Base Down Prism - follow up

[optimensch]

I've thought about this Base Down Prism Thinning situation, and here is a follow up question.

When you measure a seg height all carefully, with a ruler , or with your Visi Office, either no lenses or just demo lenses in the frame, you get a number. You measure, remeasure to make sure it is accurate, frame on and off a few times, to check how it is worn etc.

Now the lab prism thins it, adds a couple of degrees of base down prism. Now this has the effect of bending the light up, hence your measured seg height is now out the window, the seg is too high all of a sudden, no?

So should vertical base down yoked (not yolked) prism be accounted for when measuring seg heights? Obviously, I think, the answer is YES> now , do YOU do so? Does your Visio Office do so?

Thanks for your inputs.

[optimensch]

sorry, actually the base down prism "bends" the light down, not up. so your seg height should be lowered a bit accordingly? Please correct me if this is wrong.

[Darryl Meister]

Base down prism "bends" the light down, but displaces the image up, so the eye will also rotate up very slightly to compensate for this effect. The difference is small though, about one-quarter millimeter for every diopter of prism. I wouldn't bother compensating for this effect with prism-thinning, which is generally less than 2.0 prism diopters, but it is certainly worth considering with larger amounts of prescribed vertical prism.

Best regards,
Darryl