Thinking about the different shades of welding glass for solar eclipse viewing

[wevets]

It’s been a little while since the recent North American solar eclipse, but I did have a question about the welding glasses often used for viewing it:

@https://eclipse2017.nasa.gov/safety

Viewing with Protection – Experts suggests that one widely available filter for safe solar viewing is welders glass of sufficiently high number. The only ones that are safe for direct viewing of the Sun with your eyes are those of Shade 12 or higher. These are much darker than the filters used for most kinds of welding. If you have an old welder’s helmet around the house and are thinking of using it to view the Sun, make sure you know the filter’s shade number. If it’s less than 12 (and it probably is), don’t even think about using it to look at the Sun. Many people find the Sun too bright even in a Shade 12 filter, and some find the Sun too dim in a Shade 14 filter — but Shade 13 filters are uncommon and can be hard to find. The AAS Reputable Vendors of Solar Filters & Viewers page (link is external) doesn’t list any suppliers of welder’s filters, only suppliers of special-purpose filters made for viewing the Sun.To find out more about eyewear and handheld viewers go to Eyewear & Handheld Viewers | Solar Eclipse Across America (link is external).

Does anyone know why the cutoff at Shade 12? Is there a difference between Shade 11 and Shade 12 that should be kept in mind or is it just a case of ‘the cutoff has to be somewhere’?

[beowulff]

It’s simple - shade 11 isn’t dark enough.
I’m not going to dig through the web to find the recommended maximum illumination at the retina, but looking at the sun through a shade 11 filter exceeds it.

[watchwolf49]

I used #11 glass last August … I can see why everyone recommends #12 or better … the #11 was slightly uncomfortable over prolonged time periods, say minutes of viewing the sun … I’m not going to bother upgrading to darker glass, the partial phases just aren’t interesting enough to me …

Totality is viewed without eye protection and that’s what’s important …

[wevets]

@watchwolf49

I used #11 glass last August … I can see why everyone recommends #12 or better … the #11 was slightly uncomfortable over prolonged time periods, say minutes of viewing the sun … I’m not going to bother upgrading to darker glass, the partial phases just aren’t interesting enough to me …

Totality is viewed without eye protection and that’s what’s important …

How long was the prolonged time period? Any afterimages when you were done looking?

[watchwolf49]

@wevets

How long was the prolonged time period? Any afterimages when you were done looking?

Say two or three minutes until it became uncomfortable, so I looked away after only a half minute or so … no afterimages at these time spans …

If you want to sit and stare at the full hour-and-a-half partial phase, use #13 glass …