How eyeglass lenses are made

The law that sets how thin a lens may be is a steel ball dropped on it from 50 inches.

stations: 6
things upstream: 31
raw sources: 13
steps from the ground: 7

The short version

Most eyeglass lenses today are molded plastic blanks. Opticians grind the front and back surfaces to the prescription curve, polish them clear, trim the edge to fit the frame, and add a hard scratch-resistant coating. In the US, lenses must survive a ball-drop impact test, which sets a minimum thickness.

Mold the lens blank

Lenses are ground, polished or molded from clear material. Plastic is the most commonly prescribed type, mainly CR-39, polycarbonate or Trivex. Glass lenses are less common because they are heavier and can shatter.

Goes in
Polycarbonate Glass
Grind the prescription curves

The power of a lens comes from the difference in curvature between its front and back surfaces. A lab grinds the back surface to match the wearer's prescription, usually starting from a front curve that is already set.
Polish to clarity

Fine abrasives polish the ground surface until it is smooth and clear. Spherical surfaces are by far the easiest shape to grind and polish accurately.
Add the hard coat

Polycarbonate scratches easily, so a scratch-resistant coating is typically applied after shaping and polishing. Anti-reflective coatings matter more on higher-index lenses, which bounce back more light.
Trim to fit the frame

The round lens is edged to the outline of the chosen frame. Very thick lenses for strong nearsightedness are sometimes beveled to reduce the flare of a thick edge.
Drop-test the thickness

Lens samples face a test in which a 0.56-ounce steel ball is dropped from 50 inches. That test effectively sets the minimum thickness: 2.0 mm for glass or CR-39, but 1.5 mm or even 1.0 mm for some newer materials.

ball dropped from 50 inches

End of the lineEyeglass lens Next it goes into eyeglass frame.

Worth repeating

Trivex was developed in 2001 by PPG Industries for the military as transparent armor.
Trivex, at 1.1 g/cm3, is the lightest common corrective lens material, while optical crown glass, at 2.56 g/cm3, is the heaviest.
Glass lenses can now be made with a refractive index up to 1.9, for extremely high prescriptions.