It hurts, and Jon's eyes are watering by the time the other man is back to normal, but his expression has shifted to something akin to awe. That... it's like the opposite of the Extinguished Sun in terms of the nature of the 'light.' Jon's momentarily blinded to anything apart from the person in front of him.

He completely ignores Aziraphale's question to ask, "What are you? You're... that was beautiful." The Archivist is just a little breathless. He's seen terrible things and that was a bit terrible, but awesome, as well. He doesn't know what to do with it.