Fingerprint, special watermark .... or defect , on freeform PAL

[mauroventura]

Hi to everyone.
Sometime , looking freeform PAL ,coming from different customers ,(coated and uncoated), near at the lens geometric center ,shows some regular deformations shaped like concentric circles very difficult to see but presents , especially and more evident on positives ones.
This circles coincide with the geometric center of the lens.

Because this strange thing comes from different kind of PALS i think it could be originated by a construction limit.

No one has ever encountered this type of defect?

[shanbaum]

You may be seeing the bane of digital surfacing - the "center artifact". All generators that cut in a spiral pattern - which includes all freeform generators, now that OptoTech has abandoned their raster-cut approach in favor of spiral turning - are vulnerable to a problem at the center of the spiral, where the surface velocity of the cutter approaches zero. In traditional surfacing, the artifact is eliminated during the fining step; but in digital surfacing, there is no fining step. Digitial generators, therefore, have to be very carefully calibrated, or you'll end up with a peak or a divot (created in an effort to avoid creating a peak) at the surface block center. That's one good reason to locate the PRP (and the other design features) somewhere else, as many designs do.

[mauroventura]

Thank you Shanbaum , for the interesting explanation.
It seems ,a problem , without solution.

thanks

[optical24/7]

Many/most labs use a heavy lacquer hardcoat to mask FF surfacing marks.

[scriptfiller]

Trivex is good for this. Whatever happened to good surfacing technique. I've always been a little leery about the generator-to-polish systems.

[mauroventura]

Many/most labs use a heavy lacquer hardcoat to mask FF surfacing marks.

Could be a choice !
Anyway , we must remember that normally a good hardcoat thickness using commercial laquers must be around 2.5-3 microns, and i suspect that a hardcoat layer of 3 o 4 microns (.. or more) it's not enough thick to cover this problem.
We must remember also that to play with hardocoat layer could have strong impact on final lens quality and then this way can require time to develop a new coating structure.