I'm pretty new to level 20 myself but I've found that all of the rare items I've found/received after getting the message about light have had it. The best way I've found to get rare items is to do Crucible matches or farm one of the spots were two enemy types fight each other. The crucible doesn't always give you anything but most of what I've had has been rare items or engrams. I even got a legendary engram earlier, unfortunately it didn't contain a legendary item.