I use to used welding glass. I heard that the eyes can heal from the visible damage, but the UV was the danger. So UV protection is a must. But that's just what I hear.
It's my understanding that retina damage is often permanent.
The UV is the worst but even IR can damage the retina. The ISO for IR is a minimum of 97% blockage, thus far more tolerant. For very short periods, I would assume this isn't a big issue. I've looked at a setting Sun on a dusty day where it appeared orange due to extra atmospheric scattering. It was easy on the eyes. But, for enjoyment and better protection when the Sun is dim, I will sometimes use binoculars. [
Warning... don't use binoculars on the Sun when it's remotely bright, or anything bright for that matter without proper filters. ]
Why binoculars? Even when a setting Sun, on rare occasions, is dim enough to be comfortable looking at it, people forget or are unaware that most of the IR is
not blocked by our atmosphere. NASA has a warning about this, though they don't mention using binoculars. *cough* Glass, as in binoculars, reduce the IR problem. Also, binoculars and telescopes never, due to optical laws, increase the surface brightness of an object. The total brightness increases, which is spread out more over your retina (magnification). Of course, the change in the focal plane when adjusting the binoculars would be an issue, but unlikely a problem as long as the Sun is easy on the eyes, meaning a brightness less than a full moon.
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