Using double "fresnel prism"

[smac9daisy]

This is a 1st for me, HELP. My MD sent me a pt RX OD pl +.50 x045 OS +1.50 +.75 x180 +3.00 AD OU.
He wants to us 10 base down OS WITH 6 base in OS. IS THIS EVEN POSSILE? If I have done this it's been YRS and I have forgotten

[Judy Canty]

My suggestion would be to go to www.opticampus.com and use the optical calculator for compounding prisms. Also, bookmark that site for future reference. BTW, it's 11.66 at Base 239.

[fjpod]

...but he/she is not going to be able to order an 11.66 prism diopter Fresnel. You might want to call the prescriber and see if placement of one of the prisms in the other eye would be ok. So the 10 BD OS would become 10 BU OD, and the lateral prism would remain in the same direction.

[Judy Canty]

Check with the prescriber for permission to use either a 10 or a 12. the 239 degree placement is easily found on a protractor. The other issue is that the prescription is written for a multifocal, so the decision needs to be made for SV DVO or SV NVO, or both. There isn't really enough information in the post to make any firm decision at all and most, not all, MDs are not that comfortable with lens fabrication.

[Robert Martellaro]

This is only a moderate amount of prism. I would usually split and surface the prism. Expect better acuity (about two lines in the eye with the Fresnel prism) and optimally positioned optics.

The prescribed prism will cause the right eye to point down, towards the prism apex, about 1.5mm, the left eye up 1.5mm, and both in about 1mm. Anticipate the eye turn and position the lenses to account for this new eye position (if you use a Fresnel in the left, the left eye will turn up more than 3mm, and in about 2mm, mispositioning the underlying optics).

Consider segmented multifocals if they're presently wearing PALs. Use a low dispersion material. Evaluate after two weeks of constant wear.