Any directional lamp emits light energy in the shape of a cone.
Our eyes perceive this portion of the lamps cone as one intensity of light even though, at its edges, the intensity has dropped to half. This portion of the total cone of light is termed the Beam Angle. It is the Beam Angle we are concerned with when it comes to spacing one directional lamp to another. The Beam Angle is found in the lamp manufacturers' Lamp Data Chart. Candela See CandlePower.