Protection lenses sit between the cutting head’s optics and the workpiece, shielding the expensive focus lens from spatter, dust, and condensation. They’re consumables — they need regular replacement. But not all protection lenses are equal, and the specifications matter.
Transmittance: The Number That Actually Matters
Transmittance is the percentage of incident light that passes through the lens. At 1064nm (the wavelength of fiber lasers), you want 99%+ transmittance per surface. Lower transmittance means heat absorption inside the glass — which degrades the lens faster and can cause catastrophic thermal fracture at high power.
Our AR-coated protection lenses achieve <0.2% reflectance per surface, meaning 99.8% transmittance. Uncoated fused silica sits around 96% — acceptable for low-power (≤3kW) applications but not recommended for 6kW+.
Damage Threshold: What “15 J/cm²” Means in Practice
Damage threshold is measured at a specific pulse width (typically 10 nanoseconds). Our Series A protection lenses are rated at >15 J/cm² at 10ns. For CW (continuous wave) fiber lasers, the relevant parameter is power density in W/cm².
As a rule of thumb: if your cutting head uses a 30mm-diameter nozzle at 6kW, the optical intensity at the protection window is well within the rated threshold. At 12kW+, specify our dual-AR Series C lenses which are rated to 24 J/cm².
Surface Quality: 20/10 vs 40/20
Surface quality in the MIL-PRF-13830B specification (scratch/dig notation) describes allowable surface defects. 20/10 is tighter than 40/20 — fewer and smaller scratches. For protection lenses in high-power laser heads, specify 20/10 or better. Our standard range uses 20/10 for all AR-coated lenses.
Size Selection
Protection lens diameter must match your cutting head’s lens seat. Common diameters and their typical head fitments:
- D30mm: Raytools BM110/BT240S, WSX NC12, BOCI standard
- D32mm: Precitec ProCutter 2.0, Bodor B3015
- D37mm: Raytools AT300, Precitec YC52, WSX NC30
- D38mm: Trumpf TruDisk series (requires specific fitment)
Thickness affects the optical path length. Standard thicknesses are 3mm and 5mm; heavier contamination environments sometimes use 7mm for extended service life.
Replacement Interval
Under normal operations: 8–24 hours depending on material and contamination level. Replace immediately if you see visible pitting, coating delamination, or a sudden drop in cut quality that isn’t explained by nozzle condition or gas pressure.
All NEXORA protection lenses ship in individual dust-free clamshell cases with a QC test card. Available in all standard sizes. Browse our optics catalogue.
