1. A printing device, comprising: a feed mechanism operable to convey an article; 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, wherein each of the groups of data points relates to scatter caused by surface structure from the respective different regions of the surface of the article; a processor operable to determine a digital signature of the article from the set of groups of data points and to determine a printable label pattern that encodes the digital signature according to a machine-readable encoding protocol; and a print head operable to print the label pattern onto the article.