The sunglasses provide protection against ultraviolet rays in sunlight. Ultraviolet (UV) light damages the cornea and retina. Good sunglasses can completely eliminate UV rays. Sunglasses offer protection against strong light. When the eye receives too much light, the iris closes naturally. Once you have closed the iris as much as possible, the next step is to squint. If there is too much light left, as can happen when sunlight reflects off snow, this causes damage to the retina. Good sunglasses can block 97% of the light from entering the eyes to prevent damage.
Components:
1) Lens
2) Mirror coating
3) Scratch-resistant coating
4) Polarizing film
5) Anti-reflective coating
6) Photochromatic coating
Sunglasses offer protection against glare. Certain surfaces, such as water, can reflect a lot of light, and bright spots can distract or hide objects. Good sunglasses can completely eliminate this type of polarization glare. Sunglasses eliminate certain frequencies of light. Certain frequencies of light can blur vision and others can enhance contrast. Choosing the right color for your sunglasses can make them work better in certain situations.
1) Lens:
The lens is made up of both Crown glass and plastic glasses. Crown glass is a type of optical glass that is used in lenses and other optical components. It has a relatively low refractive index (≈ 1.52) and low dispersion (with Abbe numbers around 60). Crown glass is made from alkali-calcareous silicates with approximately 10% potassium oxide and is one of the first low dispersion glass.
Polycarbonate lens:
Transparency, excellent toughness, thermal stability, and very good dimensional stability make polycarbonate (PC) one of the most widely used engineering thermoplastics. CDs, protective screens, vandal-proof glazing, baby bottles, electrical components, protective helmets, and headlight goggles are typical applications for PCs. Polycarbonate is most commonly formed by reacting bisphenol A (produced by condensing phenol with acetone under acidic conditions) with carbonyl chloride in an interfacial process. PC belongs to the family of polyester plastics.
Polycarbonate, the main material, is produced by the reaction of bisphenol A (BPA) and phosgene COCl2. In the first synthetic step, bisphenol A is treated with sodium hydroxide, whereby the hydroxyl groups of bisphenol A are deprotonated. Diphenoxide (Na2 (OC6H4) 2CMe2) reacts with phosgene to form a chloroformate, which is then attacked by another phenoxide. About 1 billion kg of polycarbonate is produced each year.
CR-39 lens:
CR39 or polyallyl diglycol carbonate (PADC) is a plastic monomer that, along with the other material PMMA (polymethylmethacrylate), is often used in the manufacture of spectacle lenses.
The abbreviation stands for "Columbia Resin # 39", the 39th formula for a thermoset plastic, which was developed by the Columbia Resins project in 1940 during World War II to reduce weight and increase the range of the bomber. After the war, the Armorlite Lens Company of California is credited with manufacturing the first CR39 lenses in 1947. CR39 plastic has a refractive index of 1,498 and an Abbe number of 58. CR39 is now a branded product from PPG Industries
CR39 is manufactured by polymerizing diethylene glycol bis allyl carbonate (ADC) in the presence of a diisopropyl peroxydicarbonate (IPP) initiator. The presence of the allyl groups allows the polymer to form crosslinks; therefore, it is a thermosetting resin. The ADC monomer polymerization program using IPP is generally 20 hours with a maximum temperature of 95 ° C. Elevated temperatures can be achieved with a water bath or convection oven. To be delivered.
2) Mirror coating:
Sunglasses with a reflective optical coating (called a mirror coating or flash coating) on the outside of the lenses to make them look like small mirrors. Contact lenses typically impart a brown or gray tint to the wearer's vision. The mirror coating reduces the amount of light that penetrates through the tinted glass by an additional 10–60%, making it especially suitable for sand, water, snow, and high altitudes.
Mirrored sunglasses are unidirectional mirrors The simplest version of a mirror coating is a single layer of a deposited thin film of a suitable metal, usually made by ion beam deposition, sputter deposition, or vapor deposition. . However, this type of coating is very susceptible to scratches and deterioration, especially in a corrosive environment such as saltwater.
Newer reflective coatings generally consist of several alternating layers of certain thicknesses made of dielectric materials and sometimes metals. The metal layer can consist of titanium, nickel or chromium, or an alloy such as nichrome or Inconel and has a thickness in the range of 0.5 to 9 nanometers. The dielectric layer comprises a suitable oxide such as chromium oxide, silicon dioxide, or titanium dioxide; its thickness determines the reflective properties of the resulting dielectric mirror.
3) Scratch-resistant coating:
As an anti-scratch coating for polycarbonate (PC) or poly (methyl methacrylate) (PMMA), they do not impair the optical properties of these materials. Under normal circumstances, the thickness of this coating is very small, around 130 nm, which does not affect the optical properties of the polymers.
Aluminum oxide nanoparticles as an anti-scratch coating have 37% non-volatile substances with 30% nanoparticle content, which is essentially surface-modified polysiloxane. These are efficient and effective scratch-resistant coatings. They are suitable for rugged and demanding scratch-resistant coating applications such as military ballistic panels, window glazing.
Silica Nanoparticles as scratch-resistant coating are surface-modified silicas with 50% non-volatile substances and 50% nanoparticle content. In the organic group, silica tends to increase the crosslinking density of all reactive groups. It is a popular anti-scratch coating for plastics that combines ease of processing and flexibility.
4) Polarizing film:
Polarizers are made in many ways. One of the most common polarizers is known as a Polaroid and it is made up of iodine crystals embedded in a polymer. To create the polarizer, the polymer film is stretched, causing the polymers to line up.
The film is then dipped in an iodine solution and the iodine molecules adhere to the polymer. The ordered structure of the Polaroid allows light to be absorbed parallel to the polymer chains and light to pass perpendicular to the chains. Researchers are trying to create even better polarizers by using aligned nanowires instead of iodine-coated polymer chains.
5) Anti-reflective coating:
Anti-reflective coating, or AR coating, is a special technology that removes unwanted reflections from the front and back of your lenses so you can see with crystal clarity. When you wear glasses with an anti-reflective coating, there is more room for light to pass through your lenses and this allows you to see smoothly. This special coating is ideal not only for glasses but also for sunglasses. Even when in the sun, its AR coated bezel prevents glare from sunlight from reflecting off your eyes so you can see better.
The closest materials with good physical properties to a coating are magnesium fluoride, MgF2 (with an index of 1.38), and fluoropolymers, which can have indexes up to 1.30 but are more difficult to use. MgF2 on a corona glass surface gives a reflectance of approximately 1% compared to 4% for bare glass. MgF2 coatings work much better on higher refractive index glasses, especially those with a refractive index close to 1.9. MgF2 coatings are widely used because they are cheap and durable. If the coatings are designed for a wavelength in the middle of the visible band, they will give reasonably good antireflection properties throughout the band.
6) Photochromatic coating:
Photochromic lenses were developed in the 1960s by William H. Armistead and Stanley Donald Stookey at Corning Glass Works Inc. The glass version of these lenses achieves their photochromic properties by embedding microcrystalline silver halides (usually silver chloride) in a glass substrate. Photochromic plastic cups use organic photochromic naphthopyrans to achieve the reversible darkening effect.
These lenses darken when exposed to UV light at the intensity of sunlight, but not when exposed to artificial light. In the presence of UVA light (wavelengths 320 to 400 nm), the electrons in the glass combine with the colorless silver cations to form elemental silver. Since elemental silver is visible, the lenses appear darker. Back in the shadows, this reaction is reversed. The silver returns to its original ionic state and the lenses become transparent.
Because the photochromic material is dispersed on the glass substrate, the degree of darkening depends on the thickness of the lens, which is problematic for lenses with varying thickness in prescription glasses. In the case of plastic lenses, the material is typically embedded in the surface layer of the plastic with a uniform thickness of up to 150 µm.
Metal oxide coatings reduce the amount of UV radiation transmitted through sunglasses and therefore protect your eyes. Organic dyes can stain plastic lenses. The exact chemicals used are kept under lock and key. Silver atoms form groups that absorb ultraviolet and visible light. The Cu + ions in the glass reduce the Cl atoms and prevent them from escaping.
Photochromic glass lenses can use copper-doped silver halide salts, which generate elemental silver in ultraviolet light and darken. Plastic cups are based on organic compounds that reversibly isomerize in ultraviolet light to produce dark tints.
Reference:
1) https://www.piedmont.org/living-better/what-are-sunglasses-really-doing-for-your-eyes
2) https://en.wikipedia.org/wiki/Crown_glass_(optics)
3) https://en.wikipedia.org/wiki/Polycarbonate
4) https://en.wikipedia.org/wiki/CR-39
5) https://en.wikipedia.org/wiki/Photochromic_lens
6) https://en.wikipedia.org/wiki/Anti-reflective_coating
7) https://www.fivepointseyecare.com/eyeglasses/polarized-lenses-and-anti-reflection-coating/
Nice article
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