The spot-size measurement method introduced by Rishton et al. has been successfully adapted for use in any Philips Beamwriter. As the spot is scanned across an etched silicon edge mounted on a substrate holder, the current passing the edge is recorded by a waveform analyzer which then computes the spot size. Apart from initial calibration, the measurements are independent of the Beamwriter. Computer simulations show the influence on the final result of response speed, noise, and stage vibration. Good qualitative agreement between the simulations and measurements allows rules for obtaining accurate results to be formulated. Consistent measurements of spots from 21 to 500 nm have been obtained from several Beamwriters at a number of facilities. It is also shown that the Beamwriter’s internal measurements are accurate when allowance is made for the width of the edge of the marker used.