Laser engraving is really physical material removal: the heat of the concentrated beam vaporizes the material. But this is a chemical process — burning needs time, even at high energy.
You use the same process for cutting and for engraving — only the speed-to-power ratio differs. In engraving, the goal isn't depth, but reproducing the original image as faithfully as possible.
The chemistry's message: if the burn time isn't enough, image detail is lost. If it's too long, the material overburns and details bleed together. Both result in poor engraving.
The speed figures in a machine's specifications can be real — but the chemistry won't have the time it needs. A NEMA stepper motor can manage 64 meters per minute with a GT2 16T pulley. At that speed the laser light merely grazes the material.
Burn time is too short. The light barely touches the material — faint, detail-poor engraving, or none at all.
Burn time is too long. The material burns deeper, the dither pixels bleed into each other — a blurred, overengraved result.
The basics: find the speed and power at which burning already occurs, but cutting doesn't yet occur. This is material-dependent — there's no universal recipe.
Organic materials (wood, leather, paper, MDF) are generally soft and easy to ignite — less energy is needed, but they too need burn time. There can be differences between materials — and even between identical-model lasers from the same manufacturer.
For more easily ignitable materials (paper, thin wood) choose a smaller line interval / higher DPI — otherwise your dither pixels will merge together. For harder materials (thick wood, MDF, leather) you can lower the DPI. Think this way up to the 5W module — at higher power the dynamics change.
The laser fundamentally thinks in black and white. Grayscale engraving would need a material whose change of state visually produces shades of gray — practically no such material exists.
The laser varies the S value (power) pixel by pixel. On most materials this doesn't give reliable gray shades — the result is unpredictable.
A pattern of black and white (unengraved) areas creates the sense of gray. The laser only burns or doesn't burn — an approach that matches the material's physics.
Always choose dither for photo engraving. The dither algorithm (Floyd-Steinberg, Jarvis, Stucki) determines the pixel pattern — Jarvis and Stucki generally give a more detailed result, but with slower processing.
Engraving contrast depends on how completely the black areas burn away, and how intact the white areas stay. If the power is too high — it also affects the white areas. If too low — the black areas don't burn away properly.
To get it precise: the power needs to be just enough for the dither pixel to burn, but not enough to bleed into the neighboring pixel. This is the "narrow range" that has to be found for each material and module — through testing.