We have been asked by an ophthalmologist to surface a set of bifocals for a patient with slab off prism only in the reading segment.
Can this be done?
We have been asked by an ophthalmologist to surface a set of bifocals for a patient with slab off prism only in the reading segment.
Can this be done?
vertical or horizontal?
Vertical.
Could a horizontal prism correct aniseikonia?
The prism would not fix the aniseikonia only the imbalance caused by the anisometropia, all standard slab offs are vertical and in the reading only, younger optics even sells semi-finished slab bifocals.
So there would be no prism in the distance portion of the bifocal?
That's what we're looking for. About 4 base up but only in the reading seg.
Younger optics has them premade
PRISM AVAILABILITIES: 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0 & 6.0http://www.youngeroptics.com/default...e-939d9278861b
So is this not a big deal then? Because when we called around different surfacing labs they all told us it was impossible, that you had to slab off the whole lens.
Thank you very much for your help though!
I must be miss-reading something, slab-offs are always in the bottom half of a lens. If you slab-off the whole thing you just have a lens with prism base up.
What am I missing?
Chip
Robert, Just to be clear the Younger lens slab extends past the bifocal; If you're thinking of a slab in the bifocal itself only I think X-cel glass would be your answer.
I'm with you Chip and quite confused.Originally Posted by chip anderson
A traditional surfacing process can produce a "bi-centric" surface with a line of demarcation separating the two different sections of the lens. The difference in prism will occur at this "slab line." Slab-off prism is obviously most frequently done to correct induced vertical prism imbalance at near, resulting in a horizontal slab line.Because when we called around different surfacing labs they all told us it was impossible, that you had to slab off the whole lens
It is not really possible to produce a slab-off in a only limited region of the surface with conventional surfacing equipment. Generally, the slab-off produces a second curve over the entire lower half of the lens, at least beyond the location of the slab line, which is customarily placed at the top of the bifocal segment.
Years ago, glass prism segments were available that provided prism corrections within the bifocal segment, offering either vertical or horizontal prism. However, these lenses have been only a specialty item for years. Some specialty glass manufacturers or glass laboratories may still supply or at least custom grind a few prism seg styles though.
That said, there is typically little reason to restrict a vertical prism imbalance correction for near vision to only the segment area. In cases of anisometropia, at least, the induced prism imbalance occurs over the entire lower field of the lens, with or without the bifocal segment.
I could imagine only a handful of instances in which it would truly be beneficial to offer the vertical prism correction only within the bifocal segment, and not over the remaining lower half of the lens, but these would certainly be out of the ordinary and seldom encountered in practice.
So I would call the ophthalmologist to confirm his or her request. The ophthalmologist most likely didn't necessarily mean to specifically restrict the vertical prism correction to the segment area, only, which would be both unusual and very difficult to do without the use of highly specialized lenses that may no longer even be available.
Best regards,
Darryl
Darryl J. Meister, ABOM
I should add that, if the ophthalmologist really does want to restrict the vertical prism correction to the bifocal segment, you will need to either find someone who supplies or at least can grind prism segments in glass or use dissimilar bifocal styles in plastic or glass. Or use a Fresnel Press-On optic for temporary corrections.
Dissimilar bifocal styles with a separation of the segment optical centers can be used to correct vertical prism imbalance only within the segment, particularly in higher add powers. The prism produced within the segments will be essentially equal to the add power multiplied by the difference in segment optical center positions in centimeters (Prentice's rule).
Best regards,
Darryl
Darryl J. Meister, ABOM
Mike Aurelius might be able to do something like what you're describing in glass, although I agree with the suggestion to re-discuss this with the prescriber as I have a hunch that he just wanted slab-off and wasn't clear or was maybe misunderstood.
Best of luck.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Slabs Plus had made a number of complex, compound, prism-only in segment using a glued-on technique. Not always cosmetically appealing, but definitely worked as promised!
B
If prism is only wanted in the read area, anyone with a DAC freeform system can do it. Actually anyone with a 3-axis system can do it too. This providing that all prism is vertical.
Although it is easy enough to generate traditional slab-offs using free-form surfacing, I'd be surprised if common free-form surfacing systems can generate prism only within the segment area.If prism is only wanted in the read area, anyone with a DAC freeform system can do it. Actually anyone with a 3-axis system can do it tool
You would have to either generate a fairly complex surface that would "blend" the prism correction into the remaining distance region in the lower half of the lens, since the cutting tool produces a continuously smooth surface without discontinuities, or attempt surfacing a second, smaller "button" into the lens surface over the reading area, which would also be a bit of a pain in the rear.
Best regards,
Darryl
Darryl J. Meister, ABOM
Darryl, think about a bi-centric on the minus side. It's a pretty easy trick to surface a slab-off on a 3-axis by surfacing the read first with prism and then generating and polishing the distance, without prism, to create the line where it should be. DAC (now Chemat) software has a program on it that creates blended slab-offs and slab-ons.
I think we're talking about two different things. Yes, you can surface a slab-off onto a lens blank using a free-form generator but, like conventional slab-offs, the entire lower half of the lens still provides the prism correction. It is not restricted to the bifocal segment, which was the original request here.Darryl, think about a bi-centric on the minus side. It's a pretty easy trick to surface a slab-off on a 3-axis by surfacing the read first with prism and then generating and polishing the distance, without prism, to create the line where it should be. DAC (now Chemat) software has a program on it that creates blended slab-offs and slab-ons
Also, as far as I know, these are referred to as "blended" slab-offs only because the slab line is blended instead of sharp, not because the slab-off is blended around the perimeter of the bifocal segment.
The slab line must be blended when using a free-form surfacing process; this is unavoidable, since soft lap polishing and single-point diamond turning cannot produce surface discontinuities like a sharp slab line, at least if processed in one pass.
Best regards,
Darryl
Darryl J. Meister, ABOM
ALso remember that you will slab-off only one lens, and leave the other 'un-slabbed'. Make sure that your patient knows this.
what about a press-on
Yes, but $$$ka-ching$$$
A better option, which Barry mentioned, would be a cement-on power/prism segment. And, as he says, they are not pretty, but they do function quite well.
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