Prism Thinning in Other Lens Styles?

Today, most labs will add prism thinning to progressive lenses. The reason behind prism thinning is to create a more cosmetic appealing pair of glasses for the patient.

So why is prism thinning added to progressives and not any other lens styles?

First we have to return to where an O.C. is placed in a pair of glasses most of the time (excluding progressives, aspheric lenses and a select few of others). O.C.s are located along the 180˚ meridian (moving the O.C. right or left) and along the 90˚ meridian (moving the O.C. up and down). Let’s look at the location along the 180˚ meridian. Where the O.C. is located has to do with the Patient’s P.D. The lenses are edged so the O.C. will be the same width of their P.D. (this means in the same plane but not necessarily the same height as the pupil) when mounted. But where is the O.C. placed along the 90˚ meridian? In most cases, the O.C. is located ½ B measurement in the lens boxing method.

What does this mean to the lens thickness? Along the 180˚ meridian, it depends. The edge thicknesses along this meridian are determined by how much decentration is needed to place the O.C.s at the same width as the P.D. With Plus lenses- the more inward decentration= more nasal thickness. With Minus lenses- the more inward decentration= the more temporal thickness.

Along the 90˚ meridian, the O.C. placement is ½ B. The thicknesses along the top and bottom of the eyewires should be fairly close to the same because the O.C. is the same distance away from each eyewire. Hence, the O.C. height may or may not be in line with the distance gaze of the pupil. Don’t worry yourself, this has been going on since any of us were even born.J  There are exceptions to this however, but that is for another post sometime in the future.

Robert Hughbanks ABOC, HOAA- Manager of Training and Development