One aspect of this invention relates to an illumination device for an optical printer, the illumination device comprising: a first array of light sources (20) comprising a plurality of light source elements (14) that are each operable to emit light within a red band of the visible electromagnetic spectrum; a second array of light sources (21) comprising a plurality of light source elements (14) that are each operable to emit light within a blue band of the visible electromagnetic spectrum; and a third array of light sources (22) comprising a plurality of light source elements (14) that are each operable to emit light within a green band of the visible electromagnetic spectrum; wherein said first, second and third arrays of light sources (20, 21, 22) are generally linearly arranged along respective substantially parallel notional lines (34) on a substrate (15); and said plurality of light source elements (14) of each said array (20, 21, 22) are arranged such that respective adjacent light source elements of said plurality are offset from one another with respect to the associated notional line (34). Other aspects relate to printing heads, printing head assemblies, optical printers, and methods and apparatus for calibrating an optical printer.