Eye protection from ultraviolet radiation
You can't see, hear, or feel UV radiation, but can actually feel its effects on the body, including the eye. Many publications in professional journals devoted to the study of UV exposure on the eyes, and from them, in particular, should be that prolonged exposure to them can cause a range of diseases.
What is ultraviolet light?
Ultraviolet radiation is invisible electromagnetic radiation occupy the spectral region between visible and x-ray radiation within the wavelengths of 100-380 nanometers. The entire region of ultraviolet radiation (or UV) is divided into near (l = 200-380 nm) and far or vacuum (l = 100-200 nm); the latter name is due to the fact that the radiation of this area is strongly absorbed by air and its study is made using a vacuum spectral instruments.
The main source of ultraviolet radiation is the Sun, although some sources of artificial light have ultraviolet in its spectrum component, it also arises when gas welding works. The middle range of UV rays, in turn, are divided into three components – UVA, UVB and UVC, which differ in their influence on the human body.
When exposed to living organisms to ultraviolet radiation is absorbed by the top layers of plant tissue or skin of humans and animals. At the heart of its biological actions are chemical modifications of the molecules of biopolymers caused as the direct uptake of radiation quanta, and – to a lesser extent, the interaction formed during irradiation radicals water and other low molecular weight compounds.
UVC is the most short-wave and high-energy ultraviolet radiation with a wavelength range from 200 to 280 nm. The regular impact of this radiation on living tissues can be quite destructive, but fortunately, it is absorbed by the ozone layer of the atmosphere. Note that this radiation is generated ultraviolet germicidal radiation sources, and occurs during welding.
UVB covers the wavelength range from 280 to 315 nm and an average radiation energy representing a danger to the bodies of man. It is UVB rays causing sunburn, photokeratitis, and in extreme cases cause a number of skin diseases. UVB radiation is almost completely absorbed by the cornea, but a part of him, in the range of 300– 315 nm, can penetrate into the inner structures of the eye.
UVA is the wavelength and the least energy component of UV radiation with l = 315-380 nm. The cornea absorbs some UVА radiation, but a substantial part is absorbed by the lens. This component and must take particular account of ophthalmologists and optometrists, because it penetrates deeper in the eye, and has potential danger.
The eyes are exposed just wide enough UV-range radiation. Its shortwave portion is absorbed by the cornea, which can be damaged with prolonged exposure to radiation of waves with l = 290-310 nm. With the increase of wavelengths of ultraviolet light increases the depth of its penetration into the eye, with most of the radiation absorbing lens.
The lens of the human eye is an excellent filter, created by nature to protect the internal structures of the eye. It absorbs UV radiation in the range from 300 to 400 nm, protecting the retina from exposure to potentially harmful wavelengths. However, if long-term regular exposure to ultraviolet light develop damage to the crystalline lens, over the years, it becomes yellow-brown, turbid and unfit for operation on the destination (ie the cataract). In this case, is assigned to the operation for cataract.
The light transmission material of spectacle lenses in the UV range
The eye protection is traditionally made with the use of sunglasses, clip-ons, shields, hats with visors. The ability of glass lenses to filter out potentially dangerous component of the solar spectrum is associated with the phenomena of absorption, polarization or reflection of the radiation flux. Special organic or inorganic materials are introduced into the composition of the material of spectacle lenses or coatings are applied on their surface. The degree of protection of spectacle lenses in the UV region cannot be determined visually on the basis of hue, or color spectacle lenses.
Although the spectral properties of materials glass lenses are regularly discussed on the pages of professional publications, including the magazine "Veko", there are still misconceptions about sustainable transparency in the UV range. These wrong judgments and views are reflected in the view of some ophthalmologists and even splash on the pages of mass media. So, in the article "sunglasses can provoke aggressiveness" optometrist assistant Galina Orlova, published in the newspaper "St. Petersburg Vedomosti" on may 23, 2002, we read: "Quartz glass does not transmit ultraviolet rays, even if it is not darkened. Therefore, any glasses glass eyeglass lenses will protect the eyes from ultraviolet radiation". It should be noted that this is absolutely wrong, because quartz is one of the most transparent in the UV range of materials, and quartz cuvettes are widely used to study spectral properties of substances in the ultraviolet region of the spectrum. Ibid.: "Not all plastic eyeglass lenses protect from ultraviolet radiation". This claim could be accepted.
With the aim to finally clarify this question let us consider the transmission of the main optical materials in the ultraviolet region. It is known that the optical properties of substances in the UV spectral region are significantly different from those in the visible region. A characteristic feature is the reduction of transparency with decreasing wavelength, i.e. the increase in the absorption coefficient of most materials are transparent in the visible region. For example, normal (not glasses) mineral glass is transparent at the wavelengths greater than 320 nm, and materials such as violenoe glass, sapphire, magnesium fluoride, quartz, fluorite, lithium fluoride, transparent at the shorter wavelengths [TSB].
The lenses made of different materials:
1 - less crown glass
2, 4 - polycarbonate
3 - CR-39 with light stabilizer
5 - CR-39 UV absorber in the weight of the polymer
In order to understand the effectiveness of the protection from UV radiation of different optical materials, refer to the spectral curves of transmittance of some of them. In Fig. presents a light transmission in the wavelength range from 200 to 400 nm five of spectacle lenses made of different materials: mineral (cron) glass, CR-39 and polycarbonate. As can be seen from the graph (curve 1), most mineral spectacle lenses of glass cron depending on the thickness in the center begin to pass ultraviolet light with wavelengths 280-295 nm, reaching 80-90% of light transmission at a wavelength of 340 nm. On the border of the UV range (380 nm) light absorption mineral glass lenses is only 9% (see table).
|The refractive index
|Absorption of UV radiation, %
|CR-39 - traditional plastics
|CR-39 - with UV absorber
This means that mineral spectacle lenses of conventional glass cron unsuitable for reliable protection from UV radiation, if the composition of the batch for the manufacture of glass is not entered special additives. Spectacle lenses of glass cron can be used as a sunscreen only after applying high-quality vacuum plating.
Light transmission CR-39 (curve 3) corresponds to the conventional plastics used for many years for the production of spectacle lenses. Such spectacle lenses contain a small amount of light stabilizer that prevents the photodestruction of the polymer by ultraviolet light and oxygen. Traditional spectacle lenses from CR-39 is transparent to UV radiation from 350 nm (curve 3), and their light absorption at the boundary of the UV range is 55% (see table).
We draw the attention of our readers how much better from the point of view of protection against ultraviolet traditional plastics in comparison with mineral glass.
If the composition of the reaction mixture add a special UV-absorber, the spectacle lens transmits radiation with a wavelength of 400 nm and is a great means of protection from ultraviolet radiation (curve 5). Eyeglass polycarbonate lenses have high physical and mechanical properties, but in the absence of UV absorbers begin to pass UV light at 290 nm (i.e., similar to cron the glass), reaching 86% light transmission at the border of the UV-region (curve 2), which makes them unsuitable for use as a means of UV protection. With the introduction of the UV absorber spectacle lenses cut off the UV radiation up to 380 nm (curve 4). In table. 1 also shows the values of transmittance modern organic spectacle lenses made of different materials – high refraction and the average values of the refractive index. All these eyeglass lenses pass light radiation, starting only from the border of the UV range is 380 nm, and reach 90% of light transmission at 400 nm.
Be aware that a number of characteristics of ophthalmic lenses and design of frames affects the effectiveness of their use as means of UV protection. The degree of protection increases with the square eyeglass lenses – eyeglass lens area of 13 cm2 provides 60-65% degree of protection, and area of 20 cm2 96% or even more. This occurs by reducing the lateral flare and the possibility of penetration of UV radiation into the eye due to diffraction at the edges of spectacle lenses. Increase the protective properties of points and promotes the presence of side pads and wide temples, and the choice of a more curved shape of the rim corresponding to the curvature of the face. You should know that the degree of protection decreases with increasing vertex distance by increasing the penetration of the rays under the rim and, consequently, getting them in the eyes.
If the boundary of the ultraviolet region corresponds to a wavelength of 380 nm (i.e., the light transmission at the wavelength of not more than 1%), why are many branded sunglasses and eyewear lenses specified cutting up to 400nm? Some experts say that this method of marketing because the provision of protection above the minimum requirements like buyers, besides "round" the number 400 is remembered better than 380. At the same time, in the literature there is evidence of potentially dangerous exposure to light of the blue region of the visible spectrum on the eye, so some manufacturers and installed a slightly larger border at 400 nm. However, you can be sure that the remedies are not transmissive to 380 nm, offers sufficient protection from ultraviolet radiation in accordance with the standards of today.
I want to believe that we finally convinced everyone that the usual mineral glass lenses, and especially quartz glass, significantly inferior to the organic lenses on the effectiveness of cutting off ultraviolet light.