With digital lens surfacing (a.k.a freeform) one or both surfaces of a lens can be completely specified by the Lens Design System (LDS). For example the LDS can provide a height map, giving points on a grid of some chosen spacing. This surface will be produced with very high accuracy by the generator.
The optical performance of a freeform lens when used in spectacles, is difficult to predict. Either the lens must be manufactured and analyzed with a complex apparatus that can measure optical properties for a range of realistic view directions, or the lens must be simulated computationally using sophisticated ray-tracing algorithms.