Lens prism in RGP manufacturing and its effect on edge quality.

Unwanted prism in lathe cut RGP lenses is most frequently caused by misalignment when re-blocking the button for cutting the front surface. The geometrical relationships in three types of alignment errors and the resulting variations in edge thickness were mathematically analysed and experimentally confirmed. The edge thickness variations in a prismatic lens along the circumference follow a cosine function. For a typical tri-curve lens design, an eccentric offset 30 microm between the front and the back surface can lead to a difference in edge thickness of more than 50 microm Edge thickness variations are greatest for high power minus lenses with prominent lenticulation. The polished edge shape is significantly compromised by inconsistent edge thickness. Typically, the thinner side has a sharp and pointed edge, while the thicker side is blunt.