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Message: <blockquote data-quote="guywhofishes" data-source="post: 181940" data-attributes="member: 337"><p>you can't imagine how little light shade number 14 filter glass (the recommended sun viewing shade number) lets into your eye: </p><p></p><p><strong>What do the numbers in welder's glass specifications mean (e.g., #14)?</strong></p><p></p><p><span style="color: #000000"><span style="font-family: 'Arial'">The shade number (SN for short) is based on the optical density of the glass in the visible region. If the fractional transmission of light through the glass is T, then</span></span></p><p> OD = -log T</p><p> SN = 1 + (7/3) OD</p><p></p><p><span style="color: #000000"><span style="font-family: 'Arial'">Consider, for example, a filter that blocks 99.9% of the visible. T = 0.001 and hence OD = 3, and SN = 1 + (7/3)*3 = 8. This would thus be typical for a #8 welder's glass.</span></span></p><p><span style="color: #000000"><span style="font-family: 'Arial'">If you plug SN=14 into the formula above, you can show that the OD is 39/7, or 5.57... A #14 welder's glass transmits about 3 millionths of the light hitting it.</span></span></p><p><span style="color: #000000"><span style="font-family: 'Arial'">Note that this formula <em>only</em> applies to transmission in the visible; for IR and UV the specifications are a little more complicated, and not given by a nice neat formula like this. In the US the requirements for welders' glasses are given in the ANSI publication listed in the references. In general the tolerances for UV and IR are not quite as strict as those in the visible, which is part of the reason astronomers recommend the darkest shades of welders' glass for solar viewing.</span></span></p></blockquote><p></p>

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