one atom shows only the 589 D-line. crowd them and two emerge: trapping (a 589 photon reabsorbed before it escapes) and pooling — Na(3p)+Na(3p) → Na(3s)+Na(3d) — which lights the 819 channel silicon wants. the 819/589 escape ratio below is measured, not asserted.
3p (excited)
3d
3s ground
589 photon
819 photon