful beam, which is then spread and cast on the imaging device, which is either a Digital Micromirror device (DLP) or selective-reflective LCD device (LCoS or similar). A good 3P/6P laser presentation should only have a minimal amount to no visible speckle. IMAX (with Lasers) is an outlier.  |  IP: Logged Tom Bert Expert Film Handler Posts: 134 From: Belgium Registered: Apr 2010 posted 03-25-2019 02:44 PM                     Some facts: - shaking the screen helps to mitigate speckle. Shaking anything else (the projector, your seat, your head, ...) does not help. Speckle is created at the screen (surface) - Christie Extended Wavelength diversity (mentioned somewhere here) was actually 9P (not 3, not 6 but 9 primaries) - speckle is never 0. Even a Xenon lamp projector has speckle. It's a matter of getting the number below the visibility threshold for the majority of the population. Looking forward to seeing you at CCON 2019: cool new laser stuff to be expected on the Barco booth :-)  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 03-25-2019 03:23 PM                     Speckle is not created at the screen surface. Speckle is an interference pattern produced on the imager - a camera will show speckle, created at the imager surface by the incident light waves interfering. In your eye it's created on the retina. Speckle is much more intense with coherent laser light but any narrowband monochromatic light can produce some speckle.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 03-25-2019 08:36 PM                     I've seen one manufacturer shake the light source to mitigate speckle.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 03-25-2019 09:34 PM                     I've also been thought that speckle, like we perceive it, is essentially created on your retina as an interference pattern of coherent light. In simplified terms, it's multiple light waves with the same frequency hitting your retina around the same position at a slight phase-shift, creating a wave pattern. By shaking the screen, you're shaking the instrument that's creating the reflections that eventually end up interfering in your eyes. By shaking it fast enough, the interference patterns in your eyes also "shake" sufficiently, you don't perceive them anymore. I'm not a scientist in this particular field, but it conforms with the elementary science I've been thought. So if this isn't correct, please elaborate. The fact that it's created at the point of "imaging" is also why speckle is perceived differently by people with different sight. I was aware that most projectors, especially DLP projectors, already produce a small amount of speckle. It's mostly visible in large, totally white surfaces, but most people aren't aware of it. It's also almost impossible to photograph or film. I didn't know that that particular Christie solution was a 9P solution. I guess they used a polarized 3D solution for this install, instead of a primary color filter solution.  |  IP: Logged Tom Bert Expert Film Handler Posts: 134 From: Belgium Registered: Apr 2010 posted 03-26-2019 08:50 AM                     quote: Dave Macaulay Speckle is not created at the screen surface. Speckle is an interference pattern produced on the imager - a camera will show speckle, created at the imager surface by the incident light waves interfering. In your eye it's created on the retina. Speckle is much more intense with coherent laser light but any narrowband monochromatic light can produce some speckle. True. I just wanted to make clear that shaking things apart from the screen (the projector, your seat,...) does not help  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 03-26-2019 09:31 AM                     Well, I'm sure a seat shaker will certainly aid in helping patrons ignore speckle. - Carsten  |  IP: Logged

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