Generally, the light source cannot be infinite, and the light emitted by the light source or the lamp is divergent (except for the parallel light-emitting spotlight), and the light emitted will not cover the entire space. Even the spherical incandescent lamp has a blind spot in the lamp holder (the filament emits light in all directions, but the bulb has the lamp holder to block light). Therefore, judging from the luminescence of the light source or lamp, there is a range where the light shines. Seen from the tangent plane passing the optical axis, the angle formed by the boundary line of the light range is the actual beam angle. From a physical point of view, it is the angle formed by the boundary between light and no light at all.
