Introduction

In 2008 almost 45% of corrective spectacles sold worldwide were given anti-reflection treatment. In every country the number of wearers of anti-reflection treated spectacle lenses increased dramatically. Vision comfort provided by these treatments is particularly well appreciated, specifically with improved contrast and a reduction in visual fatigue. Studies confirm¹ the fact that when their spectacles are renewed, 95% of wearers of anti-reflection spectacle lenses wish to retain this benefit.

Since 1993, Crizal coatings have contributed to this development, becoming on the various continents a benchmark in terms of performance and quality. Crizal Forte, the latest generation in the Crizal range, launched in 2009, illustrates the technological progress that has been made in this field, for the increased benefit of wearers.

Increased demands on vision

Long hours spent working at the computer screen, watching television, playing video games and night driving all make daily demands on our vision. In all these cases, the benefits of anti-reflection treatments have been clearly established for spectacle wearers. They offer concrete advantages such as better visual acuity, a reduction in ghost images and quicker recuperation from glare. A wearer test² has shown that for night driving, anti-reflection coated lenses were preferred over standard lenses by 70% of wearers.

Maintenance of optimal performance over time is another challenge for anti-reflection coated lenses. Everyday hazards, scratching, finger marks, dust and droplets of water that affect vision, mean that antireflection coating used today must offer easier maintenance and be longer lasting.

Crizal Forte: new technologies for optimum efficiency

This observation is behind the development of the new anti-reflection coating, Crizal Forte. The aim was to reach the best level in each of the 5 dimensions identified: scratch resistance, anti-reflection efficiency, efficiency against dirt, protection against dust and protection against droplets of water.

Scratch resistance

To meet this important demand from spectacles wearers, a new intermediate layer (SR Booster ^TM) has been developed by our research teams. Situated between the polymerized varnish and the anti-reflection layers evaporated under vacuum, it enables better integration of the entire system with gradual increase in hardness, of the supple organic substrate towards the mineral layers on the surface. The end result is that laboratory tests indicate a level of resistance to abrasion (fine scratching) equivalent to that of a mineral lens with anti-reflection coating (Fig.1).

Fig. 1: SR Booster^TM  diagram.

Anti-reflection efficiency

Improvement in transmission and the reduction of visible reflections is a basic expectation by users of antireflection lenses. Thanks to technical expertise accumulated by Essilor on Crizal coatings over the last 15 years, Crizal Forte now offers the highest degree of transparency, combined with more discreet residual reflections.

Efficiency against dirt

Sometimes, a cause of disappointment amongst wearers of the previous generation of anti-reflection treated lenses , was the visibility of dirt, and specifically finger marks, on an anti-reflection coated lens compared with an un-treated lens. To combat this problem, a perfectly smooth surface must be developed to which dirt cannot adhere. Thanks to specific fluoride molecules and optimisation of deposition, with Crizal Forte perfect lens surface cover is achieved along with better resistance to dirt.

Efficiency is measured by the angle of contact of a drop of liquid on the surface of a lens placed horizontally. The greater the value of the reading, the better the anti-dirt efficiency. With Crizal Forte, the contact angle of a drop of water is 116°, the highest value ever recorded on an ophthalmic lens.

Efficiency against dust

To resist dust that can accumulate on lenses, Essilor has developed a new ionic assistance process in the production of the anti-reflection coating that integrates a conductor layer. Wiping the lenses no longer generates electrostatic charges. The lens resists dust and remains more transparent.

Protection against droplets of water

Drops of water that remain on the surface of spectacle lenses can have a major impact on wearer vision. Essilor has developed a new dynamic test to quantify the ease by which droplets of water slide over the surface of a lens. A lens with a Crizal Forte coating only has to be tilted through an angle of 6° for a droplet of water to disappear. This is twice as good as with Crizal Alizé and 3 to 4 times better than with the latest coatings produced by our competitors. Thanks to this unique performance Crizal Forte has created a new category of water-resistant coatings (Fig.2).

Fig. 2: Photo of the slide angle. 
 

Benefits perceived by wearers

During the development of new generations of Crizal coatings, technical measurements are continually compared with wearer perception. A pioneer in the ophthalmic industry Essilor has been using sensorial analysis methodologies for several years now, in order to evaluate user satisfaction closely.

These studies, which have been undertaken by independent institutes working in a wide variety of industries, are used to isolate each of the functions of an anti-reflection coating. A rigorous scientific approach guarantees results coherence. Controlled environments are used specifically (light level, temperature, …) and products are compared at least twice to confirm reproducibility of measurements.

During a study undertaken with the Eurosyn Institute in 2008³, wearer preference for Crizal Forte was established in areas such as: scratch-resistance (better transparency of lenses after an abrasion test), cleanliness and ease of lens cleaning, reduction of parasitic reflections.

Conclusion

Making use of the latest technologies and capitalising on the unique experience accumulated by Essilor in the field of anti-reflection coatings, Crizal Forte offers a particularly complete response to the aspirations of corrective lens wearers. It achieves the best performance level in the 5 main areas: scratch-resistance, anti-reflection efficiency, efficiency in terms of dirt resistance, protection against dust and protection against droplets of water.

Since being launched Crizal Forte has received an excellent reception worldwide from both professionals and spectacle wearers alike.

Footnote page:
1. Study carried out by the Sylab-Ypsis Institute in 2005 in 8 countries amongst 1704 wearers of corrective lenses.
2. Study done on 92 wearers in the USA by an independent body in 2005.
3. Study undertaken with the EUROSYN institute in 2008 involving 100 people.