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Thread: Scanners--How exactly...?

  1. #1
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    Default Scanners--How exactly...?

    This may be a stupid question but here goes..
    When making a ILDA test pattern do the scanners deflect the beam making a raster like a TV or do they just go to the spots needed? Also, what is the maximum physical/ mechanical limitations of a typical scanner? I can only imagine that they could move back and forth a couple of hundred hertz before inertia and mass would limit the speed--(I can picture parts breaking and mirrors shattering..)
    Thanks
    Steve

  2. #2
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    I presume the scanners run near the pps, after all the beam is static and it is the scanner that makes it 40kpps.

    Jim

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    The scanners normally deflect the beam like a vector graphics, that is, the scanner makes the circle, then blanks the laser to go to a corner and draws the square, etc.

    There are, however, some programs out there that will make raster images with faster scanners (think pangolin here), but you will need faster scanners as the requirements increase a lot in this mode (a lot more points to draw)
    Remember the future?, That'd today, as you imagined it yesterday.

  4. #4
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    Hey Steve-o;

    Nah... It's a good question!

    All ilda files are vector graphics files. The picture is "drawn" by the galvos much as one might "draw" a picture with a pen and paper.

    If you've ever created an image with Autocad, or better yet with Adobe Illustrator, then you understand how vector graphics work. If not, then just remember that the galvos have to trace the beam around every line that you see on the image. It's not a raster scan, it's a continuous line tracing, and there can be overlaps and line crossings. (Of course, the laser has to be blanked at times - just as you might lift the pen off the paper when drawing something by hand. Otherwise you'll have connecting lines between all the features of the picture.)

    Pangolin does have a special feature that will allow you to actually do TV-like raster-scanned images using a laser projector, even in real time from a live video source, but because galvos can't move fast enough the resolution and frame rate will suffer. (Remember that the horizontal sync oscillator for standard NTSC video runs at 15.7 Khz. That's a lot faster than any galvo!)

    If you replace the X galvo with an AOM you can get mich faster scan rates, and thus higher frame rates and higher resolutions. But then you will only have a few degrees of deflection, so there's always going to be a trade-off. I'm pretty sure Pangolin is the only company that offers the raster-scan feature. (Edit: Clarification - the "live video" raster scan, that is. There are other solutions for raster scan playback of previously saved material.)

    The ILDA test pattern has some elements in it that are designed to be beyond the scanner's capability. For example, the central circle inside the square is designed so that the scanners are "ballistic". Bill Benner explained this in detail at the recent FLEM gathering; I'll try to paraphrase as best I can:

    Essentially, what the central circle is doing is sending points to the galvos that are so far apart and changing so fast that the galvos never really reach the proper position before the next point is sent to them. Thus, they are under constant acceleration at near the maximum slew rate. (That is, they never really catch up to where the signal is telling them to go.) The resulting shape and size of the circle tells you a lot about how the galvos are performing under maximum load. It also can tell you if one galvo is running faster than the other.

    Bill also explained that an early technique of ILDA artists was to put the scanners into a ballistic arc in an effort to reduce flicker in an image. Remember that when a scanner is ballistic it is traveling as fast as it can. So if you artificially expand some of the points in your ilda file, you can "trick" the scanner into drawing a circle at max speed while only using a few points.

    Bill called this technique "pulling points", because to accomplish this feat you delete most of the evenly-spaced points in that section of the drawing and then "pull" the few remaining points farther out, or away from each other, such that the galvos will never actually reach the points but the traced arc between them will end up where you want it. I gather that the process involved a lot of trial and error, and was really only done in the old days because galvos were so dog slow that it was the only way to get a complex picture to be drawn without having a lot of flicker...

    So, the bottom line is that ILDA files are like line drawings that the galvos have to trace out point-by-point. If you have a series of points that are very far apart, it is possible that the galvo won't be able to get to the current point before the signal changes to the next point in the file and the galvo heads off towards that point. This is why most ilda file editors will allow you to set the maximum distance between points to some constant value. It keeps the galvos on track...

    As far as how fast the galvos can actually move - well, that's a *really* good question. I've never been able to find a straight answer to that question either. You are correct that the resonant frequency of the galvo is a limiting factor, but those numbers are hard to find - even when reading the galvo spec sheets from the Cambridge website! But the point per second speed rating of the galvos does not mean they can cycle full-travel back and fourth that many times per second. It's more of an emperically-derived guideline that gives you an idea as to the types of ilda files you can display (in terms of complexity) at a given optical scan angle. The actual full-travel maximum rate is at least an order of magnitude slower.

    Adam
    Last edited by buffo; 02-10-2007 at 05:39.

  5. #5
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    My LDS Pro software has a raster function in the effects menu. Simple files work best but doesn't appear the best it could. Faster galvos and a smaller scan area would produce finer details....
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

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    Nicely put Adam and thanks for the test pattern interpretaion.
    I guess also thanks to Bill for explaining this at FLEM

    Rob

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    Thumbs up

    <blush>


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    Thanks Y'all!
    My understanding of scanner/galvo principles is much improved now.
    Appreciate it!
    Steve

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