s switched on, i.e. at the second input of the AND gate 28 is a signal, the output signal of the regulating unit 27 determines the pulse width repetition rate of the pulse signal at the electronic power switch 29, so that the luminous film 2 isdriven with the regulated signal by the voltage converter 24.Naturally a.c. voltage control units other than those described in FIGS. 7 to 9 are conceivable