How Corrective Lenses Work

by Bob Broten
Making a Lens: Part 2
A compound grinder, called a generator, grinds the required curves into the back of the lens blank. The two large dials on the console set the spherical and cylindrical curves that will be ground into the lens.
A compound grinder, called a generator, grinds the required curves into the back of the lens blank. The two large dials on the console set the spherical and cylindrical curves that will be ground into the lens.

Steps 4 through 6

Step 4: Depending on the type of equipment, the lens must be prepared to fit onto the generator, which is commonly a compound surface grinder capable of grinding two curves at once.

A chuck receiver (called a block) is placed on the front of the lens over the protective tape. If there is a cylinder curve, the lens is oriented so the cylinder axis matches the cylinder sweep axis of the generator.

A chuck receiver, called a lens block, must be attached to the front of the lens so it can be mounted in the generator.

The center of the block will become the optical center of the lens. Depending on the equipment, the lens may be held in place by special adhesive pads, with a special alloy that "glues" the lens to the block or with plastic.

Step 5: The lens is inserted in the generator.

The lens blank, attached to the lens block, is inserted in the generator. The generator has pins that align the lens.

The lens might need other processing besides the compound curves produced by the generator, so the lens may also be tilted in the chuck. This tilt will offset the optical center (called induced prism) often used to allow thinner lenses or to accommodate special requirements of the prescription.

The lens is ground within a rubber-lined grinding chamber. The cone-shaped quill, or grinding wheel, is at the center. The quill has a diamond cutting surface along its outer edge and is angled so only the outside edge touches the lens.

Step 6: The curves are set on the machine and the lens is generated (ground). This step may either be fully automated or operated by hand, where the operator manually sweeps the quill (grinding wheel) across the lens, gradually advancing the lens until the desired lens thickness is achieved. Lens thickness is determined by curve type (plus or minus), lens material (some plastics are tougher and may be ground thinner), or other considerations (safety glasses, for instance, are made thicker than lenses for everyday use). If the lens gets too hot during the operation it may warp or tear, so it is cooled by water, which also washes away the cut material (called scarf).