Technical Info
metals
Metals info [ PDF ]
plastics
Acetate
Cellulose acetate is one of the most versatile of all plastic frame materials and the most commonly used. It is obtained from cotton flock and wood pulp, and it can have a great variety of colors, with either a single color or multiple colors. Safilo S. Maria di Sala factory is specialised in the production of cellulose acetate frames. Rims and ear-pieces are obtained by milling different thicknesses acetate slabs with numerical-control-precision milling machine.
Characteristics | Advantages |
---|---|
Very adaptable | Frames easy to adapt and lenses easy to insert |
Lustre of the material and wide range of possible color combinations | Great variety of styles in shapes and surface manufacture |
High mechanical strength and chemical resistance | Hypoallergenic material |
Working with Acetate
edging and inserting lenses
Lenses must be inserted by heating the frame with an air warming system at approximately 140-160F.
temples adjustments
Acetate temples have a metallic core that allows them to be adjusted without pre-heating. When adjusting strong temples, a pre-heating at 160F is recommended.
front adjusting
Front must be adjusted by pre-heating it with an air warming system at approximately 140-160F.
occasional fixing
It is advisable to use acetone to stick together acetate parts. S.M.: do not heat the frame after repairing it.
Epoxy
Epoxy or polyepoxide is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives.
Chemistry
Epoxy is a copolymer; that is, it is formed from two different chemicals. These are referred to as the "resin" and the "hardener". The resin consists of monomers or short chain polymers with an epoxide group at either end.
The process of polymerization is called "curing", and can be controlled through temperature and choice of resin and hardener compounds; the process can take minutes to hours. Some formulations benefit from heating during the cure period, whereas others simply require time, and ambient temperatures.
Temple Adjustment: (For Temples made from Epoxy Resin)
Temples can be adjusted by using a hot air warmer at temperatures between 122°F - 140°F. (50°C - 60°C.) for around 30 - 50 seconds.
Warning - Do not over heat the temples. This will damage the epoxy resin, causing it to bubble.
* Hot water can also be used to adjust these temples. Use the same time length, and temperature as for the hot air warmer.
Do not adjust the temples cold, this could cause the epoxy to crack.
Polyamide (Nylon)
Safilo uses two kinds of polyamide (nylon) materials: TROGAMID and GRILAMID. Both these plastic materials have a high performance and do not cause any type of allergies.
Trogamid
Trogamid is a thermoplastic material which in the eyewear sector is processed using injection-moulding techniques. The use of this material, which is classed as a nylon, gives products a transparent quality coupled with high chemical resistance given by the particular structure of the polymers. Trogamid is in fact available in a number of structural forms and goes by different names, depending on the base polymers of which it is comprised. The type most commonly used in the optical sector is Trogamid CX7323.
Characteristics | Advantages |
---|---|
Crystalline luminosity and permanent transparency | Good chromatic and aesthetic results |
Superior chemical resistance compared to other plastics | Durability of the mechanical characteristics of the material and consequently of the glasses |
Extremely high UV resistance | Good light reflection capacity |
High dimensional stability | Frames are unalterable and durable |
Resistance to high temperatures | The shape of glasses is non-deformable |
Excellent resistance to abrasion and cracking | Possibility to obtain reduced thicknesses/sections. Resistance to perspiration, cosmetics and detergents |
Grilamid
Grilamid is a thermoplastic material for injection moulding. Research in the field of plastics has recently led to the use of Grilamid TR 90 LX, a grade of nylon that meets all the requirements needed for a material used for spectacle frames. In addition, Grilamid is plasticizer-free, it is light - and atmospheric agent s- stabilized and has low flammability.
Characteristics | Advantages |
---|---|
Extremely high resistance to breaking. Very lightweight | Possibility to obtain reduced thicknesses/sections Glasses are especially strong and comfortable |
Transparent, even when considerably thick | Good aesthetic results both in terms of color and design |
Extremely high UV resistance | Good light reflection capacity |
Good chemical resistance | Resistance to perspiration, cosmetics and detergents |
Working with Polyamide (Nylon)
edging and inserting lenses:
Lenses must be inserted without pre-heating. This indication can be also seen on the presenting lens
temples adjustments:
Temples can be adjusted by heating the part to be adjusted with an air warming system at approximately 175°F.
occasional fixing:
It is not recommended to use commercial adhesives for occasional fixing
cleaning:
Use lukewarm water and a normal, neuter detergent in liquid form and dry with a soft clean cloth Avoid using solvents (e.g. alcohol and acetone) and aggressive chemical detergents that can damage the glasses functionality.
Quality and Training
Safilo Eyewear Manual [ PDF ]
Safilo Quality Brochure [ PDF ]
sun lenses
Special Treatments
Anti-Reflection Coating
In 1935 Zeiss patented the first single anti-reflection coating on ophthalmic lenses. This is how it works: part of the light that is filtered by a lens normally creates a reflection off the back of the lens and into the eye, which diminishes contrast and visual acuity. This reflection is eliminated by applying a very thin layer of a special substance on the lens, thus reducing glare and protecting the eye from direct reflection.
The vision improves greatly both during daytime and at night.
To obtain anti-reflection coating, glass, CR 39 and polycarbonate lenses are treated with vacuum-evaporated layers of fluoride or magnesium chloride or sodium or lithium.
Three types of coating are possible:
- single: almost no longer in use today, it gives the lens a typical magenta reflection
- double: it reduces glare but lets about 1,5% light filter in
- multiple: it is the most currently used and definitely the most effective. It also improves scratch resistance of CR 39 lenses
Safilo sunglass lenses all have a multiple anti-reflection coating.
Anti-scratch Treatment
It consists of applying a protective varnish on the lens that hardens its surface.
There are two types of varnish:
- acrylic - it is sprayed on the lens and dried with UV radiation. This treatment is the less expensive one and is usually found on cheap lenses
- silicone - is obtained by dipping the lens into silicone and then hardening the silicone layer in the oven. CR 39 and polycarbonate lenses are usually treated with this varnish.
Among plastics, only CR 39 is a naturally hard material. The other plastic materials, including polycarbonate, always need to be varnished with anti-scratch substances.
All CR 39 and polycarbonate lenses used by Safilo undergo this treatment.
Mirror Coating
With mirror coating, a very thin, almost transparent metallic layer is deposited on the lens surface. Apart from a certain degree of UV and IRprotection in the case of glass lenses, this treatment has mainly an aesthetic function.
It is relatively long lasting on glass lenses, but also on CR 39 and polycarbonate lenses, provided that they are previously varnished with anti-scratch products.
Mirror coating may cause internal reflection, but the application of an anti-reflection coating on the internal side of the lens can solve this inconvenience.
Polarization
The active element is a micro-thin film made of polyvinyl alcohol, crystallized iodine and other coloring substances. Its molecules form a sort of net capable of blocking out the reflection produced by the sunlight when it strikes a reflective surface. This film is sandwiched between two thin glasses or CR 39 lenses and fixed to them with special glue that is dosed automatically. The horizontal component of reflection is completely absorbed whereas the vertical component is naturally minimized. The aesthetic result is outstanding, but the optical performance is unique: reflection is almost completely eliminated even in the brightest sunlight conditions.
Sun Lens Materials
CR 39
It is also called hard resin or organic glass. CR 39 has been used in the optical field since the Fifties and represents a valid alternative to glass to obtain good quality sunglass lenses. Safilo uses CR 39 lenses for approximately 30% of its sunglasses. CR 39 is the name of the initial liquid compound that is catalysed with peroxide and injected in glass moulds. After a twenty-hour thermal cycle lenses can be extracted: they will be graduated or afocal depending on the internal curvature of the moulds.
Why using CR 39 lenses?
- they offer good protection from UV radiation
- they have a high impact resistance
- in comparison with other plastics CR 39 is much more non-scratch even without a non-scratch surface treatment
- they are lightweight: thickness being equal, their weight is half the weight of glass lenses
- they assure minimum chromatic aberration.
Glass NM 589
A glass lens created by Barberini exclusively for Carrera by Safilo. Glass and some precious natural elements are fused together to create a special type of crystal that is optically wrought to obtain exceptionally thin lenses. These lenses are then chemically tempered, treated with an anti-reflection coating and with a water repellent coating called Clearseal™. Some types of NM 589™ are also polarized.
The result is a top performance lens. Here are the advantages of this lens:
- it filters out radiations, eliminates chromatic confusion between red and green and enhances color contrast within the visible spectrum, thus improving image definition. This makes it particularly suitable for driving, when a rapid response to sudden situations is required
- it is shock resistant, thanks to chemical tempering
- the water repellent treatment makes the surface of the lens incredibly smooth and slick: lenses are easy to clean; water stains, dirt and skin oils slip away without leaving trace. Ideal for water sports
- the anti-reflection coating blocks the sun glare and prevents light from reflecting off the back of the lens and into the eye
- a polarising lens normalizes the perception of visible light when it is deformed by reflective surfaces such as water or sand: the vision is thus enhanced and less tiring for the eye
Glass
Glass has always been considered the ideal material in manufacturing lenses for the correction and protection of eyesight. Glass lenses consist of silica fused together with other inorganic oxides, and their color is obtained by adding other metallic oxides such as nickel, copper, chrome and carbon. The result of fusion is a rough mass that is machined with different particle-size grinders to obtain a 2 mm thick finished lens.
Due to their fragility, glass lenses are usually tempered, even if the European directive does not set this treatment as mandatory. Safilo sunglasses carry chemically tempered glass lenses manufactured by Barberini, who has achieved ISO 9002 certification.
But what exactly are the characteristics and advantages of glass lenses?
- Up to day glass is the material that still guarantees the best optical quality
- tempering renders the glass lens completely anti-scratch and impact resistant.
Polyamide (Nylon)
Polyamid (Nylon) lenses are produced through injection-moulding and they are directly colored in the moulding mass or in a second moment with uniform or shadowed colors. Nowadays about 30% of Safilo sunglasses carries this kind of lenses. The development and the use of this material has recently developed thanks to its tenacity and resistance to cracking under stress. These lenses are mostly used in 3-piece s glasses with pierced lenses/masks.
Here are the advantages of this material:
- It has class 1 optical quality
- It offers excellent protection against UV radiations
- It undergoes an anti-scratch treatment
- It is shock resistant
- It offers better resistance to cracking under stress.
Polycarbonate
It is a thermoplastic material. Lenses are produced by injection moulding and colored directly in the mass in case of uniform tint, or on the surface of moulded lenses, in case of shaded tint.
At present 40% of Safilo sunglasses carry polycarbonate lenses both in the fashion and in the sport sector. Safilo has only recently extended the use of this type of lens. The latest technological advances have eliminated problems such as micro-cracks, caused by the rigidity of the material, and chromatic aberration, deriving from the lack of homogeneity of moulded product.
Here are the advantages of polycarbonate lenses:
- they offer excellent protection from UV radiations
- they all undergo anti-scratch treatment
- they have high resistance to impact
- they are dimensionally stable up to 130°C and do not suffer from fragility at low temperatures
Polyurethane (NXT®)
Nowadays about 5% of Safilo sunglasses carries this kind of lenses. The development and the use of this material has recently developed thanks to its tenacity and resistance to cracking under stress. These lenses are mostly used in 3-piece glasses with pierced lenses/masks.
Here are the advantages of this material:
- It has class 1 optical quality
- It offers excellent protection against UV radiations
- It undergoes an anti-scratch treatment
- It is shock resistant
- It offers better resistance to cracking under stress
- It is a light material
- It offers excellent workability
Directives & Requirements
All lenses used by Safilo comply with the following directives:
EN 1836-2005 for Europe
ANSI Z80.3-2001 for the USA
These directives guarantee the quality of sunglass lenses to the consumer and establish both their technical characteristics and the tests lenses must undergo. The requirements fixed concern:
- light transmittance and filter categories
- diffused light
- refractive power
- quality of material and surfaces
- static robustness (anti-shock resistance)
- flammability
Let us examine the European directive in particular.
Light transmittance and filter categories
Lenses, or filters, are divided into 5 categories based on their tint and relative luminous transmittance, that is, on the percentage of visible radiations and UV rays that filter through them and reach the eye:
Catagory | Tint | Transmittance | Use |
---|---|---|---|
0 | very light | 80%-100% | cosmetic |
1 | light | 43%-80% | protects from light sun luminosity |
2 | medium | 18%-43% | protects from medium sun luminosity |
3 | dark | 8%-18% | protects from intense sun luminosity |
4 | very dark | 3%-8% | protects from exceptional sun luminosity |
The recommended use derives from this classification.
As concerns the visible spectrum, lenses must allow a clear distinction of traffic lights, so filters suitable for road use and driving shall be of categories 0, 1, 2 and 3.
As regards UV, a lens complies with this standard if it blocks the transmission of this radiation.
Diffused light
The lens must provide proper color contrast enhancement and image definition.
Refractive power
Sunglass lenses shall not provide corrective effects, that is, they shall have no refractive, spherical, astigmatic or prismatic power. In other words, they should not distort images. According to the European directive, only class 1 lenses provide precise optical clarity and are suitable for continual use. Their quality is equivalent to that of prescription lenses and complies with the European directive on security CE 89/686.
Standard lenses used in wraparound frames can blur the vision causing eye fatigue and headache. Safilo eliminates this problem through the use of prismatic compensation lenses. By displacing the optical centre of the lens and tapering the lens thickness the prismatic compensation lens compensates the distortion caused by the inclination.
Quality of material and surfaces
The materials used by lenses manufacturers must be free of any machining defects that could alter the vision such as bubbles, scratches, inclusions, notches and dull spots.
Static robustness (anti-shock resistance)
The test foreseen by the European directive consists in the application of 100 Newton pressure (equal to approximately 10 kg mass) on the lens. The drop ball test provided by the American standard includes a series of tests using a small metal ball dropped onto lenses from a fixed height. In both cases tests are successfully passed when lenses neither break nor lose their original shape. The lenses used by Safilo comply with both normatives.
Flammability
According to European standards, a metallic shaft is heated to 1200 F and placed on the lenses for ten seconds. When the shaft is removed, combustion, if any, must stop and no phenomena of running or dripping from the product should occur. Safilo lenses comply also with the Australian and American directives.
The Spectral Composition of Sunlight
Talking about sunglass lenses, here are some notes on the spectral composition of sunlight.
The solar radiation spectrum includes radiations in a band between 190 to 2300 nanometres (nm)*. The human eye perceives the band between 400 and 780 nm:. In the following graph this band is divided by wavelength and their corresponding perceived colors:
* is the wavelength unit of measurement corresponding to one billionth of a metre
The invisible spectrum: ultraviolet (UV) and infrared (IR) radiations
UV is high-energy radiation in the 190-400 nm band. They are dangerous for all biological tissues and in particular for the eye which does not "see" them since they do not reach the retina.
This type of radiation is usually divided into three intervals:
- UVC (190-280 nm): they are blocked by the ozone layer and hence not reaching the ground level
- UVB (280-320 nm): they reach the ground level and are absorbed by the cornea and the crystalline lens of the eye. They can be dangerous even after brief exposures
- UVA (320-400 nm): they penetrate to ground level to greater extent than UVB. These radiations pass through the cornea and are absorbed by the crystalline lens protecting the retina.
And furthermore:
- the intensity of UV rays increases one time every 10 meters of elevation above the sea level
- it is 180 times more intense on the beach because of the great reflection caused by the sand
- vulnerability to UV rays increases with aging
- overexposure to UV rays causes the eye to adjust slower to the dark
IR radiation lays in the 700-2300 nm band. It is dissipated at all levels by the eyeball and does not concentrate on the lens of the eye (like UV) or in the retina (like blue light, see further down). Since it is not damaging to the eye, absorbing filters are only required in industrial applications.
The visible spectrum: a color world
In the case of the visible spectrum, protective eyewear attempts to totally or partially absorb the high-energy visible radiation called blue light (380-500 nm). The unprotected eye strains to focus on blue light causing chromatic aberration and aging of the eye, as the retina is ill equipped to perform this function. The sunglass lens helps to accelerate this process without eliminating completely the blue light, as this would alter the authenticity of the colors.
When Does a Lens Really Protect the Eye
When it absorbs the UVA and UVB rays. Their elimination is beneficial to the eye and does not affect our perception of visual images.
However, be careful. Any sunglasses reduces the visible light and thus causes the pupil to dilate, potentially allowing a greater quantity of UV radiation to enter the eye. A good quality lens must reduce as much as possible the radiation that damages the lens of the eye.
When it efficiently and progressively absorbs the blue light and filters enough light to avoid distorting the other colors. This makes sunglasses suitable to be worn for extended periods without tiring the eye.