Abstract: The invention is an apparatus that measures dirt density on steel plates, characterized by including a pulsed laser that impinges on the inspected plate, and a microphone that detects the sound produced by the sudden ablation of the dirt, the amplitude of the microphone signal being the measure of the dirt on the plate. The microphone is connected to an electronic signal processing system, which is a computer or a programmed digital processor that determines the average value and standard deviation of the signal over the number of hits specified by the operator.