Raytools Original Protective Lens D27.9 x 4.1mm for 6KW

Key Features

• Original Raytools protective window, not an unbranded near-size substitute
• High-purity fused silica (quartz) substrate, plane-parallel and polished both faces
• AR coating on both faces, centred on the 1064-1080 nm fiber laser band
• Over 99% transmittance, so beam power reaches the sheet instead of heating the glass
• Exact 27.9 mm diameter × 4.1 mm thickness for Raytools drawer-type lens holders
• Focusing-side (lower) protection, dimensioned for 6 kW-class cutting heads
• Both faces coated identically - no orientation to get wrong during a shift change
• Bulk supply across India with GST invoice on every order

Specifications at a Glance

Check these against the window currently in your head before you order. Diameter and thickness are the two that decide fitment; everything else describes how the optic behaves once it is in.

Is 27.9 × 4.1 mm the Right Window for Your Head?

Yes, if your head takes a focusing-side protective window of that size - the 27.9 × 4.1 mm is the lower protective window on Raytools heads in the 6 kW class. Confirm by reading the size off the old window or measuring it with a vernier, because fitment is decided by diameter and thickness, never by the model badge on the head.

The Raytools protective window range covers four standard sizes across two positions in the head. Find your stage and power class here, then verify against the part you are replacing:

The range fits Raytools BM109, BM111, BM114S, BT210S, BT240S and BT240S-RC heads with drawer-type lens holders. If your head is not on that list, or the machine came rebadged from an integrator, do not guess from the model name - take the old window out and measure it, or send us a photo of the head label and the old glass on WhatsApp at +91 92740 95891 and we will confirm the size before you order.

Why the 4.1 mm Thickness Matters as Much as the Diameter

Because a flat window in a converging beam pushes the focus further from the lens by roughly t(n-1)/n. For 4.1 mm of fused silica that is about 1.27 mm of focal shift - a fixed offset your machine was calibrated around. Change the thickness and you change that offset.

This is the part of protective-window selection that gets skipped, and it is pure optics rather than opinion. Fused silica has a refractive index near 1.45 at 1064 nm, so a 4.1 mm plate displaces the focal point by about 1.27 mm. That displacement is not a fault - it is baked into how the head was set up. Fit a 5 mm window into the same seat and the displacement becomes about 1.55 mm, moving your focus roughly 0.28 mm deeper into the sheet than the program expects. On 10 mm mild steel you might get away with it. On 1 mm stainless at production speed you will not: the edge goes rough, the operator starts chasing focus offsets, and the actual cause is sitting in the drawer.

A 0.1 mm error - a 4.0 mm window sold as "the same thing" - is optically trivial, around 0.03 mm of shift. It still matters, for a mechanical reason instead: the drawer seat and its seal are machined for 4.1 mm. An undersized window sits loose and the seal no longer closes, so shop-floor dust is drawn into the head and settles on the focus lens you were trying to protect. An oversized one loads the glass when the drawer is closed and can chip the edge. Order the exact thickness; there is no useful tolerance to trade here.

What "For 6 kW" Means on a Piece of Glass

It is a cleanliness and coating-quality requirement, not a mechanical one. A window that is fine at 1 kW can fail within minutes at 6 kW, because absorbed power - and the damage it does - scales with the beam behind it.

Nothing about a protective window "handles" power in the way a cable handles current. What changes with power is the consequence of imperfection. A film of oil, a fingerprint or a scatter of spatter freckles absorbs a tiny percentage of the beam. At low power that percentage is a warm spot. At 6 kW it is a heat source inside the glass, and heat inside glass means thermal lensing - the window quietly starts behaving like a weak lens and drags your focal position with it as the shift goes on.

Damage almost always starts in the AR coating rather than in the silica bulk, and it starts at a defect: a pit, a scratch, a bonded spatter ball. Once absorption concentrates at that point, the process runs away - the coating burns through, the local heating cracks the substrate, and at 6 kW the window can go from serviceable to destroyed in a single cut. This is why an original, properly coated window with clean polished faces earns its place at this power level, and why the habit of running "just one more shift" on a marked window is the most expensive habit on the machine.

When Should the Protective Lens Be Replaced?

There is no fixed hour count - window life depends on material, assist gas, pierce frequency and nozzle condition. Replace on evidence: visible pitting or burn marks, haze that does not clean off, coating discolouration, or cut quality that degrades across the shift and recovers after a window change.

Inspect at every shift change. Hold the window at an angle under a bright light rather than looking straight through it - damage that is invisible face-on shows up clearly as scatter when the light rakes across the surface. Match what you see here:

Handling and Cleaning - Getting Full Life Out of Each Window

• Hold it by the rim, never by the faces - skin oil burns in permanently, so use lint-free finger cots or clean gloves
• Clean a film, but replace damage - a wetted lint-free tissue drawn across the face once and discarded clears fume deposit
• Anything still visible after a wet pass is in the coating, not on it, and no further cleaning will bring it back
• Never dry-wipe a gritty window - bonded spatter dragged across the glass cuts the scratches that become the next absorption sites
• Change it in the cleanest spot available - wipe the head exterior first and keep the drawer open for as little time as possible
• Check the seal and the seat every time - a perished O-ring or a window sitting proud lets dust reach the focus lens continuously
• Store spares sealed and dry - windows kept loose in a toolbox drawer arrive at the head already scratched
• Keep a set on the shelf, not one spare - a machine stopped waiting for a low-cost optic costs far more than the pack

Features and Benefits

How to Order the Right Window First Time

1. Take out the window you are replacing and read the size off it, or measure the diameter and thickness with a vernier. The old part is the most reliable specification you have.
2. Confirm which stage you are ordering for. The focusing side (lower) takes 27.9 × 4.1 mm; the collimation side (upper) takes the thinner 24.9 × 1.5 mm window. They are not interchangeable in either direction.
3. Check your laser source power. This size covers 6 kW-class heads; larger heads step up to 30 × 5 mm and 37 × 7 mm.
4. Match the thickness exactly. A near-size window will physically go in and will still be wrong - it changes the focal offset your machine is calibrated around and compromises the drawer seal.
5. Order in a pack sized to your consumption rate, not one at a time, and ask for bulk rates if you run multiple machines. Request a GST invoice at checkout so the input credit is documented properly.

Applications & Industries

• Fiber laser sheet metal cutting machines running 6 kW-class Raytools heads
• Automotive and auto-component manufacturing lines
• Industrial machinery, fabrication units and laser job-work shops
• Electrical enclosure, switchgear and control panel manufacturing
• Tube, pipe and profile cutting operations
• OEM machine builders, service engineers and spare-parts dealers

Original Raytools Window vs Generic Substitute

Every protective window looks identical in the packet. The difference only appears on the machine - usually at the worst possible moment in a production run:

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