  This paper presents the results of an investigation on the spectral losses of
photovoltaic (PV) soiling. The transmittance of a glass coupon exposed to
natural soiling outdoors in Ja\'en, southern Spain, has been measured weekly
and used to estimate the soiling losses that various types of photovoltaic
materials would experience if installed in the same location. The results
suggest that measuring the hemispherical transmittance of the soiling
accumulated on a PV glass coupon can give enough information to quantify the
impact of soiling on energy production. Each PV technology is found to have a
preferred spectral region, or a specific single wavelength, for which the
transmittance through a PV glass coupon could be used for the best estimation
of soiling losses. Overall, considering the average spectral transmittance
between the extreme wavelengths of the material-specific absorption band, or
the transmittance of soiling at a single wavelength between 500 and 600 nm
yields the best estimations for different PV technologies. The results of this
work can lead to innovative approaches to detect soiling in the field and to
estimate the impact of spectral changes induced by soiling on PV energy
production.
