This is why I love the Straight Dope. I just spent 20 minutes Googling around at sites that want to sell me the things, or tell me about things I already understand.
So. How do auto-darkening welding helmets work? I understand a good bit of physics and even own one of the little bastards in question. Anybody know?
WAG: LCD screen activated by a photoelectric sensor. Bright light causes LCD to darken. Or is that not what you were looking for?
There are photochromic filters, and electrochromic filters.
Many photochromic filters work via a light sensitive oxidation reduction reaction between silver and copper chlorides. Silver chloride is reduced to (dark) silver when it is hit with ultraviolet light. In the dark, copper reoxidizes the silver, and thus allows more light to pass.
Electrochromic filters work on the same principle, but switch oxidation (and light transmission) states depending on an electric current. Titanium<->Titanium oxide is one reaction that is used for this purpose. There are others, and it gets complicated fast.
From what I found on welding filters, it appears that yours are of the electrochromic variety. Do you have to hook them up to the welder to control the darkness, or do they come with a built in sensor?
Thanks, Squink. At the very least I now have the right terms on which to Google further.
My helmet has a sensor and darkens with the first birght arc light that hits it, FWIW.
Thanks, again.
Squink, my helmet, as well as all others I have seen, has its own sensor. An arc will trigger it, as will a spark from a striker and unfortunately so will the sun as it does not have adjustable sensitivity. It is electrically powered and has a photovoltaic cell to keep its own battery charged. Even the cheap one I have is the best welding accessory I ever bought.
I’m going to disagree with Squink, while admitting that I’m not an expert in this area, and that I could be wrong.
Photochromic material is what turns eyeglasses dark when you go outside. It’s a chemical process that takes way too long to be the method for the helmets.
Electrochromic materials change as the result of application of an electric charge, but based on my experience with the first application of that process that I personally came in contact with (the autodimming mirror in my new car) and some quick googling I’ve just done, I don’t think electrochromic can work fast enough either. For instance, this site says “…absorbance changes reaching more than 2 are acheivable [sic] in 0.1 to 0.5 seconds…” The helmets I’ve seen worked much faster than half a second, and I suspect that even a tenth of a second would be too slow to prevent eye damage.
Whereas I know for a fact that LCDs can cycle on and off more than 48 times a second. (They’re used in 3D glasses for IMAX theaters and other applications.)
So I stand by my assertion that they use LCDs.