in apparatus including a cathode ray tube , electrical circuitry for preventing &# 34 ; burn &# 34 ; spots in the phosphor of the cathode ray tube . the protective apparatus includes means to measure the rate of change of deflection of the electron beam of the cathode ray tube and means to measure the brightness level of the electron beam . the absolute value of the rate of change of the deflection of the beam is divided by the brightness level of the beam to produce a signal which is compared to a reference value . if the dividend exceeds the reference value , the crt is shut down as in the preferred embodiment by blanking the cathode ray tube beam .