The instrument consists of a light source and a stereo camera. Particles pass through the measurement volume defined by the viewing angles of the cameras as well as minimum and maximum distance from the cameras.
All particles cause extinction of the light seen by the cameras. Particle sizes are deduced from the area seen by the cameras and their position within the measurement volume. Particle speeds are deduced from the movement of the particle during a predefined time. Furthermore, the characteristics of the particle image allow the system to distinguish between rain, snow, hail, graupel, seeds, and insects.
The calculated data are memorized over one minute, and then transmitted via serial interface, Ethernet (TCP/IP connection, up to 5 clients at a time) or store on the SD card as a file.
The type of precipitation is determined from the statistic proportion of all articles referring to diameter and velocity. These proportions have been tested scientifically (e.g. Gunn, R., and Kinzer, G.D., 1949, ”The terminal velocity of fall for water droplets in stagnant air.” J. of Meteorology, Vol. 6, pp. 243-248). In addition, the temperature is included in order to improve the identification.
The instrument is almost maintenance-free. Only the glasses of camera module head and LED pane should be cleaned, if necessary. For application in areas of extreme weather conditions (for example high mountains), we recommend a model with “extended heating”. By using a flash-memory the internal software can be updated any time via Ethernet connection.