This part of ISO 8503 specifies the optical microscope and describes the procedure for calibrating ISO surface profile comparators conforming to the requirements of ISO 8503-1. This part of ISO 8503 is also applicable to the determination of the surface profile, within the range hy = 20 µm to 200 µm, of essentially planar blast-cleaned steel. The determination can be carried out on a representative section of the blast-cleaned substrate or, if direct observation of the surface is not feasible, on a replica of the surface (see Annex E). NOTE Where appropriate, this procedure can be used for assessing the roughness profile of other abrasive blastcleaned substrates. An alternative procedure is described in ISO 8503-4.