We feel your pain when you have to tell a patient that you can't put their prescription in the designer, high-wrap frame that they just picked out. Until now, there wasn't any option for getting a high-minus prescription into a high-wrap frame. However, the latest technology enables us to help this patient.
Getting a power beyond a -2.00 sphere becomes difficult to fit in many wrap frames. This is a problem because traditional lens surfacing requires us to cut a single, consistent sphere power into the back of the lens. Using this method, we end up with very thick edges for high-minus prescriptions in high bases. Take, for example, the following scenario:
When we try to surface this prescription in a high-wrap frame, we end up with a very thick lens, which would likely be impossible to make even if we wanted to, as most lens blanks don't have enough thickness to allow us to produce this kind of an edge thickness. Below is a side-view of the resulting lens created by traditional surfacing of this prescription.
This lens, as produced by the traditional way of surfacing, would never fit into most wrap frames. Even if it did fit, it certainly isn't something that the patient is going to be happy about putting on their face. This is why we need to be able to take control over the thickness of the lens and make it something that's workable.
The same concept applied to fully-backside progressives can be used to help us reduce thickness, which is the idea that we can vary the powers across the back surface of the lens to give us the desired outcome. The process of thinning the edges of the lens is referred to as lenticulation, and it involves specifying an area of the lens as the 'visual area', which is the part of the lens the patient is intended to actually look through. This 'visual area' is also referred to as a 'bowl', and can vary in size from 35mm on up.
Outside of the visual area, the prescription power is quickly and drastically reduced, which essentially flattens the edges of the lens, resulting in the non-visual area seeming distorted. In practical terms, this means that when someone else is looking straight on at the patient, the edges are going to look 'different' than the rest of the lens, but when putting high prescriptions in large frames, it's normal to have some degree of apparent distortion when viewed by someone else. In terms of how the wearer perceives it, they won't be able to look out of the corner of their eye while wearing this type of lens, but the vast majority of their field of vision will be unimpaired.
This is because of how close the lens is to their eye, and the wrap angle involved. By the time the distortion becomes an issue, the lens is wrapping to a point where the patient isn't going to be looking through it anyway. In terms of the thickness reduction we get, we'll take a look at the same prescription we used earlier, but this time lenticulating it.
By reducing the visual zone, the edges can be thinned. The minor reduction in visual area is far outweighed by the benefits, as before this there was simply no way to make this prescription fit into a wrap frame.
FEA Industries creates Eagle customized lenses using lenticulation in single vision and progressive lenses, as well as lined bifocals and trifocals. IOT lens making software is used to calculate the curves. Lenses are processed it on a Schneider free-form system (Master 08 and Swift polisher).
Now, doesn't that feel better?