A method for monitoring a characteristic of a printed image of a thermal transfer printer. The method comprises providing a ribbon and a substrate at a printing location of the thermal transfer printer. The method further comprises printing an image on the substrate at the printing location by transferring ink from a region of the ribbon in a printing operation, a negative image being formed on the region of ribbon. The method further comprises transporting the region of ribbon, by a ribbon transport system, from the printing location towards an imaging location along a ribbon transport path. The method further comprises when a characteristic of the ribbon transport meets a predetermined criterion, obtaining, by an image capture system, a ribbon image of the negative image. The method further comprises processing said ribbon image to generate data indicative of the characteristic of the printed image.