useful luminous flux, <of a directional light source>
part of the luminous flux of a light source that contributes predominantly to the lighting task
Note 1 to entry: For non-directional light sources the useful luminous flux is the total luminous flux of the light source.
Note 2 to entry: For directional light sources the useful luminous flux is the partial luminous flux in a defined cone angle, the axis of the cone being the optical beam axis of the light source.
Note 3 to entry: The cone angle,
Note 4 to entry: The optical beam axis of a light source is the axis about which the luminous intensity is substantially symmetrical.
Note 5 to entry: The useful luminous flux is expressed in lumen (lm).