A method for automated, in-line measurement of a differentiated surface cleanliness, in terms of carbon, of a continuously-moving metal sheet or strip, having a level of surface carbon pollution lower than 100 mg/m2, includes: generating a radiation beam using a source; focusing the radiation beam using a focusing device such that an energy density deposited on the metal strip or sheet is sufficient to create a plasma and generate CN radicals in the plasma if carbon and nitrogen are present; creating a nitrogen atmosphere around the plasma using a sweeping system with a flow rate that prevents any presence of oxygen from air in the plasma; analyzing light emitted by the plasma using an optical collection device, and redirecting the light toward a spectrometer or device for separating wavelengths of the emitted light; measuring an intensity of an intense vibration line of the CN radical.