| Category | Usage |
| 0 | Not really a category - transmits 100% of visible light. Protection against grit and ice only. |
| 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 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 - however, just because the lenses are suitable does not mean that the design of the glasses is.