Whether they are made from glass or plastic – optical lenses go through a lot of processing steps before they are placed into a spectacle frame. 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, soda) are melted into a glass mixture at 1400 to 1500 degrees Celsius. Next, they are automatically fed into press and processed into a round glass block 1 to 3 cm thick – the so-called blank. It is a sort of uncut diamond.
The front face is treated in the following production step: A diamond-grinding tool gives it the exactly defined surface shape and final touch. 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 fashioned with the corresponding prescription values or are provided with are standardised, 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, they are generally the basis for spectacle lenses today: 100% synthetically manufactured plastic.
The corresponding organic glass 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 into casting moulds.
The container is 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, that is: It is heated to under the melting point and is 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 so-called high-index plastics allows for the manufacture of extremely thin organic glass lenses with refractive indexes of up to 1.665. 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 together.
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 – it is ground and polished on the front and rear side. After this manufacturing step, the bifocal lens can be refined.
The production of bifocal lenses out of organic glass is considerably less involved. Contrary to mineral bifocal lenses, the power of the near portion is not achieved with a stronger piece of glass inside the spectacle 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 obtains the characteristic bulging near portion and noticeable edge.
The development of a lens for presbyopia begins with what its wearer wants, what purpose the design should fulfil: Special sight functions, e.g. for use at a computer work station or general everyday application.
To calculate such a power design, a system of equations with hundreds of unknowns and under the circumstances of 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 – under more difficult circumstances – processes 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 spectacle lenses for presbyopia here.