Introduction
Photochromic lenses were originally developed by Roger Araujo at the corning glass works Inc. In the 1960s, and are made of glass, the process was used in the first mass produced variable tint lenses. The chemistry of plastic photochromic lenses is completely different from that of glass. The first generation of plastic lenses relied on a family of organic dyes known as blue pyridoberizoxazines, says Chris baldy, manager of photochromic performance testing at transitions optical around 1990, transitions optical Inc released a new photochromic lens.
These lenses were made from a lightweight plastic monomer based from the CR–39 resins. The material was mixed with a photochromic material, which was activated by ultraviolet rays in sunlight, turning the lens from a clear lens to a dark sunglass lens. First generation transitions lenses darkened slowly, and not very intensely. When a photochromic dye is exposed to UV radiation, a chemical bond is broken. The molecule then rearranges into a species that absorbs at longer wavelengths in the visible region, causing the lens to darken.
The word “photochromic” comes from two Greek words “photo” meaning light and “chroma” meaning colour so photochromic simply means something that changes colour in response to light. In relation to sunglasses, photochromic lenses darken or lighten dependent on their exposure to ultraviolet radiation. For example, once the UV is removed by walking indoors or reduced by cycling through a forest, the lenses will gradually return to their clear state or a lighter tint respectively. Photochromic lens are available in nearly all lens materials and designs, including high index lenses, bifocals and progressive lens.
Blodgett, a physicist at general electric laboratories, developed a method of coating glass with a soapy film that would eliminate most of those reflections. Her insight led to the practical anti-reflective coatings that now coat picture, glass, windshields, eye glasses, camera lens and much more. Blodgett received a patent for the process in March 1938, and G E announced the discovery in December of that year. Blodgett developed a way to transfer the soap film from a water surface to a solid surface such as metal or glass, and found that by repeating the process she could build up films of barium stearate layer by layer, up to about 3000 layers. These became known as Langmuir – Blodgett films.
Blodgett then began looking for some applications for the films she noticed that even the clearest glass reflected as much as 10 percent (%) of incident light, making it difficult to see through. Blodgett realized her soapy films could solve that problem since she could precisely control the thickness of the soapy films by building them up one molecular layer at a time, and could easily deposit the films on a glass surface, Blodgett figures out that she could developed a coating of just the right thickness to cancel out most reflections from the glass surface. She built up a film with thickness equal to ¼ the average wavelength of visible light (about 1388 angstroms). This way, any light that reflected off the glass surface would have travelled half a wavelength farther than light that had reflected off the film surface, so most of the reflections would cancel out. Blodgett also tweaked the chemical composition of the film to adjust its index of refraction to enhance the reflection canceling, and she was able to eliminate almost all of the reflection, making the glass nearly invisible. Anti-reflective lenses: (Also called anti-glare coating) is a microscopically thin multilayer coating that eliminates reflections from the front and back surface of eyeglass lenses. By doing so AR coating makes your lenses nearly invisible so people can focus on your eyes, not distracting reflections from your eye glasses. Anti-reflective coating also eliminate glare caused by light reflecting from your lens.
Styles statement: Eyeglass lens coating is a special additive that either bonds with the lens, or is built into the lenses themselves during the lens manufacturing process.
Conceptual framework
Renzi and Hammond (2016) state that adding filtering via a photochromic lens significantly increased subjects abilities to cope with intense broad band and short-wave lighting conditions and to adapt back to normal viewing after being presented with an intense photostressor.
Annamosterholm (2015), supported that in the case of photochromic glasses, the lenses usually respond to UV light, that is why the lenses darken when exposed to the sun (which produces UV radiation), while they remain clear under artificial lights (which do not produce much UV light).
In 2011, German lens manufacturer Carl Zeiss also said that photochromic lenses can darken up to 20% faster and lighten indoors. Photochromic lenses will get very dark in cold weather conditions which make them more suitable for beach goers and snow skiers while they were outside, once they are inside away from the light, the cold lenses take time to regain their clear colour than warm lenses. Colour can be added as solid tint, where the entire lens has the same colour density or as gradient tint where the colour density is darken at the top of the lens and gradually fade to clear or nearly clear at the bottom.
Citek (2008), noted that anti-reflective coating (ARC) provide numerous visual benefits to spectacle wearers. However, coating designers and manufacturers seem to have placed little or no emphasis on reflectance of wavelengths outside the visible spectrum. Ultraviolet (UV) radiation from sources behind the wearer can reflect from the back lens surface toward the wearers eye. Various clear lens materials with and without AR coatings were tested for their transmittance and reflectance properties. Although the transmittance benefits of AR coatings were confirmed, most coatings were found to reflect UV radiation at unacceptably high levels. Tinted sun lenses also were tested with similar results, frame and lens parameters were evaluated, confirming that eye wear that incorporates a high wrap frame and high base curve lenses can prevent UV radiation from reaching the eye. The findings strongly suggest that clear, flat lenses should not be dispensed for long-term use in sunny environments, even if clip on tints are provided.
Liz de Franco (2006) states that glass photochromic lenses was first developed by the corning glass and ceramics company in the early 1960s and was released commercially in 1964, corning however updated his findings to include thin and dark photochromic glass lenses. These glasses are set to change from clear to dark sunglasses lens in 30 seconds; this chemical reaction takes place when the lenses are exposed to ultra-violet radiation. Thin and dark lenses are also available in grey and brown.
Lakkis and Weidemann (2006), noted that photochromic lenses can be used to reduce discomfort glare and lenses with ultraviolet radiation (UVR) absorbing properties in the range of 340nm to 360nm can help minimize symptoms of disability glare caused by florescence of the crystalline lens.
In 2001 Edith in her literature said that photochromic lens has it merit and demerit. Photochromic lenses when exposed to ultra-violet rays from the sun are darken into sunglasses. The demerit of photochromic lens is that they do not adjust immediately when exposed to bright glare or UV radiation. It takes up to two minutes before the lenses will completely change from light to dark or vice-versa.
Kriss and Kriss (1998), stated that anti-reflective coatings helps to make the eye behind the lens more visible. They also help lessen back reflections of the white of the eye as well as bright objects behind the eyeglasses wearers (e.g., windows, lamps). Such reduction of back reflections increases the apparent contrast of surroundings at night, anti-reflective coatings help to reduce headlight glare from oncoming cars, street lamps and heavily lit or neon signs.
Kriss and Kriss (1998), also state that photochromic lens is used to reduce the transmission of light in the ultraviolet spectrum. UVB radiation increases the likelihood of cataracts, while long-term exposure to UVA radiation can damage the retina. DNA damage from UV light is cummulative and irreversible, some materials such as trivex and poly carbonate naturally block most UV light, they have UV- cut off wavelengths just outside the visible range and do not benefit from the application of a UV coating.
Donna (1998), said that certain factors must be considered when producing photochromic lens; the temperature and the lens material that is used in respect of technological advancement in manufacturing. John (1924), stated that the simplest form of anti-reflective coating was discovered by Lord Rayleigh in 1886. The optical glass available at the time tended to develop a tarnish on its surface with age, due to chemical reactions with the environment. Rayleigh tested some old, slightly tarnished pieces of glass, and found to his surprise that they transmitted more light than new clean pieces. The tarnish replaces the air-glass interface with two interfaces: an air-tarnish interface and a tarnish glass interface. Because the tarnish has a refractive index between those of glass and air, each of these interfaces exhibits less reflection than the air-glass interface did. In fact the total of the two reflections is less than that of the naked air-glass interface.
Elmer and Vincent (1937) stated that photochromic lenses are optical lenses that darken when exposed to ultraviolet rays going from a lens that is virtually clear or has a light shade of colour to a dark sunglass like colour photochromic lenses will block out 100% of uv light automatically when going from indoor to outdoors without needing to change eye wear. They will adjust to the varying light conditions you encounter every day.
References
Annamosterholm (2015) ACS Appl mater. Interfaces, 7 (30) pp 1413. 1421.
Carl, Z. (2011) German lens manufacturer, introduction of self. tinting photochromic lens.
Citek, K. (2008). Anti-reflective coatings reflect ultraviolet radiation. Optometry, 79, pp 143 -148.
Donna, W. (1998). The irelen lens, dyslexic and scotopic sensitivity Optician, 194, pp 22-25.
Edith (2001). Merit and demerit of photochromic and tinted lenses whiting PR. Improvement in reading and other skills using green coloured lenses. Aust J Remed Edu, 20, pp 13-15.
Elmer, V. & Vincent, C. (1937). Xcel optical company Minnesota U.S.A.
John, W. (1924). Third baron Rayleigh, OM, F.R.S sometime president of the royal society and chancellor of the university of Cambridge, E. Arnoid and company, pp 307.
Kriss, T. & Kriss, V. (1998). History of the operating microscope; from magnifying glass to micro neurosurgery. Neurosurgery, 42 (4), pp 899-907.
Lakkis, C. & Weidemann, K. (2006). Evaluation of the performance of photochromic spectacle lenses in children and adolescent age 10-15 years. Clinical experimental optometry, 89, pp 246-252
Liz de, F. (2006). The use of photochromic lens, early photochromic 23, pp621 – 626.
Renzi, H. & Hammond, B. (2016). The effects of photochromic lenses on visual performance pp 10:11 cxo 12394. Australia.
