Whether they are made from glass or plastic, optical lenses undergo a complex process before they are placed into an eyeglass frame. Here are some insights into the development of a product some consider an everyday tool and others a luxury object.
Mineral lens production begins with a blank lens. Natural raw materials (such as quartz, potassium carbonate and soda) are melted into a glass mixture at 2500to 2700degrees Fahrenheit. Next, they are automatically fed into a press and processed into a round glass block 1 to 3 cm thick – the so-called blank. It is like an uncut diamond.
The front face is treated in the following production step: A diamondgrinding tool gives it the exactly defined surface shape and final touch. Then the required transparency is achieved through polishing. A see-through blank that is ground on the front side is thus created.
After further processing steps, the finished mineral glass lenses are now either equipped with the corresponding prescription values or are provided with standardized, common strengths for fast orders such as laser lenses. Both variants can be processed in many different ways.
Produced for the first time in 1940, 100% synthetically manufactured plastic lensesare the basis for eyeglass lenses today:
The corresponding high-index plastic is manufactured using a casting method: The liquid source materials (so-called monomers) are mixed with special additives to harden the material and increase the UV absorption of the finished lenses. Next, they are placed in casting molds.
The molds are then submitted to temperature treatments for hours: First the material is hardened, and then the finished plastic lens is removed from the casting die and “annealed,” i.e., heated to just below melting point and maintained at a preset temperature for a certain amount of time. The result: The internal stress of the material is relieved.
By the way: Enormous progress in the field of high-index plastics facilitates the manufacture of extremely thin high-index plastic lenses with refractive indexes of up to 1.67. These plastic lenses are also made using the casting method. The process is considerably slower, however, and can take up to 48 hours.
Glass and plastic lenses are as diverse in their production as they are individual in their further processing – take the manufacture of bifocal lenses, for example. An additional lens is incorporated into the blank lens for mineral bifocal lenses. Its upper portion – the supplementary part – has the same power as the base glass; the near portion on the bottom is stronger. On the surfaces of the later visible separating edge, both fields are precision ground and are then fused.
After the rear surface of the additional lens has been ground and polished, it is placed on the base glass with the curved side inside the recess of the base glass, and both are then fused under high pressure. After cooling, the supplementary part forms one homogenous mass with the base glass and is completely invisible. Next, the overlaying part of the additional lens is ground and the resulting blank bifocal lens undergoes further processing – the front and back sides of the lens are ground and polished. After this manufacturing step, the bifocal lens can be refined.
The production of bifocal lenses out of high-index plastic is considerably less involved. In contrast to mineral bifocal lenses, the power of the near portion is not achieved with a stronger piece of glass inside the eyeglass lens, but rather with a deeper curvature of the additional surface. For production, the concave interior of the casting die (see graphic) is hollowed out in the shape of the near portion, whereby the cast blank is given the characteristically bulging near portion and noticeable edge.
The development of lenses for presbyopia begins with what its wearer wants and what criteria the lenses must meet, depending on whether they are needed for general everyday use or for a particular function, such as use at a computer workstation.
To calculate such a power design, a system of equations with hundreds of unknowns and thousands of equations must be resolved. And this is in combination with constant adjustment to the individual customer’s data. Only after the conclusion of numerous processes – accomplished under even more difficult circumstances – do the first prototypes emerge. Finally, they are tested by test subjects for quality and compatibility. If the glass type is not accepted, a new power design is calculated – and the production cycle starts again from scratch.
You can find out more about the design of eyeglass lenses for presbyopia here.