  Reconstructions of past irradiance variations require suitable data on solar
activity. The longest direct proxy is the sunspot number, and it has been most
widely employed for this purpose. These data, however, only provide information
on the surface magnetic field emerging in sunspots, while a suitable proxy of
the evolution of the bright magnetic features, specifically faculae/plage and
network, is missing. This information can potentially be extracted from the
historical full-disc observations in the Ca II K line. We have analysed over
100,000 historical images from 8 digitised photographic archives of the
Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland,
and Wendelstein observatories, as well as one archive of modern observations
from the Rome/PSPT. The analysed data cover the period 1893--2018. We first
performed careful photometric calibration and compensation for the
centre-to-limb variation, and then segmented the images to identify plage
regions. This has been consistently applied to both historical and modern
observations. The plage series derived from different archives are generally in
good agreement with each other. However, there are also clear deviations that
most likely hint at intrinsic differences in the data and their digitisation.
We showed that accurate image processing significantly reduces errors in the
plage area estimates. Accurate photometric calibration also allows precise
plage identification on images from different archives without the need to
arbitrarily adjust the segmentation parameters. Finally, by comparing the plage
area series from the various records, we found the conversion laws between
them. This allowed us to produce a preliminary composite of the plage areas
obtained from all the datasets studied here. This is a first step towards an
accurate assessment of the long-term variation of plage regions.
