Mineral Lenses

Manufacturing

Glass ingredients

Pure silica (SiO2) has a melting point of about 2000 °C (3600 °F), and while it can be made into glass for special applications (see fused quartz), two other substances are always added to common glass to simplify processing. One is soda (sodium carbonate Na2CO3), or potash, the equivalent potassium compound, which lowers the melting point to about 1000 °C (1800 °F). However, the soda makes the glass water-soluble, which is usually undesirable, so lime (calcium oxide, CaO) is the third component, added to restore insolubility. The resulting glass contains about 70% silica and is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.
As well as soda and lime, most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant' because the increased refractive index causes noticeably more "sparkles", while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion, and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern glasses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).
Glasses that do not include silica as a major constituent are sometimes used for fibre optics and other specialized technical applications. These include fluorozirconate, fluoroaluminate, and chalcogenide glasses.

manufacturing techniques

Corning Incorporated invents the first complete continuous production line
The Second World War brought paralysis to the European glass industry, depriving the allies of German-made optical glass products. Across the challenge, the baton was taken up by Corning Glass Works, now Corning Incorporated, who developed a revolutionary continuous furnace in which the refractory material was largely platinum. The use of this furnace resulted in the production of glass of perfect quality for precision optics and spectacle lenses. Moreover, the automation of a number of stages of preparation and manufacturing accelerated the arrival of continuous production on a large scale. Thus the modern ophthalmic glass industry was born. For France, its arrival was enhanced by the creation in 1955 of Sovirel, which became Corning France in 1978, and Corning S.A. in 1996.
Preparing the composition

Precise weighing of ingredients

This is a vital operation, since the composition determines the homogeneity and refractive index of the finished glass. It entails weighing the raw materials and blending them carefully to obtain the appropriate composition for the furnace.
Raw material checking
Raw materials for glass composition arrive from the supplier with an analytical data sheet attached. The suppliers are selected the world over in accordance with the requisite variety of products. Additional controls are applied in the plant laboratories.
Weighing
Glass components are weighed with the utmost precision to obtain the right proportions of materials required on the weighing sheet. This may entail an addition of 1 part per 10,000 for a composition in which a minute quantity of a particular ingredient can substantially affect the properties of the glass.

Glass composition blender Mixing

The various materials are blended in an industrial mixer. A predetermined time cycle is observed for each glass type. The batch is then transferred to the furnace area for melting.
Glass production
This is a four-stage process, involving :
* melting,

Cullet
* fining,
* conditioning,
* glass delivery.

Melting

The batch is placed in the portion of the furnace where melting takes place, in mixture with cullet*. Cullet* is recycled glass of identical composition with the batch, which has been retrieved from a previous operation and crushed.

Loading the furnace

A certain amount of cullet facilitates the melting operation.
To melt the composition and produce a homogenous result, the temperature of the furnace must be high enough for the glass batch to become liquid.The temperature may vary according to the type of glass from 1,100 to 1,500°C.
Various types of refractory material are used in furnace construction, in accordance with their placement inside the tank and the temperatures they must withstand. Heating to high temperature is achieved using different forms of energy : gas, fuel and electricity, or a combination of the three.
Where electrical heating is used, the glass bath acts as a resistor. It is a lesser known property of glass that, at very high temperature, it is a conductor of electricity. Glass which is a perfect insulator at room temperatures, may have a resistivity of several ohms per cm when heated to its melting point.

Glass melting Fining

At the fining stage of production, the temperature is raised to render the glass more liquid so as to allow the escape of gases still present in the melt. This operation is carried out in a second chamber of the furnace called the fining tank.
Because the temperatures involved are so high (up to 1,600°C) conventional refractory materials cannot be used ; they would be adversely affected by the heat, and therefore contaminate the glass with impurities, such as colorants.
This is the reason for employing platinum, a material virtually unaffected by hot glass.

Gob of glass, or parison onditioning

On completion of fining, the glass is at too high a temperature to be used for blank forming ; it is too fluid and insufficiently homogeneous.
To condition it, a bank of indicator and regulator thermocouples are distributed within the conditioning zone.
In addition, to attain the required optical quality - that is, total homogeneity - the glass must undergo non-stop blending, employing a stirring process, Guinandage (named for its inventor, Mr Guinand).

Cutting the parison

Following the blending and controlled temperature reduction, the glass is ready to exit the delivery tubes to the press at a working viscosity which varies from 100 to 10,000 poises, according the category of glass.
Glass delivery to the press
The objective within this operation is to deliver to the press glass gobs of constant weight. To achieve this, the stream of glass leaving the delivery tube at a stable flow rate is cut automatically by shears made from a special steel ; the cycle is synchronised with the rotation of the press.
These gobs of constant weight are also termed paraisons.

Manufacturing tool for spectacle lens moulding

Automatic pressing

Glass gobs of required viscosity are vital to produce good-quality lens blanks. Each paraison slides into a mould on the press turntable. Each position of the press corresponds precisely to a phase of the operation : loading, pressing, cooling and blank takeout.
This continuous pressing system makes possible production rates of several thousand blanks per hour.The tooling of the press determines the dimensions of the blank.

Blank pressing

A press is comprised of four main parts :
* the mould, which determines the overall diameter,
* the valve, which gives the convex curve,
* the plunger, which compresses the mass of glass to form the concave surface,
* the ring which closes the whole assembly and determines the peripheral shape of the blank.

Different types of press are used, according to the blank required and the physical properties of the glass.

The annealing lehr

Annealing

When the blank leaves the press, it is carried by a conveyor belt into an annealing lehr. For standard glass blanks, the lehr is used to anneal the glass ; the aim of this operation is substantially to reduce internal stresses induced by heat. For this purpose the blank is brought up to a temperature ranging from 550 to 700°C, according to the nature of the glass, then recooled at a controlled rate.Stress reduction within the blank makes later lens surfacing operations easier.

Glass blanks

A special case : photochromics

With photochromic lenses, the annealing operation has the additional effect of activating, in potential, the silver halide crystals, their size and number precisely determined, which will later give the lens its photochromic properties (transmission change, colour and speed of reaction). The process is carried out in lehrs where the temperature is very closely regulated, taking into account the characteristics required to be developed.

Packing

Initial packaging of the blanks into cardboard trays is effected when they leave the lehr. Containers designed to house the trays are used to despatch the blanks safely to their destination, wherever it may be.

SunSensors® in-mass technology

The most unique attribute of Corning SunSensors lens material is the in-mass photochromic technology, which disperses photochromic molecules throughout the lens material. This is the first in-mass photochromic material to optimize a wide variety of properties including darkening, speed of reaction, attractive colors and longer life. This breakthrough technology raises the standard for how plastic photochromic lens materials should perform.
Molecules in all plastic photochromic lenses lose their darkening and lightening ability due to UV exposure. Corning's 30+ years of photochromic innovation has culminated in SunSensors in-mass technology, in which the photochromic molecules are distributed evenly throughout the lens material. As UV degrades some molecules, others replace them, providing a longer lasting, more consistent color over the life of the prescription.

plastic solutions

Corning's advanced lens technology is available in fixed tint or photochromic sunglass lens material for use in premium sunglass applications. Choose from a multitude of colors and processing technology to achieve that special look.
See our plastic photochromic range ...
Contact your optical laboratory, lens manufacturer or a Corning sales office for additional information.

high index

Hi Index Refraction Criteria

The refractive index of a lens (n) is measured for the yellow ray of helium (d) or the green ray of mercury (e), at the midpoint of the visible spectrum.
In ophthalmics, lenses of index nd = 1.523, 1.6, 1.7, 1.8 and 1.9 are used for single vision, solid bifocal and progressive prescription requirements.
Another range of higher index glasses is used for near vision correction in multifocal lens segments.
Among the advantages of glass as an ophthalmic material is its structure, which enables lenses to be produced in a range of refractive indices, without altering its intrinsic optical, physical or chemical properties.
In practical terms, this variation of index is helpful because it allows the lens designer to optimise surface geometry, selecting an index according to the type of refractive error to be corrected and the power range required.
Learn more ...

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