Photochromatic lenses R.D.Gopinath Supervisor – Optical Sales Aravind Eye Hospital

Introduction PC lenses are darkens and lightens automatically when exposed to different (UV) sun light intensities. The rate of darkening depends on the ambient temperature. Available in glass /plastic ; either brown or gray in color. All photochromic lenses block 100 % of the harmful UVrays.

History The term photochromic refers, changing color – chroma ; in response to light - photo - First Glass lens developed by corning in 1960. Photolite lens sold in the early 1980’s by American optical corporation. Standard plastic lens by transition optical in 1991. Early photochromics were strictly glass lenses, but today,regular plastic, polycarbonate and high-index glass and plastic.

Process Glass lenses achieve their photochromic properties through the embedding of microcrystalline silver halides. Plastic lenses rely on organic photochromic molecules (for example oxazines and naphthopyrans) to achieve the reversible darkening effect.

Photochromic lens Temperature sensitive & good UV absorbent lens. Fully-darkened lens are best UV absorbers. Also known as transition or sun-sensitive lenses Not become completely clear in indoors. Available in gray, brown and pink colors.

Color changes in PC lens Occurs depends on the intensity of UV light. Converts silver ions to elemental silver, when short-wave length light (300-400nm) interacts. Similar process like photo -film is exposed to sunlight. Silver halide micro crystal is responsible to get darken within a minute by absorb 80% of incident light lighten by absorb about 20% of incident light in about 3 minutes.

Photochromic Lenses Ideal for people who don’t want to switch between prescription eyeglasses and prescription sunglasses. Provide the convenience of having a pair of sunglasses and transparent glasses in one pair of lenses. PC lenses have millions of light absorbing molecules that are transparent in artificial light, which is free of UV rays. When exposed to UV rays from natural sunlight, however, the molecules undergo a chemical process, causing them to absorb light and to darken. PC lens darken automatically after approximately 30 seconds of natural light and lighten after approximately 5 minutes of artificial light.

Photochromism Is a reversible transformation of a chemical species induced in one or both directions by absorption of electromagnetic radiation between two forms, A and B, having diferent absorption spectra. The phenomenon was discovered in the late 1880s, including work by Markwald, who studied the reversible change of color of 2,3,4,4-tetrachloronaphthalen-1(4H)-one in the solid state. He labeled this phenomenon "phototropy", and this name was used until the 1950s when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel, while studying the phenomenon proposed the term "photochromism", which we use today.

Photochromic materials Reversible photochromics Irreversible photochromics

Grades

Most common processes involved in photochromism are: Pericyclic reactions, Cis-trans isomerizations, Intramolecular hydrogen transfer, Intramolecular group transfers, Dissociation processes and Electron transfers (oxidation-reduction).

Photochromic molecules May belong to various classes: triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans, naphthopyrans, spiro-oxazines, and others. For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by a sp3-hybridized "spiro" carbon. After irradiation with UV light, the bond between the spiro carbon and the oxazine interrupts, the ring opens, the spiro carbon achieves sp2 hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals with the rest of the molecule, and a conjugated system forms, with ability to absorb photons of visible radiation, and therefore appear colorful. When the UV source is removed, the molecules gradually relax to their ground state, the carbon-oxygen bond renews, the spiro atom becomes sp3 again, and the molecule returns to its colorless state.

Photochromic dyes Photochromic dyes tend to be inherently unstable in their open, colored form. Their lifetime is therefore significantly negatively affected by exposure to UV light, as the open form is susceptible to degradation by oxygen and free radicals. Incorporation of the dyes into a polymer matrix, adding a stabilizer, or providing a barrier to oxygen and chemicals by other means prolongs their lifetime. Photochromic dyes usually have the appearance of a crystalline powder. In order to achieve the color change, they have to be dissolved in a solvent or dispersed in a suitable matrix.

Photochromic colors Usually, four basic colors (magenta, cyan, yellow, black) are available commercially. Other colors can be made by combinations. Spot colors are available from some vendors. Exact colors are difficult to achieve, as the exact hue depends on many factors, including the intensity of the UV radiation and temperature.

Photochromic colors change Usually between a leuco form in base state to a colored form after exposure to UV radiation. The rate of change varies by color. In some formulations, the yellow color takes the longest time to fade back to colorless and requires exposure to visible light for such transition. Transition between two colors can be achieved by mixing the photochromic dye with an UV-transparent permanent dye.

Photochoromatic lenses not darken in… At indoors (room light does not contain UV) While driving the car ( windshield absorbs some of the UV light.)

Reversible photochromics Work by changing their chemical structure after absorbing UV light. The UV light causes the photochromics to absorb colour and then change back to clear when the UV source is removed. Can cycle thousands of times depending upon the application. Also can change from one color to a different color by combination with a permanent pigment.

Reversible photochromics One of the most famous reversible photochromic applications in color changing lenses, also found in novelty applications such as toys, cosmetics, and industrial applications. The largest limitation in using PC technology is that the materials cannot be made stable enough to withstand thousands of hours of outdoor exposure so long-term outdoor applications are not appropriate at this time.

Irreversible photochromics A term is used to describe materials that undergo permanent color change during exposure to ultraviolet radiation. Because by definition photochromics are reversible, there is no such thing as an "irreversible photochromic

Silver halide A silver halide is one of the compounds formed between silver and one of the halogens, usually Silver bromide (AgBr), Silver Chloride (AgCl) and Silver Iodide (AgI). As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation states of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which Silver (II) fluoride is the only known stable one.

Silver halide Used in photographic film and photographic paper where an emulsion of silver halide crystals in gelatin is coated on to a film base, glass or paper substrate. The gelatin is a vital part of the emulsion as it contains trace elements which increase the light sensitivity of the emulsion. Silver bromide and silver chloride may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride.

Silver halides Silver halides are also used to make some corrective lenses darken when exposed to ultraviolet light. When a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image). If the speck of silver contains approximately four or more atoms, corresponding to an absorption of four or more photons, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver.

A sensitivity speck is an imperfection (dislocation) in the crystal lattice of silver halide (usually silver bromide), where a positively charged cluster of silver ions is exposed to the crystal surface. When the silver halide grain is exposed to light, the photons interact with the halide anions and knock out their electrons. The electrons then travel through the lattice, until they are captured by the dislocation. There they recombine with silver ions, forming neutral silver atoms. Tiny clusters of these silver atoms form the latent image. If such speck of silver contains approximately four or more atoms, corresponding to an absorption of four or more photons, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver.

Glass Photochromic Lenses  Photochromic process is different for glass and plastic lenses. Glass lenses contain millions of silver halide crystals embedded throughout the lens material.  When exposed to sunlight, UV rays transform the crystals into light-absorbing particles.  As these particles absorb light, the lens darkens and the amount of light passing through the lens is reduced. When the lenses are removed from sunlight and the stimulating UV radiation is no longer present, the crystals return to their original orientation and the lenses become clear.

Plastic Photochromic Lenses Owes its darkening to the presence of spiro-indoline organic molecules instead of silver halide crystals.  When activated by UV rays ,the spiro-indoline molecules scatter, absorbing sunlight and reducing the amount of visible light that penetrates the lens. when the lens is removed from sunlight the chemical reaction reserves and the lens returns to its clear state. Not recommendable to use more than 2 years because it gradually loses its darkening ability with time. Most popular plastic pc lenses, made by Transitions* Optical. 

Plastic photochromic lensesLinksIt was only very recently that manufacturers succeeded in producing plastic lenses with photochromic properties comparable to those of glass lenses. Organic photochromic lenses are produced from a special plastic which is optimized for the absorption of photochromic dyes. Unlike glass photochromic lenses, they are not given their photochromic properties until the semi-finished or finished stage. In this process – known as photochromization – millions of photochromic molecules are incorporated in the front surface of the lens at a depth of approx. 0.15 mm.

The Photochromic molecules Are primarily indolino-spironaphthoxacins. These change their chemical structure when they are irradiated with UV and short-wave blue light. Much like a flower when exposed to sunlight, they unfold and cause the lens to change colour. If the energy level of the UV radiation decreases or – as is the case when the wearer goes indoors – no UV is present at all, the molecules close again. The lens clears and adopts an attractive filter tint. As the molecules are only found on the lens surface, the darkening process takes place evenly over the entire surface, irrespective of the dioptric power.

Plastic photochromic process "imbibing" technology. Instead of distributing light-sensitive particles, Transitions applies a thin layer of photochromic material to the front surface of its lenses.  This material penetrates the lens surface evenly to a depth of approximately 0.15 mm.  "in-mass" technology. Light-sensitive molecules distributed throughout the lens material (just as silver halide crystals are in glass photochromatic lenses). 

"Break-In" Period It takes several exposures to sunlight before a new pair of photochromic lenses will darken and lighten as designed.  Allow your lenses to be exposed to sunlight at least five periods of 30 minutes or longer to "break-in" the photochromic process. The Effect of Temperature All photochromic lenses perform better (i.e. get darker and change faster) in cold weather.  For standardization purposes, photochromic lenses are usually tested at an ambient temperature of approximately 72 degrees Fahrenheit.  At higher temperatures, photochromic lenses may take more time to change and may not darken to the same degree. 

Photochromic Lenses and Driving Photochromic lenses require exposure to UV radiation from the sun to darken.  Since automobile windshields block some UV rays, PC lenses do not darken appreciably inside many cars.  For the best vision and comfort while driving in bright sunlight, Consider a pair of clip-on sunglasses to wear over photochromic lenses or a separate pair of polarized sunglasses.

Reducing Glare from Reflections To eliminate glare from light reflecting off the back surface of photochromic lenses, A-R coating can be applied. AR coating does not adversely affect the photochromic process.  to any PC lens will increase the light transmittance in both the indoor (clear) and outdoor (dark) states.  Exact percentages will vary depending on the brand of AR coating.   When applied to both surfaces, AR coating can also prolong the effective lifetime of some photochromatic lenses.

How long do they last?  Most plastic photochromic lenses gradually lose some of their ability to change in sunlight after 18 to 24 months of use.  For the best performance, plastic photochromic lenses should be replaced every two years.

Photochromics in power lenses… The whole lens changes when exposed to sunlight. In power lens, the thickest part of the lens will be darker than the thinner parts. If there is a large difference in prescription between the two eyes, the stronger power lens will be darker than the weaker one. Corning Thin & Dark photochromic glass lenses, which are up to 30 % lighter in weight than traditional glass lenses and also thinner

SunSensors* -lens Corning has recently introduced a lightweight, high index plastic PC lens called SunSensors(R).  It has photochromic material uniformly distributed into the front surface of the lens, but has a penetration depth that is significantly thicker than the photochromic layer in Transitions lenses.  This extra-thick photochromatic layer enables SunSensors lenses to change faster and get darker than other photochromic lenses, according to the company

Rodenstock*-lens ColorMatic Extra from Rodenstock is another mid-index plastic photochromic line. A photochromic dye is mixed throughout these gray lenses. ColorMatic Extra lenses can be tinted to alter the color or darken the lenses. In addition, Hoya Lens makes Sungray Lenses, mid-index plastic photochromics with UV-activated color-changing molecules blended throughout the lens material. They can also be tinted.Transitions lenses are available for nearly every lens design, refractive index and prescription. Performance features include advanced variable-tint technology that allows rapid darkening when you go outside (and rapid return to clear when you go in), and 100% UV protection.

Vision enhancement with PC lenses Beyond the convenience of having lenses that can be worn both indoors and outdoors, some photochromic lenses also have medical applications. Corning's CPF family consists of glass photosensitive lenses with colors in the red family. They are used to enhance the vision of patients who have various ocular pathologies. Low vision Practitioners are familiar with these lenses and can help patients determine which color will be the most helpful. Another glass photochromic lens that is designed for use by individuals who have beginning-to-moderate light sensitivity because of various ocular conditions is Corning's GlareCutter. This is a darker, more cosmetically pleasing brownish-red that allows a precise level of blue light to "leak" through the lens, thus preserving a more natural color scene for the wearer than the more reddish CPF lenses. Lenses with Tints

Brand names… Though photochromic is the proper generic term for all lenses that darken in sunlight, these lenses are often referred to by a specific brand name, such as Transitions(R) (made by Transitions Optical) or PhotoGray(R) (made by Corning).  Other terms used to describe photochromic lenses include variable tint lenses and lenses with sun-activated tints.

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