When a patient orders a pair of glasses they first choose a set of frames that suits them. The patient must have a prescription must be written by an optometrist which states the needed lens power and pupil distance (PD). Lenses in the needed power are either ordered or taken from stock. These lenses
come from the factory as uncut round blanks approximately four inches across. Edging is the process of cutting these optical lens
blanks to fit frames, producing a pair of glasses.
There are four pieces of equipment used in the lens edging process. A lensometer is used to check the power in the lens. The tracer is used to determine the dimensions and shape of the frame. The blocker is used to prepare the lens for handling by the edger. The edger cuts the lens to the dimensions specified by the tracer.
Lensometer (Source: Topcon)
Tracer (Source: National Optronics)
Edger (Source: National Optronics)
Step 1 Checking the Lens
After receiving the lens blanks the lab technician first checks them visually for any scratches or imperfections that would be cause for rejection. Next using the lensometer the power of the lens is verified. The lens is rotated to the correct axis (lenses with astigmatism or bifocal correction only) and the optical center (point where the prescription is strongest in the lens) and horizontal axis are marked.
Step 2 Tracing the Frame or Pattern
After lens checking the technician must trace the frame. Tracing determines the needed dimensions of the finished lens. There are two methods of doing this, tracing the frame and tracing the lens. The technician determines which method will work best with a particular frame based on experience and the lab's equipment. In frame tracing the demo lenses or the patients old lenses are removed from the frame and the frame is placed face down in the tracer and held in place with specialized clips. In pattern tracing either a pattern of the needed lens shape, a demo lens, or the patients old lens is used. Using a double sided adhesive the pattern or lens is attached to a stand that will hold it in the tracer. The technician will enter relevant information into the tracer such as a job number, patient's name, or other identifying information and also specifies whether he will be tracing a frame or a pattern, and whether he is tracing the right or left side. Next, the machine will ask for the DBL or Distance Between Lenses measurement, which the technician will input manually. After all of the needed information is input a motorized probe on the tracer will follow the inside of the frame or the outside of the pattern for one complete revolution and record or "memorize" the dimensions for use later.
Step 3 Blocking
After verifying the tracer measurements the technician must block the lens to prepare it for the edger. Using the optical center and horizontal markings from Step 1 the lens is aligned with markings on the blocker machine's tray. The blocker arm places a special metal chuck with adhesive backing on the center of the lens' front surface.
Step 4 Edging
The blocked lens is placed into the edger and held in place by pressure on the chuck and also from a pad on the back side of the lens. Using information received from the tracer, the edger now shows an actual-size outline of the lens to be cut on a small screen, giving the technician one last chance to verify the measurements are correct. The edger now asks the operator to input variables such as the lens material (plastic, polycarbonate, or glass) and the patient's PD, as specified on the prescription. A motor slowly rotates the lens and high speed motors begin to rotate a steel cutting blade. The high speed blade is slowly brought towards the slowly turning lens and removes material from the edge of the blank, layer by layer, until the shape of the lens matches the measurements from the tracer. Finally the edger allows the technician to place either a bevel or a groove around the outside of the lens to hold it in the frame.
Step 5 Quality Control
After the lens is removed from the edger it is put into the frame and cleaned. The technician visually checks once again for any scratches or nicks in the lens, and uses the lensometer to once more check that the power matches the prescription. If no problems are found the new glasses are ready for the patient to pick up.
The process of producing a pair of prescription eyeglasses requires very sophisticated equipment and an experienced technician but the steps involved in the edging process are fairly simple. Using modern equipment an experienced technician can produce a complete pair of glasses in just 15-20 minutes.