Cutting a custom foam tool

**SurfaceDummy** · 05-01-2023, 08:24 AM

Background: Processing lab that has lost a LOT of old school expertise in the surfacing dept, I am the Surface Dept Mgr for 9 mos. 99.99% digital lab.

Making a high prism/low power job for a patient. Dr reports last pair resulted in 20/40 while phoropter was 20/20. Suspect center aberration due to 4.33dpt OD and 4.29dpt OS generated prism on top of 5.00dpt OU blocked prism.

I am reprocessing this job on our hard tool line to fine it out after generating to ensure a uniform back surface. One of the tools I require has gone missing. I've got a stash of DAC's foam tool blanks. We've got a V75 generator so I can easily cut a temporary replacement. I'm just not sure if I need to compensate for pad thickness, and if so: How do I calculate this compensation, my google-fu is lacking.

Tool requested by DVI is 1.80base and 4.00cross. Do I need to compensate for the pad thicknesses? or can I just plug these right into my v75 as-is and process? For the sake of completeness DVI is calculating the toric cut for this lens at -1.80base and -3.98cross

Thanks!

**drk** · 05-01-2023, 10:53 AM

This is way over my head, and I'm not going to derail the original poster, but I would appreciate someone else explaining this.

4.33 dpt (diopter?) prism ON TOP OF a 5.0 dpt prism for a total of 9.33 diopter?

**SurfaceDummy** · 05-01-2023, 12:00 PM

Originally Posted by drk

4.33 dpt (diopter?) prism ON TOP OF a 5.0 dpt prism for a total of 9.33 diopter?

Yes :D
I have 5 diopters of prism being blocked, and the rest being generated. that's the 9.33, add in prentice's rule and the decentration from the block center and I end up with the 10.00dpt prism required by the script. Along the axis of decentration I have 1.68dpt of power and 4mm of decentration making up the remaining 0.67dpt required. At least that's how it works according to the previous manager I trained under for 9 weeks before taking over surface (from inspection/QC, this handover was NOT handled well at all LOL)

**drk** · 05-01-2023, 12:04 PM

Again, I hate to waste your time because this is not an answer to your question, but is it realistic for the prescriber to expect non-reduced acuity with 10^ monocular prism? 20/40 is pretty bad, I guess, but there's going to be some decrease. What kind of material was used?

**SurfaceDummy** · 05-01-2023, 12:18 PM

Material: 1.67, previously 1.74. 20/40 is completely unacceptable, but as far as reasonable expectations, I don't think the Dr. is upset about the reduced acuity, just the severe degree. myself and our sales were unable to get the doctor to agree to return the lenses so that I could personally investigate the failure mode, but I'm very strongly inclined to think it was simply surface aberration caused by the generator and having all that generated prism. Typically with high prism jobs I'll toss them on a hard tool with a real gentle cutting pad just to hone the back surface for a little extra TLC.

**drk** · 05-01-2023, 02:58 PM

You sound like you are a fantastic professional. I hope someone can help.

**optical24/7** · 05-02-2023, 01:48 AM

The problem is Lateral Chromatic Aberration. (LCA). The Abbe of the material coupled with the prism is causing the reduced VA. Scroll down from the below page to LCA discussion for more info for yourself and you can reference for the prescribing Dr to read too.

http://216.144.236.77/cecourse.php?u...ic_aberration/

**PRECISIONLAB** · 05-02-2023, 10:57 AM

Pad compensation is usually .018 inches.

With that amount of prism there might be an issue with parallax @ the finish blocker.

Or the actual prism lens layout/marking at the lensometer.