The mechanisms of solar influence on Earth's climate are not yet entirely understood. Direct measurements of the solar irradiance exist only since 1978, which is too short for Sun-climate studies. Models assuming the irradiance changes to be due to the changes in the spatial coverage by the solar surface magnetic features have been successful in reproducing the measured irradiance variations. However, suitable high-resolution magnetograms required for this modelling are only available for about four decades, and the sunspot number usually employed to reconstruct solar irradiance on centennial time scales does not accurately describe the evolution of the bright magnetic features (faculae and the network). A potential source of this information over the last century is provided by the historical Ca II K spectroheliograms. Their employment is not straightforward, however, as they suffer from numerous problems and lack photometric calibration. We have developed a technique to process and photometrically calibrate these images and will present preliminary results from selected historical datasets.