In reply to stewieatb:
http://www.needlesports.com/Catalogue/Accessories/Glacier-Glasses-Goggles
1 43% - 80% visible light transmitted. Suitable for use in cloudy conditions with virtually no direct sunlight eg blizzards and whiteouts.
2 18% - 43% visible light transmitted. Suitable for use as general sunglasses and in cloudy conditions in the mountains, but not direct sunlight in mountain areas where there will be a lot of reflection off snow and ice.
3 8% - 18% visible light transmitted. Suitable for Alpine and Himalayan mountaineering for most uses, but for prolonged exposure at high altitude Category 4 is recommended.
4 3% - 8% visible light transmitted. For prolonged exposure at high altitude. Will be found too dark for everyday use and should not be used whilst driving a car.