Photography

In passing, this might read as a suggestion to go buy one of these filters. I would actually suggest thinking long and hard before doing that. Really, their only use is photoing the sun on a clear day, and so:

• Eclipses - a very slightly jagged dark circle out of a bright circle
• Sunspots - black blotches on a bright circle
• Conceivably solar flares, but I've never even tried for them (and probably even at their strongest, at the best angle, less than 5% of the diameter of the sun - I'm worried by my google results on that!)

On the flip side, these things are expensive (needing to be optical quality)and likely limited to one diameter of lens.

There is something deeply satisfying about making your own solar observations, but you may feel replete after very few photos!