Abstract: A method of registering one image (6) of a reticle plate (5) formed by a projection lens (21) of an optical system on a coated substrate (1) with features created previously on the substrate by an earlier image projection, comprises forming a reference pattern (7) on the substrate and monitoring the changes in position, caused by relative movement of the reticle plate (5) past the substrate (1) in a scan (or x-) direction, of the reference pattern relative to the optical system prior to forming the said one image on the coated substrate. The method is characterised in that the reference pattern (7) is interrogated by optical means (8, 9, 10, 11) operating through the projection lens (21) as the reticle plate (5) approaches a position in which an image (6) thereof will be in register with said features created previously to permit creation of said image correctly on the coated substrate without stopping the movement in the scan direction.