Abstract: This invention relates to a control device, suitable for use to control the exposure of a light-sensitive material in a copying apparatus for the preparation of a printing plate, which comprises program store means for predetermining a basic exposure, input line means for connection to a photocell for measuring the luminous intensity of a light source in the copying apparatus, comparator means having its input connected to an output of the program store means and connected by a transmission path means to the input line means, the output of the comparator means being connectable to a control switch for the light source, and the transmission characteristics of the path being variable in steps, adjacent pairs of which have transmissivities in the ratio .sqroot.2:1.