Making a Lens: Part 4
Steps 10 through 12
Step 10: A pattern in the shape of the frame is inserted in the edger. Patterns are commonly plastic and may be supplied by the frame manufacturer or made in the lab.
Newer edgers do not use patterns; instead, the shape is determined by a probe that measures the frame and stores the information in a computer, which in turn controls the edging operation. As it operates, the slowly turning lens is brought into the fast turning cutting surface, which is either a grinding wheel or steel blades, until a guide contacts the pattern, which is rotating to match the lens. If the frame has a complete rim surrounding the lens, a bevel, or ridge, is cut along the edge of the lens that will fit into a groove in the frame; otherwise, the edge is left flat.
Step 11: The lenses, now cut to fit the frame, are prepared for inserting into the frame.
- If the lenses are to be tinted, the dyeing is done at this point. Special dyes are kept in heated containers and the lenses are immersed. The density of the tint is determined by how long the lenses are left in the dye. Lenses may be only partially tinted (fade), tinted different colors at top and bottom, or tinted a custom color by combining different colors. Also, special UV blocking dyes may be applied in the same way.
- If the frame is rimless, a groove is cut along the edge of the lens to receive the string that holds the lens to the frame. Any sharp edges are trimmed and smoothed and, if desired, the edge is polished on a buffing wheel.
Step 12: The lens is inserted into the frame. Fit and orientation is double checked, any worn screws or hinges are replaced as needed, and the frame is made square. The finished eyeglasses are then thoroughly cleaned and packaged for delivery to the patient.
Glass lenses are ground and polished much the same way as plastic except that diamond cutting surfaces are used, and some details may vary. The blanks are made of relatively soft glass and must be tempered, either by chemicals or heat, to strengthen them before inserting into the frame.
Advances in automation are rapidly changing how lenses are made. For example, the vast majority of labs now use computers to determine curve parameters and lens choice, and equipment is available that will combine several steps or even do the entire operation automatically.
For more information about corrective lenses and related topics, check out the links on the next page.