Optical Physics

What’s the difference between optical glass and normal glass?

Optical glass has the characteristic of being transparent and pure in a noticeably different way, also the production process and the components are completely different from normal glass.

Are the rays of the internal and external curve of a plano lens the same?

No, they are not. The two surfaces of a plano lens are different.

What is refraction?

It is the deviation that a ray of light is submitted to when passing through a surface that separates air from another transparent part.

What is light “transmission” of a lens?

It is the quantity of global light, in the visual area, which goes through a filter. If the ray of light that passes through a lens is 100 and the lens has 15% transmission, the light that will reach our eye is equal to 15%. The remaining 85% is rejected or dispersed in the lens.

What is a transmission curve?

A sun lens does not filter all colors (wave lengths) in the same way, therefore the transmission curve represents the color filtering of a particular lens.

What are UV rays?

They are electromagnetic waves coming from the Sun which reach the Earth naturally filtered by atmosphere. In general they are dangerous for Man’s health and for all living beings.

What are IR rays?

They are electromagnetic waves coming from the Sun or from other heated bodies. They have in common the transportation of thermal energy. So far no pathologies have been associated to these radiations.

What are coatings and how can a coating modify the color or the transmission curve of a lens?

Coatings are micrometric layers of material with different refraction index compared to a lens (glass or organic). They have the power to intervene on the transmission curve and the apparent color of a lens.

What is polarized light?

Light is an electromagnetic wave that oscillates throughout its propagation axis on infinite oscillating surfaces that pass the axis. The polarized electromagnetic wave (light), oscillates only on one surface.


What is the difference between plastic sun lenses and glass sun lenses?

From a technical point of view, glass sun lenses are more scratch-resistant and maintain their optical geometric shape in time as well as their characteristics of transparency and coating adherence. They have an excellent resistance to impact after being chemically hardened. Organic lenses vary in their performance. In general they are known to be lighter weight and some (polycarbonate) are more impact resistant. They are easier to cut to shape and to adapt to the styles that fashion presents. From a commercial point of view, organic lenses are usually cheaper.

Is it true that glass is fragile?

Glass is a fragile material and this we know from everyday experience. But it is also true that there is optical glass and hardening treatments that have granted exceptional performance to this material from an impact resistance point of view. A chemically hardened glass lens can be more resistant than many organic material lenses, and however largely superior to what is required by International norms concerning sun lenses.

Is it true that a glass lens breaks easily?

Absolutely not. Glass lenses do not break easily at all. In glass lens production, very special glass materials are used, both for transparency and for resistance. Our lenses pass experimental tests with impact energy up to three times greater than those imposed by norms. Technology has very much improved glass lens production and processing. All our lenses are chemically hardened.

Is it true that glass is heavy?

Glass is a material with a specific weight superior to other materials used in producing lenses. Given the hardness of glass it is however possible to produce very thin sun lenses which noticeably reduce the difference in weight between a glass lens and an organic one.

Can any type of frame be used with glass lenses?

The only type of sunglasses that are practically not obtainable with glass lenses are the mask ones.

Are there any characteristics and performances that only glass can give?

Transparency, purity, shape-retaining, and scratch resistance: these are characteristics that only glass can give. Furthermore it is a unique material in terms of coating adherence.

Which are the advantages of a glass lens compared to a CR39 lens?

Glass lenses are much more scratch resistant compared to CR39 as well as any other plastic material used to produce lenses. Coatings have a much better adherence on glass. Among all materials used to produce lenses, glass is the one with the highest Abbe number. This number indicates the extreme transparency of a material. Glass weighs more than plastic, but glass can be processed into very thin wafers and thanks to the new technologies, it can be hardened in such a way to compensate these disadvantages. Glass is shape retaining in all conditions and withholds its original optical precision unaltered in time.

What are the advantages of a glass lens compared to a polycarbonate lens?

Polycarbonate is more impact resistant but it has the same limits of all plastic lenses (see CR39).


What characteristics does glass have?

The glass used for sunglasses is a special kind of “optical” glass. It stands out for its transparency and brilliance. Optical glass:

  • Is shape-retaining;
  • It leaves the surface of lenses brilliant after being processed;
  • Anti-scratch and inalterable;
  • Resistant to chemical agents;
  • Heat-resistant;
  • Antistatic;
  • Impact-resistant;
  • Light weight in the extremely thin thickness used for sunglasses;
  • Perfect coating adhesion.

What type of glass is used for sun lens production?

It is a particularly transparent and pure special glass. It is the same glass used to produce optical equipment, like cameras, microscopes, binoculars etc.

How is glass colored?

Glass can be colored during the melt with special chemicals, in this case the color is inside the glass mass. Glass lenses can also be colored with coatings applied to the external surface.

What does it mean to harden a lens?

Hardening is a physic-chemical treatment that lenses undergo to make them more impact-resistant. It can be thermo or chemical. Thermo hardening is obtained by heating a lens making it reach very high temperatures and then submit it to a cold air jet. Chemical hardening is obtained by dipping the lenses in a bath of melted salts containing potassium for 10-12 hours.

What is the difference between chemical and thermo hardening?

Thermo hardening induces a tension in the glass mass. To not deform the geometrical shape of the lens it is necessary for it to have a thickness of 2,5-3 mm. It has the advantage of being reduced into thousands of little pieces in case of breakage (like car shields). Chemical hardening induces a state of tension only on the surface. Compared to thermo hardening, it is more resistant, even with very thin lenses without any deformation.

Can a lens be cut after it is hardened?

Particular technologies are necessary in this case to avoid that the lens loses its resistance obtained during the hardening process.

What is a coating?

We call coatings the deposition of one or more layers of chemicals (in general oxides) on the surface of the lens. Coatings can change the color of a lens, the transmission curve or grant anti-reflection or transform the lens into hydrophobic etc.

What is anti-reflection treatment?

A lens having anti-reflection treatment on the surface means that the reflection of incident light on that surface is eliminated. In sunglasses only the inside of the lens is treated while with prescription glasses both surfaces are treated.

What is the clearseal oleophobic treatment?

It is a protective treatment for the lens and its coatings. It makes the lens become waterproof (drops of water slip off) and it does not allow dust to adhere. In general it makes the lens easier to clean. It is an excellent protection for antireflection coatings which are particularly sensitive to fingerprints.

What does decentered lens mean?

A decentered lens is a lens whose optical center and geometric center do not coincide.


Which is the best color for a sun lens?

This is a difficult question to answer. Lens manufacturers suggest an infinite range of colors according to aesthetic factors and to preferences in general. However we can say that a gray lens is usually known to be the one that better reflects the natural colors of the environment while a brown lens is known to be the most comfortable for the eye and the orange-yellow well adapts to scarce lighting conditions.

Of how many colors can a glass lenses be obtained?

There are practically no limits.

What gives color to glass?

First of all it can be given directly in the melt and we say colored in the mass. Then there are the coatings that can be applied to the surface with special processes. Also there is the H2F process that can be applied to photochromic lenses. In this case the color is given in an oven at very hight temperatures in an atmosphere of pure hydrogen . Even in this case the color is transformd in the mass of the material.

Does the color of a lens change in time?

No, not in the case of a glass lens as well as the glass itself which will never alter in time. Only polarized lenses as they contain a polarized plastic film inside which can with time slightly lose their intensity in color.

Norms and Safety

How can one recognize a pair of sunglasses that has passed all norms?

A pair of sunglasses that respect norms is recognizable from an indelible print CE on the frame. This means that the manufacturer who produced the sunglasses certifies that the product is conform to the specifications required. Only a conform pair of sunglasses guarantees the purchaser on the quality, on the performance and on the protection. The European Directive makes sure that all commercialized sunglasses are conform to the norms.

What does DBT mean?

DBT stands for Drop Ball Test. This test is requested by ANSI. The lenses must be impact resistant and are certified as such dropping a 16 diameter steel ball from a height of 1.3 meters on them. The lens must remain unal

What does CE mean?

CE stands for European Community. When printed on sunglasses it means that they are conform to the European Norms.

What is FDA?

FDA stands for Food and Drug Administration. In our case it is the American Institute that inspects and controls sunglass importation and in particular observes that the imported product is conform to American Norms.

What is meant by category of sun filters?

The category represents the level of protection from the sun. There are five levels in all where 0 corresponds to the lightest filter and 4 to the darkest. Each category is defined by a transmission field which determines the filtering capacity of the lens.

What is meant by optical class?

In the European norms there are two levels of lens quality production which differ in the tolerances requested. Optical Class 1 is stricter and Optical Class 2 is for more standard productions.

Which filters are not suitable for driving?

Category 4 filters (very dark) are not suitable for driving and this must be declared on the instructions or on the frame with a symbol informing the user that recognizing colors can be difficult with filters of this category, therefore dangerous when driving.

What is meant by UV400 lens?

Normally protection against UV rays refers to UV that is contained between 280 and 380 nm. UV protection can reach also up to 400 nm.

Are there norms which must be respected when producing sun lenses?

Yes, norms for sunglasses have come into force since 1997 and they declare the following:
"The main aim for sun filters is that of protecting the human eye against high solar radiation, to reduce eye strain and increase perception. Eye protection must be assured also in the ultraviolet spectrum field."

What does en 1836:2005-A1 mean?

This is the European norm explaining the production standards for Individual Protection Devices which obviously include sunglasses. Sunglasses which are not produced in conformity to this norm cannot be commercialized.

How are Barberini lenses inspected?

Barberini lenses are destined to a worldwide market. It was therefore a precise choice to inspect all our lenses according to the main norms in force: UNI EN 1836 (Europe) ANSI std. (USA) Australian std. (Australia). All our lenses must pass the DBT test even if the European norm defines it as optional.

How should sunglasses be taken care of?

Sunglasses are optical tools and must be well kept and correctly used in order to maintain their characteristics in time. When they are not being worn, they should be kept in their case protected from abrasion. Even if glass lenses are more resistant to abrasion it is a good habit to clean them with water and neutral soap and wipe them dry with a soft cloth. For polarized lenses it is a good idea to avoid excessive heat (leaving your sunglasses on the car dashboard under the sun for example) the plastic film inside could get ruined.

How should lenses be cleaned?

A lens is an optical device and as such it must be treated. We suggest not wiping them dry with a cloth because dust can act as a very thin abrasive. It is preferable to wash them with water and soap and dry them with a soft cloth.

What is a spectrophotometer?

It is a tool used to measure the quantity of light that passes through a lens at various wave lengths. It is also the tool that allows us to know the “transmission curve” of a lens.

Which are the norms that rule the production of glass sun lenses?

In Europe sunglasses can be commercialized only if they are conform and pass the tests requested by the norm UNI EN 1836. The manufacturer or the distributor is responsible for the certification of the sunglasses and must print an indelible CE on the product. This print indicates that the sunglasses pass all the safety tests required by the norm. Other States such as USA and Australia have their proper norms that must be respected to commercialize sunglasses.

What prints or logos or marking should we observe to be sure that we are buying a good pair of sunglasses?

Sunglasses cannot be commercialized if they do not pass the European standards. After having done all the necessary tests the manufacturer must print CE on the product to demonstrate that it has observed all the procedures requested by law. Avoid sunglasses that do not have this print. Category 4 filters (very dark) also have a warning printed on them saying that they are not suitable for driving as they do not allow good color recognition.

Which norms rule the sun lens field?

The main one is the European Directive EN 1836/97 respected by all the countries who are members. This Directive establishes all the minimum safety requirements that a pair of sunglasses or better so: an Individual sun Protection Device, must have in order to be sold to the public. The Manufacturer must respect this Directive by following all the procedures to certify and guarantee the final product and after doing so he must print the CE mark on the frame.

Can I feel sure about impact resistance where glass lenses are concerned?

Absolutely yes. All our lenses are submitted to the impact test and are guaranteed on resistance as requested by the International norms.


Lenses can be of almost any color. However it is not correct to say that a color is more protective than another. It is possible to say that a certain color is more adapt for a certain situation compared to another.

Some lenses increase the contrast in the colors we see, others highlight certain colors compared to others.

Yellow-Orange lenses for example are excellent in dim lighting conditions, Grey lenses are usually thought to be more neutral in colorinterpretation, brown on the other hand is known to be more comfortable because of its warm shades etc.

What is important is to pay attention to the filtering quality of the lens (transmission), lenses that are too dark should never be preferred otherwise in dim lighting conditions our sight is penalized. Mainly, for a general use, it is preferable to choose an average dark pair of sunglasses. Protection from UV rays however never depends on how dark the lens is.

Why is it preferable to reduce blue light?

Our eyes focus on blue light with difficulty, therefore to reduce its quantity improves acuity in vision. Furthermore, blue light tends to expand itself in our ocular globe and obscures the image.

If it is true that it is better to reduce blue light, why not eliminate it completely?

Norms prescribe a minimum transmission level of blue light for the correct recognition of colors.

What are the advantages of a gradient lens?

Lenses are produced also gradient especially to satisfy an aesthetic need. It is also true that these lenses can be useful in some situations when one prefers to have a part of the lens, usually the top part, more protective and darker while the other part remains lighter to better see for example the street, car dashboard etc.

Is it true that UV rays can cause cataract?

UV rays can be very dangerous, it depends on the type of radiation (A, B or C) and how much we are exposed. They are known to be one of the main causes for this pathology.

Which are UV rays?

UV rays are not visible to the human eye. They are even more dangerous because we do not have a perception of the condition of danger. They are classified in categories based on their wave length. The shorter the wave length the more dangerous. They divide themselves in a decreasing order of danger: UVC, UVB, e UVA. The UVC and UVB are very dangerous but fortunately they are almost completely blocked by the atmosphere. UVA reach Earth and are dangerous only if the exposure to them is for long hours. These are the rays that determine the color of our skin or commonly said our “tan”.

Why must we protect ourselves from uv rays?

Because they are dangerous for our eyes and can be the cause of even serious diseases. Obviously the danger is based on the amount of time we expose ourselves to them. The easiest way to defend ourselves is by wearing a pair of UV protection sunglasses. The CE marking on the sunglasses guarantees that the producer passed all certified tests.

What are the damages these rays can cause?

Diseases to our eyes, going from a simple irritation to cataract or even more serious cases.

Which are polarized rays (or polarized light)?

When a ray of light, at a certain angle, hits a shiny surface, it bounces back polarizing more or less in function of the angle of reflection. Reflected light, polarized or not, is particularly annoying and tiring because it hits our eyes directly from the point we are looking at: sea, street, ice etc. From direct light we can defend ourselves with hats, umbrellas etc but reflected light is everywhere! Only a pair of sunglasses can protect us, better if polarized because these lenses can eliminate all reflected polarized light.

What is a "decentered" lens?

Very wrapping sunglasses, therefore with very curved lenses, can give problems by deviating the image, uselessly tiring our eyes. Norms prescribe limits of acceptance to these deviations. Quality lenses (wrapping) are produced with spherical curves calculated to bring the optical axis back to being parallel with the visual axis. This way vision is perfect and there will be no deviation not even if the sunglasses are very wrapping.

What features must a good pair of sunglasses have?

First of all it must have excellent lenses that do not tire our vision, absolutely neutral (in terms of optical power) without any aberrations, deformations etc. These of course are the basis for a good pair of sunglasses. Glass sun lenses from this point of view are excellent.

Any open air activity presents peculiarities for lens performance and sunglasses performance. It is too long to discuss all that is suggested on the market but in general we can say:

For driving it is recommendable to use a category 2 (not too dark) if they are polarized they can eliminate glare from the street asphalt;

For general use a category 3 (dark) to be worn only in bright day light;

For general use a photochromic pair of sunglasses, when light conditions are changeable: in the woods, cloudy days, in areas which are not all equally illuminated;

In windy days which are not very sunny a pair of photochromic lenses that pass from a category 2 (average) to 3 (dark);

For use in the mountains: category 3 (dark) lens or 4 (very dark) perhaps polarized to eliminate glare coming from the glaciers etc. It is very important to choose a lens with maximum protection towards UV rays, with protection up to 400 nm and with a good protection also towards IR radiation. In the mountains the natural filter is noticeably reduced;

For use at sea or in water : polarized lenses are ideal because they strongly reduce glare coming from the waves and consequently reduce eye strain. It is important that the lens has an excellent UV filtering (and eventually IR) as not only direct light hits us but also indirect light.

Which is more protective between a plastic lens and a glass lens?

A good sun lens can be produced in plastic or glass, both can have excellent protection features, the important thing is to choose quality lenses guaranteed by the producer.

Are dark lenses more protective than light colored ones?

A light colored lens can be as protective against UV rays (Norm CE) although it has a limited filtering power towards light. They are not recommended in full sunshine.

Does the protection given by a lens wear out in time?

Not glass lenses. Glass lenses are made of natural material such as silicon, boron, carbonated calcium etc. Coating adhere to the lenses at very high temperatures in vacuumed coaters with oxides. Because of this we can say that a glass lens is eternal in maintaining its characteristics.

Lens Characteristics

Which lenses are indicated for skiing? (snow)

In the mountains radiation is particularly high for two very important factors: the atmosphere, natural filter, is reduced because of the altitude and the strong glare reflecting off the surface of the snow, ice, clouds etc. Different factors must be taken into consideration:

  • Ultraviolet radiation
  • Blue light
  • Invisible infrared light
  • Reflected light
  • Cold and wind
  • Rarefaction of the air at high altitudes

All these factors could cause disturbance, strain and reddening or dryness to our eyes. It is recommended to use sunglasses with lenses belonging to filtering category 3 (8% - 18%) and in some circumstances of very strong radiation even category 4 is recommended (3% - 8%). We must remember that in full light, very dark filters are comfortable but when under covered skies or in the shade, our visibility can be reduced.

It is very important that the lenses give full guarantee of protection against UV rays (in the mountains UV radiation is more aggressive) up to 400 nm. Even “blue light” must be reduced, in conformity with the norms, to improve protection and to increase visible acuity and focus.

It is a good idea to have lenses that protect even against IR which bring undesired heat to our eyes.

Protection against UV, IR and blue light does not depend on how dark the lens is or on the color of the lens but on how the lens is produced.

The color of the lens can change as there is not one ideal color. Yellow-orange lenses are however recommended on cloudy days or when it snows as they help increase contrast in scarce visibility conditions. On sunny days colors such as gray or brown are comfortable and they do not alter colors.

Furthermore lateral protection can better defend us from indirect radiation as well as from the wind.

Particularly advanced technological lenses with coatings applied at refraction interference or with special glass are capable of protecting our vision even in extreme conditions.

Which lenses are recommended for golf?

There are lenses that have selective filtering of colors, which highlight the color of the golf ball on the golf field for example. A good lens must improve contrast and visual acuity, therefore the lens should not be too dark. Photochromic lenses can be more adapt for the light transmission on the field in order to have always the maximum visible perception.

Which lenses are more adapt for water sports? (fishing, sailing, surfing etc.)

Polarized lenses are particularly indicated for water sports as they protect us from polarized light that reflects off shiny surfaces. In fact the light that reflects off a surface, such as water, polarizes. The effect of this is glare. A polarized filter eliminates almost completely this reflection, which is very relieving for someone who practices water sports or spends a lot of time near water surfaces. The almost total elimination of glare allows to see beneath the surface of the water. This is the main reason why polarized lenses are especially appreciated by fishermen.

Which lenses are more adapt for water sports? (fishing, sailing, surfing etc.)

Polarized lenses are particularly indicated for water sports as they protect us from polarized light that reflects off shiny surfaces. In fact the light that reflects off a surface, such as water, polarizes. The effect of this is glare. A polarized filter eliminates almost completely this reflection, which is very relieving for someone who practices water sports or spends a lot of time near water surfaces. The almost total elimination of glare allows to see beneath the surface of the water. This is the main reason why polarized lenses are especially appreciated by fishermen.

Which lenses are more indicated for a general use?

A lens that adapts to all lighting conditions is a photochromic lens because it darkens or lightens according to the light in the environment. Photochromic lenses are excellent because they can be worn in dim light, in shady areas, or under overcast skies and this way, we are protected not only from radiation but also from wind etc.

Photochromic lenses are available also in various fashion colors thanks to the new technology called Hydrogen Firing.

Which lenses are more indicated for hunting or shooting?

A yellow lens gives the maximum light transmission, it increases contrast and filters part of the blue light and is particularly indicated in this sport as it is necessary to have the maximum perception and visual acuity. A glass lens, optically perfect and free from aberration or distortions is the most adapt to this sport. A photochromic lens can also be used.

Does a lens for all situations exist?

Photochromic lenses adapt to all situations because they darken or lighten according to the lighting conditions of the environment we are in. Photochromic glass lenses are available today in all fashion colors thanks to the technological innovation called Hydrogen Firing.

I spend a lot of time driving, what lenses should I wear?

For driving it is important that the filter is not too dark. Inside the car it is already darker than outdoors and it is important that although we wear sunglasses to protect ourselves from the sun we should however be able to see the dashboard clearly. A category two filter (medium) would be suitable. Polarized lenses are indicated when driving during the hours when the sun is high and reflected light causes glare to disturb our vision. Even a photochromic filter can be useful when driving for the entire day because it adapts to the lighting conditions of full sunshine as well as dawn or sunset or when driving in shady areas. A photochromic lens however reacts less quickly when worn inside the car as the windshield already filters part of the light.

What happens if our eyes are overexposed to the sun without protection?

Our eyes are exposed to dangerous radiation constantly, first and far most UV radiation, “blue” light and infrared IR. In conditions of very strong and prolonged radiation without protection, the immediate effects are well known: burn, strain, erythema etc. but continuous exposure without protection could bring to far more serious problems.

What is done to a lens to make it more resistant?

A glass lens does not need any particular treatment to be resistant to scratches. Glass is a very hard material however for a more extreme use like in sports such as rock climbing or in environments, where lenses can be scratched more easily such as sand, protective coatings can be added to make the lens even harder and resistant to scratches.

What treatments are done to a lens to make it more impact-resistant?

Optical glass is formulated in such a way that it becomes extremely resistant after chemical hardening. After the lens has gone through all the processes of grinding and polishing, they are put into melted salt for about 8-12 hours. The salts are held in a temperature of about 360-400°C. There is a chemical reaction between the glass and salt which grants the glass lenses with a resistance to impact.


Can glass lenses be used for very curved frames?

Base 9 lenses are produced regularly for strongly curved frames. These lenses are also available in decentered version to correct any eventual defect of deviation. The same curves are obtainable in the polarized and photochromic versions.

Can round lenses be purchased at barberini?

No. Barberini guarantees their product and is responsible for the lenses that are delivered completely finished and ready to be assembled into the frames.

Can barberini supply bifocal lenses?

It is technically possible but at the moment our production does not foresee this production.

Filter Categories

What type of protection is necessary in front of the computer?

A category 0 or 1. It is important that it filters UV rays up to 400 nm.

In which case are anti-reflection lenses more adapt?

Sunglass lenses should always have an internal anti-reflection coating to avoid the bounce back effect of radiation hitting the internal surface of our lenses.

In which cases are photochromic lenses more adapt?

A photochromic lens is an excellent choice in all situations in which the lighting conditions is changeable. For example think of city streets (shady areas), trekking in the woods etc. Furthermore photochromic sunglasses are useful every time one wishes to wear sunglasses throughout the whole day even during the evening or to protect oneself from the wind.


Does the sales contract require a minimum quantity?

Yes, our General Sales Conditions require a minimum of 300 pairs per color and style.

Who can purchase lenses from Barberini?

Sunglass producers can purchase sun lenses from Barberini. For commercial reasons we cannot sell directly to opticians or distributors etc.

How much time is required from the moment of the purchase order to the delivery?

Normally about 4 weeks, but for programmed or scheduled orders this time can be reduced

Does barberini offer to assemble the lenses into the frames?

Yes, we have an assembling department who performs the assembling directly into the frames that our customers send to us.

How are the lenses packaged?

In boxes of 50 pairs. The lenses are packaged already in pairs and should not be changed or matched differently. The lenses are then put in boxes that contain 1200-1500 pairs of lenses. It is important to not unpair lenses after packaging because during inspection the lenses are paired also based on the color shades.

Which styles assemble barberini lenses?

Barberini glass lenses are assembled into the frames of prestigious sunglass brands such as: Revo, Ray-ban, Persol, Oliver Peoples, Mosley Tribes, Bulgari, Versace, Dolce & Gabbana, Maui Jim, Loro Piana, Carrera, Giorgio Armani, Gucci, Celine, Police, Chopard, Serengeti, National Geographic, Ralph Lauren, L.G.R., Von Zipper, Electric, and many more.


How do sun lenses protect our eyes?

They protect us by decreasing the quantity of light energy that reaches our eyes. In particular sun filters must assure that a certain amount of light pass the lenses. This amount is established by norms which regulate the production of sun filters. Sun filters can also be selective towards different types of radiations.

When are polarized lenses more indicated than standard lenses?

In all cases in which shiny surfaces such as water, snow, glaciers, asphalt or sand etc. dominate the scene and cause reflected light and glare under the sun.

How does a photochromic lens darken? how exactly does this happen?

In a photochromic glass lens particular atoms are dispersed having the property of changing shade (darker) when activated by sunlight. The opposite happens when in the shade, they become lighter.

How dark can a photochromic lens get?

There are many photochromic lenses on the market and it is good to keep in mind that the minimum transmission also depends on the temperature. In general Barberini’s production allows us to obtain photochromic filters of category 3 in the activated state. (T=20% )

How long does it take for a photochromic lens to go from a dark state to a light state?

This depends on various factors however we can say that on average this happens in seconds (20-30).

What is the maximum transmission of light that can go through a polarized lens?

The darker the polarized filter is the more efficient it will be against polarized light. Normally we use filters with light transmission within 15 to 30%.

How can the difference between a polarized lens and a normal one be shown?

Polarized light is everywhere around us therefore to show someone the effect that polarized light produces is quite simple: for example if you stand in front of a reflection (on a table, on the pavement, on asphalt or glass etc), make sure that the angle of the reflected ray of light with the surface is about 45-50°, then with a pair of polarized sunglasses look through the lens at the spot where the reflected light is, rotate the sunglasses 90° and you will see the reflection disappear or reduce noticeably. Another way to observe polarization is by putting two polarized lenses one in front of the other and cross, when the angle between the two lenses is about 90°, the two lenses become black and light does not pass through.

How can the effect of anti-reflection on a lens be shown?

Barberini sent to all the members of the Sun-wise Optician club a lens with anti-reflection treatment in the center. It is very easy to see the effect of anti-reflection in the part treated which appears enormously more transparent. If one does not have the demonstration lens, the best way is to look at the internal surface of the lens and look for a small reflection on it. The reflection will appear attenuated and the reflected light slightly colored, on the contrary the lens is not treated with anti-reflection.

How fast does it takes for a photochromic lens to go from the light state to the dark one?

Photochromic lenses are sensitive to ultraviolet light which is contained inside the solar spectrum and in special artificial lamps. Apply a post-it on half the lens so that only one part of the lens surface is covered, expose the lens to sun light or under a UV lamp that has good intensity for at least one minute then remove the post-it and you will see the difference. Remember that the photochromic effect also depends on the temperature in the environment, the colder the lens the better you will see the effect.

How fast does it takes for a photochromic lens to go from the light state to the dark one?

Photochromic lenses are sensitive to ultraviolet light which is contained inside the solar spectrum and in special artificial lamps. Apply a post-it on half the lens so that only one part of the lens surface is covered, expose the lens to sun light or under a UV lamp that has good intensity for at least one minute then remove the post-it and you will see the difference. Remember that the photochromic effect also depends on the temperature in the environment, the colder the lens the better you will see the effect.

How can i verify the quality of a lens?

The first characteristic that a good sun lens must have is that of not distorting the image because the consequence would be strong headaches and eyestrain etc. To verify it simply take a pair of sunglasses and keep them at a distance of 30-40 cm in front of you. Look through the lens and move the sunglasses in small circles. If the image deforms it means the quality is not very good.

Can a good pair of sunglasses at a low price be found?

Quality sunglasses are complex to produce and require high technology and noticeable investments. Their cost is therefore proportional to the quality proposed.

How can I evaluate price/quality of a pair of sunglasses?

The choice of a pair of sunglasses is as delicate as the sense it protects: our sight. We know that in different lighting conditions we may need a different pair of sunglasses and the modern technologies give us the possibility to find very sophisticated lenses adapt to protect us from radiation even the most dangerous ones. Ask your optician and remember that very protective sunglasses cannot be cheap!