A method and apparatus for identifying surface relief features of a substrate processed in a printing machine, and adjusting the machine parameters as a function thereof, before the substrate is processed through the machine. The apparatus includes a light source positioned adjacent to the substrate surface. A light sensing device, in a light receiving relationship with the substrate surface, detects light reflected from the substrate surface. A signal is generated indicative of sensed light intensity with a first magnitude range indicating ridges and a second magnitude range indicating depressions. Control circuitry, electrically connected to the light sensing device, receives the signals of ridges and depressions, and discriminates between them to generate a set of signals indicating surface relief features of the substrate.