PHOTOCHROMIC LENSES

With improving photochromic technology, the sector is experiencing a substantial turnaround after the market stagnated in the '80s. Traditionally a glass product, wearers are enjoying more and more the convenience offered by various plastic photochromic lenses and by an increasing choice of colours.

Photochromic lenses are made by adding substances which are sensitive to UV to the base lens materials; thus treated, the lenses darken as the light intensity increases. The process reverses as the light diminishes and the lens returns to its original deactivated state. It is important to dissociate their function from that of sunglasses. Photochromics are intended as a versatile option rather than full solar protection. There are occasions when wearers would be better off with a solid tint Rx lens, such as in prolonged sunlight, however photochromics, most of the time, obviate the need to switch between two pairs of glasses-white Rx and plano sunglasses.

COLOURS
All photochromic lenses have a colour tint. Until recently, most have been offered in grey or brown, a limited range which has been dictated by the high development costs of specially coloured photochromic lenses. Recently, however, there have been moves to stimulate consumer demand by introducing more colour variations. Transitions 'Green' was launched recently in Europe. Following its success,
Transitions has introduced a wide colour range of variable tint lenses, drawing on fashionable consumer colours.

In most cases, photochromics will pass the traffic signal recognition criteria and are generally considered suitable for daylight driving \{where the minimum permissible
transmission level is 8%]. However, with a lower limit of 75%-80% transmission for night driving, wearers should be warned against their use when illumination levels are low.
The important consideration is that wearers should be aware that they will not activate while driving and so may not give protection from intense sun glare.
Since all current photochromics are activated by UV the wearer benefits from UV protection in their activated state.
This protection is not total: a small amount of UVA (between about 380 and 40Onto) is still transmitted. However, the amount of transmission is reduced in proportion to the light intensity. Photochromics also reduce the amount of visible (low-spectrum) blue light, about which there are long-term exposure concerns.

Reactions times
The speed at which photochromic lenses changes and the amount of transmission change are both affected by temperature. At lower temperatures the reaction speed is slower, but the lenses darken further. Conversely, at higher temperatures, the reaction time is quicker, but the lenses do not go as dark (see figure 1).

 These characteristics are more noticeable with plastic photochromics than for glass.
Photochromic glass has been available for many years. Perhaps the best known are the Photogrey and Photobrown from Corning. These glasses have a more complex composition than standard white glass and have high proportions of Boron and Silicon oxides, hence the name borosilicate glass. They include silver halide microcrystals, which in response to light, disassociate into silver particles and colloids that are responsible for the photochromic properties. Various versions are available, including standard and high index.

The nature of silver halide crystals means that lifetime performance is guaranteed with glass photochromics. In the case of plastics, however, the very nature of organic molecules means that there is some slowing down of
performance over a period of time.
In the case of most photochromic lenses the transmission varies typically by 60%, startTng at about 85% transmission and darkening to about 25% for prescription use. Special photochromic materials are, however, available in darker versions for sunglass use.

The transmission characteristics for Coming PGX shown in figure 2 demonstrate a typical photochromic transmission range, with a change from about 90% to about 25%. Photochromic plastics are a more recent development. In 1990, Transitions Optical was the first company to successfully make and commercialise plastic variable tint lenses. Since then, this sector of the ophthalmic market has continued to grow rapidly. Already available in a wide range of indices and lens forms, Transitions products are now expanding into polycarbonate and high index materials.
Coming has now entered the plastic photochromic market with their Sunsensors product. Unlike Transitions, which processes the photochromic chemicals into a previously cast lens, Coming provides chemicals to the lens manufacturers.

And there is more technology to look forward to. PPG Industries, which developed the technology for Transitions Lenses, is testing an entirely new photochromic process. In the US, the company is unveiling an "electrochromic" product, 'Intelligent Optics Eyewear'. The result of more than nine years of research, these battery-powered frames can be lightened or darkened by pushing a pair of small buttons located on the hinge.

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