Abstract: A fast method of determining the position of an electron beam in a shadow mask display tube and has particular application to measuring the convergence of electron beams in color display tubes. A camera is positioned against a color display tube faceplate, the objective lens of the camera covering 10 to 12 triplets of phosphor elements on the screen. The electron beams are modulated with a video signal having a spatial frequency which is slightly different from either the spatial frequency of the color selection electrode or, preferably, half the spatial frequency of the color selection electrode, and the luminance values of the microspots of each color are sampled at the phosphor pitch. Measuring the geometrical positions of the respective beat waveforms of the three colors provides an indication of the misconvergence of the electron beams at the screen.