To understand the influence of the Sun on Earth's system, long and
accurate measurements of solar irradiance are a prerequisite. The
available direct measurements of solar irradiance since 1978 are clearly
not sufficient for this purpose. This stimulated development of models
used to reconstruct past solar irradiance variations from alternative
observations. The main driver of the irradiance variations on time
scales of days to millennia is the evolution of the solar surface
magnetic field in form of dark sunspots and bright faculae and network.
Therefore, models require input data describing the contributions of
these various magnetic regions on the Sun at earlier times.
Unfortunately, records that can be used to describe the facular and
network contributions are barely longer than the direct irradiance
measurements. Thus, irradiance reconstructions to earlier periods have
to rely on sunspot data alone. Data that have hardly been used for solar
irradiance reconstructions until now are full-disc solar observations in
the Ca II K line. Such data exist since 1892 from various observatories
and include all the needed information describing faculae and the
network. However, they are plagued by a bunch of various problems and
artefacts, and recovering the non-linear response of the photographic
material to the radiation is non-trivial since the required information
is usually missing, too. We have developed a method to process Ca II K
observations from various sources and demonstrated the higher accuracy
achieved by our method compared to other techniques presented in the
literature. Here we use the carefully reduced Ca II K observations from
multiple archives to reconstruct solar irradiance variations.