Ghosting and Double Lines in Laser Engraving
Ghosting, also called double lines or a shadowed engrave, is easy to spot once you know what to look for. A letter has a faint twin. A filled shape has a fringe on one side. Thin lines look thicker than the file, or they split into two paths that almost line up.
This is not the same as uneven engraving, where one area is lighter because the bed is out of level. Ghosting is a registration problem. The head is not tracing the exact same path every time it should, or the left-to-right pass does not land on the same line as the right-to-left pass.
What Ghosting and Double Lines Look Like
On text, you may see a light "echo" beside the strokes. On a fill, alternating scan lines can look like a zipper. On a scored outline, a single vector can appear twice, a fraction of a millimeter apart. If the shadow only appears in one corner of a large job, also consider focus and bed level in Uneven Laser Engraving: Level the Bed and Check Focus. If the double image is consistent across the whole design, keep reading.
Mechanical Causes of Double Lines
Before you change software settings, put your hands on the machine. With the power off and the head free to move, gently test for slop.
Check the belts. A loose belt on the X axis is a classic cause of doubled vertical edges, because the head can lag or overshoot when it changes direction. The belt should be taut, not guitar-string tight, and it should not jump teeth on the pulley. Look at the pulley set screws too. A pulley that has slipped on the shaft will throw positioning off.
Check the head and rails. If the head wiggles, a bearing, wheel, or mounting screw may be loose. A mirror mount that can shift mid-job will also throw a second image, and it can look a lot like a software problem. Tighten what is loose, clean packed debris out of the rails, and make sure nothing on the air line or cable chain is tugging the head at the end of a travel.
If the double line is worse at higher speeds and almost disappears when you slow down, mechanics or scan offset are both still in play.
LightBurn Scan Offset and Overscan
If the machine is tight and you still see a regular, even shadow on fills, look at LightBurn. Bi-directional Fill saves a lot of time: the laser engraves in both directions. If the controller or the mechanics fire a hair late in one direction, every other line is shifted. Scanning Offset Adjustment is the LightBurn setting that corrects that timing error. See LightBurn Overscan, Scan Offset, and Camera Alignment.
The usual test is a small filled rectangle at the speed you actually engrave (LightBurn's guide uses a 50 mm by 10 mm fill with a wide line interval, such as 0.5 mm). If the edges look like a zipper, measure the distance between the mismatched line ends, then enter half that distance for that speed in Device Settings → Basic Settings → Scanning Offset Adjustment. Turn Enable Scanning Offset Adjustment on. Offset often changes with speed, so a value that is perfect for a slow photo fill may not be perfect for a fast wood fill. Do not copy a number from a forum post and assume it fits your machine.
Overscan matters here too. If the head is still accelerating when it starts firing, the edge of a fill can look darker, lighter, or slightly smeared. LightBurn can add overscan so the head is already at speed before the beam comes on. That is not the same as a double line, but the two issues get blamed for each other all the time. We go through both settings in LightBurn Overscan, Scan Offset, and Camera Alignment.
When the Artwork Is the Problem
Sometimes the file really does contain two lines. A logo that was outlined, or a shape that was offset and also left in place, will engrave twice no matter how tight the belts are. Zoom in in LightBurn, click the paths, and look at the line count. If you see stacked vectors, fix the artwork first.
Imported SVGs from other programs are frequent offenders. Weld or delete the extras, and keep a single clean path for a score or cut. For fills, make sure you do not have a fill layer and a hairline outline that almost, but not quite, matches it unless that is what you want.
A camera overlay can also make a job look "off" when the engrave itself is fine. If the preview on the camera is shifted but the finished piece is sharp, that is an alignment issue, not ghosting on the material.
How We Track Down Ghosting in the Shop
This is the order we use when a job comes off the bed with a shadow:
Zoom the file and confirm there is only one path where there should be one.
Run a small filled square at the same speed as the job, in the same area of the bed.
If the square's edges zipper, check belts, pulleys, and head play before you change software.
If the machine is tight, run a Scanning Offset Adjustment test at that speed, enter half the measured gap, and save the values.
If you are on a GCode-based machine, confirm Overscanning is enabled for that fill or image layer so the head is at speed at the edges. On DSP controllers such as Ruida (common on OMTECH CO2 machines), overscan is handled automatically and the switch will not appear in LightBurn. If a DSP job errors with "Not Enough Extend Space," slow down or move the art inward so the controller has room to overscan.
Only then rerun the real job on a scrap of the same material.
Ghosting is annoying because it makes a good design look cheap, and on a bag tag or leather patch it shows up immediately. The fix is usually mechanical or a single LightBurn adjustment, not a new machine. Test a square and let the edge tell you whether you have slop in the hardware or an offset in the software.
If the whole fill is pale or muddy rather than doubled, skip this checklist and go back to focus, optics, and material settings. Those problems need a different path.
Comments