The Matlab code used for data analysis, raw data, simulations, graphs, photos and videos of the experiments is available from the following website: Raindrop measurement with event camera.This supplementary material has the following sections:• S.1 provides details on the Dynamic Vision Sensor (DVS) event camera.• S.2 describes our Hard Disk Droplet Generator (HDDG) and Intravenous Dripper Droplet Generator (IVDG).-S.2.1 details our droplet needle.-S.2.2 details construction of the HDDG.-S.2.3 provides additional useful methodology for testing the stability of the HDDG -S.2.4 describes unsuccessful attempts to develop hydrophobic coatings to reduce the IVDG droplet size.• S.3 details our experimental setups.• S.4 describes our experiments and the methods we used to measure droplets and estimate uncertainty, in particular -S.4.2 details our error analysis.-S.4.3 explains how we optimized our droplet illumination.-Secs. S.4.7, S.4.4, and S.4.5 explain our calibration of the optical magnification of the camera M , angle ↵, and droplet fall height.-S.4.6 explains how we measured the droplet diameter and speed in the image plane -S.4.10 provides formulas to compute the physical diameter and speed from the image plane measurements.-S.4.11 describes how we measure droplet flow rate for ground-truth droplet size estimates.-S.4.12 further details our ground truth (GT) measurement of droplet mass. • S.5 explains our model of the droplet speed versus fall height and droplet diameter, which we used to obtain our GT speed from mass and to ensure that droplets fell at close to terminal speed.

Abstract. Hydrometers that can measure size and velocity distributions of precipitation are needed for research and corrections of rainfall estimates from weather radars and microwave links. Existing video disdrometers measure drop size distributions, but underestimate small raindrops and are impractical for widespread always-on IoT deployment. We propose an innovative method of measuring droplet size and velocity using a neuromorphic event camera. These dynamic vision sensors asynchronously output a sparse stream of pixel brightness changes. Droplets falling through the plane of focus create events generated by the motion of the droplet. Droplet size and speed are inferred from the stream of events. Using an improved hard disk arm actuator to reliably generate artificial raindrops, our experiments show small errors of 7 % (maximum mean absolute percentage error) for droplet sizes from 0.3 to 2.5 mm and speeds from 1.3 m/s to 8.0 m/s. Each droplet requires the processing of only a few hundred to thousands of events, potentially enabling low-power always-on disdrometers that consume power proportional to the rainfall rate.