A new system has been developed for determining the writing speed of sweeping image cameras. In this system the time intervals between light pulses from the image of a small objective slit as seen by a photomultiplier tube through a pair of slits at the object plane are measured by a time interval meter. Several features of this system allow the writing speed of an individual measurement to be determined to better than a tenth of a percent so that very precise corrections can be determined for nonideal behavior of the camera’s optical system.