useful luminous flux, <of a directional light source>

Φ_v,u,α ; Φ_u,α

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, α, which can be expressed either in degrees (°) or radians (rad), is given by the full conic angle and has to be specified, e.g. Φ_u,90° or Φ_u,π/2.

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).