Line density vs DPI in laser engraving

Line density is how many scan lines the laser writes per millimetre (some programs use per centimetre). Line interval is the distance between two scan lines. DPI counts the same lines per inch. They are three views of one setting: a higher line density, a smaller line interval and a higher DPI all mean the lines sit closer together. The conversion is exact because an inch is 25.4 mm.

• Line interval (mm) = 25.4 ÷ DPI
• Lines per mm = DPI ÷ 25.4 (lines per cm = × 10)
• 254 DPI = 10 lines/mm = 0.1 mm interval

If one program asks for DPI and another for line interval, convert with 25.4. A 0.1 mm interval is 254 DPI and 10 lines per mm. A 0.05 mm interval is 508 DPI and 20 lines per mm. A 0.08 mm interval is 317.5 DPI and 12.5 lines per mm. Software that works in lines per centimetre simply multiplies lines per millimetre by ten, so 10 lines per mm becomes 100 lines per cm. The numbers here are arithmetic, not recommendations for your machine.

• 0.1 mm ↔ 254 DPI ↔ 10 lines/mm
• 0.08 mm ↔ 317.5 DPI ↔ 12.5 lines/mm
• 0.05 mm ↔ 508 DPI ↔ 20 lines/mm

Each scan line is as wide as the laser's focused spot. When the line interval gets smaller than the spot, lines overlap: the material receives more energy per area, tones darken, fine gaps close up and photos turn muddy. When the interval is much larger than the spot, you see visible stripes between lines. The sweet spot sits near the spot width of your laser, which depends on the laser type, the lens and the focus — and it differs between a diode laser and a CO2 laser.

• Interval smaller than the spot → overlap, darker, muddy
• Interval much larger than the spot → visible stripes
• Spot width depends on laser type, lens and focus

Do not copy a line density from another machine. Engrave a small test grid on the material you will use, with a few line intervals side by side at otherwise identical settings, and pick the one where solid areas are even and fine detail stays open. In Vextrify, Photo Studio can link DPI and line interval so the image resolution always matches the lines the laser will write, and Laser Control builds the material test grid as real objects on the bed.

• Test grid on the real material, one variable at a time
• Pick the interval where solids are even and detail stays open
• Link DPI and line interval so image and laser match