A device to measure the characteristics of droplets within a stream of liquid droplets used in spraying includes a light source supplying light across the stream of droplets; a detector to detect change in the light level caused by a passing droplet, the detector generating a signal according to the change in light; and a processor to analyze the characteristics of the droplets in the stream based on the signal produced by the detection means. The light passes through a slit in a panel disposed between the droplet stream and the detector, so that the change in level of light detected by the detector is proportional to the diameter of the droplet.