A color cathode ray tube with a shadow mask has a screen partitioned into a plurality of elemental screen areas and electron guns positioned and corresponding to the respective elemental screen areas, the electron guns generating electron beams to scan the screen. The shadow mask includes effective regions having a number of apertures passed through by the electron beams to impinge on the screen and a non-effective region adjacent to the effective regions. Phosphors as a signal source are deposited on the non-effective regions or the boundaries between the elemental screen areas to emit a light signal by electron beam excitation. A photo-electric transducer is provided facing the shadow mask and to detect the light signal for feed back to a deflection system to control the electron beam deflection.