Sunglass lens quality test needs to be revised, study shows

Researchers warn that parameters of the test used as the basis for technical standards in countries like Brazil are unsafe.

By Elton Alisson  |  Agência FAPESP – The quality and safety tests for sunglasses that serve as a basis for technical standards in Brazil and other countries at similar latitudes should be revisited, especially to improve the acceptable limits for ultraviolet (UV) radiation.

According to a study by researchers at the University of São Paulo’s São Carlos Engineering School (EESC-USP), with support from FAPESP, the current tests do not ensure that sunglasses available in Brazil provide adequate protection from UV radiation.

The results of the research have been published in BioMedical Engineering OnLine.

“The standard UV protection test doesn’t reliably confirm that the lenses of sunglasses sold in Brazil protect the user against solar radiation within limits considered safe by the World Health Organization,” said Liliane Ventura, a professor in EESC-USP’s Department of Electrical Engineering & Computing and principal investigator for the project.

Some studies have shown that UV protection can deteriorate over time, given that the lenses eventually become less dark after a certain amount of exposure to sunlight. This may change the category they are classified into, which depends on transmittance, a measurement of the amount of visible light that can pass through the lenses.

In addition, exposure to high levels of UV radiation can make lenses less impact-resistant and more liable to shatter.

To ensure the quality of sunglasses sold, the technical standards in force in Brazil (based on international standards) require products to be submitted to a test that verifies the category of the lenses based on exposure to sunlight during a period of simulation.

“These standards, however, make no reference to an analysis of UV protection degradation, which should be one of the main factors measured in this test,” Ventura told Agência FAPESP.

Adjustment of parameters

In the test, sunglass lenses are exposed for 50 hours to a solar simulator consisting of a 450 W xenon arc lamp. The lenses are 30 cm away from the lamp, which has a spectrum similar to that of the sun.

Before and after the test, the lenses are analyzed by spectrophotometry to compare their transmittance of visible light and to determine the category to which each lens belongs. The goal is to see if they have changed category, such as by becoming lighter, and to measure the extent to which their UV filters deteriorate during the artificial aging process.

This process is equivalent to two summer days or four winter days of exposure to natural sunlight in a Brazilian city such as São Paulo, explained Mauro Masili, also a professor at EESC-USP and co-author of the study.

“Research has shown it’s not effective to analyze degradation in two days of sunlight,” Masili said. “Worse, there are no tests to ensure that UV protection lasts during a long enough period of use.”

An online survey conducted by the researchers showed many Brazilians use sunglasses for two to four hours per day on average and buy a new pair every two years.

To find out whether the standard quality test ensures sunglasses can be used in Brazil for this length of time, the researchers developed a mathematical model that estimates how sunlight reaches ground level in 27 major cities in Brazil and in 110 northern-hemisphere national capitals, taking into account the geographical characteristics (latitude, longitude and altitude) and typical atmospheric profile of each city.

They used this mathematical model to calculate sunrise-to-sunset solar irradiance in watt-hours per square meter at ground level in the 137 cities analyzed.

They then compared these solar irradiance values with the spectral irradiance of the 450 W xenon lamp used in standard tests and found that solar irradiance in the cities evaluated is actually far more powerful than the lamp’s irradiance. Moreover, to reproduce actual conditions in terms of exposure to UV radiation, sunglass lenses should be tested for 134.6 hours at a distance of 5 cm from a 450 W xenon lamp.

“The parameters for the standard quality test required by international standards need to be adjusted,” Masili said. “Currently, they don’t reproduce real conditions of exposure to sunlight.”

Revision of technical standards

The researchers at EESC-USP contributed to the drafting of the first Brazilian technical standard for sunglasses – NBR 15111, published in 2003 – and to its 2013 revision, suggesting parameters better suited to local conditions.

Until 2013, the Brazilian standard faithfully copied the European standard (BSEN 1836), requiring sunglasses sold in Brazil to filter UV radiation in the wavelength range of 280-380 nanometers (nm).

A 2010 study by the World Health Organization (WHO), however, found that ocular health could be damaged by UV radiation of up to 400 nm.

“We concluded that there was a significant gap between the solar irradiance limits used in Brazil and those considered safe for ocular health by the WHO,” Ventura said.

Through a study also supported by FAPESP, the researchers succeeded in having the Brazilian standard revised in 2013, requiring sunglasses sold in Brazil to protect users against UV radiation in the 280-400 nm range.

In 2015, this standard was revoked and replaced by ISO 12312-1, which required sunglasses sold in Brazil to filter UV radiation in the 280-380 nm range. This was identical to the requirement set by the European standard and the first version of the Brazilian standard.

“The standard needs to be revised. The limits set for UV protection are insufficient to effectively protect Brazilians’ ocular health,” Ventura said.

In May of this year, Ventura presented the results of the study to the technical committee of the US National Institute of Standards & Technology (NIST), which agreed that the standard quality test for sunglasses and hence national standards for the product should be revised.

“Although our calculations of solar irradiance are based mainly on Brazilian cities, cities in other countries can benefit, especially at similar latitudes,” Ventura said.

According to the researchers, exposure to UV radiation varies by latitude, but levels in the tropics are extremely high in both summer and winter. Sunglasses worn in the southern hemisphere may therefore have to be replaced more often than those worn in the northern hemisphere, where most countries are at higher latitudes than in the south.

The article “Equivalence between solar irradiance and solar simulators in aging tests of sunglasses” (doi: 10.1186/s12938-016-0209-7) by Masili and Ventura can be read in BioMedical Engineering OnLine at biomedical-engineering-online.biomedcentral.com/articles/10.1186/s12938-016-0209-7.