During laser operation with several kilowatts, the focusing lens is heated because it absorbs a small portion of the laser power. Absorption takes place mainly in the AR (Anti Reflection) coatings and at dirt on the lens.
At a new clean lens with standard AR coating, absorption is typically 0.2% of the incoming laser power. A lens with Ophir's Black MagicTM coating initial absorption is guaranteed to be less 0.15%.
In our newest type of lenses the Clear Magic, the Max absorption is lower than 0.13% and the typical absorption is 0.10% and lower
Every lens absorption is changed over time:
During use in a laser cutting machine, absorption increases gradually due to increasing amounts of dirt on the lower surface. When the lens needs to be replaced, absorption usually is in the range 0.3 to 0.4%.
The accumulation of the absorption is gradule increasing
Heating of the lens causes additional surface curvature due to thermal expansion and increases the refractive index of the lens material. As a consequence of these effects, the lens focal length becomes shorter, and the focus position cannot be predicted exactly because it depends on many parameters like laser power, intervals laser on/off, cleanliness of lens, and others. Therefore, use of lenses with reduced absorption can make the focal length more stable and therefore improve reliability of the cutting process.
If there are dirt particles on the lens, the lens material is not heated uniformly, but mainly at the areas close to these dirt particles. As a consequence, focusing properties become worse, focus diameter increases, and cutting quality decreases. So if a certain "critical" amount of dirt has accumulated on the lens, it needs to be replaced. However, it might still work fine at reduced laser power.