Interesting question. I've never seen such a chart.
I prefer sub 10% transmission for any mountain travel. Most important: the glasses need 100% UV cutoff. While the sun is definitely more intense up there, it's only a true problem when traveling on snow, where you need to deal with reflection from below in addition to the direct sun. For that I like gradient tint lenses that are darker below and above, as well as more complete coverage (larger, wraparound lenses). Even better if they're also polarized (polarized
and tinted lenses can be difficult to find, but some makers have them). That and a dark brimmed hat or cap and I'm good to go.
I have done some searching to see how much light transmission is ok at a certain elevation for your eyes, but can't come up with a comparison or chart. I see where "mountain climbers" need 5% to 8% light transmission in glacier glasses/sun glasses, but would like a chart or comparision table for any given elevation. I just found out, that permissible light changes by 5% percenage points for every 1000'. I would like some eye protection good for an elevation of around 8000' or less, since my winter trips are at a lower elevation than that, but could go close to that if going to Lassen Volcanic National Park. I'm going to AK in a little bit and I guess we may visit a glacier while there and I want something finally to give my eyes decent protection that I can use in the future. Does anyone know, or have a chart? If I knew what the elevation was where a maximum of 5% light transmission starts at and could figure downward, I could figure this out.
Also, I need something to fit over my small glasses, someone mentioned Cocoons, but I can't find any info on their light transmission.
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