The fluorescence process illustrated by plotting emerging versus emitted wavelengths (λ_out and λ_in, respectively) for all escaping photons that were absorbed by a line at least once. The intensities are normalized in each λ_out-bin (Δλ/λ = 0.03), so that the dominant λ_in for each λ_out can be seen as the yellow/red regions. Contours are at 0.1, 1, 10, 50 and 90%. Very little fluorescence occurs for λ_in > 7000   Å, which is therefore excluded from the plot.