Luminators are fairly good at producing light, but not so good at making mobs catch fire. The mob more or less has to be in the /exact/ same cube as the light in order to catch fire.
It would be much nicer if the 'fire' area of effect were a 3x3 box with the luminator in the center; any obstructions of course blocking the box by definition. Among other improvements, this would allow for different hallway configurations than luminator on the floor before a door.