Photochromic lenses are lenses

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 that darken on exposure to ultraviolet (UV)

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation. Once the UV is removed (for example by walking indoors), the lenses will gradually return to their clear state. Photochromic lenses may be made of glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, polycarbonate

Polycarbonate

PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

, or another plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

.

Technical details

The glass version of these lenses achieve their photochromic

Photochromism

Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra. Trivially, this can be described as a reversible change of colour upon exposure to light...

 properties through the embedding of microcrystalline silver halide

Silver halide

A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide , chloride , iodide , and three forms of silver fluorides. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX...

s (usually silver chloride

Silver chloride

Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...

), or molecules in a glass substrate. Plastic photochromic lenses rely on organic photochromic molecules (for example oxazines and naphthopyrans) to achieve the reversible darkening effect. The reason these lenses darken in sunlight but not indoors under artificial light, is that room light does not contain the UV (short wavelength light) found in sunlight. Automobile windows also block UV so these lenses would darken less in a car. Lenses that darken in response to visible (rather than UV) light would avoid these issues, but they are not feasible for most applications. In order to respond to light, it is necessary to absorb it, thus the glass could not be made to be clear in its low-light state. This correctly implies photochromic lenses are not entirely transparent, specifically they filter out UV light. This does not represent a problem, because the human eye does not see in the UV spectrum.

With the photochromic material dispersed in the glass substrate, the degree of darkening depends on the thickness of glass, which poses problems with variable-thickness lenses in prescription glasses. With plastic lenses, the material is typically embedded into the surface layer of the plastic in a uniform thickness of up to 150 µm.

Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and then continue to darken very slightly over the next fifteen minutes. The lenses fade back to clear along a similar pattern. The lenses will begin to clear as soon as they are away from UV light, and will be noticeably lighter within two minutes and mostly clear within five minutes. However, it normally takes more than fifteen minutes for the lenses to completely fade to their non-exposed state. A study by the Institute of Ophthalmology at the University College London

University College London

University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...

 has suggested that even in dark conditions photochromic lenses can absorb up to 20% of ambient light.

Because photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions, which makes them more suitable for snow skiers than beachgoers while outside. Once inside, away from the triggering UV light, the cold lenses take longer to regain their clear color than warm lenses.

A number of sunglass manufacturers/retailers (Intercast, Oakley

Oakley, Inc.

Oakley, Inc., based in Foothill Ranch, California, makes sport equipment including sunglasses, sports visors, and ski goggles, as well as watches, clothing, bags, backpacks, shoes, prescription glasses, football and hockey eyewear, Golf gear and other accessories...

, Serengeti Eyewear, Persol

Persol

Persol is an Italian eyewear company specializing in sunglasses. The name is derived from "per il sole," which, in Italian, means "for the sun." Formed in 1917 by Giuseppe Ratti, Persol originally catered to pilots and sports drivers. Presently, the company is famous for its durable sports sunglasses...

to name a few) offer products that use photochromism to make lenses that go from a dark to a darker state. Because these products are tinted in the bleached state, they are typically used only outdoors and are not considered general-purpose lenses.

Advantages and disadvantages

There are both advantages and disadvantages to photochromic lenses. The main advantage is that they will darken into a sunglass tint when exposed to U.V. thus obviating the need to carry a separate pair of sunglasses for protection against harmful U.V. rays from the sun.

The main disadvantage of photochromic lenses is they do not adjust immediately. It could take up to two minutes for the lenses to completely change from light to dark or vice versa. Another disadvantage for some users is that they will not darken when worn inside vehicles with glass that blocks U.V. light.

External links

• Photochromic lenses How stuff works.
• Optometry Glossary