Laser optic still marked after cleaning? How to tell residue from damage

If a fiber laser window or lens still shows marks after you have cleaned it, the first job is to tell residue from damage. Clean it once more with a fresh sheet of lens tissue and fresh solvent. A streak, ring, smear or fibre that moves, fades or changes shape was left by the cleaning, and a better pass removes it. A dark pit, a fine line, a fused bead of spatter or a stain that looks exactly the same after two clean passes is in the coating or the glass, and no amount of rubbing will shift it. Replace that optic. If the optic now looks clean but the cut is still poor, the fault is somewhere else in the head.

Inspect before you clean again

Most repeat cleaning goes wrong because nobody looked properly the first time. Newport's optics cleaning note describes the inspection: hold the optic close to a bright light and look at it from several angles, so dust and films show up as scattered light. A torch held at a low angle across the face works on the bench.

• Check both faces. Fiber protective windows are coated on both sides, and a film on the underside looks the same as one on top.
• Hold it by the edge, in powder free gloves or finger cots. A new fingerprint at this stage sends you round the loop again.
• Look at the holder and seal ring too. Dirt on the seat transfers back onto a clean window as soon as it goes in.
• Note where the mark is. A mark that follows the line of your last wipe is residue. A mark in the middle of the beam path, round and dark, is usually a burn.

Then do one test clean with the drop and drag method from our lens tissue cleaning guide: dust blown off first, a fresh sheet, a few drops of solvent, one slow pass. Look again. What is left after that pass tells you which list below you are in.

Marks the cleaning left behind

These come off. Each one points to something in the method, so fix that, or the mark comes back after every clean.

• Streaks along the direction of the wipe. The solvent did not evaporate evenly. Edmund Optics notes that with the drag method done correctly, the solvent evaporates uniformly without leaving streaks or spots, so streaks mean the pass was too fast, the tissue too dry or the solvent too slow to dry. Newport warns that isopropyl alcohol is safe but its slow evaporation can leave drying marks. Use a fresh sheet, a slower pass, and ethanol of at least 99.5 % or acetone if your head maker lists them.
• A ring near the edge. Solvent pooled at the rim of the holder or the edge of the glass and dried there, carrying the dissolved dirt with it. Shake the excess off a folded pad before it touches the glass, and on a window out of its holder, clean the edge before the face.
• Fibres or lint. Dry tissue, a reused sheet, or a wipe that is not lens tissue. Blow them off with oil free canned air and finish with one wet pass.
• A smear that spreads and gets wider. Oil. It comes from bare fingers, a pad dipped back into the solvent bottle, or shop compressed air, which Precitec says must never be used on optics because it carries oil and dust. Change gloves, decant fresh solvent into a clean container and use a new sheet for every pass.
• A dull film or water spots after drying. The cleaner was wrong. Precitec warns that spectacle wipes leave an anti fog residue that damages coatings, and that camera and contact lens cleaners can contain water. Stick to reagent or optical grade ethanol, isopropyl alcohol or acetone.

Newport's own advice for drying marks is to blow the remaining solvent off from one direction and to wipe in one continuous motion, so the drying edge never stops on the glass.

Marks no cleaning will remove

These stay put however many times you clean, and pressing harder only risks scratching the coating around them. The window is a consumable: replace it.

• Black pits or dark spots in the beam path. Spatter or a speck of dirt absorbed laser power and burned into the coating. Lasvio's inspection guide describes these as burn pits that cannot be wiped off.
• Fine lines. Scratches, usually from wiping before the dust was blown off. Edmund Optics warns that particles trapped under the tissue scratch the surface, and Newport that dry lens tissue can scratch.
• Raised beads that catch a fingernail through the tissue. Fused spatter. It is welded to the glass.
• A yellow or brown film, or a stain shaped like a fingerprint. Edmund Optics notes that a fingerprint left on a coated optic can leave a permanent stain, which is why prints should be cleaned off straight away.
• Cracks, chips or coating lifting at the edge. Mechanical damage. Precitec warns that pressing the fitting tool too hard when seating the seal can break the window.

Never turn a marked window over and refit it. Precitec is clear on this: the dirt then faces the lens above and burns it, and the focusing or collimating lens is far more expensive than a window. Our guide on when to replace a fiber laser protective window shows how to read the size off the old one. Windows such as the Raytools 211LCG0125 D37 x 7 mm, 30 kW come in 1, 5 and 10 packs, and the full range of 54 cutting head and handheld windows is in our laser protective windows collection.

The optic is clean but the cut is still bad

A clean window that does not fix the cut means the window was not the problem, or not the only one. Work through these before you open the lens housings.

• Window frame not screwed fully home. Precitec's manual says a frame that is not firm lets the window vibrate under cutting gas pressure, puts the focal position out and spoils the cut. Refit it to the mechanical stop.
• Nozzle worn, damaged or off centre. Bodor traces incomplete cuts to the beam and nozzle being out of line. Check the nozzle bore and run the tape shot centring check from our nozzle changing guide. Replacement nozzles are in the laser nozzles collection.
• Dirty or damaged lens above the window. If a window was ever refitted dirty side up, check the focusing lens next. Open it only once the window and nozzle are ruled out.
• Contaminated cutting gas. Precitec warns that oil residue from compressed air feeding the head causes hot spots on the optics, and Bodor names moisture, dust and oil in the gas. A window that hazes again within a shift, evenly across the face, points here rather than at your cleaning.
• Condensation. TEYU, a laser chiller maker, notes that moisture can form on cutting heads and optics when the chiller water runs below the workshop dew point. A fog that appears on a cold, humid morning and is gone by the afternoon is water, not dirt. Set the optics circuit water temperature above the dew point rather than cleaning the window again.

Handheld welding guns

The same split applies to the protective lens in a handheld gun, with two extras. Durma's guidance on welding gun lens damage traces black dots on the lens to contamination getting in while the lens holder was out, so wipe around the holder before you pull it and tape the slot while it is open. It also lists light that starts strong and then weakens as a sign of lens damage, and notes that a red light left off centre burns out the seal ring.

If the lens is clean and the seal ring is flattened, hard or discoloured, replace the ring as well. Our 20 x 3 mm protective lens for Ospri and Relfar guns comes in packs of 20 and 50, and the matching rings for five gun makers are in our laser seals collection.

When the marks keep coming back

If the window cleans up but is spotted again within hours, the cleaning is fine and the cut process is sending spatter up into the head. Mazak Optonics notes that window spotting normally happens during piercing, with low pierce gas pressure and a small nozzle gap. Our guide to making protective windows last longer covers the pierce height, gas and delay settings that reduce it.

Keep a closed booklet of lens tissue at each machine, so nobody reaches for a shop rag or a spectacle wipe. Each booklet holds 50 sheets of 100 x 150 mm; the 3 pack is see current price and the 10 pack is see current price. If you cannot find the window or seal ring for your head, send us the dimensions through our quote request page.

Frequently asked questions

Why does my protective window streak after I clean it?
The solvent dried unevenly: the pass was too fast, the sheet too dry, or the solvent too slow to evaporate. Newport notes that isopropyl alcohol can leave drying marks. Clean again with a fresh sheet and a slow, continuous pass.

How do I tell a burn mark from dirt?
Clean it once with a fresh wet sheet. Dirt moves, fades or changes shape. A burn pit looks exactly the same afterwards, usually dark and in the beam path, and means the window needs replacing.

Can I polish out a scratch or burn on a protective window?
No. The marks are in the anti reflection coating or the glass. A damaged window absorbs power at that spot, so replace it.

The window fogs up on cold mornings. Is it dirty?
If the fog clears as the shop warms up, it is condensation. Set the chiller's optics circuit water above the workshop dew point, as chiller makers such as TEYU advise.

Can I just turn a marked window over?
No. Precitec warns that the dirt then faces the lens and burns it. Clean the window properly or fit a new one.