Traditional vs Freeform Surfacing
Traditional lens making tools can work only on the backside of the lens. They cannot create aspheric shapes or the complex curves required for PALs (progressive addition lens). This is why the progressive channel of designs has traditionally been pre-molded by the manufacture on the front surface of the lens.
Freeform or direct surfacing makes it possible to grind the front, back or both sides of a lens blank. A computer numerically controlled (CNC) freeform generator creates the lens surface according to the desired parameters, which include optics-influencing variables. The lens is then polished using a computer controlled “soft sponge” system to insure optical clarity is achieved while maintaining the surface curve integrity.
There are several ways that freeform technology is currently being adapted to provide next generation PAL designs.
- Fine tuning a semi-finished lens blank that has a pre molded surface progressive design. The backside can then be ground with complex freeform curves that will adapt the lens precisely to the visual needs of the patient. Freeform technology allows use of optimal back surface curves rather than good approximations.
- Adding new prescriptions parameters. The new design can incorporate the patient’s visual habits into the lens design in order to provide a progressive surface that is uniquely suited to that individual’s prescription and viewing behaviors. To achieve this data an instrument called the Vision Print System is used to measure how the patient moves his head and eyes when shifting his gaze from straight ahead to the periphery. This data and the standard prescription are used to create a lens optimized for the way the patient uses his or her eyes.
- Making adjustments for the frame and its wearing position freeform PALs can be further customized by selecting different progression lengths based on the selected frame shape.
It is important to remember that freeform surfacing is a technology freeform surfacing cannot improve an optical design. In the end, a lens is only as good as the lens design on which it is based. Therefore freeform technology is only valuable when it is paired with excellent lens design.