  Studies and reconstructions of past solar activity require data on sunspots
as well as faculae/plage and network. Such data are also important for
understanding the magnetic activity and variability of the Sun and Sun-like
stars. The longest available direct faculae/plage datasets are white-light
facular and Ca II K observations going back to 1874 and 1892, respectively.
Prior to that time, the only direct data available are for sunspots. We
reassess the relationship between plage areas and sunspot records (areas and
numbers) since 1892, to allow reconstructions of facular/plage areas which can
be employed for studies going further back in time, i.e. over the period when
solely sunspot observations are available. We use the plage areas derived from
38 consistently processed Ca II K archives as well as the plage area composite
based on these archives. We find the relationship between plage and sunspot
areas and numbers to be well represented by a power law function. We further
find that the relationships depend on the bandwidth and the solar cycle
strength. The reconstructions with a power law relationship are in good
agreement with the original plage area series, whereas employment of a
cycle-strength-dependent relationship improves the reconstructions only
marginally. Performing the same analysis on other previously published plage
area series, usually derived from a single archive with diverse processing
techniques, returns different results when using different time series. This
highlights the importance of applying a consistent processing to the various
archives and demonstrates the uncertainties introduced by using previously
published series. Our results have implications for past solar activity and
irradiance reconstructions as well as for stellar activity studies, which
sometimes assume a linear dependence between plage and sunspot areas.
