Is stacking welder's glasses a safe way to watch at the eclipse?
You can find in many place on the Internet that welder's glass #14 is good for looking at an eclipse. Tomorrow (March, 20th 2015 at 10:45 CET) there's a solar eclipse and yesterday I could only find glasses #11 and #9. I tried briefly this morning, and combining a #11 and a #9 was giving me a good view of the sun. Using 2 #11 gave a too dark image. Now, I read different opinions on the internet.
Against (Perkins Observatory):
Be careful that you use the right kind of glass! Welder's glass is
numbered from 1 to 14 with 14 being the darkest. It is only number 14
glass that is dark enough for solar viewing! And NO STACKING! A pair
of number 7's or a 10 and a 4 together DO NOT have the same protection
as a single piece of number 14 (see unsafe methods for more details).
Favorable (Royal Astronomical Society of Canada):
If SN14 filter is not available, it is possible to combine lower shade
numbers to get roughly the same level of eye protection from solar
radiation, e.g. combining SN 6 and SN 8 filters. However the image
quality may be considerably poorer than that seen through the single
I could not find a table or something explaining which kind of protection gives each number; according to the Canadian website, the only concern is about how much infrared light goes through, ultraviolet does not seem to be a problem in almost any case (I was surprised to read that).
Note: It's not my intention to open here the discussion on what could be other safe methods to watch the eclipse, this is well explained everywhere around. I read too late about the eclipse to order specific glasses.
13 or darker is safe enough. Also, you CAN add up welding glass, using
the formula S(sum) = S1 + S2 -1. S(sum) should be greater than or
equal to 13
Stacking is fine. Just don't expect the numbers to be simply additive, that's all. E.g., indeed a 10 and a 4 together are not the same as a single 14. What you really need is bring the brightness down to the level of a TV screen showing a white image, that's all. Don't stack too many glasses - if it doesn't work with two of them, don't stack more than that.
You have summarized pretty much everything available on the subject. There are loads of people saying you cannot, but fortunately for us, the Royal Astronomical Society of Canada, that you quote, includes evidence to back up their claims. They are a respected institution and the author of this article appears immensely qualified. The article itself does not include evidence that welding goggles can be stacked, but the inclusion of such for all the major points adds credibility to the entire article.
It does appear like the glasses are not exactly additive, this Reddit post goes through some calculations, but they are pretty close such that aiming for 1 tint higher will more than offset it and if anything is overkill.
I know it's a bit late for the UK eclipse, but for future reference:
Welders goggles do not cut out the same frequency of light that Mylar/Black Polymer eclipse glases do.
Arc welding glass #14 is much darker than brazing welders glass but there is no guarantee that they will block the Sun's ultra-violet and infrared rays.
The problem is there are no pain sensors in your retina so you don't feel your eyes cooking and the dark lenses (as do sunglasses) cause your pupils to dialate letting in more ultra-violet and infrared rays so they could make things worse.
I have used arc Welders glass on a camera and it worked fine but gave a picture of a green Sun. Project the image on paper to take it.
Do NOT look through a camera viewfinder even with the correct eclipse glasses on if you do not have the correct solar filer on the LENS end.
I don't think what you say about the #14 welder glass is correct: it's really said everywhere that they are safe, reporting also frequency analysis of the filter (check the Canadian website linked in my question)
The welding arc generates much more harsh radiation compared to the Sun, simply on account of its much higher temperature. The only thing to keep in mind is that the filter must be dark enough. Welding glass #14 should be very safe for viewing the Sun.
Yes #14 glass is considered safe on many sites because it filters similar wavelengths. The only completely safe viewing device is an approved CE marked (in Europe) viewing device. I am erring on the side of caution as some people seeing the answers here may not know if the glass is truly #14 or not. If you are a professional welder then you will probably know but a DIY welder may not. I am going by health and safety advice in the UK. The question was on stacking welder’s glasses and the safest advice would be to get the correct equipment, eclipse glasses are not expensive. Getting it wrong is.