Aspherics and Prism in surfacing!

[LENNY]

I need help in understanding possibilities of using aspheric blanks for surfacing prism or larger decentration in them.
I am a little confused!

[HarryChiling]

I might catch flack for this but here we go when surfaceing prism in an aspheric blank you want to use the same decentration as you would for a progressive with prism.

[shanbaum]

Harry, did you mean a decentered progressive (one on which the Prism Reference Point is decentered from the geometric center of the blank)?

[Darryl Meister]

Just keep in mind that, while surfacing prescribed prism is fine, lens manufacturers generally recommend against surfacing prism for decentration in aspheric lens blanks, since doing so offsets the aspheric optics and degrades optical performance...

[LENNY]

So how do we know what decentration can be ground on an aspheric blank without compromising the optics?

[shanbaum]

So how do we know what decentration can be ground on an aspheric blank without compromising the optics?

Easy: none.

[LENNY]

Easy: none.

Is that how the surfacing program setup? No dec on Aspheric lenses?

[optical24/7]

You don't grind prism into an aspheric for decentration. All decentration is done at finishing layout. The OC stays in the uncut blank's center. If you can't decenter the blank at finish layout enough to cut out of your given frame, you need to use a smaller frame or use a spherical lens you can grind prism into for decentration.


What Harry was referring to ( correct me Harry if this isn't what you were saying). Is that with prescribed prism in an aspheric, you need to adjust your decentration by .26mm away from the base per diopter of prism, just as you should for a PAL to keep the pupil centered in the optimal part of the aspheric design.

i.e. 4 diopter base out prism OU...old PD 63....new compensated PD 61. Using .25 instead of .26 for easy figuring..( .25mm X 4D = 1mm X 2 (R&L) = 2 mm change of PD) Prism not only moves light, but pupil location. This comp is to keep the pupil in the "sweet spot" of a PAL or aspheric SV.

[DocInChina]

Prism can be incorporated into free form aspherics without compromising the optics of the aspheric design.

[optical24/7]

Prism can be incorporated into free form aspherics without compromising the optics of the aspheric design.

Yes, where the asphericity is ground inset. ( i.e. not in the center of the blank.) Your OC will still be in the center of the aspheric design.

[shanbaum]

Is that how the surfacing program setup? No dec on Aspheric lenses?

That's right - no prism for decentration on aspheric single vision, unless the center of the aspheric field is decentered (like the KBCo wrap lens).

And yes, if you're doing a freeform aspheric surface, you can "incorporate prism", but in fact the lens design systems I'm familiar with effectuate decentration by decentering the surface matrix, and effectuate prism by tilting it, which is sorta the obvious way to do it.