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.
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 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.
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