LightBurn settings for granite portraits

LightBurn is common among headstone engravers, and the questions on its forum repeat. This guide covers the image settings that matter for a portrait on black granite, and the mistakes that come up most. It does not give speed and power numbers. Those depend on the machine, the lens and the stone, and you find them with a test.

The short version

1. 1.Import a positive grayscale PNG at the final size.
2. 2.Set the image mode to Jarvis or Stucki.
3. 3.Tick Negative Image.
4. 4.Set the DPI you have tested on this stone. A CO2 often starts at 200.
5. 5.Run a test strip on an offcut before the memorial.

Why Negative Image

On black granite, the laser makes the stone lighter. Where it fires, you get a pale grey mark. Where it doesn't, the polished black stays. So the laser has to fire on the light parts of the photo: the face and the highlights. A normal photo has to be inverted for that. LightBurn's own docs point to Negative Image for this case, "for slate or granite."

Don't invert twice

The most common mistake is a double invert. Someone inverts the photo in Photoshop because black granite "needs a negative," then ticks Negative Image in LightBurn too. The portrait then etches like a film negative. PhotoGrav works the same way: its Black Granite material setting inverts automatically, so it also expects a positive file.

Jarvis or Stucki

Both are error-diffusion dithers. They turn grey tones into patterns of dots. A memorial tutorial from Thunder Laser recommends inverting, then dithering with Jarvis or Stucki. Try both on the test strip and keep the one that looks better on your stone. Trotec notes that error diffusion "increases the contrast," so start with a file that has a little less contrast than you would print.

DPI and line interval

In LightBurn, DPI and line interval are two views of the same setting. 200 DPI is a 0.127 mm interval. For a CO2 on black granite, sources report 120 to 250 DPI. LightBurn's developer has suggested 120 to 180 for a 120 W tube and about 250 for 80 W, and lower for a longer lens. See the DPI guide for other lasers.

Image adjustments

LightBurn has its own image adjustments. For granite, LightBurn's developer has suggested a gamma of about 0.8 and Enhance at 200 with a radius of about 4 or more, so a small visible halo survives. If your file already has tone and sharpening built in, leave LightBurn's adjustments neutral. Otherwise you apply them twice. Pick one place to adjust and stick with it.

Keep pure white out of the file. Trotec's guidance is to compress the output range so the extreme levels are dropped. A forum engraver described mapping the grayscale to the range between where the stone starts to mark and where it burns. A test strip shows you both points.

Pass-Through for pre-dithered files

If the file is already dithered (a 1-bit dot file), use Pass-Through. LightBurn then sends the dots as they are, and it ties the DPI to the image's pixel size. That means:

• The file must have been made at your machine's exact DPI.
• It must not be resized, in LightBurn or anywhere else.
• You need to know which pixels fire. In most software, black pixels fire. Check the polarity before you run.

Pre-dithered files are also hard to judge on screen, and engravers on the forum report confusion previewing them. Use them only when you have a reason, such as a dither you have already dialed in for one machine.

Power and speed

We don't publish power and speed numbers here, on purpose. Dynamic Stone Tools gives 60 W as the minimum CO2 power for black granite. One forum headstone job reports a 100 W fiber at 20% power. Granite is easy to over-power. Over-burn shows as dots that merge into flat white. Start low and work up on scrap.

After the etch

Wipe the stone clean. Some shops use a stone polish or mineral oil to bring up contrast, and some fill the etch with white ink.

How Stone Portrait does it

Stone Portrait's main download is a positive grayscale PNG at final size, with the DPI stored inside and POSITIVE in the file name. Import steps come with it: for LightBurn, Jarvis or Stucki, Negative Image and your tested DPI; for PhotoGrav, Black Granite. If you need one, there is also a pre-dithered dot file for Pass-Through, with the polarity in the file name. Tone defaults keep the image off pure black and white. These are starting points, not tested settings for your machine, so etch the included test strip first. Start a free trial.