Nuclear bombs are harmful in proportion to their distance from you when they explode. I believe that, at short distances, the radiation (visible or not) from the bomb is so powerful and destructive that it burns things up right away before they're even hit by the shockwave. Is there a word for this destructive light?
Nuclear weapons emit large amounts of thermal radiation as visible, infrared, and ultraviolet light, to which the atmosphere is largely transparent. This is known as "Flash". –Wikipedia
It depends on what exactly you're talking about. The initial explosion creates a nuclear flash. It's this flash that blinds people miles away, who are far enough to survive the blast itself. In fact, blindness resulting from a nuclear explosion is called flash blindness. The most dangerous emission from within a nuclear flash, though, is called gamma rays. It's light in this spectrum that is most lethal to all life. Long after all of the searing heat of the nuclear flash has dissipated, gamma rays are still doing immense damage.
This is a physics question. The atmosphere is opaque to radiation of charged particles and heavy particles. 'Luminance' or 'illumination' often implies just photons, and 'flash' implies visible photons.
Honestly it's just sunlight. ;) Try to remember the next time you enjoy the sun that you are voluntarily basking in the light of an ever ongoing nuclear explosion, the scale of which we can barely comprehend. Trust me on this.