FoundationsModule 1.3~5 min read

CO2 Lens Guide

CO2 lenses are described in INCHES (focal length), unlike fiber's millimeter F-thetas. Entry and prosumer machines (Glowforge, xTool, most Chinese imports) ship with a 2" lens; Thunder ships every model (Bolt, Nova Plus, Titan) with a 2.5" as the most versatile all-rounder. Either is a sound default. Specific work calls for shorter or longer focal lengths.

The five common CO2 lenses

Lens	Spot size (≈)	Depth of focus	Best for
1.5"	~125 µm	±0.5 mm	Fine engraving, photo, small text. Tightest-fitting puzzles and inlays (smallest kerf). Best on thin, intricate cuts such as fabric and paper. Common on Glowforge and xTool entry models.
2"	~170 µm	±1.0 mm	Default lens. Cuts up to 6mm wood / 5mm acrylic. Best general-purpose.
2.5"	~210 µm	±1.5 mm	The most versatile all-rounder: Thunder's stock lens on every model. Covers cutting under 1/2" and any engraving that doesn't need ultra-fine detail. If you own one lens, this is the one.
4"	~340 µm	±2.5 mm	Thick cuts (8-15mm acrylic, 12-18mm plywood) with cleaner straight walls. Also the best engraving lens for two- and three-layer acrylic (smoother, cleaner sign faces) and for tapered tumblers and bowls, where its long depth of focus absorbs the height change. Thunder's 4" has a removable spacer so the head can sit back from a bowl's rim and still be at focal distance.
6"	~510 µm	±4 mm	Very thick stock (20mm+); rare. Industrial use.

"Laser head," "lens," and "lens kit" all mean the same thing; the only difference is how the lens is mounted. A Thunder Bolt lens kit is a plate with a metal mount and the lens attached; the Nova Plus and Titan use an elongated cylinder head with the lens inside. Same sizes, same job.

Three things people get wrong about CO2 lenses

There is no "cutting lens" and "engraving lens." Every lens both cuts and engraves. Each is simply optimized for one end of the range: small lens = detail, big lens = thick cuts and depth of focus.
You cannot resolution-crank a wide lens into a fine one. Run the same photo on a 1.5" and a 4" with identical settings and the 1.5" wins outright; raising DPI on the 4" doesn't close the gap because the spot is physically too wide. If photos or small text are core to your work, own a 1.5".
Fit on puzzles and inlays is a lens question. A smaller lens removes less material, so pieces fit tighter; the 1.5" gives the tightest fit. You can dial fit on any lens with Lightburn's kerf offset, the 1.5" just leaves less to adjust. See Inlay Technique.

Why focal length matters more on CO2 than fiber

CO2 lenses focus a far larger spot than fiber lenses (170 µm vs 20-50 µm). The DOF advantage at longer focal lengths matters dramatically for thick cuts, a 4" lens keeps the beam tight through the full 12mm of acrylic, where a 2" lens loses focus halfway down and the cut walls bow.

A short lens bends the beam hard: a tiny spot, but it only stays tiny for a sliver of height. A long lens makes a bigger spot that stays usable over a much deeper range.

Decision rules

Thin work (under 6mm): 2" or 2.5". Sharp engrave detail; plenty of cutting depth.
Medium thickness (6-10mm): 2.5" for cleaner walls; 2" still cuts it.
Thick cutting (10mm+ acrylic, 12mm+ plywood): 4". Mandatory for clean walls.
Photo engraving or small text as a core use: 1.5". A 2" is acceptable; a 4" is not.
Puzzles, inlays, anything that must fit together: 1.5".
Thin intricate cuts (fabric, paper, lace-like files): 1.5".
Layered acrylic sign engraving: 4".
Norton tile method: 2". Standard for the technique.
Tapered drinkware on rotary, bowls, curved surfaces: 4". DOF tolerates the taper; see Tapered Tumblers.

Think of lenses like drill bits: nobody buys just one. With all four (1.5", 2", 2.5", 4") you cover fine detail, thick cuts, and everything between. With one, you spend your time working around what it can't do.

Lens swap procedure

Power off laser. Wait for beam to be cold.
Loosen the lens collar at the bottom of the head. Some heads use a thumb-screw; others a small Allen.
Slide the lens cell out (usually drops out the bottom).
Inspect the new lens, both sides clean of fingerprints, debris.
Slide new lens cell in. Note orientation: most CO2 lenses have a CONVEX side that faces UP toward the beam. Check the manual or look for an etched arrow.
Re-tighten collar. Don't overtighten, finger-tight + 1/8 turn.
Refocus. Different focal length = different working distance from material to lens.
Run a calibration cut to verify performance.

Lens orientation matters

CO2 lenses are usually plano-convex (one flat side, one curved). Installing upside down causes severely degraded performance, the beam focuses BEFORE the working surface, defocuses by the time it hits material. If the manual is unclear, look for the etched arrow or "↑" mark on the lens edge.

Lens material & cleaning

Most CO2 lenses are ZnSe (zinc selenide). Burning a ZnSe lens releases toxic selenium oxide fumes, if a lens is scorched or cracked, ventilate the area, retrieve the pieces wearing nitrile gloves, bag, and dispose as hazardous waste. Never sand, machine, or otherwise pulverize ZnSe.
Some premium lenses are GaAs (gallium arsenide), more durable, more expensive.
Cleaning: blower bulb first to lift dust. Then lens tissue + 99% reagent IPA, single-direction wipe, fresh tissue per swipe.
Replace if you see scratches, hot spots (visible burn marks), or coating delamination.

Where to source replacement lenses

American Photonics is the community-standard US source for premium CO2 optics, ZnSe focus lenses (1.5", 2", 2.5", 4", 6"), Si and Mo mirrors, and beam combiners. Tighter quality control than the bargain replacements bundled with Chinese machines and dramatically lower scatter loss, which translates to cleaner cuts and longer tube life.

Lens roles, the head-vs-lens-kit terminology, and the Thunder stock-lens note are from Thunder Laser USA's "what each laser head does" walkthrough (YouTube, 2026), covering the Bolt, Nova Plus, and Titan. Spot-size and depth-of-focus figures are community approximations; verify with a ramp test on your machine.