Blurry or doubled engraving: diagnosing three causes

Engraved text that comes out fuzzy, ghosted or visibly doubled has three quite different causes. They look similar enough that people often spend a long time fixing the wrong one.

They respond differently to one test, which separates them: run the same engraving at a much lower speed.

The triage test

Engrave the same small block of text twice: once at your normal speed, once at roughly a quarter of it. Then compare.

What you see	Cause	Go to
Sharp at low speed, doubled at high speed. Alternate scan lines shifted in opposite directions, giving a comb or zigzag look on vertical edges.	Electronic, timing	Scanning offset below
Equally bad at both speeds. A constant, parallel ghost line next to every line, same offset everywhere.	Optical, beam path	The shadow line article
Irregular and wavy, worse at the start and end of lines or where direction changes.	Mechanical, play or vibration	The vibration article

That single test resolves most cases. The rest of this page covers the electronic one, because it is the one without an article of its own.

Electronic: scanning offset

Firing the beam is not instantaneous. The command goes out, and the laser source and its power supply take a short time to respond. At low speed that delay is irrelevant. At high speed the head has moved a measurable distance in the meantime, so the burn lands slightly late.

Raster engraving scans alternately left-to-right and right-to-left. “Late” means to the right on one line and to the left on the next, so consecutive lines shift in opposite directions. That is what produces the characteristic doubled or combed look on vertical edges, and it is why it gets worse the faster you go.

The compensation is a table of speed-versus-shift values that tells the software how far ahead to fire at any given speed. LightBurn calls it Scanning Offset Adjustment; other software calls the same idea reverse interval or reverse compensation.

The measurement is straightforward in principle: engrave a test pattern at two or more known speeds, measure how far the line ends are offset at each, and enter those pairs.

Check the mechanics before the offset table

Loose belts produce almost the same symptom, and a scanning offset table will mask a mechanical fault rather than fix it. The result is a machine that engraves acceptably at one speed and badly at every other, with a table of numbers compensating for something that should have been repaired.

Check belt tension before you touch the offset table: adjusting and replacing the belts.

The values belong to your specific machine

A scanning offset table is specific to the laser source and power supply fitted. If either is replaced, the table is no longer valid and must be measured again. This catches people out after a service: the engraving quality changes and nobody connects it to the table.

Where to find the procedure

BRM supplies LightBurn with its machines, and LightBurn provides the in-depth software support under our partnership with them. Rather than keep a second-hand copy of their procedure here, use the source:

• Scanning Offset Adjustment (official LightBurn documentation)
• LightBurn forum

The simplest fix of all

If you only need sharp engraving occasionally and do not want to calibrate anything: engrave more slowly. Scanning offset error scales with speed, and at moderate speeds it is often small enough to be invisible. Slower is not always practical for production work, but for a one-off it is the fastest route to a clean result.

On a BRM machine

Before concluding anything is wrong with timing, rule out the physical causes; they are more common and they affect cutting as well as engraving:

• Adjust and replace the belts of the X and Y axes
• The laser head is no longer perpendicular (Original) / Pro
• Everything you need to know about the guides of the BRM Original
• Mirror alignment

If the engraving is muddy or low in contrast rather than blurry or doubled, that is a different topic entirely (image preparation and dithering).