You can treat the light beam as a geometrical object, as you have started to do in your explanation.
You could go with something of the like "Imagine that every point on the surface of the sheet is emitting a light beam with angle Q ... as the sheet rotates, the light emission rotates also."
You could then continue describing all the relevant properties of the light (its intensity, maybe?) and the "shape" that it forms around the sheet. If the sheet keeps rotating, you could compare your rotating sheet to the upper side of a toroidal solid. Also consider that if the light is emitted from any point of the surface, the angle Q won't matter much.
Keep in mind that anything you mention should either A) made the description clear or B) be relevant to the topic of your article.