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 onto an article conveyed by the feed mechanism and a detector arrangement arranged to collect a set of data points from signals obtained as the light scans over the article, wherein different ones of the data points relate to scatter from different parts of the article.