Glacier Glasses & Goggles

Whilst Sunglasses are not a necessity for most people, Glacier Glasses and/or Goggles are essential for high altitude mountaineering. Without them you would soon become snowblind (a very painful condition) due to the reflection off the snow and ice. Good quality dark glasses are categorised according to their suitability for mountain use. The best glacier glasses and goggles (if you can afford them) are Photochromic which adust to the light conditions, generally over 3 Categories. Fixed category glasses and goggles are cheaper but not as convenient.
 

Category        Usage
0 Not really a category - transmits 100% of visible light. Protection against grit and ice only. 
1 43% - 80% visible light transmited.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 in 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 and especially if you have weak eyes. Will be found too dark for everyday use and should not be used for driving a car.

*Categories 3 and 4 are the only categories suitable for Alpine and Greater Ranges mountaineering.

The Julbo Lens Chart gives further information.