UIBs and hydrogen recombination lines in the 2–12 μm wavelength range. Ticks on each horizontal line segment mark transitions T_p → n (calculated from Rydberg’s formula) from level p > n to level n for each n = 4, 5, ... 11. The bottom line gathers all hydrogen transitions between 2 and 12 μm. The ticks’ thickness is 0.01 μm. Moving from red to blue on each “n”-line, there is a quick convergence to a quasi-continuum of transitions from high Rydberg states, bordered on its blue side by T_{∞ → n}. Shaded areas indicating the extent of UIBs are justified in the text. UIBs’ conventionally agreed upon central wavelengths are represented by vertical dotted lines.