UV danger to the eyes

Chronic exposure of the eyes to UV radiation is a widely established ¹ public health problem (cortical cataract, pterygium, pinguecula, eyelid cancers…), and over 40% of our exposure to UV occurs during low to moderate sunshine situations [1], in which we can wear our colourless spectacle lenses comfortably. However, due to the lack of information regarding the dangers of UV radiation and in the absence of a recognised protection factor for lenses which could help in their choice, it is still rare for consumers to take protection of their eyes into consideration when purchasing lenses for their spectacles.

Indeed, the foremost expectation expressed by spectacle wearers is clarity of vision. 

Therefore, to meet this requirement, anti-reflective lenses have gradually become the standard lenses offered. 

What level of UV protection is really offered by the lenses which are currently on the market? 

Organic materials, which absorb UV rays, offer near-complete protection against all frontal UV exposure. But recent studies ² show that the UV rays arriving from the sides and back of the lens, where they are reflected strongly by the anti-reflective treatment on the inner side, can represent up to 50% of the UV exposure suffered by the eye and its surroundings.

Indeed, although the anti-reflective lenses on the market are designed to be efficient against the reflection of visible light, they reflect on average 25% [2] of the ultraviolet spectrum!

Crizal UV lenses were therefore created from the need to develop a new AR treatment ensuring protection for the wearer against UV light arriving on both sides of the lens!

Designing the first anti-reflective lens that protects against UV 
Spectral considerations: The Thelen formula

The foremost function of an anti-reflective treatment is to improve the transparency of the spectacle lens, reducing reflection from both sides of the lens.

Anti-reflective lenses, as designed and made in the ophthalmic industry, are based on the laws of interference. The principle consists of alternating layers of low index and high index materials in order to create destructive interference and therefore reduce as far as possible the level of reflection for the desired spectral range. Optimisation to wavelengths close to the visible involves depositing thin layers, the thickness of which is around a few tens of nanometres.

The main parameters used to improve the efficiency of anti-reflective treatment are now well known in the business. There is a mathematical formula, defined empirically by Thelen [3], which shows their respective impact on the average reflection level of a stack of anti-reflective layers. In this formula it appears that reflection is an exponential function of the spectral band width on which one is seeking to optimise an anti-reflective coating. This shows that it is all the more difficult to reduce average reflection because it has to be optimised across an extended spectral range.

In the case of Crizal UV, the aim is specifically to achieve reduced UV reflection whilst maintaining the optimal level of transparency that characterises Crizal, Essilor's premium range of anti-reflective lenses. To achieve this we have succeeded in identifying a limited number of groups of multi-layer stacks characterised by highly specific combinations of thicknesses of these layers. Identification of these groups of stacks has resulted in an application for an international patent.

Geometric considerations

In addition to spectral considerations, optimisation of the performances of Crizal UV also meets considerations of a geometric or angular nature.

Figure 1 clearly illustrates that the share of light coming from behind the wearer and reflected by the rear side of the lens is contained in a solid angle of between 30° and 45°. This angular range has been defined by measurements made in experimental conditions representative of real life wearing conditions, and corresponds to the values given in scientific literature [4, 5].

Fig. 1: Diagram illustrating, as seen from above, the share of UV radiation transmitted by the lens when the light source is opposite the wearer and the share reflected by the rear side of the lens when the source is behind the wearer.

In summary, Crizal UV is a multilayer antireflective stack whose optical performance meets a twofold requirement, spectral and angular. This product is characterised by an optimal level of visual transparency in the direction facing the wearer, typically between 0° and 30° and by minimum reflection in terms of UV light arriving on the rear surface of the lens, at an angle of between 30° and 45°.

In order to explain and demonstrate the innovation brought by Crizal UV, we have designed a new demonstrator, which has been made available to the group's various subsidiaries (see Fig. 2).

Fig. 2: Photos of the model used as a demonstrator by Essilor subsidiaries.

UV reflection factor

The requirement for low UV reflection implies being able to quantify it properly, taking account of the health risks associated with UVA radiation (315 nm - 380 nm) and UVB radiation (280 nm -
315 nm) on the human eye. To do this we used the existing international standard (standard ISO 8980-3 :2003) which proposes a calculation of the UV transmission factor applied to ophthalmic
lenses. In this standard, the UV performance calculation is carried out using a weighting function W(λ) (fig. 3) which depends on:

•  the direct sun radiation spectrum ES(λ) received at the Earth's surface – small amount of UVB compared to UVA, due to absorption by the ozone layer of rays between 200 and 300 nm,
   
• the relative efficiency spectral function S(λ) [6] or "function of UV risk", which shows that UVB is more dangerous than UVA. This latter function S(λ) expresses the biological risk linked to photochemical deterioration of the cornea, when it is exposed to UV.

We have therefore applied this function to evaluate reflection R(λ) in UV, using the formula: 

This factor is used in the calculation of the E-SPF ³ which is used to evaluate the level of UV protection offered by ophthalmic lenses. (Fig. 3)

Fig. 3: Sunlight energy spectrum function ES(λ)[orange] in W.m-2.nm-1 and spectral relative efficiency function S(λ)[pink] in arbitrary units in UV. Normalised weighting function W(λ)[black] (x5), resulting from the product of ES(λ) and S(λ).

Characterisation of performances

The development of Crizal UV has required new characterisation methods. Firstly in the R&D phase, spectral ellipsometry and variable angle spectrophotometry, in both UV and visible, were used to characterise all materials, from the substrates to the thin layers. Measurement methods based on the same principles were adapted and deployed at production sites in order to guarantee the performance levels of this new product, from both a spectral and an angular point of view.

The UV protection provided by low level UV reflection, (R_UV), from 5 to 10 times less than that measured on the anti-reflection coated lenses of the main manufacturers ⁴, thus means an E-SPF protection factor of 25 for colourless Crizal Forte UV lenses, and 50+ for Crizal Sun UV sun lenses.

The usual optical characterisations confirm that anti-reflection efficiency remains unchanged in the visible spectrum for Crizal Forte UV lenses compared to previous generations of Crizal lenses.

Conclusion

Associated with organic materials, Crizal UV lenses bring to the market for the first time protection against UV radiation incident at the back of the lens, whilst ensuring optimum visual clarity for the wearer.

Crizal Forte UV colourless lenses are associated with an E-SPF protection factor of 25 ⁵, the best on the market.

In sun lenses, Crizal Sun UV offer an even higher level of protection, with an E-SPF factor of 50+.

With a complete offer available and based on an E-SPF factor that is explicit for consumers, vision professionals can convey an important prevention message and help wearers to make the right choice in terms of protection for their vision health.

Footnote page:
1. See the articles in this issue referring to the dangers of UV for the eye and its surroundings.
2. Read the article in this issue by Karl Citek.
3. Read the article in this issue by Karl Citek. 
4. Best UV protection for Crizal Forte UV lenses according to the E-SPF factor compared with colourless anti-reflective lenses in equivalent materials with the best anti-reflective properties produced by other main manufacturers on the market. Lens performance measurement only: the E-SPF factor does not include UV radiation that enters the eye directly without interaction with the lens, which depends on external factors (the wearer's morphology,frame shape, wearing conditions, etc.). E-SPF measurements: independent body, USA, 2011.
5. E-SPF of 10 for Essilor Orma® Crizal Forte UV lenses