Overscanning
When your laser engraves graphics set to Fill or Image Mode, the line-by-line, back-and-forth movement across the graphics is sometimes referred to as scanning.
Overscanning refers to additional moves added to the beginning and end of each line to give the laser space to speed up before firing, and slow down afterward, which in turn prevents the edges of engraved areas from burning darker than the centers.
Note
Only gantry-style lasers with GCode-based or DSP controllers use Overscanning.
Galvo lasers compensate for overburning at edges using Delay settings.
How Overscanning Works¶
Engravings made without Overscanning are prone to overburning at the edges due to the relationship of Speed and Power output in laser engraving.
The longer your laser spends passing over any part of an engraving, the more effective power it will output over that area. That means that the same Power setting will burn more at a slower Speed than a faster one.
When your laser approaches the end of a scan line, it needs to decelerate so it can change direction, and, after turning around, accelerate to reach the Speed you told it to engrave at. It can only slow down and speed up so quickly — just how quickly depends on the laser, but all lasers have physical limits.
That means if you tell your laser to engrave an area, and it's not allowed to travel beyond the bounds of that area, it will be engraving as it slows down and speeds up, and so the edges will absorb more power than the center, and burn darker.
By giving your laser extra room to accelerate and decelerate with the laser off, you can ensure that it only fires once it's reached the full Speed you set it to engrave at, resulting in an even burn across the entire engraved area.
Applying Overscanning¶
DSP controllers apply Overscanning automatically, so DSP users never have to do so manually, and do not have the option to disable it.
Users with GCode-based lasers have the option to enable or disable Overscanning in the Cut Settings Editor, and to specify the distance.
Overscanning distance is calculated as a percentage of Speed. You'll see the amount of required distance for your combination of Speed and Overscanning percentage to the right of the percentage field.
If you increase Speed, Overscanning percentage, or both, the amount of required distance will go up.
Speed, Overscanning Distance, and Job Time¶
Although you can't control the percentage through LightBurn, Overscanning distance varies based on Speed for DSP lasers as well, and in the same way — as you increase engraving Speed, Overscanning distance increases as well.
This is, again, due to the physical limits of your laser — while every laser is different, there is a limit to how quickly any laser can slow down or speed up.
No matter what that limit is, it will always require more space and time to slow down from or speed up to a relatively high speed than a relatively low one.
Imagine driving a car and quickly pressing the brakes to bring it to a halt. The car will require a longer distance to stop if you are traveling at 60 miles per hour than if you are only traveling at 10 miles per hour. It will also take more time to halt the faster-traveling car than the slower-traveling one.
Because high Speed settings require more distance and time for acceleration and deceleration, increasing Speed does not always lead to a much faster overall job time.
Increasing the Speed you set your laser to engrave at will yield diminishing returns in time savings the higher you go, and, in some cases, increasing past a certain point can actually lead to an increase in job time, rather than a reduction.
You can use the Preview window to check the your project's estimated run time, and see the effect of a range of Speeds on overall time.
Troubleshooting¶
Error Messages
GCode-based lasers
If you enable Overscanning for a GCode-based laser and there isn't enough room at the edge of the job to accommodate the extra travel distance, you may see LightBurn's Cut may be out of bounds due to overscan settings error, or your laser's controller may return an ALARM:2 error.
DSP lasers
If a job on a DSP laser requires Overscanning and there isn't enough room at the edge of the job to accommodate it, your DSP controller may return the Not Enough Extend Space error.
Because Overscanning requires your laser to travel extra distance outside the exact boundaries of your graphics, if your graphics aren't positioned properly, or you are too close to the edge of your machine when using Current Position or User Origin as your Start From mode, you may accidentally command your laser to travel outside of its physical limits, once the Overscanning distance is included.
To correct excessive Overscanning:
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Reduce Speed or Overscanning percentage (if you have a GCode-based laser).
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Move your graphics in toward the center of your Workspace, if you are using Absolute Coordinates positioning.
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If you are using Current Position or User Origin to set a custom Origin, set it closer to the center of your laser's work area.
Overscanning Distance in the Preview Window¶
You can check for excessive Overscanning using the Preview window.
Make sure Show traversal moves is toggled on, and you'll see the additional Overscanning moves indicated as red lines (black lines indicate engraving or cutting moves).
The maximum boundaries of your laser's work area are indicated by a green outline. To see the outline you may need to zoom out by pressing - on your keyboard, or by using your mouse's scroll wheel (or two-finger dragging if you're using a trackpad).
If the red lines cross the green border on one side, and you've selected Absolute Coordinates as your Start From mode, the Overscanning moves will command your laser to travel outside its work area. If you're using Current Position or User Origin, it may still be ok, depending on where you establish your custom origin.
If the red lines cross the green border on two sides, no matter which Start From mode is selected, it means the Overscanning distance is excessive, and will command your laser to travel outside its physical bounds.
Related Topics¶
For more help using LightBurn, please visit our forum to talk with LightBurn staff and users, or email support.