AbstractThe objective of this study is to develop a sensor which is able to monitor spray processes by measuring a mean droplet size as well as the droplet concentration. To minimize the manipulation of the spray by the sensor, an optical measurement principle which is called statistical extinction method is used. The sensor can be constructed lanceolate because only one laser beam is necessary to illuminate the spray, and the intensity of the laser beam has to be measured at only one place behind the measurement volume. Consequently, the sensor can be inserted in the spray tower through only one notch with a diameter of about 50 mm. The laser beam can be shielded using adaptable sheaths, which can spatially limit the measuring volume. In addition, the optical components of the sensor can be protected against fouling by a rinse volume flow through the sheaths in direction of the measuring volume.To prove suitability of this measuring principle for the monitoring of spray processes, droplet sizes of different sprays are measured by a prototype of the sensor. The measuring principle, the construction of the sensor, and measurement results are presented and discussed in this paper.