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Thread: Will LCoS projector with laser illumination source have spekcle?

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    Default Will LCoS projector with laser illumination source have spekcle?

    Will LCoS projector with laser illumination source have spekcle?
    LCoS is basically bunch of tiny mirrors for each pixel for forming an image with an external illumination source. With lasers as with LEDs the beam will need to be expanded, unlike a scanner based projector where the beam is thin.
    In this case, will the image still appear wih speckle? Or will it be reduced due to the very wide laser beam and the micromirrors blocking parts of it?
    My only concern is getting a 720p or even 4K video projector but the speckle ruining the high definition and completely discoloring some pixels.

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    ... AFAIK there are lens-arrays in the laser beam path to "wreck" the coherency - so no speckles to expect

    Viktor

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    I wouldn't be so sure there is. With LEDs there is to make the beam profile more uniform. With lasers they use single mode diodes so might just be cutting the edges and using a rectangular section from the middle of the beam profile.
    Otherwise I would have no explanation how the laser pwoered LCoS projectors are focus free.

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    Makers of laser illuminated projectors do everything they can to eliminate laser speckle. It's usually done with a spinning diffuser. A second LCoS display can also be used to despeckle without moving parts, by displaying a randomly changing phase pattern. Choice of screen material affects the subjective speckle.

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    Quote Originally Posted by joec View Post
    might just be cutting the edges and using a rectangular section from the middle of the beam profile
    With a single-mode source, this would be the worst case for *causing* speckle. (As in, most likely to cause lots of it.) So, yeah, they've got to have something to kill the speckle.

    Quote Originally Posted by Eidetic View Post
    Makers of laser illuminated projectors do everything they can to eliminate laser speckle. It's usually done with a spinning diffuser.
    Agreed. In fact, isn't that the exact method that Casio used for their slim-green projectors? Worked pretty well, although those were multi-mode blue diodes to start with, so not quite as bad as a single-mode source...

    Adam

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    Hm, well I ordered one to disassemble and check. They are always in focus so I can only assume the light coming out of the light modulator is still coherent to have such a high f-number and large depth of focus. Wouldn't a diffuser kill the coherence, and as a result the focus free nature of these projectors? I'm not familiar with Casio projectors and don't knpw if they are focus free. These pico LCoS ones cost about USD400, don't know if they could even afford to put a de-speckler in one, but we'll see soon.

    EDIT: an somewhat related question/idea: perhaps the de-speckling can be done on a special kind of the projection screen, such a s retro reflective?
    Last edited by joec; 07-27-2018 at 10:12.

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    The spinning wheel inside a Casio projector couldn't have cost more than a buck or two.

    As for the focus problem, you're confusing spacial coherence (which causes speckle) with low divergence (which allows for a large depth of focus). You can get rid of one without altering the other.

    Casio projectors needed an adjustable projection lens because only the blue light was from the laser diodes. Green light was created via fluorescence after being illuminated by some of the blue lasers (the fluorescent material was mounted on the same spinning wheel that killed the spacial coherence of the blue beams), and red light came from a huge LED.

    Adam

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    Quote Originally Posted by buffo View Post
    you're confusing spacial coherence (which causes speckle) with low divergence (which allows for a large depth of focus). You can get rid of one without altering the other.
    I did not know that, interesting.
    What is the diffuser made of? There seems to be an alternative diffuser which uses oscillating diffuser membrane rather than rotation. I'll ask for bulk order price as the samples they quoted me were over the price of a good 1080p LED projector. https://www.optotune.com/index.php/p...eckle-reducers

    The projector I ordered is "UO Smart Beam" and uses laser illumination with single LCoS chip to generate the image. The lens focus ring is glued in place and focus range is from 50cm to infinity. I think they went with lasers as I don't believe this high depth field could be achieved with LED sourced and were competing with others which used Microvision's laser scanning technology instead of DLP/LCoS to achieve infinite focus. I know for a fact that the latter doesn't fix speckle and the upcoming dev kit which can do 4K scanning has no plans to solve that as well. A bit skeptical how fine 4K will look with laser speckle but time will tell.

    Casio projectors needed an adjustable projection lens because only the blue light was from the laser diodes. Green light was created via fluorescence after being illuminated by some of the blue lasers (the fluorescent material was mounted on the same spinning wheel that killed the spacial coherence of the blue beams), and red light came from a huge LED.
    Interesting. Is that were those laser bars come from from which we extract some NUBM diodes?

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    Quote Originally Posted by joec View Post
    What is the diffuser made of?
    I'm not sure, as I've only ever seen videos of the insides of the unit from people who were ripping apart the Casio projectors to harvest the A140 diodes. I've never pulled one apart myself. But from the videos, it looked to be some sort of frosty glass or plastic.

    If you can find someone who still has the guts from one of those projectors that has already had the diodes removed, you might be able to buy just that wheel from them and experiment with it. I'd imagine they'd be willing to sell it pretty cheap, as they probably don't have any other use for it. (But I suspect most people who harvested those didoes just trashed the left-over parts when they were done, so it may be hard to find that stuff now...)

    There seems to be an alternative diffuser which uses oscillating diffuser membrane rather than rotation.
    Yeah, that would make sense. Any rapidly-changing refractive medium would do the trick. You're trying to average the interference pattern so it "smears" out the bright and dark spots.

    The lens focus ring is glued in place and focus range is from 50cm to infinity. I think they went with lasers as I don't believe this high depth field could be achieved with LED
    Yeah, laser diodes have super-low divergence, which is where you get the fixed focus from. Not really an option for non-coherent light sources though. Not unless you had an LED with .1 mrad divergence. (And if you did, you'd be rich!)

    Is that were those laser bars come from from which we extract some NUBM diodes?
    No. The diodes used in the Casio projectors were the A130 and A140 diodes, and were only good for about a watt or two of output. The NUBM diodes can produce 7 watts, and as far as I know people have been buying them in trays, not harvesting them from a commercial product.

    To the best of my knowledge the Casio Slim Green projector was the only product ever cannibalized on a large scale to harvest blue diodes. (Well, that's not counting the 405 nm blue-ray diodes, which were often harvested from Blue-ray burners back in the early days. But 405 is more violet than blue.)

    Adam

    EDIT: This old thread from 2011 has several good pictures of a diode block that was harvested from a Casio projector.

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    Many thanks. I just found some photos of rotating diffusers and they are a bit big too bit in my modified pico projector. I'll probably try out the oscillating speckle reducer from Optotune first.

    Quote Originally Posted by buffo View Post
    Yeah, laser diodes have super-low divergence, which is where you get the fixed focus from. Not really an option for non-coherent light sources though. Not unless you had an LED with .1 mrad divergence. (And if you did, you'd be rich!)
    TO be fair to LEDs I've seen some surface mount LEDs with about 1mm or maybe less dimension. May not be as low divergence as a laser but maybe good enough to have a depth field of 50cm - 4 meters?
    Last edited by joec; 07-28-2018 at 04:43.

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