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 blast-cleaned substrates. An alternative procedure is described in ISO 8503-4.