We previously developed a measurement system of spark length in air discharges of electrostatic generator (ESD gun) using laser Doppler effects. The spark length can be obtained through time integration of the velocity data from spark occurrence time to collision time when moving the grounded plate to the ESD gun, while the measurement accuracy has not yet been validated. In this study, we compared the measured spark lengths using two methods that identify the collision time by detecting field emission and by detecting discharge of gun remaining charges, respectively.