There are two different basic materials and two different production techniques. Customised precision lenses – also known as prescription lenses - are now produced using ZEISS freeform technology, regardless of whether they are made of plastic or glass. Stock lenses – pre-finished lenses – are normally produced using a casting process.
For simple, low-priced solutions, the optician normally selects pre-finished lenses, e.g. for single vision or reading lenses. The optical powers of these stock lenses are not adapted to each wearer, but they do correspond to his or her vision profile. Customised lenses are produced to prescription. This means that the optician measures the wearer's personal profile, on the basis of which the prescription lenses are then produced. These differences account not only for the different delivery times, but also for the different prices of the lenses. It all starts with a visit to the optician, during which the prescription needed by the wearer and his or her personal vision profile are determined. This process is known as refraction. But what happens after this? How is a modern, customised lens produced?
|Plastic (organic material)
Single vision, multifocal, progressive, special purpose lenses
Single vision, multifocal, progressive, special purpose lenses
|Refractive index||1.5 - 1.74||1.5 - 1.92|
|Customised lenses (prescription production)||Freeform technology
Grinding and polishing
|Stock lenses||Casting process
||Casting process and grinding
Hard lacquer, anti-reflective coating, Clean Coat and other coatings
Tinting, graduated tints and tinting in the dipping process - any colour is possible
Anti-reflective coating, Clean Coat and other coatings
Tinting, graduated tints and tinting in the vacuum deposition process - limited choice of colours
After the optician has sent the individual order to ZEISS, the order preparation process begins: each lens is first calculated individually and the data required for production is generated. The data is saved on a chip integrated into a transport tray. The two semi-finished lenses – for left and right – are automatically removed from inventory and placed in the tray. The journey now begins: conveyor belts transport the tray from one station to the next.
Blocking is needed to ensure that the lens is mounted securely and in the correct position in the machine tools. Before the already finished front surface of the semi-finished lens is connected to the blocker, a protective lacquer is applied to the surface. The material used to bond the lens and the blocker is a metal alloy with a low melting point. The semi-finished lens is therefore "welded" into position for the subsequent machining process (forming, polishing and engraving).
In ZEISS freeform technology the front surface of the semi-finished lens has already been finished and displays an optical power. The adaptation of the lens design to the needs of each individual wearer is performed on the back surface in order to provide the wearer with optimum vision. This forming process is conducted in an automated 5-axis CNC machining process. The shape and the optical power are generated in around 90 seconds. The data required for the forming process are received from a central server. The shaping process includes diameter reduction, bevelling using a milling technique and fine turning with a natural diamond. The minimum surface roughness resulting from the fine turning process enables direct polishing without compromising the lens shape or radii.
In the "soft" polishing process developed by ZEISS, the surface is polished in 60-90 seconds in such a way that no marks of any kind remain. The freeform surface previously generated remains intact – a quality feature of ZEISS. And to ensure that every optician and wearer can recognise that they are receiving a genuine ZEISS lens, a laser is used to engrave a "Z" on the lens surface. The wearer's initials can also be engraved on request.
The blocking material is melted in hot water, allowing the lens to be gently removed from the tool. The metal alloy is completely recycled.
The cleaning process is performed in several steps in a brush washing unit which is much like a car wash. Various cleaning agents and high-purity water are used. Finally, the lenses are dried with a device resembling a hairdryer.
Whether single, graduated, customised or medically prescribed tints are required, plastic lenses can be tinted in any desired colour. The dyes used correspond to those used for textiles. In a dipping process the dyes penetrate deep into the surface of the plastic material. In glass lenses the tints are applied in layers consisting of metal oxides, therefore limiting the choice of colours.
In the dipping process a lacquer is applied to the lens to make it scratch-resistant. Specially adapted lacquers are used for the various plastic materials with different refractive indices. The hard protective coating is about two microns thick. After ultrasonic cleaning the anti-reflective coatings are applied in a vacuum deposition process. The individual lenses are arranged in special racks known as calottes. A modern coating has up to 9 individual layers – the overall package is around 400 nanometers thick. The last layer makes the surface of the lens extremely smooth, making it resistant to both dirt and water (the "lotus effect"). This coating is only about 10 nanometers thick – that's how much grass grows in roughly 10 seconds!
Each lens is inspected thoroughly before delivery. The quality check includes a visual inspection for dust, damage and the right residual reflection colour. Machines are used to check whether each individual lens meets the specifications: refractive power, axis, cylinder, thickness, design, diameter, etc. Using the micro-engraving with the "Z" and the measurement of the dioptric powers in transmitted light with an electronic focimeter, the lens is precisely aligned to ensure that the stamp is correctly positioned. This is important for correct insertion of the lens in the frame by the optician.
Printing of the delivery note, allocation of the lenses to the delivery note, automated packaging, insertion of the warranty card …. The packaging and despatch processes are steered by means of a bar code. The automated assignment of the orders into boxes for collection by a transport company is the last process to be performed at ZEISS Vision Care. The finished lenses are despatched twice a day via an express delivery service, by road or air.
Glazing – the term used to describe the insertion of the lenses in the frame – is performed by the optician. Here, too, micrometer precision is required, as incorrectly centered lenses impair visual quality and can even lead to visible aberrations. The spectacles would then be worthless.
The starting point for freeform production is a semi-finished lens, also known as a puck due to its resemblance to an ice hockey puck. These are produced in a casting process that is also used to manufacture stock lenses. The semi-finished lenses are produced in a casting process. Here, liquid monomers are first poured into moulds. Various substances are added to the monomers, e.g. initiators and UV absorbers. The initiator triggers a chemical reaction that leads to hardening or "curing" of the lens, while the UV absorber increases the UV absorption of the lenses and prevents yellowing.
Moulds are required for the casting process. The moulds feature the negative shape of the resulting surface of the plastic lens. The two moulds are held together by a sealing ring known as a gasket. The gasket ensures that a defined distance between the two moulds is maintained. The liquid monomer is poured into the moulds and hardens to form a polymer, i.e. the plastic lens.
Lenses with different dioptric powers can be made from one semi-finished lens. The curvature of the front and back surfaces indicates whether the lens will have a plus or minus power.
After filling, the moulds are exposed to a thermal process for many hours. In other words, the semi-finished is "baked." The input of heat activates the initiator and triggers a chemical reaction (polymerisation or polyaddition) which, with the release of heat (exothermic reaction), causes the material to harden or "cure." Here, the starting materials bond to become long molecular chains. A catalyst has the same function as baking powder.
Depending on the refractive index required, different starting materials and chemical reactions are used. While only one starting monomer is used for CR 39, for example, two monomers are needed for lenses with the indices 1.6 to 1.74. These two monomers join in a chemical reaction known as polyaddition. The manufacturing process for high-index materials is considerably longer and lasts for up to 48 hours.
During the curing process the volume between the moulds shrinks. In plastic lenses with the index 1.5 this shrinkage is as much as 14%, while it is around 9% for a refractive index of 1.67. The elastic gaskets must adapt to this shrinkage and cannot be used for any more than one production process. For environmental reasons, therefore, the gaskets are made of recyclable material.
After polymerisation the gasket is removed and the plastic lens is separated from the moulds. A heat treatment process called tempering reduces the inner strain in the lens.
After thorough inspection and cleaning, the moulds can be re-used. Depending on the production technique, a mould can be used for up to 100 times.
Stock lenses have been so named as they are not produced to prescription but are kept in stock until an appropriate order is received from the optician. These lenses can be produced using the casting technique. The technology is essentially the same as that used for semi-finished lenses. After the lens blanks or "pressings" have been cured, they are provided with a hard protective lacquer and coating.
Glass lenses are produced much like plastic lenses. Here, once again, there is the possibility of using freeform technology to produce customised lenses or to produce stock lenses using the casting method. The traditional technique of grinding and polishing semi-finished lenses is still used is in some cases. The hot, liquid glass melt is used to produce a lens pressing, the front surface of which is then machined. This front surface is first pre-ground or "roughed" to give it an exactly defined form using a diamond grinding tool. A finer tool is then used to grind the surface again. It is then as smooth as silk. The surface receives its transparency through polishing. Only then is it optically effective. The back surface of prescription lenses (prescription surface) is not finished until the lens order is received. The back surface is also roughed, finely ground and polished. Here the front surface is protected against damage by a layer of lacquer. The resulting lens features the required dioptric power and can then be provided with an anti-reflective coating or otherwise enhanced.