Sunglasses degrade under UV radiation. More specifically most of the sunglasses you buy are made out of polycarbonate. With more UV radiation over an extended time, light transmission and color changes can be seen.
This degradation can be related back to the changes in the polymer microstructure. The lenses of the sunglasses become less glossy and the light rays can not pass the material as smoothly when the material ages. This leads to the lenses becoming hazy or dull.
Old sunglasses oftentimes have a yellow tint to them which can oftentimes be seen best in sunglasses with a gradient which have a lighter tint in the lower half of the lenses.
The question is does photodegradation under ultraviolet (UV) radiation lead to an increased risk for the wearer? Because UV light is well known for increasing the risk for cataracts damaged structures inside the eye that can lead to macular degeneration. Surprisingly research does not exactly tell how long sunglasses will protect your eyes from harmful radiation.
This is why Dr. Ventura from the University of Sao Paolo and her team developed an “aging test” to see how well sunglasses deal with the wear and tear to their UV protection. Because sunglasses need to meet certain criteria to get certifications but the protection is not tested in the years after the sunglasses were used. In this test, 20% of sunglasses did not meet proper UV protection.
Of course, some of the sold sunglasses do not even pass the test for UV protection when they are new. But the question is how often should you get a new pair of sunglasses to protect your eyes optimally?
How Long Should You Use a Pair of Sunglasses?
According to Dr. Liliane Ventura most people in brazil wear their sunglasses for two hours per day. She and her team recreated the teardown due to the UV radiation with a lamp positioned at 5cm in front of the tinted sunglasses. Now she recommends sunglasses should be replaced every two years when they are worn for approximately two hours daily.
This of course highly varies from person to person. Some use their sunglasses a lot more and others less. The amount of UV exposure also varies based on world latitudes too. Finding a general norm here is that includes every aspect is not easy. Equatorial countries have higher UV indexes in the summer and remain higher in the winter compared to western countries. This is why sunglasses worn in tropical countries should be replaced more often.
How Do You Know Your Sunglasses Have Already Started to Degrade?
Without special equipment, you can not tell if your sunglasses already started to degrade. A pair of sunglasses with and without UV protection looks exactly the same. UV measurements are possible with UV–Vis spectroscopy. Those measurement devices work with LEDs.
A number of LEDs shine at specific wavelengths in the UVB, UVA, blue and visible light range. The LEDs gradually shine through the lens, and the light passing through is detected by a sensor. In this way, it can be precisely measured to two decimal places whether the transmission of a pair of glasses is also below the UV400 limit curve required by many countries.
In a lot of optical shops, opticians can measure if UV protection is still given in combination with your sunglasses.
How to Keep Your Sunglasses From Degrading
Even if they are called sunglasses, they should not be left behind the windshield in hot places such as in a parked car for a long period of time. If you do, you run the same risk with sunglasses as with purely corrective glasses that the coatings on the lens surface may suffer a defect.
The coatings on the lens can also contribute to UV protection. Especially very high-quality sunglasses provide coatings on the back surface of the lenses for increased UV protection. If these coatings suffer damage, not only the UV protection is poorer but also the view can be clouded by this.
When you undertake an extended walk on the beach in midsummer you do not have to worry about your sunglasses.
Also avoid bringing your sunglasses into contact with chlorine. This often causes surface defects in sunglasses. Even a few dips in the pool can break sunglasses. Also, use only the special microfiber cloths to clean the sunglasses. If you have the possibility use running water and detergent. The water flushes small particles and dust from the lens surface. This ensures you will not rub the particles on the lens surfaces which leads to micro scratches in the coatings.
However, the temperature of the running water should not be extremely cold or warm, otherwise, the coatings on the sunglasses could crack again which causes a breakdown of the material, loss of toughness, and embrittlement.