How should the protective window and optics on the laser head be properly maintained?

If the protective window is replaced too late, cleaned incorrectly or fitted carelessly, it can itself cost power and cause damage.

This article describes signs, causes and the usual procedure for maintaining the protective window and optics in accordance with manufacturer specifications. What is binding is the operating instructions for your machine and the risk assessment for your workplace.

What the protective window in the laser head is for

The protective window sits behind the focusing optics in the beam direction, i.e. between the lens and the workpiece. The patent literature on laser optics clearly names its purpose: it protects the lenses, because metal spatter and fumes from processing would otherwise contaminate the optics.

Laser cleaning produces ablation products directly in front of the head. The DGUV guidance document FBHM-139 provides for extraction that captures them as close as possible to the point of origin. Some of the particles and back spatter can nevertheless reach the optics.

The protective window has one limit: according to Precitec, protective windows largely prevent dirt particles from entering, but do not help against particles that are already inside the head. Anyone opening the head should therefore do so as cleanly as possible.

How to recognise contaminated optics

Manufacturers name these signs in their instructions:

• dark spots on the surface, caused by dust, oil or residues
• Burn marks, scratches or a damaged coating
• Cracks or breakage
• declining effectiveness or irregular results with unchanged parameters, in laser welding for example a lower weld penetration depth

For visual inspection, an optics guide recommends bright light and, where appropriate, a magnifying glass, because many contaminants and defects are small. One manufacturer of cleaning lasers advises looking at the window from several angles.

Why contamination costs power

Dirt on an optical surface scatters and absorbs radiation. Both are then missing at the workpiece. One manufacturer of laser cleaning machines puts it succinctly: if the protective window is contaminated, the cleaning effect is lost.

There is also a less obvious effect. If particles are deposited on optical elements, these heat up due to increased absorption. As a result, the refractive properties and, with increasing contamination, the focal length can change — the focus shifts. This effect is known as thermal focal shift.

Contaminated optics can therefore not only let less power through, but also change the position of the focus. The result then gets worse even though the same values are set on the machine.

How consequential damage to the optics occurs

Absorbed radiation turns into heat. A guide to handling optical components describes how contaminants thereby create hot spots on the surface that can lead to permanent damage. In the case of the protective window, this can go as far as destroying the window, as a laid-open patent application on laser welding optics notes.

If the window cracks or breaks, the optics behind it lose their protection. According to Precitec, particles that settle on optics can contribute to failure through destruction of the optics.

Anyone who compensates for declining effectiveness with more power sends more radiation through a window that is already absorbing more. It makes more sense to check the optics first. If protective windows repeatedly burn, one manufacturer of cleaning lasers names back spatter and insufficient assist gas flow as possible causes.

The operating instructions define the procedure

For maintenance and inspection, FBHM-139 refers to the manufacturers’ information in the operating instructions. The maintenance intervals are expressly mentioned in this context, including cleaning of the optical equipment.

There are no uniform intervals. One manufacturer of laser cleaning machines specifies mandatory replacement after 200 operating hours and in the event of cracks or breakage; if contaminated, the window can also be cleaned. By contrast, one manufacturer’s instructions for cutting heads recommend checking the condition regularly instead of replacing according to a fixed schedule. An interval therefore cannot be transferred from one machine to another.

Before the first change, the following should be known from the operating instructions:

• which replacement window fits, with dimensions and wavelength
• whether and with what the window may be cleaned
• which parts of the head the operator is permitted to open
• which check is intended after the change

Preparation before opening the head

For maintenance work on the machine, FBHM-139 states that it may only be carried out with the laser system switched off. Accordingly, one manufacturer’s instructions begin the protective window change by switching off the machine and shutting down the laser source completely.

Further preparations from manufacturer instructions:

• clean the outside of the head with a lint-free cloth so that no dust gets into the optics compartment when it is opened
• work in an environment that is as clean and low in dust as possible
• have clean gloves, a new protective window, lint-free cloths or lens cleaning tissue, an approved solvent, a blower bulb and the appropriate tool ready

Changing the protective window step by step

The procedure depends on the design — some heads have a retaining ring, others a removable window holder. The sequence summarises what manufacturer instructions describe. If in doubt, your own operating instructions apply.

1. Switch off the machine as specified in the instructions and clean the outside of the head.
2. Loosen the retaining ring or window holder without excessive force.
3. Hold the old window only by the edge — touching the surface leaves fingerprints or oil that impair transmission.
4. Remove loose dust in the holder with a blower bulb.
5. Hold the new window by the edge, remove any protective film and insert the window flat and without tilting — according to manufacturer instructions, a tilted fit can lead to an uneven focus.
6. Tighten the retaining ring evenly and without excessive force.
7. Check for fingerprints, streaks and a secure fit, then carry out a test run.

The same protective measures apply to the test run as in normal operation. Anyone who records the date and operating hours of the change can actually comply with an interval specified by the manufacturer.

When cleaning is sufficient and when replacement is necessary

One manufacturer’s instructions make a distinction: light contamination can be cleaned if the surface and coating are intact. With burn marks, scratches or a damaged coating, cleaning does not restore the performance. In the case of cracks or breakage, another manufacturer states that replacement is mandatory.

Nor is cleaning useful any number of times. An optics guide points out that overly frequent contact with solvents and cloths can itself damage the surface. Every cleaning is therefore preceded by checking how heavy the contamination is and what kind it is.

Whether the operator may work on lenses inside the head is defined in the operating instructions. FBHM-139 provides for maintenance work only by specialist personnel, for example from manufacturers, or by persons trained for this purpose.

Suitable cleaning materials

The optics guide starts without contact: loose dust is blown off with a blower bulb or inert compressed gas. Compressed gas cans should not be shaken and should be held upright so that no propellant gets onto the surface.

If that is not enough, cleaning is carried out with solvent:

• Cloths: lens cleaning tissue, cloths made of pure cotton or optical cotton swabs, not ones from the chemist’s
• Solvents of optical grade, such as acetone, methanol or isopropanol; manufacturer instructions for laser heads also mention high-purity ethanol
• Gloves or finger cots throughout the entire procedure

What matters is which agent the machine manufacturer approves. Most solvents are toxic, flammable or both and must be handled with appropriate care.

The drop method

For flat surfaces, the guide describes the drop-and-drag method: a fresh sheet of lens cleaning tissue is held over the surface, one or two drops of a fast-drying solvent are applied to it, and the moist tissue is then drawn slowly and evenly across the surface. Each sheet is used only once. One manufacturer of cleaning lasers recommends wiping in one direction.

Typical mistakes when changing and cleaning

These points contradict the instructions mentioned:

1. The optical surface is touched with bare fingers; skin oil can damage it permanently.
2. The window is wiped dry, although cloths should always be moistened with solvent.
3. A cloth is used more than once.
4. Dust is blown away with the mouth — this deposits droplets of saliva on the optics.
5. A window with burn marks or scratches is cleaned instead of replaced.
6. The new window sits tilted, or the retaining ring is overtightened.
7. The head is opened in a dusty environment.
8. Declining effectiveness is compensated for with more power without checking the optics.
9. A replacement interval is adopted from a different machine.

Safety and responsibility during maintenance work

According to DGUV Information 203-093, hand-held laser equipment for material processing is predominantly class 4 lasers. The document lists maintenance and servicing as operating modes that must be taken into account in the risk assessment. The operator determines the type, scope and intervals of the required inspections as part of the risk assessment.

For maintenance work, FBHM-139 states: only with the laser system switched off and only by specialist personnel or persons trained for this purpose. Even after switching off, considerable electrical hazards can arise. Tampering with the system, particularly with safety devices, must be prevented. The document expressly does not cover work on the laser equipment itself, such as maintenance and adjustment; for this, the manufacturer’s specifications remain decisive.

This article does not replace the operating instructions or the risk assessment for your workplace and does not constitute legal advice.

Sources

• DGUV – FBHM-139 “Blasting work – cleaning and decoating with laser radiation” — As of 20 June 2024. Maintenance in accordance with the operating instructions, including cleaning of the optical equipment; prerequisites for maintenance work.
• DGUV Information 203-093 – Guidance for the risk assessment when operating open laser equipment for material processing with hand guidance or hand positioning (HLG) — Edition April 2019. Maintenance and servicing in the risk assessment; inspections and inspection intervals.
• Precitec GmbH & Co. KG – laid-open patent application DE102021123617A1 “Laser processing head with adhesive trap” — Heating due to particle deposits, thermal focal shift, limits of protective windows.
• Inpro Innovationsgesellschaft für fortgeschrittene Produktionssysteme in der Fahrzeugindustrie mbH – laid-open patent application DE19605018A1 — Consequences of contaminated protective windows on laser welding optics.
• SCANSONIC GmbH – utility model DE20314918U1 — Protective function of the protective window against metal spatter and fumes.
• Handling and Cleaning Procedures for Optical Components (guide, wavelength-oe.com) — Handling, visual inspection, blowing off, the drop-and-drag method and solvents.
• STRION LASER – How to Replace the Protective Lens of a Laser Cleaning Machine — Manufacturer instructions for changing the protective window on a laser cleaning head.
• HARSLE – Laser Cutting Head Protective Lens Replacement — Manufacturer instructions for cutting heads with criteria for cleaning or replacement.
• PULSAR Laser – accessories for laser cleaning machines — A manufacturer’s replacement criteria for protective windows.

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