Photochromic glass is a type of smart glass that darkens and changes tint when exposed to UV light. This technology can be employed for protecting interior windows from UV rays while decreasing energy consumption on sun screens and interior windows.

Photochromic smart glasses differ from thermochromic smart glasses in that they change tint when exposed to heat, as opposed to thermochromic glasses which use dyes which undergo chemical changes when exposed to UV light.

Transparency

Photochromic smart glass is a type of passive glass technology that tints when exposed to shorter wavelengths of ultraviolet (UV) light. When no UV rays are present, the smart glass will return to its normal transparent state. Depending on the manufacturer, transmittance values can range from 15%-30% when exposed to sunlight and 60%-75% when no solar radiation is present.

Photochromic lenses’ transparency is heavily determined by the composition of silver halide used. Oxidation of silver ions to oxides creates an optical defect state, leading to reduced transmittance. This defect state can then be eliminated during photolysis of silver halide crystallites.

These crystallites are only 2 to 3 microns across and contain a very small number of atoms, giving silver halide particles an extremely fine size. Their small shape allows light to pass through but still absorb it; making indoor visibility excellent but tinted when exposed to direct sunlight.

However, these glasses won’t completely darken in high temperatures and may take longer to adjust to a dark state in cold climates. Furthermore, they may be harder to adjust while driving if your vehicle doesn’t have UV protection built-in.

Different manufacturers produce various types of photochromic glass, such as sunglass lenses and transitional lenses. Both options have their own benefits and drawbacks.

Transitional lenses offer a wider variety of uses than traditional sunglasses. They can provide shade indoors and add some flair outdoors. Plus, their slight tint helps reduce glare from digital device screens, fluorescent lights, and other harsh lighting sources.

These lenses can also be worn while driving a car, thanks to their polarized nature which makes them effective in situations where headlights may cause eye strain.

Photochromic sunglasses come in a range of materials, such as plastic or metal. For those seeking more durability and lightweight options, these can be made to meet individual needs with various styles, colors, and designs to suit any taste or preference. It’s best to consult your eye doctor to decide which kind of photochromic sunglasses is most suitable for you before making a purchase.

Light Transmission

Photochromic glass is transparent indoors, but darkens when exposed to ultraviolet (UV) radiation from the sun. This makes it a great option for those who want both prescription glasses and sunglasses in one frame.

Transitions lenses utilize a silver chloride compound that alters its tint when exposed to UV rays. They come in an array of lens colors and are light-intelligent, meaning they darken and clear automatically without sacrificing vision quality or comfort.

They’re also incredibly lightweight, so you won’t feel like you’re wearing bulky lenses. Additionally, these lenses provide protection from harmful UVA and UVB rays of the sun which may lead to various eye diseases as well as some forms of cancer.

Furthermore, these lenses can block some blue-spectrum radiation. This is beneficial for individuals suffering from eye disorders like macular degeneration and cataracts.

Photochromic glasses require an uncompromising level of precision during their annealing process, where the temperature of molten glass must be precisely controlled to activate its silver halide crystals.

Before wearing photochromic glasses, it’s wise to test them out in the dark. This way, you can ensure the tint is suitable for your needs and the eyeglasses are an accurate reflection of your personal style.

If you plan to drive with these glasses, it is wise to wear them in a car or behind a windscreen that blocks UV rays. For daytime driving, you can always offset the glasses with clip-ons or fitovers, or opt for separate prescription sunglasses.

Photochromic lenses exhibit a change in optical properties when exposed to UV rays and other wavelengths between 400-600nm, triggering the transition from transparent, light-intelligent state to darker tint. This change is reversible and plays an integral role in maintaining the overall optical properties of photochromic lenses.

Fading can occur in photochromic glasses even when they are exposed to long wavelengths of radiation that lack UV content, known as optical bleaching. This fading occurs at a slightly faster rate than darkening does due to the electrons in silver ions having to travel a greater distance when trying to move from one lattice site to another than their doubly charged copper ions must.

Fading

Photochromic glasses feature an automatic fade mechanism that darkens the lenses when exposed to UV rays, then fades back to clear lenses when no more UV light is present. This makes them the ideal solution for those who find switching between sunglasses and regular eyewear tiring.

Glass photochromic lenses fade due to a chemical reaction within the lens. When UV light hits a silver metal embedded within, an electron transfers from it to another compound (usually copper chloride) also embedded within the glass.

This second compound absorbs the transferred electron and, once lenses are no longer exposed to UV rays, returns the electron back to silver metal, making the glass clear once more.

However, photochromic glasses take much longer to reverse their chemical process than those using dyes to change color due to differences in lens temperature which affects how quickly molecules react.

Some modern photochromic glass brands such as Transitions Optical have developed methods to accelerate fading. These involve using a thinner coating of organic, carbon-based molecules called naphthopyrans instead of traditional silver crystals and sandwiching these dyes between multiple layers of polyurethane on lenses made out of thin plastics like polycarbonate or thiourea.

These lenses are much simpler to create than their silver-based crystal predecessors, which were the original technology for photochromic lenses. Molecules with much less mass than microscopic silver crystals allow them to be distributed more evenly throughout the lens.

They are less vulnerable to breaking down when exposed to abrasives or hard-to-clean surfaces such as mirrors and glasses frames, making them a great option for those on a budget. What’s more, photochromic glasses tend to be cheaper and easier to produce than their predecessors – making them an attractive option for those who want photochromic glasses without breaking the bank.

Darkening

Photochromic glass differs from photographic film in that it can be reversed between clear and dark states through exposure to light. This transition occurs automatically.

Photochromic glasses undergo reversible darkening when exposed to light, depending on factors like their basic composition and temperature during annealing. As such, precision in product manufacturing is paramount for successful outcomes.

Photochromic lenses work by embedding microcrystalline silver halide crystals within the glass substrate, creating a reversible darkening effect. Through chemical reactions, these crystals undergo transformations that result in darkened lenses.

Therefore, when exposed to UV radiation, glass will darken. This darkening is caused by the interaction between photochromic molecules and UV (ultraviolet) rays from sunlight.

Photochromic lenses also darken on overcast days and in sunny weather due to UV rays penetrating the atmosphere, though they may not darken well inside a car due to its windshield’s UV protection.

Thanks to recent technological advancements, some photochromic lenses now activate in both UV and visible light conditions, darkening more accordingly. This can be especially advantageous for drivers who need to protect their eyes from the sun’s UV rays while still having a clear view of what is going on around them on the road.

Another advantage of photochromic lenses is their versatility; they can be used in a range of environments, from the office to outdoors. Furthermore, photochromic lenses may even be prescribed as eyeglasses or sunglasses with prescription capabilities if designed to darken in sunlight and illuminate indoors.

Lenses can be made of a range of materials, such as polycarbonate, polarized and trivex. Some of these lenses filter blue light which may be beneficial for computer users who need to block UV rays from entering their eyes.

Photochromic lenses come in an array of tints and styles to meet your vision and lifestyle needs. Popular brands include Transitions, XTRActive and Drivewear; consult with your optometrist for more details about the lenses that best suit your vision requirements.