In this paper we present a new sensor that can measure on-line the material wear in laser material processing. To meet the demands of industrial production optical rough surfaces have to be measured at low apertures within the rough environment generated by the production process. Hence, common sensors have a quite poor performance. It is hardly possible to measure the material wear through the plasma occurring in the working zone at temperatures above 3000 K. We solve this problem by utilise just this plasma spot as a signal from which we evaluate the distance between sensor and work piece. The specific features of the sensor are: it is insensitive against the spot size and shape, and, hence accurate, even with low aperture and within the turbulent atmosphere. The time for one measurement is one millisecond.