A thick lens blank showing its glossy molded front and its rough unfinished back.

Making and checking a lens

Semi-finished blank

Optical glossaryMaking and checking a lens

A lens with a factory-molded front surface, finished by surfacing the back to the prescription. Every CLIFF Rx lens starts as one: freeform surfacing is how its back is finished.

A semi-finished blank arrives with its front already molded. The back is cut away to the prescription, leaving the finished lens inside the outline of the blank.

Illustrative · the shape of the principle, not measured data

In more depth

A semi-finished blank is a thick disc of lens material with its front surface already molded to a chosen curve and its back left deliberately unfinished. Labs keep blanks in each material and front curve, ready to be made into any prescription within their range.

To make a lens, the blank is held by its front and the back is cut away to the depth and shape the prescription needs, then polished, coated and finally edged to the frame. Every CLIFF prescription lens starts this way; the front is never worked on.

Related terms

Freeform surfacingManufacturing in which a lens surface is cut point by point by a computer-guided tool from a map calculated for one order, rather than molded. On made-to-order CLIFF lenses it is the back surface — the surfacing and the design both — on every made-to-order tier. Versatile, the stock lens, is molded, not freeform.Base curveThe curvature of a lens's front surface. Must suit both the prescription and the frame's wrap.EngravingPermanent laser markings applied at manufacture, readable under angled light. Every stock lens and every progressive lens carries the CLIFF mark — the CLIFF icon, or the letters CLF.

Go deeper on this site