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Written by Administrator
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Thursday, 11 June 2009 17:42 |
About High Index Lenses
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. Lenses of index nd = 1.5, 1.6, 1.7, 1.8 and 1.9 are used for single-vision, solid bifocal and progressive prescription requirements; another range of high-index barium glasses is used for near-vision correction in multifocal lens segments. Index 1.5 is often called “low index”; indices up to 1.56 are generally described as “mid-index” and those including 1.59 and above are called “high-index”. Plastic materials (such as nylon, polycarbonate, Sunsensors®,…) can be found within the 1.5 – 1.71 range of indices, whereas glass reaches 1.9. Benefits of choosing higher indices
In simple terms, the higher the refractive index of a lens, the more it will allow thickness reduction when designing its surface geometry, according to the type of refractive error to be corrected and the power. High-index lenses are more complex than mid-index ones and logically more expensive. This is why ophthalmic professionals will also consider the cost-benefit ratio when determining which index is most suitable for which correction. Our two animations (based on Corning ophthalmic glass data) will show you in detail the benefits of using high-index lenses. Corning High-Index glass
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 significantly altering its intrinsic optical, physical or chemical properties. Corning’s premium clear glass lenses are available in a wide range of refractive indices up to 1.9, to provide the ultimate in thin, optically superior prescription lenses. |
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Last Updated on Sunday, 14 June 2009 19:26 |
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Written by Administrator
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Thursday, 11 June 2009 17:42 |
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Glass Products
Our Glass product range covers photochromic glass, sunglass lenses, high index lens blanks, glass for molds and finished molds You will find in this section the main features, customer benefits and technical data of our well-known range of high quality glass solutions. Glass Lenses
Corning Inc. invents the first complete continuous production lineWorld War II 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. Preparing the compositionThis 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 |  | Cullet |
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 productionThis is a four-stage process, involving : - melting,
- 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 |
Conditioning
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 parisons.  | Manufacturing tool for
spectacle lens moulding |
Automatic pressingGlass gobs of required viscosity are vital to produce good-quality lens blanks. Each paraison slides into a mold 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 mold, 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 | | |  | Glass blanks |
AnnealingWhen 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. 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. PackingInitial packaging of the blanks into cardboard trays is effected when they leave the lehr. Containers designed to house the trays are used to dispatch the blanks safely to their destination, wherever it may be. |
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Last Updated on Sunday, 14 June 2009 19:36 |
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Written by Administrator
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Thursday, 11 June 2009 17:30 |
Sunglass
Sunglass lenses |
A basic definition and a bit of history
As everyone knows, sunglasses are made with darkened, usually tinted glass or plastic lenses to filter the quantity of light reaching the eyes. Their purpose is mainly to eliminate UV rays, to reduce direct light to the desired level of comfort and to eliminate or decrease glare.
But most of us are less familiar with the motivations of those who pioneered this concept two centuries ago. At a time when no one had heard about damaging UVs and everyone was naturally adapting to strong summer light with hats or visors, the aim of tinted glasses was initially to correct vision impairment (better techniques were soon developed), and later on to protect patients who had developed an abnormal sensitivity to light due to specific diseases. Not too surprisingly, the extended use of sunglasses by a much larger share of the world population came around the 1920s, from the fashionable imitation of movie stars who had to protect their eyes from extremely bright lights on the sets (bright lighting was used to make up for the low sensitivity of the first black & white films).
Not forgetting other occasional use, like hiding our emotions or physical defects, avoiding eye contact or going unnoticed, we nowadays add outdoor eye comfort, style and fashion benefits to the protection of our eyes from excessive ultraviolet radiation, which can lead to various diseases such as photokeratitis or cataract, as reiterated by health care professionals. Sunglass materials and technologies
Both glass and plastic have the required properties to make sunglass lenses. The plastic materials mostly used for sunglasses are acrylic, nylon and polycarbonate, but glass lenses have the best optical clarity and scratch resistance, and they remain the high-end market leader, even though they are heavier than plastics.
Thanks to this material versatility, the sunglass prescription lens market covers most prescriptions. It is a fast-growing market segment in most regions. Tinted sunglass Depending on the color, the lens will eliminate specific frequencies of light (some frequencies or wavelengths can blur vision, and others, like green, can enhance contrast). For example, while gray and brown are the most widely-sold multi-purpose tints, yellow is used in snow glasses to enhance sharpness/ details on ski slopes. Similarly, purple and rose tints may offer more contrast in a green or blue environment, useful for hunting or water skiing. | Lens Color | Properties | Transparent
(UV filter but no tint) | Protects from UV/spatter/wind/dust/insects | | Gray | Neutral, all-purpose, tint which does not enhance contrast or distort colors
Offers good protection against glare
Good choice for driving and general use | | Green | Excellent visual acuity - good contrast enhancer
Minimal color distortion, reinforces natural green
Reduces glare | | Brown | Warm, popular, multi-purpose tint.
Minimal color distortion - Reduces glare - Enhances contrast and clarity
Absorbs higher frequency colors, such as blue
Useful in aquatic environment (best for fishing) and under hazy sky. | | Amber/Yellow | Best contrast enhancement in faint to moderate light (ideal for skiing under cloudy sky)
Color distortion
Improves depth perception (ideal for golfing) |
Polarized lenses
In simple terms, polarized sunglasses eliminate most of the annoying glare coming from flat reflective surfaces (roads, snow, lawns, lake water,…)
This effect is obtained by the presence of a polarizing film between two thin layers of glass, one being a tinted glass (external side, with all necessary sunglass treatments), and the other one being a neutral glass. As the glare from reflected light is by and large horizontally polarized, it will be blocked by the vertically oriented polarizing film. The result: an increase in sunglass performance, especially when driving, fishing and sailing ... This explains why, although more expensive, polarizing sunglasses are one of the fastest-growing segments in eyewear. 
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Photochromic Sunglasses
These lenses incorporate photochromic molecules which undergo a reversible chemical process when exposed to UV rays. This change of shape (gain/loss of an electron for silver atoms in photochromic glass, or opening/closing of the organic molecule in photochromic plastics) results in the absorption of visible light, causing the lenses to darken. These sunglasses will darken when exposed to UVs and fade back indoors – ideal for changing environments. See our detailed Photochromism page. Other treatments
Finally, apart from the usual anti-reflective AR and Hard Coating HC treatments, sunglasses can also be coated with a complementary waterproof treatment (also active against condensation and stains) as well as with a very thin reflective coating (mirror effect), sometimes with a gradient effect so that added reflectivity/protection occurs on the upper half of the lens while better visibility is reserved for the lower half (ideal for driving in a sunny environment – reading the dashboard). 
Sunglass lenses |
Protection standards
As sunglasses should protect the retina from intense sunlight, improve perception and reduce eye fatigue overall, the standards below have been designed for patients to know precisely which level of protection and visibility they are buying (based on regulated labelling, similar to UV filters in cosmetics).
On the manufacturers’ side, knowing the exact specifications of the standards allows not only full compliance with the rules, but also the creation of a range of products which will best address different categories and market expectations.
Three regulatory standards coexist worldwide, which specify the detailed safety and performance requirements (including the expected physical properties) for non-prescription sunglasses and fashion eyewear in order to obtain a specific lens category/classification: RANGE OF LUMINOUS TRANSMITTANCE (from over… to…)
| SUITABLE
LIGHT CONDITIONS
| LENS CATEGORY EN-1836 and AS-1067 | DESCRIPTION EN-1836 | DESCRIPTION AS-1067 | 80 to 100 % | Indoor | 0 | Clear or very light tint | Fashion spectacles - not sunglasses
Very low sunglare reduction
Some UV protection | 43 to 80 % | Limited sunlight | 1 | Light tint | Fashion spectacles - not sunglasses
Limited sunglare reduction
Some UV protection | 18 to 43 % | Moderate sunlight | 2 | Medium tint | Sunglasses
Medium sunglare reduction
Good UV protection | 8 to 18 % | Strong sunlight | 3 | Dark tint | Sunglasses
High sunglare reduction
Good UV protection | 3 to 8 % | Extreme sunlight – not suited to driving | 4 | Very dark tint | Sunglasses – special purpose
Very high sunglare reduction
Good UV protection |
Example of a category 4 label for sunglasses (EN-1836) in French, with its self-explanatory pictograms: 
RANGE OF LUMINOUS TRANSMITTANCE | DESCRIPTION ANSI-Z80.3 | Greater than 40 % | Cosmetic lens or shield, Light | 8 to 40 % | General purpose lens or shield, Medium to dark | 3 to 8 % | Special purpose lens or shield, Very dark | Up to 3 % | Special purpose lens or shield, Strongly colored |
Comments: - Choosing a lens category should take into account the local intensity of light, how sensitive the sunglass wearer is to glare and the need for UV protection (especially at high altitudes).
- The highest categories (cat 4 or less than 8% transmission for both European and Australian standards) being devoted to specific applications (such as mountaineering or protection of hypersensitive eyes), they are not suited to driving, an aspect which international standards also take into account (traffic signal vision tests data).
- Regulators are actively working on a new, global standard.
- Corning has chosen to offer only medium or high levels of protection, thus not to address category 0 or 1 sunglasses.
Corning Ophthalmic offers a wide variety of glass solutions, including fixed tints, photochromics and glass lenses for the manufacturing of polarized sunglasses, as well as photochromic coating for plastic lenses and in-mass photochromic monomers. Our products, which you will find in some of the world’s leading sunglass brands, comply with all international standards and beyond. - Outside on a sunny day, the brightness of the light ranges from about 1,000 lumens in the shade to more than 6,000 lumens. When the brightness of the direct or reflected light gets to about 4,000 lumens, our eyes begin to have difficulty absorbing the light. What we see when we try to look at these brighter areas are flashes of white ; this is glare.
Source http://science.howstuffworks.com/sunglass
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Last Updated on Sunday, 14 June 2009 19:29 |
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Written by Administrator
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Thursday, 11 June 2009 17:30 |
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