The long answer is "it depends".
Is the beam diameter more than 70% of the diameter of the lens? (a 12mm lens is ok up to an 8mm beam)
The ZnSe crystal is coated at up to 300deg C. The lens and coating can handle more temperature than your O-ring or my glue.
A lens fails when:
1- the coating gets damaged;
2- The lens is dirty;
3- The crystal is under stress.
I have had my Co2 optics professionally tested and they passed the 300 watts per mm diameter energy density test.
Obviously if alignment is an issue, a bigger diameter lens will be more forgiving.
Another question might be "Why does a lens fail?"
Including the 3 causes above.
It might also depend on the quality of the crystal being used or the quality of the coating applied to manufacture the lens.
The measurement of this is called absorption (how much of the Co2 laser energy is being absorbed by the lens and converted to heat).
This # combined with the other 3 causes might make your lens fail when a higher quality optic will not, but this again has very little to do with lens diameter and more to do with your operations and the lens quality.
By Natalia Pereira
