In certain circumstances, the wavelengths corresponding to the Fraunhofer absorption
lines in the stellar continuum may be preferred for communications because they may
slightly increase the obtainable signal-to-noise ratio.
Table 9006-025 and the summary charts of 9008-073 & 9008-074 indicate some
of the more significant Fraunhofer lines. It will be recalled that for analysis purposes,
the H line at 656 nm was chosen for the modelled visible laser frequency. The typical
absorption profile of this line from a quiet Sun is such that the minimum intensity in the
dark line is about 20% of the intensity of the general continuum. The so-called dark line
is really not that dark, and in the case of the H line, amounts to a contrast ratio of
only 5:1. Thus if a ETI transmitter operates at this wavelength, the Signal-To-Planck
Ratio (SPR) within the dark line is increased by about 7 dB.
Except in the case of incoherent communication links, and coherent communication links
operating within the Solar System where the Planckian noise is very intense, this SPR
improvement has no effect since the noise-floor is set by signal quantum noise.