Ground beam, made from light.
Sky beam, made from steel
Ground beam. This beam was projected from a crack in the door of the shed. Being that the door is so high, the beam was this long. It is as thick on the ground as light is and only fog and daylight can erase it. When it is erased, there is no waste.
Sky beam. This beam is part of a steel sculpture. Unlike the ground beam, the sky beam is not erased by daylight. The ground beam requires our presence in the shed whereas the sky beam projects in our absence.
The word beam has been applied to steel and to wood and to light. The only requirement of a beam is to travel in (any) one direction, like a crow.