Sketch representing the reduction of a bundle of streamlines to a single line with an effective emission angle. Left: bundle of lines with narrow emission angles (cones) that all have a tiny portion (thickness of the tick at the base of the cone) simultaneously visible by the observer (light goes along dotted lines). Right: this particular bundle was reduced to a single line with a wider emission angle such that the length of line along the vertical (altitude) axis visible by the observer is the same as the cumulated portions of lines of the bundle on the left.