How much thinner is high-index?
For determining thickness, a lens can be thought of as two curves: a front curve and a back curve. By using 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.
Shape #1: Standard
Shape #2: Biconcave
Concave front curve, standard back curve with no effort to mitigate edge thickness.
Freeform HD Wrap
This lens design is used for unusual base curve requests. For instance, a -7.00 in an 8 base wrap 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.
Shape #3: Standard Myodisc
Flat or convex front curve, with a concave bowl cut into the center of the lens The rest of the lens is cut on a convex curve to drastically reduce edge thickness. The typical bowl size is approximately 40 mm
Shape #7: Back Fade
Where the myodisc cuts a convex curve on the back of the lens, the back fade cuts a flatter concave curve on the back of the lens. This thins the lens, but not as drastically as the myodisc.
Blended Myodisc Shapes
Blended Myodiscs are surfaced on the backside using mutiple different curves. The back curve is cut in in the center, and then a series of curves are cut to blend the bowl’s transition into the edge of the lens.
Shape #4: Blended Myodisc
Processed much like a traditional myodisc, but with an additional finishing process to blend the bowl to the rest of the lens.
Biconcave Blended Myodisc
Used when the RX exceeds machine limitations on Shape 4. A concave curve is cut into the front of the lens to make a lens with more power.
Biconcave Double-Blended Myodisc
Used to further thin a Shape 5 lens. The lens is run through the blending process on both the front and the back of the lens.