The goal is to get all the heat to the ground.  Superior Radiant tube heaters utilize 10-sided deep dish optical reflectors to reflect all, not just some, of the infrared heat energy to the "people zone" at ground level.  Infrared (IR) energy travels in straight lines.  The above picture represents how IR energy bounces away from the hot 4" diameter tube, with 0% "bounceback", then downward to the floor below.  Compare to other tube heater brands where up to 30% of infra-red heat energy impinges on the tube emitter, resulting in higher top clearances and wasted heat at the ceiling level.