Eyewear Fabrication, Finishing, and Verification
Fabrication sequence
Surfacing runs through generator, fining, and polishing, then coating; edging, mounting, and adjustment follow. Layout positions the optical center or major reference point in the frame relative to the pupil and segment height. Blocking secures the lens during generating and edging, so blocking errors become power and axis errors.
Verification tools
A lensometer confirms sphere, cylinder, axis, prism, and add power. A lens clock or sag gauge checks base curve, calipers check center and edge thickness, a PD ruler or pupillometer checks PD, and a distometer checks vertex distance. Confirm major reference point placement, segment or fitting cross height, coating integrity, and frame alignment before delivery.
Traps
- Reading a lens from the wrong side; the lensometer reports back vertex power.
- Misreading the axis scale because the drum and the axis protractor rotate together but are read differently.
- Skipping a binocular PD and optical center check after edging, which shows up later as unwanted prism.
Worked example: neutralization
A lens focuses at −3.00 D in one meridian and −5.00 D in the perpendicular meridian. The sphere is −3.00, and the minus cylinder axis lies on the −3.00 meridian: transcription is −3.00 −2.00 × (axis of the −3.00 reading). Ordinary lenses rarely measure exactly; tolerances in the applicable standard decide pass or fail, and power range determines the allowed deviation.