Initially, Luminosity is pretty much light. It's intangible, at least in the beginning. That's why the preferred defense for most Luminous Knights is to either avoid the spell completely, destroy it before it reaches it's target, or block it with a physical object which is sheathed/cloaked with Luminosity. Using Luminosity against a non-mage is pretty much the same as shining a flashlight on someone. A very bright flashlight. 

But as I've mentioned many times, this is only initially. Just like warm sunlight can be focused into something deadly and destructive, so can Luminosity. And there's quite a lot more advanced applications, I just believe the characters discovering them by themselves is a bit more exciting.