Townes raises the question of where in the electromagnetic spectrum should the search for extraterrestrial intelligence be conducted? While SETI has historically been radio-centric (especially at the time this paper was published), the author suggests that certain assumptions are required to arrive at the conclusion of the radio as the optimal region to search. If these assumptions are appropriately relaxed, one could argue that the infrared is an equally viable part of the spectrum to search for signals of extraterrestrial origin.
One aspect that must be considered in choosing the optimal spectral region is the power requirements of the transmission. This depends on whether the signal is isotropic or beamed, which, of course, we can only guess. A second point that must be considered is the technology of the transmitter and, more broadly, the technology of the transmitting civilization as a whole. In particular, it is conceivable that other forms of communication technology could be dominate on a distant world, e.g., lasers. This relates directly to the suggestion by Schwartz & Townes (1961) to search for nanosecond light pulses in the optical.
Ultimately, the authors recognize that the optimal SETI search would be include efforts across the electromagnetic spectrum. After all, arguing for one particular spectral region over another involves making a set of assumptions that could turn out to be false.