Why acrylic is different
On wood or slate the laser darkens the surface, so a photo lands as tone you can see immediately. Acrylic works the opposite way: engraving does not darken it, it frosts it. The beam disrupts the smooth polished surface into a fine white micro-texture that scatters light, and that pale frost against the clear body is what gives you contrast. Every decision on this page comes back to one question - are you making a bright, even frost, or are you melting a shiny groove that disappears?
Because the contrast comes from frost, not burn colour, acrylic actually needs less laser than most beginners think. Too much power and you stop frosting and start melting: the surface goes glossy, the edges of your dots reflow into each other, and detail vanishes. On acrylic, restraint is the whole game.
Cast vs extruded acrylic
Acrylic (chemically PMMA) is sold in two manufacturing grades, and for photo engraving the difference decides everything. They can look identical on the shelf, so read the spec sheet before you buy.
| Property | Cast acrylic | Extruded acrylic |
|---|---|---|
| Engraves to | Bright, opaque white frost | Shiny, near-clear melted groove |
| Photo contrast | Excellent | Poor - almost invisible |
| Cutting edge | Slightly matte | Clean, flame-polished, glossy |
| Thickness tolerance | Varies more | Very consistent |
| Cost | Higher | Lower |
| Best use | Engraving photos | Cutting parts and clean edges |
The takeaway is simple: buy cast acrylic for anything you plan to engrave. Cast sheet is often labelled "GS" or "cast" by suppliers; extruded is labelled "XT" or "extruded". If a sheet is suspiciously cheap and dead-flat in thickness, it is probably extruded - great for laser-cut keychains, wrong for a portrait.
Engrave from the back and mirror it
For clear acrylic, the professional look comes from engraving the back of the sheet and viewing the frosted image through the glossy front face. The clear body acts like a lens: the image gains depth, the polished front stays protected from scratches, and the frost reads brilliantly against the smooth surface in front of it.
There is one non-negotiable step that trips up almost everyone the first time. Because you are looking through the material at the back, the engraving is seen in mirror. You must flip the image horizontally before you send it, or every face, logo and line of text comes out reversed.
The back-engraving checklist
- Mirror the image horizontally. EngraveBot has a one-click flip; in LightBurn use
Ctrl/Cmd + Shift + H. - Peel the mask off the back only - the side that faces the laser - and leave the front mask on to protect it.
- Refocus on the back surface. If you flipped a masked sheet over, your focus height changed; set it on the new top face.
- Remember the beam travels through nothing. On a look-through piece the frost is the image; a dark backing card behind the finished panel makes it pop.
For opaque, mirrored or colour-core acrylic you do the opposite - engrave the front and do not mirror, because you are looking directly at the marked face.
Clear acrylic on a diode laser
Here is the single most common disappointment on the forums: someone puts clear acrylic under a blue diode laser, runs a perfect file, and nothing happens. The sheet comes out untouched. This is not a broken machine - it is physics.
A blue diode emits around 450 nm, and clear acrylic is transparent to that wavelength. The beam passes straight through without depositing energy, exactly as visible light does. A CO2 laser at 10.6 µm is absorbed strongly by the same material, which is why CO2 engraves clear acrylic effortlessly. If you own a standard diode, you have three real options:
- Use a tinted or coloured cast acrylic. Black, coloured, or "one-sided mirror" cast acrylic absorbs the blue beam and frosts nicely. The clear-on-clear look is what defeats a diode, not acrylic in general.
- Paint the back and engrave through the paint. Coat the back face with a thin, even layer of matte paint (spray works). The diode marks the paint layer readily, giving a crisp image on a clear body - a paint-fill in reverse.
- Use an infrared (1064 nm) diode module. Some newer machines offer an IR laser head specifically for clear plastics and metals; that wavelength behaves more like a fibre laser and will mark clear acrylic where the blue diode cannot.
LED edge-lit acrylic portraits
Edge-lit acrylic is the trick behind those glowing signs and photo plaques that light up when you flip a switch. Clear acrylic pipes light internally - shine an LED into the edge of a sheet and the light travels through it, staying invisible until it hits an engraved mark, where it scatters and lights up bright. Your engraving becomes the only thing that glows.
That changes how you prep the image. A solid, filled photo engraving frosts a large area and looks cloudy when lit. Edge-lit pieces want line and dot work - the engraving should be mostly clear with fine marks that catch the light:
- Convert the portrait to a stipple or line style, not a solid raster. Sparse dots and thin contours light up crisply; large black fills go milky.
- Keep the marks shallow and clean. Each frosted dot is a tiny light source, so consistency matters more than depth.
- Feed the light from the edge. A slot LED base sends light along the sheet; the engraving glows and the clear areas stay dark.
To turn a photo into line-and-dot art, run it through the Dithering Lab at a lower density, or vectorise the main contours with the Image to SVG tool and engrave those lines. A high-contrast portrait with a removed background - clean it up in the Background Remover - gives the brightest, most legible glow.
Prep the photo: dither, grayscale, DPI
Acrylic frost is essentially two-state - a spot is either frosted white or clear - so it behaves much more like a black-and-white surface than a smooth-tone one. That makes dithering the reliable default here, just as it is on most materials.
Dither vs grayscale on acrylic
Grayscale (power-scaled) engraving asks the laser to make a lighter or darker frost by varying power. On a well-tuned CO2 this can produce a genuinely photographic, continuous-tone result on cast acrylic, and it is worth trying if your machine holds linear power. On diodes it usually bands or melts in the dark areas. Dithering - thousands of on/off frosted dots your eye blends into tone - sidesteps that entirely and is the safer bet for a first attempt. Floyd-Steinberg gives a fine grain for detailed pieces; switch to Jarvis if your dots merge and the image clouds over. Our dithering settings guide walks through the choice in full.
DPI on acrylic
Acrylic takes a fine dot well, but the melt risk means you should not overpack lines - dense lines pump too much heat into one strip and reflow the frost into a shiny smear.
| Laser | Sensible DPI | Interval |
|---|---|---|
| 5-10 W diode (on tinted/painted acrylic) | 254-300 DPI | 0.085-0.10 mm |
| 40-60 W CO2 | 300-380 DPI | 0.067-0.085 mm |
If the frost looks grey and glassy instead of white, you are melting - lower the DPI first, then the power. The Laser Calculator converts DPI, interval and physical size so you can match the image resolution to the dot your machine actually makes.
Starting settings for CO2 and diode
These are starting points, not guarantees. Acrylic varies by brand, grade, thickness and even colour, and your lens, air assist and machine differ from mine. Always burn a test grid on the same sheet before committing to the real piece. The goal on acrylic is a clean matte-white frost - so aim low on power and high on speed, and creep up only if the frost is too faint.
| Machine | Power | Speed | Notes |
|---|---|---|---|
| 40-60 W CO2 - cast acrylic engrave | 15-25% | 300-450 mm/s | Frost, don't melt. Air assist on. Back-engrave, mirrored. |
| 10 W diode - black/tinted cast acrylic | 25-40% | 2500-4000 mm/min | Only on absorbing acrylic; clear won't mark. |
| 10 W diode - clear acrylic, back painted | 30-50% | 2000-3500 mm/min | Marks the paint layer, not the acrylic. |
| Any - edge-lit line/dot work | Low | High | Shallow, consistent marks beat deep ones. |
Two settings matter as much as power and speed on acrylic:
- Air assist. A steady stream of air clears fumes and stops the surface reflowing glossy. It is close to mandatory for a clean white frost on CO2.
- Focus. A slightly defocused beam can give a softer, whiter frost on some cast sheets; a tight focus gives sharper detail. Test both on your grid.
Prep your acrylic photo now - free, in your browser
Masking and cleaning
Cast acrylic ships with a paper or film mask on both faces, and how you handle it changes the result. For an engraved photo you generally want the mask off the face being engraved, because engraving through paper can trap debris in the frost and dull it. Leave the mask on the opposite face to protect it from scratches and stray reflection.
- Back engraving: peel the back mask, engrave, leave the front mask on until you are done handling the piece.
- After the burn: wipe the frosted area with a soft, damp microfibre cloth to lift residue. A drop of dish soap in water helps; avoid ammonia glass cleaners and alcohol, which can craze (crack) acrylic over time.
- Fingerprints and static: acrylic attracts dust. A final wipe with an anti-static cloth keeps the frost looking crisp, especially on a display piece.
Safety: the plastics that can hurt you
Cast and extruded acrylic (PMMA) are among the safer plastics to laser, given decent extraction - they give off fumes you want to vent, but nothing corrosive. The danger is confusing acrylic with plastics that look identical and are genuinely hazardous.
- Never laser PVC or vinyl. Cutting or engraving PVC releases chlorine gas, which combines with moisture to form hydrochloric acid - toxic to your lungs and corrosive to every metal part of your machine. Many cheap "acrylic-look" sheets and sign vinyls are PVC.
- Never laser polycarbonate (Lexan). It absorbs the beam poorly, chars, yellows, catches fire and off-gasses - it is not acrylic even though it is a clear, rigid sheet.
- When in doubt, do not laser it. If a sheet is unlabelled, treat it as unknown. Buy acrylic from a supplier who states PMMA, cast or extruded on the spec sheet.
- Always run extraction. Even safe acrylic fumes should be vented outdoors or through a proper filter, and never leave a running laser unattended - acrylic can flare.
Frequently asked questions
Do you engrave a photo on the front or the back of acrylic?
On clear acrylic, engrave the back and view the frost through the glossy front - a look-through piece with real depth. Mirror the image horizontally first, or faces and text come out reversed. On opaque or colour-core acrylic, engrave the front and do not mirror.
What is the difference between cast and extruded acrylic?
Cast frosts to a bright, opaque white when engraved - perfect photo contrast - so it is the grade for engraving. Extruded melts to a shiny, near-clear groove that shows almost no contrast, so it engraves poorly, but its clean melt makes it excellent for cutting flame-polished edges.
Can a diode laser engrave clear acrylic?
Usually not directly - a blue 450 nm diode passes straight through clear acrylic. Use tinted or coloured cast acrylic, paint the back so the beam marks the paint, or fit an infrared (1064 nm) module made for clear plastics. A CO2 laser engraves clear acrylic natively.
Is it safe to laser engrave acrylic?
Cast and extruded acrylic (PMMA) are safe with good extraction. Never cut or engrave polycarbonate (Lexan) or anything with PVC or vinyl - PVC releases chlorine gas that forms hydrochloric acid, toxic to you and corrosive to the machine. If you cannot confirm a plastic is PMMA, do not laser it.
Keep going
New to photo engraving? Start with converting a photo into a laser-ready file, then dial in dithering settings. Comparing materials? See engraving photos on wood, metal, leather or slate - or get every LightBurn option spelled out in LightBurn image settings explained.