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Thread: Galvo scanner question (method of scanning)

  1. #1
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    Default Galvo scanner question (method of scanning)

    Do galvo scanners scan line by line, or do they scan to specific point coordinates?

    So say for a tunnel effect does the scanner move in an X-Y "circle" with the laser unblanked the whole time or does the scanner move across one X line then down one Y point , unblanking the laser at specific points?


    Thanks
    JK

  2. #2
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    They are capable of both however 99% of the time they are doing vector (point to point).

    Chad


    When the going gets weird, the weird turn pro.


  3. #3
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    Cool

    Quote Originally Posted by ripe View Post
    Do galvo scanners scan line by line, or do they scan to specific point coordinates?
    As Chad mentioned above, they *can* do both. However, they're almost always moving from point to point, drawing a vector graphic. When you run them in a line-by-line configuration (called raster scanning), they will perform like a very slow television. The reason is that the horizontal scanner isn't fast enough (by at least an order of magnitude) to give you the resolution you need.
    So say for a tunnel effect does the scanner move in an X-Y "circle" with the laser unblanked the whole time or does the scanner move across one X line then down one Y point , unblanking the laser at specific points?
    Tunnels are normally scanned as a vector, so the laser never shuts off and the scanners are moving the beam in a circle. (Or, if you have a multi-color projector, then the lasers are turning on and off only when they need to color that particular pie segment of the tunnel.)

    If you tried to scan the tunnel as a raster image, you would end up with a very pixelated version of the tunnel, rather than a smooth circle. (This is due to the low resolution inherent in rastor scanning with galvos, caused by the relatively slow movement rate of the X galvo.)

    Now, if you had a magic galvo that was capable of insane speeds (IE: a small signal bandwidth of around 14.7 Khz, or roughly 176 Kpps), then you could generate a rasterized laser image that would be close to the resolution of a standard television. To get up to computer monitor resolutions (which would be like a Hi-Def TV image), you're talking about 47 Khz or better, which is comparable to 500 Kpps!

    However, there *IS* a way to generate a higher-resolution raster image using a laser projector. You simply replace the X galvo with a polygonal spinning mirror. The laser bounces off the Y scanner first, and then off the spinning mirror which scans the beam horizontally.

    Of course, the mirror has to be spinning at *very* high speed. (Even with a 45-sided mirror, which would yield a scan angle of just 8 degrees, you're still talking about spinning the mirror at 19,400 RPM just to get resolution approaching that of a standard TV!) Then too, you must have the mirror position *PERFECTLY* syncronized with the rest of the software. (This gets expensive.) But it can be done (and in fact, already has been done).

    Adam

  4. #4
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    Awesome, thanks for the detailed answer, that's exactly what I needed.

    Do raster frames have to be created as "vector" raster images, or do most laser control software packages have an option to switch to raster scanning for a specific frame?

    Thanks again,
    JK

  5. #5
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    Cool

    The software doesn't need to know the difference between a raster file and a vector file, because it's going to follow the points in order no matter what. The file creation tools, however, are different for the two types of files - even though the resulting files share the same basic format. (The primary difference being that the raster file will be much larger.)

    Not all software supports the creation of raster frames. And even among the few that do, the frames are created as raster frames right from the begining. You don't start with a vector file (like an ordinary ILDA frame) and then convert to raster. Rather, you start with a picture (like a .bpm, .jpeg, or .gif file) and then convert that directly to a raster graphic file. (Though in theory if you had a picture, or even a screen grab, of an ILDA image that you liked, you could convert that picture to a raster frame.) There's not much point in doing that though, since vector files are already a lot more compact (and easier to display) than a raster file of the same image would be.

    Note that the actual file format for a raster image is the same as it would be for an ILDA file, except that the laser is tracing from left to right and from top to bottom for every single frame. Obviously this makes for a *HUGE* ILDA file with many more points than normal, and thus it also takes much longer to scan each frame.

    The conversion tool should allow you to select how many lines of resolution you want. The goal is to get a picture that has just enough resolution to be recognizable while maintaining the frame rate high enough to eliminate flicker.

    If you've got a raster frame that was created with another software package and saved as an ILDA file, you can display it with any laser show software you want. However, you must be careful. Some raster frames are designed to be scanned at very high speed (60Kpps) over a tiny scan angle (just 2 or 3 degrees). Before you display such a file, you want to make sure you have your scan angle turned down and your speed set appropriately. Trying to scan a raster image with the scan angle set too wide will over-drive your X galvo. (And even when set correctly, displaying a raster file will *really* work the hell out of your X galvo.)

    Adam

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