How are spectacle lenses manufactured?

Lenses are the most important part of a pair of glasses and do the most to ensure you enjoy optimum vision. But first the lenses go on quite a journey before ending up in your frames. BETTER VISION explains how both glass and plastic spectacle lenses are made. What are lenses made of? And what exactly sets individualised lenses apart from those you get "off the shelf"?

There are basically two different processes: no matter the manufacturer, individualised precision lenses – also known as prescription lenses – are almost all created using freeform technology. This is a cutting-edge production technique developed by ZEISS that the entire spectacle lens industry has since licensed.

Unlike prescription lenses, "off-the-shelf" stock lenses are created in a casting process. There is a difference between these two kinds of spectacle lenses: the optician usually selects stock lenses for a simple, cost-effective pair of glasses, i.e. for reading glasses or when the wearer needs replacement lenses quickly. Unlike prescription lenses, these are manufactured in large quantities and not individually. Stock lenses are perfectly well-suited for standard vision correction, but do not offer the same level of performance as individualised precision lenses. To produce the latter, the optician creates a detailed, comprehensive visual profile of the spectacle wearer that serves as the foundation for spectacle lens manufacture.

Stock lenses and semi-finished lenses made of plastic are created in a casting process: special ingredients, such as for improving UV absorption, are added to the liquid materials called monomers. This mixture is poured into moulds, hardened and then processed to reduce residual stress. Then the semi-finished lens is ready for use. If it will serve as a stock lens, then a hard coating is applied and, if requested, additional coatings.

The process is slightly different for semi-finished lenses made of glass: first, natural materials such as quartz, potassium carbonate, soda and oxide are melted at 1400° to 1500° C to form a glass mixture. Then this is pressed to form a round glass block that is one to three centimetres thick called a pressing. The front surface is processed in the next step. A diamond grinding tool is used to give the lens its exactly prescribed shape, while polishing gives it the necessary transparency. Now we have a transparent semi-finished lens that has already been ground on the one side.

Bifocal lenses are a type of spectacle lens that have one field-of vision for viewing objects up close, and another for distance vision. Bifocal lenses can be made of either glass or plastic. However, the production process varies enormously depending on the material. With glass bifocal lenses, an additional lens is integrated into the semi-finished lens, the upper half of which has the same power as the main lens while the lower near-vision area is stronger. First, the back surface of the additional lens is ground and polished. Then, the curved side is placed in the existing indentation of the main lens. Now the additional lens is melted and ground together with the main lens until only a single lens is visible. The bifocal semi-finished lens is then further processed by grinding and polishing the front and back surfaces.

Manufacturing bifocal lenses made of plastic is significantly less complicated. Unlike bifocal lenses made of glass, the power of the near vision area does not require an additional lens. Instead, the relevant area is more strongly curved. This curvature is achieved by using a suitable mould into which the semi-finished lens is poured.

The decisive factor when developing  varifocal  (also known as progressive) lenses is knowing what they'll be used for: Will the wearer place special demands on their lenses, such as by working at the computer screen? Or will the lenses be worn while performing standard day-to-day activities? A large number of factors comes into play when calculating a lens design, much like a math equation with hundreds of unknown variables. Thus it should come as no surprise that producing the lens is a complex task. Here a special grinding machine gives the semi-finished spectacle lens the calculated design. A varifocal lens is optimised in multiple test phases before it is rolled out and batch production begins. The varifocal lens design is repeatedly modified and fine adjustments are made until the first lens prototypes are available. Test wearers put the lenses through the paces to ensure wearer tolerability. Batch production and marketing only begin once the test wearers are satisfied with the particular lens design.

At night, our eyes switch from daytime vision (photopic vision) to nighttime vision (scotopic vision). Healthy eyes need around 25 minutes to adapt to the dark. The less light that is available, the more active the eye’s sensory cells will become; they’re responsible for our light-to-dark vision and are known as rods. At the same time, the pupils widen to “let in” as much light as possible. Healthy eyes have no trouble adapting to changing light conditions. Hereditary diseases, certain medication, injuries and a vitamin A deficiency can all result in limited vision at night or dusk. This is a problem that affects many people who wear glasses. Pupils need to dilate more in reduced lighting conditions. As a result, depth of field is lost and spatial vision limited, while reflections and poor contrast tire the eyes. The  i.Scription^®  Technology from ZEISS takes into consideration the wearer’s dilated pupils at night in the lens design, helping to considerably improve visual performance in low light conditions.  

And did you know that our light-and-dark vision also plays a role when it comes to safety on a plane? During takeoff and landing, the cabin lights are dimmed so that passengers’ and crew members’ eyes can adapt immediately to the new light conditions in the event of a crash. This can save precious seconds in an emergency.

To be sure, "off the shelf" lenses are more than adequate for many different activities and visual defects. However, a perfectly customised pair of glasses can do a lot more. For example: while stock lenses accommodate the visual point of each eye, the actual fit of the spectacle lenses in front of the eye – no matter the frames – are not considered during fitting. The more lens production incorporates how a spectacle wearer looks through their frames, the more precise the manufacture, ensuring the wearer enjoys optimum, natural vision. To achieve this, the optician needs to identify many other important facial parameters in addition to the visual point – and all with an accuracy of a tenth of a millimetre. The more the optician knows about the wearer, the more precisely the ZEISS lens design can be optimised. It also doesn't hurt if the optician works with ZEISS measuring equipment, because this helps prevent "data transfer errors".

Unless you attain perfect harmony between the frames, lenses, individual visual needs and the shape of the wearer's face, then relaxed and natural vision is simply not possible, especially for people with older eyes or less common visual defects. ZEISS freeform technology accommodates a large number of personal data for the spectacle lens wearer and, thanks to complicated mathematical calculations, produces lenses featuring such precise manufacture and fit that the wearer enjoys optimum vision with their chosen frames. This is how you enjoy clear vision across the largest possible visual field for different distances as well as outstanding wearer tolerance. The more complex and unique the situation, the more significant the differences are between individualised and stock lenses.