Have you ever wondered what goes into making the lenses in your glasses? How does the eye clinic take the information from your eye exam and use it to create glasses that are perfect for you? Woodhams Eye Clinic uses top-quality lenses from Essilor; check out their video to see every step in their factory.
The Raw Materials
Essilor starts with granules of polycarbonate, a transparent plastic, in bags that weigh nearly 4,000 pounds. While lenses used to be made from actual glass, now most are made from tougher plastic materials. Polycarbonate is light, extremely impact resistant, and has a great refractive index. The refractive index measures how much a material bends light, and a higher refractive index allows a powerful lens to be made thinner.
Drying the Polycarbonate
The granules are transferred from the bags through a surge hopper (like an enormous funnel) into dryers, which heat the polycarbonate to 255°F to remove all traces of water. A vacuum tube then sucks the granules through to a press, where they will be shaped.
Injection Molding
The granules are melted in an oven at 500°F. Essilor technicians use 175 tons of pressure to inject the melted polycarbonate into the block molds. The molds are shaped to create spherical, aspherical, and progressive lenses. In less advanced manufacturing processes, this step would be used to make “lens blanks”—hockey-puck-shaped chunks of plastic that would later be shaved down to the correct prescription. Instead, Essilor creates specific prescription lenses in the molds.
The molds create clusters of connected lenses. The clusters are then degated—excess material is removed—and separated into pairs.
Hard Coating
Before the lenses are ready for shipment to the eye clinic, they must be coated with various treatments. At Essilor, the lenses are rinsed in several water solutions, dried with pulses of air, and dipped into a hard-coating solution. The coating dries to a thickness of 3–4 microns, or about 1/7,000 inch, enough to protect the lens from scratches. Next the lenses are placed into a vacuum coating chamber, where an electron beam gun adds four layers of anti-reflective treatment and one layer of water repellent treatment. This is repeated on the other side of the lens. At each stage, the lenses are carefully inspected.
Packaging and Shipping
The progressive lenses are marked with special inks to allow them to be perfectly centered for later shaping. All lenses are individually packaged in protective materials, labeled, and boxed for shipment. All that’s left for the eye clinic to do is select the right lenses and shape and bevel their edges to fit your frames. Your glasses are ready!
For questions or comments, contact Woodhams Eye Clinic.
Image source: Wikimedia Commons
