As a pilot I am always considering what is the correct darkness (density) of sunglass lenses is correct for my flying conditions.
General purpose sunglass lenses will have VLT (Visible Light Transmittance) values of between 15 and 20%. VLT is the amount of light the lens tint allows to reach your eyes.
But that much light may be too much for some people. For example, people with blue eyes may have a higher sensitivity to light and require a darker tint in their sunglasses. If you have extra sensitive eyes you might need to go down to 8% VLT, or even less.
Another factor is age. As we age our eyes gradually lose their "light gathering ability", especially after the age of 50 or so. In that case higher VLT lenses of 20-30% or even lighter may be appropriate.
Selecting a lens with the correct VLT is the first step. It is also important to select lenses that effectively block UV (ultraviolet) light as well. Since UV light is one of the causes of cataracts, the lenses should block most of the UV light.
We recommend that the UV light transmittance be less than 10%.
The other light rays to be considered are IR (infrared) light, which is mostly heat waves. It is important to block IR if you wear contact lenses, or spend a lot of time in direct sunlight.
Heat waves will cause your eyes to "dry out", thereby causing "scratchy eyes" thereby adding to your discomfort.
Important: Only glass sunglass lenses block IR light!
Don't get sunglass lenses that are too dark. It is easy to become addicted to dark lenses, and they can become dangerous when driving if you go from bright sunlight to a tunnel or heavily shaded area. You will be blinded temporarily if the lenses are too dark.
Don't judge the density (darkness) of the lenses in the first two or three minutes you put them on. The difference in light intensity between indoors and outdoors is 1,000 to 1 and no lens will compensate for that much difference immediately. It will take a minute or two for your eyes to adapt.
Be aware that some cheap plastic lenses can severly distort colors. If you stick to Dark Grey, G-15 or Kontraster lenses made of glass you will not have a problem with color distortion.
If you have an occupation that requires precise color identification then use the Dark Grey (also known as Neutral Grey) lenses. They are the most color neutral.
Have a prosperous day!
p.s. Please share "Selecting The Darkness (Density) Of Sunglass Lenses" with your friends. Thanks!