Polarized lenses protect the eyes from UV radiation, and this protection depends on their UV filter. We must demand at least a 99%.
About the light intensity that passes through the lenses, it depends on its filter. Category 0 allows almost all light to pass through, lenses are almost transparent, and Category 4 filters are so dark that they hardly allow light to pass through (they transmit only 3% to 8%).
A degraded lens is darker in the upper area than in the lower area, and normally it’s not of a high Category, because we are not looking for maximum light blocking in this case.
Mirrored lenses reflect some of the light that hits them. They are recommended for water sports and snow, and other situations in which light intensity is very high and comes not only from the sun but also from the reflection on the ground or in the water. The combination with polarized lenses is especially suitable in extreme sports (mirrored + polarized lenses).