The most common consumer question regarding sunglasses is “what is their UV protection?” The popularity of this requirement is due to various reasons – both the general danger of UV and stories about the deterioration of vision when using low-quality sunglasses (the author read in the early 90s in the “Arguments and Facts” bike about Sochi taxi drivers who massively turned to ophthalmologists). And it was this property of points that we decided to check in due time, since there was the right equipment.
At first, it was planned to test two pairs of mountaineering sunglasses that our group took to the Caucasus in 2009 (Ushba), and the third, cycling (Alpina Sibiria, Cebe SPOT 1500 MC and BBB BSG-21 Cruiser). But then several other points for different purposes were added to the comparison.
Unfortunately, the case was postponed, so you will not find the models under discussion, but the general conclusions will remain fair.
1. Solar radiation
Let us dwell on the theory and standards. According to their biological effects, solar radiation is not uniform, so the solar spectrum is divided into 3 areas: ultraviolet radiation (280–400 nm), visible radiation (400–760 nm) and infrared radiation (760–2800 nm). Excessively bright visible light causes eye strain, and direct sunlight temporarily impairs vision. Infrared radiation penetrates deeper into the tissue and can cause burns due to thermal expansion of the capillaries, as well as clouding of the lens.
Ultraviolet has the greatest biological activity. UV radiation is absorbed by the upper layers of human tissue. There are ultraviolet ranges that differ in the type of biological activity: A (UVA, 400–315 nm), B (UVB, 315–280 nm) and C (UVC, 280–100 nm). UVA is responsible for sunburn and premature aging of the eyes, and UVB is involved in the formation of vitamin D, which can cause sunburn and irritation of the cornea. UVC radiation causes mutations and is carcinogenic, as it contains DNA and RNA absorption bands (240–280 nm). Quanta of the UVC range, having the most energy from the quanta of the optical range, can damage biological membranes (for example, cell walls).
It is also known that at the height of the radiation intensity is higher. This is caused by a decrease in the thickness of the atmosphere above the observer. So, for example, at an altitude of 5 km, the equivalent thickness of the absorbing layer, like atmospheric pressure, drops approximately twice. The composition of the solar spectrum also varies with altitude, since different wavelengths are differently absorbed by the atmosphere. Thus, at high altitudes, additional bands appear in the IR range, which at sea level are completely absorbed by water vapor, and the emission of the UVC range is also enhanced. Radiation from other bands is either absorbed in the upper atmosphere, or not absorbed at all, and the spectrum is present in the same amount as at sea level.
2. Standards for sunglasses
Russian GOST R 51831-2001 “Sunglasses. General specifications visible range. In accordance with the American standard ANSI Z80.3-2101, lenses should have a transmittance of no more than 1% in the UVB range and a transmittance of no more than 30% of the visible transmittance in the UVA range.
The most common European standard is setting 5 categories of filters corresponding to the degree of transmission of visible radiation (S0 – S4).
Protection against UV radiation according to the European standard has four categories of protection and is denoted by the CE mark: 0 (ineffective protection), 2, 6 and 7 (full protection). Category 7 at a wavelength of 380 nm should pass no more than 5%. Thus, all requirements for UV standards are limited to a wavelength of 380 nm. A number of countries (including the US) and experts believe that UVA and UVB radiation should be blocked to 400 nm at 99–100%. This standard is called UV400.
3. Materials used to make glasses
Ordinary inorganic glass provides sufficient protection against ultraviolet B and C, and from the main part of IR radiation, since it only passes in the range of 300–2000 nm. However, the glass does not have sufficient strength and crumbles when destroyed, representing a great danger to the eyes, so at the present time lenses for sunglasses are made from polymers. The most common are polycarbonate (based on bisphenol A) and plastic CR-39, they weigh less and are safer with active rest. In addition, polycarbonate is a very common material, and compact discs are made from it. Therefore, its production technology is cheap and well developed.
Both plastics are similar in properties to glass – they transmit 80% in the near-IR range, are almost completely transparent in the entire visible range and have a sharp transmission boundary in the region of 300 nm.
Polycarbonate transmittance spectrum (fragment of the basis of the uncoated compact disc); Shimadzu UV3600 spectrophotometer.
To create a sun lens in plastic, additives are added or coatings are applied that reduce transmission. At the same time, all the standards cited earlier stipulate that transmission in the infrared or ultraviolet range cannot exceed a certain fraction of transmission in the visible range. This has a physiological basis – the pupil reacts to the intensity of light in the visible range, therefore, darkened lenses that transmit invisible radiation are much more dangerous than ordinary transparent lenses.
1. Alpina Sibiria. Production – Alpina, Germany (2008–2009). The purpose is climbing, the lens material is a polymer of an unknown type, and mirror coating is applied (labeled as “ceramic mirror”). According to the manufacturer, the mirror coating absorbs IR radiation. Declared protection class – S4, UV400. New.
2. Cebe SPOT 1500 MC. Production – Cebe, France (2005–2006). Appointment – sports, climbing, a material of lenses – polycarbonate. Declared protection class – S4. Year of active operation.
3. BBB BSG-21 Сruiser. The producer is BBB, the Netherlands (2008–2009). The purpose is sports, the lens material is polycarbonate, the declared protection class is S3, UV400. New.
4. Christelle 2458 320-D428. Manufacturer – Yingchang Glasses Industrial Co., LTD., China. Purpose – inexpensive fashionable glasses, the manufacturer claims full compliance with international standards and 100% UV protection. Exploited a little.
5. Diopter glasses with polymer lenses Carl Zeiss with anti-reflective coating. Lens material unknown.
6. Lenses from Polaris cyclists «Kir Royale. Black ”and“ Kir Royale. Titanium Silver (2007). The class of protection is unknown, visible rubbing of the surface. 2 years of operation.
7. Oakley “Crowbar” ski mask, type and characteristics of the lens are unknown. Year 2008. One year of operation.
8. Ski mask Swans Polarized, year 2009. Declared full protection against UV. New.
9. Polycarbonate (CD base without reflective layer) – for comparison.
The appearance of several tested points. From left to top: Alpina Sibiria, BBB BSG-21 Cruiser, Cebe SPOT 1500 MC, Polaris Kir Kiry lenses. Black ”and“ Kir Royale. Titanium Silver. Right, top to bottom: Christelle 2458 320-D428, Oakley Crowbar, Swans Polarized.
The transmission of glasses was measured on an Evolution 300 spectrophotometer (Thermo Scientific), unlike most similar instruments with a cuvette compartment, large enough to place glasses in it. In the summer of 2009, the transmittance spectra of the first batch of glasses were measured, in the spring of 2010 – the second.
The transmission spectra of the first batch of points in a linear (left) and logarithmic (right) scale.
The best results are for Depose and Alpine, but they do not quite correspond to the declared protection classes S4, the prize “for the honesty of the manufacturer” goes to the BBB.
Depose show uniform absorption in the visible range and suppress in addition all infrared light, which is prescribed by the class S4. The disadvantage is that the transmission of UV reaches 1% at a wavelength of 380 nm. It conforms to standards, but does not meet UV400 recommendations.
Alpine exhibit a slightly lower absorption uniformity in the visible range and a sharp absorption edge at the UV border (400 nm), which also corresponds to S4 class. But they miss the extra IR radiation, moving in this range to class S3 (despite the advertised deposition).
BBB is characterized by an equally clear boundary with the UV range, uniform absorption in the visible range, and a slight increase in transmittance in the IR range. Full compliance with class S3.
Christelle is distinguished by a fuzzy absorption limit in the UV range (but better than Depose), completely uneven transmission of visible radiation (which should lead to color distortion), and a significant increase in IR transmittance. Looks like casually done in class S2, and pointlessly shaded in S3. For cheap glasses it is not surprising.
Swans paints everything in reddish colors, because it transmits both red light and IR radiation well. UV blocks completely.