Method for trimming a spectacle lens

A method of accurately trimming a lens in order to enable it to be mounted in a determined frame rim, in which method the lens is held between two clamping pads in a defined position in a frame of reference associated with the pads, and grinding of the periphery of the lens is controlled along a trajectory whose programmed terminal portion corresponds on the lens to the shape of the outline of the rim, the method comprising:

• while the lens is in a lightly clamped condition, taking first measurements of a plurality of points on a face of the lens;
• while the lens is in a tightly clamped condition, as is required for trimming the lens, taking second measurements of another plurality of points on said face of the lens;
• on the basis of the above measurements, making an approximate mathematical representation of the above-mentioned face of the lens in each of the two clamping conditions;
• using the above mathematical representations to calculate the coordinates of the transformed points of the trace of the programmed shape of the rim on said face of the lens, said transform being the result of the lens being deformed in compliance with a model obtained on passing from the first clamping condition to the second;
• and
• correcting each of the points of the programmed milling trajectory by an amount defined by the difference between the programmed coordinates and the calculated coordinates.