The rectangular filters in welder's helmets that protect their eyes from the intense light of welding are sold in different degrees of “darkness,” to suit different jobs.
In the United States, welder's glasses are assigned shade numbers, which can be calculated from
where T is transmission, the fraction of visible light transmitted.
Welding produces hot metal, and hence a lot of infrared radiation, and arc welding produces large amounts of ultraviolet light. For that reason, the absorption of welding glasses isn't limited to visible light, but is carefully extended into the infrared (unlike sunglasses) and ultraviolet.
Amateur astronomers typically use a shade 14 welding glass to view the sun with the naked eye. When a solar eclipse is imminent, that shade sells out.
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Last revised: 1 February 2010.