The use of any instrument in making scientific measurements, be it a camera for collecting light, a scale for measuring mass, a straight edge for measuring length, or a clock for measuring the passage of time, requires that it be calibrated. Calibration is a process that tells you how to accurately relate the output of your instrument, like changes in electrical voltage or the changes in the length of a spring, to a physical entity like the number of photons of a particular energy or the amount of mass.
And so it is with the Cassini cameras. These complex and well-built imaging devices would be useless to us if we couldn't know how to translate the electrical signal coming out the back end into the absolute flux of electromagnetic radiation coming from the sources we image.
The calibration of the Cassini cameras has been a long and arduous process. It involves taking many images with the cameras of celestial sources whose absolute brightnesses at various wavelengths across the ultraviolet, visible and far-red regions of the electromagnetic spectrum have been determined over the years by other means. In the most reliable cases, those known sources have been the Saturnian moon Enceladus, observed from the Hubble Space Telescope (HST), and stars, like Vega, observed by HST and even Earth-based telescopes.