Maintaining optical density of images produced by a printing device

A method for maintaining the optical density of a printing device includes, with a printhead module of the printing device, printing a color region onto a first test page, the color region including a plurality of sub-regions, each of the sub-regions being printed with a different drive voltage; measuring an optical density of each of the sub-regions printed onto the first test page; determining a response curve based on a relationship between the optical density of the sub-regions and the different drive voltages used to print the sub-regions; and, using the response curve, determining a target drive voltage for the printhead module which will cause the printhead module to print at a target optical density.