The paper contributes new information about the Horologion of Andronikos Kyrristos of the Roman Agora of Athens, Greece and its continuous presence in Athens from antiquity until now. The monument is of cultural, historical and astronomical importance. It has undergone conservation work and non-destructive documentation which contribute to its sustainability. The spectral imaging technique, which allows the acquisition of high-resolution images, was applied on the stone surfaces of the monument. The acquisition of images in the visible and infrared regions, combined with the digital tracing reproduction of the graffiti and image processing, is used for the first time in this particularly interesting case study of the aforementioned monument, of which a complete description is provided. New evidence with a clearer recording of the vessels already mentioned in the literature, as well as the detection of new historical data and graffiti, is discussed in comparison with the results of previous studies.