What is the difference between Freeform and classical progressive lenses.
Traditionally, base curves ranges were determined solely by the sphere power of the Rx.
For example, if we had a progressive lens with the following Rx, the minimum base curve we would use would be a 4 base.
+3.75 -1.00 x 90 +3.00 Add
We would be grinding into the back of the lens a -0.25 back curve with a -1.25 cross curve (the back curve plus the cylinder power). We do not take in to account the add power since the add is cast on the front of the lens and has no effect on the back side.
Curvature and progression position
Back side freeform progressive requires, in general, higher bases in order to preserve the performance of the equivalent front side one.
Let’s compare two progressive lenses: +2.00 D Add 2.00 D
Excess flattening at the bottom of the lens causes:
- Bigger difference between user-power addition and focimeter measured addition
- If the lens is user-power optimized, focimeter near power present unwanted astigmatism. If the lens is curvature-optimized, unwanted astigmatism is perceived by the user.
- The lens separates from the eye
- The extension of the vertical field in near vision is reduced.
To recover performance, the back side PAL must be made with a bigger base. In the example, a base +6.00 D allows for a back surface of -2.00 even at the reading sector.
Free-form technology and user power optimization alleviates this problem, but they doesn’t eliminate it completely.
To summarize minimum base curve selection:
Conventional lenses: Base curve = Sphere power
Freeform lenses: Base Curve = Sphere power + Add power+1.00
Tags: Base Curve