We are dealing with super-enhanced Kryptonian physiology here. His reception powers of vision already amplify things to incredible levels. It stands to reason that he could also amplify his output beyond what he passively sees.
Hey, on another level, he's already amplifying his energy levels and output far beyond what he takes in from solar power.
It all holds together consistently -- just like real science -- so that makes it that much more believable.
Note that this doesn't invoke anything really out of canon, only exploiting what's published combined with real world science and extrapolated from that. (Heat vision was an extrapolation from the original passive X-ray vision power.)
[BTW, I divide my time posting here and on this site
http://s2.excoboard.com/exco/index.php?boardid=6337. If you want comics discussions beyond just Superman related stuff, check that place out. Either way, I'm always posting on Great Rao's site here.]