How much thinner is high-index?
When determining the thickness of a lens, there are two curves to consider: a front curve and a back curve. By using a high index, you’re able to use a flatter back curve to accomplish the same high minus power. This thins out the edges of the lens. An easy way to estimate how much thinner a certain lens will be in a different index is by dividing their indices of refraction. The reason this is an estimate is it doesn’t account for asphericity or the fact that certain materials can be ground to a thinner center than others.
How thin is 1.74 compared to CR-39 (1.498)? (1.498 – 1) / (1.74 – 1) = 67.3% of the thickness of CR-39.
You can use the above formula to compare any two materials. Or you can use this chart:
If you have a patient with a -20.00, using a 1.74 index lens reduces the edge thickness to roughly the same thickness as a -14.00 CR-39 lens. That thickness is often not satisfactory to a discriminating patient. To help make these lenses look better, we offer a number of different lens shapes that you can choose from to improve the appearance of a high minus RX.
The standard shapes, #1 & 2, are surfaced in the normal way, and we do not run the lens through any additional processes to thin the edges. While these shapes are the most affordable, they can also be quite unsightly in high RX’s if you’re not using an appropriate frame.
Myodiscs are surfaced on the backside using 2 different curves. The back curve in the center, and then an alternate curve to thin the edges of the lens.