When the sensor strip turns blue under the sunscreen, it's time for more protection. You may need a higher SPF value. Yes, simply hold your glasses over a portion of the sensor strip out in the sun. There are many tests you can perform. You can also check the window glass in your home, auto and business.
Artists can use it to see if the glass covering their artwork is providing UV protection.
Exposure to ultra violet rays have been shown to cause wrinkles and premature aging of the skin as well as melanoma - a type of skin cancer. These same rays have been implicated in causing cataracts of the eyes. Ultra violet rays may be present at unsafe levels even on cloudy days.