1. A printing device, comprising: a print head for printing onto an article; a feed mechanism operable to convey the article past the print head; and a scan head incorporating a coherent source and a detector arrangement, wherein the coherent source is arranged to direct light sequentially onto each of a plurality of regions of a surface of an article conveyed by the feed mechanism, wherein the detector arrangement includes a plurality of photodetector elements in which each photodetector element is arranged at a different respective angle relative to the surface of the article, such that the detector arrangement is configured to collect a set comprising groups of data points from signals obtained as the light scans over the article in which each of the groups of data points is collected by a respective photodetector element in the detector arrangement and relates to scatter caused by the intrinsic surface structure from the different regions of the surface of the article.