Prism Thinning and ProgressivesNovember 1, 2015
Unlike most other lenses. An O.C in a progressive lens is at the Prism Reference Point (PRP). The PRP is located under the fitting cross. The distance between the Fitting Cross and the PRP is determined by the lens designer. Different progressive designs have different distances. Most are 4mm but some are 0mm, 2mm, or 6mm.
When we fit progressives, we align the fitting cross over the distance gaze of the pupil. Knowing the PRP (O.C.) is under the Fitting Cross, it may not align with ½ B. Most patient’s eyes are in the top third of the frame height which means the PRP (O.C.) is above ½ B. This causes the Plus lens to be thicker at the top and the Minus lens to be thicker at the bottom compared to the other styles of lenses. Therefore to cosmetically enhance their appearance, prism thinning is added.
Plus lens without prism thinning (Thicker on top than bottom)
Robert Hughbanks ABOC, HOAA- Manager of Training and Development