Grayscale Depth Engraving on Diode Lasers: Multi-Pass 3D Relief in Wood and Acrylic

Standard grayscale laser engraving varies burn darkness by modulating power, which changes how a surface looks but not how deep the laser actually cuts. Depth engraving is a different technique: instead of mapping grayscale to power at a single pass depth, you map it to how many passes — and therefore how much material — gets removed at each pixel. The result is a genuine 3D relief carved into wood or acrylic, with raised highlights and recessed shadows you can actually feel, not just see. It's the diode-laser equivalent of a CNC 3D relief carve, and on a 20W machine like the Longer Ray5 it's slow but very achievable.

This guide covers how to convert an image into a depth map, how to structure a multi-pass job in LightBurn to follow that map, and which materials hold the detail well enough to be worth the extra time.

How Depth Engraving Actually Works

A diode laser removes a roughly fixed amount of material per pass at a given power and speed — it doesn't have the continuously variable depth control of a CNC spindle. To fake continuous depth, you break the job into a stack of passes, each one masked to a different threshold of your grayscale image. Pure white areas in the source image get zero passes (untouched, full material height). Pure black areas get the maximum number of passes (deepest cut). Mid-tones get somewhere in between. Stack enough threshold levels — 8 to 16 is a typical range — and the steps blend into what reads as a smooth relief once the material is sanded lightly or viewed at an angle.

Preparing the Depth Map

Start from a grayscale height map, not a photograph straight off the camera. A height map is an image where pixel brightness directly represents elevation — white is high, black is low — the same convention used for CNC relief carving and game-engine terrain. You can generate one three ways:

• From a 3D model: render a top-down orthographic depth pass in Blender (the Z-pass or a simple grayscale material driven by world-space height) for the cleanest, most predictable result.
• From a photo: convert to grayscale and boost midtone contrast in GIMP or Affinity Photo, then apply a moderate Gaussian blur so the depth transitions are gradual rather than pixel-sharp — a sharp depth map produces visible stair-stepping even with many pass levels.
• From vector art: assign flat gray fill values to different shapes to create stepped, geometric relief — this works well for lettering and badge-style designs where hard elevation changes look intentional rather than like an artifact.

Building the Multi-Pass Job in LightBurn

LightBurn doesn't have a dedicated depth-engrave mode for diode lasers, so you build the effect manually using threshold layers:

1. Duplicate your source image into 8-16 copies, one per depth level.
2. Apply a hard threshold to each copy at a different brightness cutoff (evenly spaced, e.g. 240, 210, 180... down to 30) using GIMP's Threshold tool or LightBurn's own dithering/threshold pass settings, producing pure black-and-white masks — each mask is a stencil showing exactly which pixels still need cutting at that pass level.
3. Import each threshold mask as its own layer in LightBurn, set to Fill mode, and assign every layer the same power/speed pass parameters for consistent per-pass depth.
4. Order the layers from lightest threshold to darkest so the shallowest cuts happen first and the deepest happen last — this keeps debris clearance consistent since later passes are cutting into already-lowered material.
5. Run air assist on every pass. Depth engraving generates far more char and debris than a single-pass engrave, and buildup between passes will scatter the beam and blur detail.

Reference Settings by Material (Longer Ray5 20W)

MaterialPasses per LevelSpeedPowerNotes Basswood / Poplar11800-2200 mm/min60-70%Holds fine detail well; light char, easy to clean up Baltic Birch Plywood11500-1800 mm/min70-80%Glue lines between veneers can shift shading slightly — test first Cast Acrylic (light colors)11200-1500 mm/min50-60%Frosts rather than chars; excellent for backlit relief pieces MDF11600-2000 mm/min70-85%Heavy char per pass — expect a darker overall relief unless cleaned between passes

Treat these as a starting point and always run a small threshold-level test strip before committing to a full multi-hour job — per-level depth varies with lens focus and material batch more than with any other engraving technique.

Time Budget and Managing Long Jobs

A 16-level depth engrave at 3-5 minutes per pass on a modest-size piece easily runs 60-90 minutes, and larger reliefs can run for several hours. Plan for this: use LightBurn's job time estimate before committing, keep the machine attended or fitted with a flame/smoke detector for anything running unattended (see this site's guide on automatic fire detection and shutdown for unattended machines), and don't walk away from a first attempt at a new depth map — early passes are where you'll catch a masking error before it propagates through the whole stack.

Finishing

Depth-engraved wood benefits from a very light sanding with 400-600 grit to knock down char fuzz on the raised highlights without flattening the relief, followed by a wipe-on oil or matte poly to bring out the shadow contrast. On acrylic, skip sanding — a soft brush and isopropyl wipe is enough, and a light diffuser or LED edge lighting behind a relief piece makes the depth variation read even more strongly than it does under ambient light.

Safety

Multi-pass, multi-hour jobs mean extended exposure to combustion byproducts and a longer window where an ember or hot spot can develop — run air assist and exhaust ventilation for the full duration, not just the final pass, and never leave a first-time depth job running unattended overnight until you've confirmed it behaves predictably on scrap material.

Depth engraving trades speed for an effect that flat grayscale simply can't produce — real, touchable relief from a machine that has no physical Z-axis feedback loop. Once you've built a threshold-layer template in LightBurn, reusing it for new depth maps is mostly a matter of swapping the source image, which makes the upfront setup worth the investment if relief work becomes a regular part of your engraving output.