“It is absolutely insufficient that glasses have dark lenses as some mistakenly believe, and as others will lead you to believe.”

Federico Bartolomei

Summer is here and so is the issue of protection from sun rays.
People love sunlight, which influences in a decisive way our activities and our state of mind, even our hormonal level.
From being a goodwill agent to a damaging agent: excessive exposure can cause serious damage to the skin and to the eyes.
Natural light is composed of electromagnetic radiations of various wavelengths, and their visible portion constitutes only a small part.
The eye is able to perceive only electromagnetic oscillations of wavelength between about 370 and 700 nm (visible spectrum). Radiations take up various bands, which are characterized by specific chromatic signals: between 380 and 430, they appear violet; between 430 and 470, blue; between 470 and 520, green; between 520 and 590, yellow; between 590 and 610, orange; between 610 and 700, red.
Accredited by the analysis of the sun's effect on the Rhesus Monkey's eyes, numerous scientific studies have demonstrated that ultraviolet radiations (UV) and visible radiations (if sufficiently intense) can be at the basis of biological damages.
Blue light is therefore more damaging than green light, and green light is more damaging than red light. In short, the danger from radiation increases with the intensification of the energy, therefore with the decrease in wavelength.

In function of biological effects, it is usual to separate ultraviolet radiations into three bands: UV-A, UV-B, UV-C.
The UV-C radiation is the most energetic one and could be quite damaging to our eyes let alone our skin, but it is fortunately almost completely blocked by our atmosphere. In fact, the UV-C radiation is absorbed, with a peak of around 150 nm, from molecules of oxygen.
The ozone is the second defense for our eyes provided by the atmosphere, since it can absorb longer UV-C radiations or part of the UV-B spectrum, with a maximum peak of absorption of around 260 nm.
For that reason, ultraviolet radiations arriving on earth always have wavelength superior to 280 nm.
Because of the well-known hole in the ozone layer, that is the reduction of the layer present in the atmosphere at heights between 15 and 35 km, it is estimated that in the next decades we will receive an additional dose of radiations.
The three bands of ultraviolet radiations are not absorbed in the same way from the various ocular means, and the absorbing factor is decisive in understanding the damages that can be caused by the radiation.
The UV-B radiation is almost completely absorbed by the cornea (Fig. 4).
It is for that reason that the UV-B radiation can be responsible for keratitis and conjunctivitis. The UV-A radiation, however the less energetic of the three, is largely absorbed by the crystalline lens (for example, 52% of the radiation at 360 nm is absorbed by the crystalline lens).
Problems caused by the infrared radiation are less important but it becomes dangerous only in situations where there are  high temperature sources: ovens, blast furnaces, etc.
Visible violet and blue photons can be damaging to the retina. The link between ocular diseases and UV and visible electromagnetic radiations was confirmed by experimental investigations, which have drawn attention to the fact that populations who are more exposed to the sun, like fishermen and farmers, are more prone to suffer from the diseases mentioned above.

Everyone should be informed about the use of lenses with UV filters, besides sunscreens, visors and hats.
Be aware not to underestimate UV on cloudy days: the heat from the sun is less perceptible because the clouds absorb it, but UV go through the clouds better than visible light.
Moreover, we are today exposed to the dangers of UV radiations coming from non-solar sources. Fluorescent lights, lasers, soldering work, tanning lights are a few of the non-solar UV sources with which we normally come into contact.
It is therefore particularly recommendable to use sunglasses with lenses of optimum quality and made according to regulations established by public organizations: that is, lenses which absorb 100% of the UV rays and up to 96% of the violet and blue rays.
The eye reacts to strong luminosity causing a constraint on the iris. Because of the reduced pupillary aperture (miosis), there are less light rays entering the eyes, which defend themselves  from the strong environmental luminosity. This a phenomenon called dazzlement.
Classical examples of such extreme situations, where the eye is confronted with very strong light sources, are: on a white beach, on the snow, on the smooth asphalt in the summer, etc.
This condition produces a loss of comfort, or even the inability to see details or objects, leading to a true disability such as to reduce the sense of direction.

Consumers buying glasses must know that there exist precise public dispositions regulating production and commercialization of sunglasses, which must by law comply with standards of quality. Since July 1st, 1995, all sunglasses must indicate on their labels their filtering capacity, from 0 to 4, as well as the CE symbol. It is important at the moment of the purchase to look for these elements. 
It is absolutely insufficient that the sunglasses have very dark lenses as some mistakenly believe. Paradoxically, dark lenses that do not have an ultraviolet filter are even worse than fully clear lenses because they reduce the mechanism of pupillary aperture (miosis) contraction, which is one of the best natural defense mechanism that the eye has. Aside from direct light, the eye needs also to be protected from sun rays that may enter by the each side or around the lenses. UV reflected by snow, lakes and rivers are an important sources of indirect radiations for the eyes. Chromed and brilliant metallic surfaces reflect up to 85% of UV, and white cotton reflects up to 50%. This means that it is not advised to choose a frame with lenses that are too small, not providing sufficient protection against these accidental rays of which the level of danger is not to be underestimated.

Mountain climbers and skiers in high altitude have always used sunglasses with a strong ultraviolet protection often with mirrored lenses, which help in reducing the passage of sun rays (the filtering capability of a lense increases by one fourth of a degree when it is mirrored). These glasses may also be laterally closed by blinkers. However, these blinkers, or side-pieces that could be too large, reduce the lateral visual field and are not always recommendable. In a driving situation for example, they can create awkward or even dangerous situations particularly when arriving at crossroads. In some cases, using polarized lenses would be useful. These contain small special crystals (dicroic) which can block a lot of the reflected light. As for the colour of the lenses, once the exigence of filtering visible and invisible rays has been take care of, it is a matter of taste, however within certain limits. Yellow or orange lenses increase contrast and are indicated in conditions of low environmental luminosity and low contrat (fog, rain). It must be remembered that strong colours (often useful in cases of low vision) tend to alterate chromatic perception of the environment and consequently of  light signals, etc. The standard colour used in aviation is neutral gray, a colour that softens luminosity without significantly altering colours in the environment.  


Cordella Marco, Lenti alla melanina quando e perché prescriverle, 1999, ED. Intercast Europe

Cristini G., Meduri R., Basi fisiopatologiche di Clinica Oculistica, 1983, ED UTET

Abati S., Farini A., Filtraggio della radiazione elettromagnetica e ipovisione, Argomenti di ipovisione supplemento 2, 2002, Ed. Fabiano

Meduri R., Scalinci SZ, Scorolli L., L’ipovisione, Edizione Martina, Bologna, 1995