How Are Eyeglass Lenses Made?
Eyeglasses restore your vision and protect your eyes from damage caused by external forces like ultraviolet radiation and blue light from electronic devices. It is an amazing eyewear invention that has aided many civilizations for several centuries. Ever wondered how eyeglass lenses are made?
It was a magnifying glass first invented in the early 1200s as an optical lens to enhance vision. It was made of transparent quartz and beryl lens. Thereafter glass lenses have mostly substituted quartz and beryl lenses due to increased demand for eyewear.
The first optical lens used in spectacles to help with farsightedness correction was the convex lens. Additionally, corrective lenses, such as the concave lens for the treatment of nearsightedness and more sophisticated lenses for the treatment of astigmatism emerged.
Until the early 1950s, when plastic lenses were first produced, the glass lens predominated. Because it was lighter and less likely to break, the plastic lens soon gained wide acceptance. Although the production of plastic eyeglass lenses is now far more than that of glass lenses, the process has mostly not changed.
The lenses are the most crucial component of a pair of eyeglasses since they have the greatest impact on your ability to see well. However, the lenses travel a long way before they are placed in your spectacles.
There are basically two processes: prescription lenses and "off-the-shelf" stock lenses. Stock lenses are typically preferred by the optician for a simple, low-cost pair of eyeglasses, such as reading glasses, or when the wearer requires replacement lenses quickly. Stock lenses are perfectly adequate for standard vision correction, but they do not perform as effectively as individualized precision lenses.
This article explains how individualized precision lenses are made. It includes the raw materials used and different stages of the manufacturing process.
Raw Materials of Eyeglass Lenses
Opticians previously depended on specialized optical laboratories to produce eyeglass lenses. There are several full-service optical stores now that make lenses on-site. Optical laboratories do deliver lens "blanks" to optical retailers, which are plastic pieces. These blanks have been precisely made and have various curves ground into the front of the lens.
The plastic blanks procured from optical laboratories are circular pieces of plastic, such as polycarbonate, that are 0.75 inches (1.9 cm) thick or thicker and similar in size to eyeglass frames. The thickness of finished eyeglass lenses is considerably decreased to at least 0.25 inches (0.63cm). However, it might vary depending on the specific optical prescription needed. Other materials required are adhesive tape, a lead allow base liquid, metal dyes, and colors.
Depending on how much and what kind of correction is needed, each lens' thickness and shape will be different. The degree and orientation of curves in a lens impact its optical power. One ground curve is needed for each of the spherical (convex and concave) lenses; however multiple curves are needed to treat astigmatism.
Prescription Data Processing
The first stage of production starts when the optical laboratory technician enters the optical prescription for a pair of plastic lenses into the lab's computer. The personalized lens is computed and the data needed to manufacture the lenses is made available.
The technician chooses the right plastic lens blanks based on the processed data. The original work order, the customer's eyeglasses, and each blank are then put on a prescription tray.
The fronts of the plastic blanks feature a variety of pre-ground curves, so the technician must choose the blank that matches the appropriate optical prescription for each lens. The remaining optical power, or prescription, has to be ground into the back of the lens.
Blocking is the next stage. Here, the surface of the lens is coated for protection. First, the lens' "optical center," or the spot that should be centered over the patient's pupil, is determined and marked with a lensometer, which the technician inserts into the lens blanks.
The front of each blank is then covered with adhesive tape to prevent scratches during the "blocking" procedure. This step is required to correctly clamp and process the lenses in the device.
A "blocker" machine, which includes a heated lead alloy that fuses the block to the front of the blank, is then used by the technician to fit one lens blank at a time into it. Each lens is held in place by the blocks while being ground and polished.
After blocking is complete, the lens is shaped to get the required shape and prescription. The technician subsequently inserts each blank into a generator, an optical prescription-set grinding machine. The back of each lens is ground with the necessary optical curves by the generator. The lenses are then polished after this stage. Only the back surface must be carved and contoured as per the prescription.
The surface of each lens is polished while the optical properties remain intact. A precise surface is required for the application of advanced lens coatings that do not peel off from the lens.
The technician places both lenses in the fining machine with the rear of each lens in the appropriate metal lens lap, a mold that corresponds to the requisite optical prescription of the lens. Then, a series of fine processes are applied to polish each lens' front surface.
Each lens is first scraped against a soft sandpaper-based abrasive fining pad. The lens is polished once again using the fining machine as water is sprayed over the lenses while a second fining pad made of smooth plastic is placed over the first one made of sandpaper.
The laps are then taken out of each lens and given a quick soak in hot water. The third and final fining pad is then fastened to the laps before they are inserted into the fining machine. Lastly, Aluminum oxide, water, and polymers are used to polish the lenses while the fining machine rotates the pads in a circular motion.
De-blocking and Cleaning
The blocks attached to each lens are gently pried off with a small hammer once the lenses have been taken out of the fining machine. The tape is then manually removed from each lens. The lenses are next cleansed with various cleaning chemicals and ultra-pure water to eliminate any residues. After that, the lenses are blow-dried and prepared for coating. Before holding more lenses, the laps are sterilized.
The left and right lenses are labeled on each lens by the letters "L" or "R" written in red grease pencil. The lenses are once more put in the lensometer to verify and mark the optical center and examine the other curves required for the accurate optical prescription.
The technician next chooses a lens pattern that complements the design of the eyeglass frames, inserts the pattern and lenses into an edge machine, and then picks the pattern. Each lens is properly shaped by the machine, which also bevels the edge of the lens so that it fits the eyeglass frames. Throughout this procedure, water is poured over the lens.
If the lenses need more grinding, it is done manually using an attached power grinder. For lenses to be placed in metal or rimless frames, which require more crisp bevels, this step is essential.
Tinting and Coating
If tinting is desired, the lenses are colored at this point. The lenses are dipped into the selected treatment or tint solution. Plastic lenses are dip dyed, whereas glass lens colors are deposited in metal oxide layers.
The final and most technically hard phase in the manufacturing process is the application of a lens coating. In the dipping process, an appropriate hard coating to shield against scratches is applied as a lacquer to the plastic lens, hardening it. The type of lacquer used depends on the material and thickness of the lens.
Each lens is meticulously tested before shipping to guarantee it meets quality assurance standards. A visual examination is undertaken to check for dust or damage, and a mechanical inspection is performed to confirm that each lens matches the required standards.
The eyeglass lenses are prepared for placement into the chosen frames once they have dried.