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Saturday / July 2.
HomemilensesEvolving Vision: Photochromic Lenses

Evolving Vision: Photochromic Lenses

Photochromic lenses offer wearers comfort and convenience whether indoors or out, while also protecting eyes against harmful UV rays. In America, according to a recent survey by The Vision Council, photochromic lenses today account for about 18 per cent of eyeglass lenses sold.

The launch of Transitions XTRActive lenses adds to the lens choices available for consumers, and is expected to meet growing demand, making this the perfect time to rewind and look at where the technology began.

Transitions Optical has devoted more than twenty years of research to photochromics and optical plastics. The company was the first to successfully commercialise a plastic photochromic lens in 1990 and is universally acknowledged as the number one-recommended photochromic lens worldwide.

Business Director for Transitions Optical Australia and New Zealand, Eric Breda, predicts demand for photochromic lenses will grow: “Currently, across developed markets, about two lenses sold in ten is photochromic, and there is no reason – given the added comfort and flexibility the lenses offer – that this ratio should not improve.”

When the millions of molecules are exposed to UV rays they undergo structural changes that modify the light transmission properties of the organic lenses

The Lenses

‘Photochromisim’, from the Greek, phôtos (light) and khrôma (colour), is a reversible colour change caused by the exposure of a material to visible or invisible light. Photochromic lenses change colour in response to a chemical reaction triggered by UV rays. When UV exposure ceases this chemical reaction moves in the reverse direction, causing the lenses to fade back to their initial colour.

Photochromic technology depends on three key elements: lens material matrix, photochromic dyes and the manufacturing process.

The key ingredient that stands Transitions lenses apart from ordinary clear lenses is the assortment of photosensitive coloured-dye molecules, each of which produce a specific colour when activated. Transitions Optical has developed and evaluated several dyes since the 1990s and is currently selling the sixth generation of its lens product.

The particular type and proportion of the different dyes are responsible for the specific properties of the different lenses, including: colour, activation speed, uniformity of colour and durability. Or if you like, the differences between XTRActive, SOLFX and original Transitions VI lenses.

Lens Material Matrix

The lens matrix is the foundation of photochromic lenses, and may be composed of either organic or mineral materials. Mineral, or glass lenses, are scratch resistant, but heavier and breakable. Organic or plastic lenses, are not so scratch resistant, but are impact resistant, light-weight and better at filtering UV rays. Because the advantages far outweigh the disadvantages of plastic lenses, mineral lens sales have dropped steadily for several years in the optical market. Photochromic mineral lenses have also steadily given way to photochromic plastic lenses.

Photochromic Dyes

Mineral lenses were the first photochromic lenses on the market. The technology used to make these lenses consists of incorporating silver halide crystals into the mineral material. When exposed to UV rays, these silver halide crystals react and darken in a chemical reaction called oxidoreduction. The UV rays induce the oxidation of the chloride ions to chlorine and the reduction of silver ions to silver atoms. The silver atoms then cluster together, blocking the transmission of light, causing the lenses to darken. The level of darkening is temperature dependant (Figure 1).

Because silver halide crystals are not compatible with organic material, Transitions Optical has used organic photochromic dyes instead. When the millions of molecules are exposed to UV rays they undergo structural changes that modify the light transmission properties of the organic lenses. This structural change is driven toward the coloured state by UV light and toward the colourless state by ambient heat. So when UV exposure ends, the molecules naturally return to their initial structure, and the lenses return to their initial colour.

In the instance of the new Transitions XTRActive lenses, the dyes contain not only UV activated photochromic molecules, but also visible light activated molecules. This causes them to tint in sunlight, making them ideal for use behind a windscreen (Figure 2).

Manufacturing Process

There are two main ways to manufacture the photochromic lenses: the ‘in-mass’ process used for mineral lenses, which incorporates silver halide crystals in the casting phase; or surface technologies which incorporate the photochromic dyes on the front surface of the lens. Manufacturers use one of two types of surface technologies: imbibition (or deep coating), and surface coating.

While most other lens manufacturers use in-mass casting or other surface coating technologies, Transitions Optical is the only company to use the imbibitions process. Because not all lens materials are suited to the imbibition process Transitions Optical also uses its own variation of surface coating called ‘Trans-bonding’ for high index and polycarbonate lenses. Transitions Optical, as a ‘Lens-Converter’, has pioneered the surface technologies area and can process lenses supplied by most lens casters – including over 70 progressive lens designs

“Significant improvements have been made in our capability and consistency to produce and deliver lenses worldwide. We have evolved to operating five facilities worldwide and brought on a totally new platform of production in Trans-bonding, to add to our traditional imbibed methodology of production. This has allowed us to apply the photochromic process to a much broader range of products and appeal to a much wider audience,” Mr Breda explains.

In one of the cleanest rooms in the world, Transitions Optical begins the manufacturing process with clear lenses. If a single piece of dust touches the lens it could cause a serious defect. While an average room contains up to one million specks of dust per cubic foot, the air filters in the Transitions ‘Class 100 Room’ remove about 99.97 per cent of airborne particles, reducing them to less than 100 per cubic foot. With the clear lenses immaculately clean, the ‘Transitioning’ process can begin.

A coating containing photochromic dye is then deposited onto the centre of the lens surface. The lens is secured on a rotating arm, which ensures that the photochromic resin is evenly distributed on the surface of the lens through the process of centrifugation.

Applying the liquid to the lenses is done in a manner similar to staining a piece of wood: you put the stain on and it soaks in. But because plastic is not porous like wood, the lens is placed in a high temperature oven and heated until the lens matrix opens and photochromic dyes penetrate into the lens material.

Once the lens matrix closes, the dyes are ‘trapped’ and are an integral part of the lens matrix. The photochromic dyes are said to have ‘migrated’, ‘diffused’, or ‘imbibed’ into the substrate. The coating, now depleted of photochromic dye, is rinsed from the surface of the lens (Figure 3).

Like other prescription lenses Transitions lenses need to be toughened up for everyday use. To make them scratch-resistant they are dipped into a special coating which ensures they are as resilient as a glass lens. Depending on patients’ requests lens manufacturers then add anti-reflection, water-repellent, anti-smudge and other coatings to the front and back surface of the lens. They are then shipped to an optical laboratory where they are cut to a prescription. Using a diamond-tipped cutting surface to gently grind curves into the plastic, the lenses are shaped to refract light in the right way. The lenses are then ground and polished to precision, within an accuracy of a single micron, or 100 times thinner than a piece of paper. Once complete a machine positions them into the frame.

Looking Ahead

By utilising photochromic technology Transitions Optical is providing key benefits such as comfort, convenience, and protection from glare and UV to patients. With innovative products such as the performance sunwear, Transitions SOLFX, and the darkest everyday Transitions available, the new XTRActive lenses, Transitions continues to tailor its technology to unique consumer needs.

The Transitions Lens Range

Transitions VI

This is the best everyday Transitions lens for a perfect balance between darkness, fade back speed and indoor clarity.

Features:

  • Darken outdoors
  • Fastest fade back
  • Clear indoors

Transitions XTRActive lenses

This is the best everyday Transitions lenses for extra darkness at all temperatures. They have a slight tint when indoors, and activate moderately behind the windshield of a car.

Features:

  • Very dark outdoors, even in warm weather
  • Activate moderately inside car
  • Slight tint indoors

Transitions SOLFX

Performance sunwear, specifically designed for outdoor activities to help enhance visual performance.

Features:

  • Polarised
  • Adjust from dark to darker depending on light
  • Available in prescription and non-prescription
  • Ideal for visually demanding outdoor activities