Not that this isn't nice and all, but it completely overshadows the existing capstone spells.
In fact, comparing it to Silver Flame, it's even a direct upgrade!
Searing Light (Maximum damage 5d4+20 (Vs Undead: Maximum damage 10d3+30))
Sun Bolt (Maximum damage 15d4+60 (or 7.5d4 + 30 on a succesful save for half- so even on a save, it's better [except against undead]. Note that it also hits multiple targets.))
All I can see to balance it is a lengthy cooldown, and those are awful for gameplay, with spells you intend to utilize often.
Heck, it's bad enough half the existing capstone abilities- and most of the Archmage SLAs- aren't exactly thrilling.