A tube using a cathode‐ray type of electron gun has been used to study the properties of five phosphor samples using four different materials. These were (1) zinc‐orthosilicate (2) calcium‐tungstate (3) zinc‐sulfide: cadmium‐sulfide and (4) zinc‐sulfide. The first three showed saturation effects with current density definitely greater than the last, and the first two have maximum potentials as limited by secondary emission of 6500 and 5000 volts respectively. For these, the light output per unit electron current increased with the square of the voltage, while it increased with the 2.8 power of the voltage for the zinc‐sulfide. A simple power law does not hold for the zinc‐sulfide: cadmium‐sulfide which shows a maximum light output at 8000 volts. All of the sulfides had such good secondary emission properties that the maximum limiting potential was well above 10,000 volts.