A 20mm CVD zinc selenide lens with the same 50.8mm focal length a stock K40 uses, ground to a meniscus shape and coated on both faces. It is a straight optical upgrade, provided your laser head has a 20mm seat.
The Cloudray lens is 20mm across, 2mm thick and focuses at 50.8mm, the same two inches most K40s ship with. Keep the focal length and you keep your focus height, so the swap changes the quality of the spot, not your setup. The difference is in the glass and the shape.
Cloudray grinds this lens as a meniscus: one face convex, the other slightly concave. Compared with a plano-convex lens of the same focal length, a meniscus bends the beam more evenly across its two faces, which reduces spherical aberration and gives a slightly tighter spot. On a 40W machine you notice that most in fine engraving and small text.
Most stock K40 lenses are budget PVD zinc selenide, and many owners find theirs hazed or spotted after a year of smoky work. A clean CVD lens will not add power the tube does not make. What it does is deliver more of that power into a smaller spot, which is where engraving detail and cutting speed come from.
| Spec | Value |
|---|---|
| Diameter | 20mm (0.79 in) |
| Focal length | 50.8mm (2 in) |
| Thickness | 2mm (edge) |
| Material | CVD ZnSe (chemical vapor deposition) |
| Shape | Meniscus |
| Transmittance | 99.5% or better |
| Coating | Double-sided solid coating, wipe resistant |
| Wavelength | 10.6 µm CO2 laser |
| Laser power | 40W to 200W |
| Model | CVD ZnSe D20 F50.8 ET2 (Meniscus) |
| Weight | 20 g |
| In the box | 1 lens |
| Use | CO2 laser engraving and cutting machines |
K40 heads do not share one lens size. Older machines often used 12mm lenses, many later ones 18mm, and some aftermarket heads take 20mm. Cloudray's own K40 air-assist head takes 12mm and 18mm lenses only, and states plainly that it cannot hold this 20mm lens.
Unscrew your lens tube and measure what is there. Calipers across the old lens settle the question in seconds.
Thickness matters as well as diameter. A 2mm lens in a seat cut for a thicker one leaves the retaining ring short of the glass, so the lens can shift. Add the correct spacer or O-ring rather than cranking the ring down harder, because a lens that moves in its seat throws the spot off-centre.
If your head seats 18mm, look at an 18mm lens instead, or change the head. Lightobject's K40 head takes 12, 15, 18 and 20mm lenses, and Cloudray sells K Series whole sets that bring their own head and mirror mounts with a 20mm lens already inside.
| Your head seat | What fits |
|---|---|
| 12mm | A 12mm lens, or a new head |
| 18mm | An 18mm lens, such as a 38.1mm or 50.8mm version |
| 20mm | This lens drops straight in |
A lens that is too small will rattle off-centre, and a thin shim will not hold it square. Never force an oversized lens into a seat; ZnSe cracks under point pressure.
Zinc selenide is soft, easily scratched and toxic. Treat the lens as a hazardous part, not a piece of glass. Wear powder-free nitrile gloves or finger cots, hold it only by the edge, and keep it off bare workbench surfaces.
Fit it the right way up. The convex face points up toward the incoming beam, and the flat or concave face points down toward the work. Reversed, the lens still focuses, but the spot is larger and less sharp.
Clean it when you see haze, smoke film or spatter, which on a K40 without air assist can mean every few hours of cutting.
Good air assist keeps smoke off the lens and extends the time between cleans. That is why air assist comes so early in the upgrade order.
Cloudray sells this lens alone and inside several bundles, and the choice turns on the head you already have. Pick the version that solves your actual fit problem, not the biggest box.
The whole sets make sense when your stock mounts are loose, cracked or out of adjustment range. Molybdenum mirrors are uncoated and hard to damage, which suits a smoky K40. Replacing the whole path at once also means one alignment job, not three.
Against the other 50.8mm options, CNCOLETECH sells a 20mm CVD lens in the same focal length with a plano-convex shape. For a shorter focus and finer engraving, the Mssoomm 18mm 38.1mm lens or the TEN-HIGH 18mm lens suit 18mm heads. For thicker stock, a 63.5mm lens such as the CNCOLETECH 20mm trades a larger spot for deeper focus.
After any lens change, check focus with a ramp test and confirm the beam still enters the head centred. The alignment and focus guide walks through both.
Only if your head seats a 20mm lens. Many K40 heads take 12mm or 18mm, and Cloudray's own K40 head takes 12mm and 18mm only, so measure your old lens first.
Convex face up toward the incoming beam, flatter or concave face down toward the work.
No. At 50.8mm it matches the stock K40 focal length, so your existing focus height carries over if the lens sits at the same height.
Zinc selenide is toxic. Handle it with gloves by the edge, and if it breaks, collect the pieces wet, bag them and dispose of them as hazardous waste.
