the video signal extends from black level corresponding to no light to white level corresponding to maximum white light a highlight occurring during the image portion of the horizontal line is shown by the time t 3 t 4 the amplitude of the video signal during the highlight is limited as illustrated in waveform 2a by the maximum beam reserve current which is typically two times the steady state normal beam current the video signal from output terminal 15 is applied to one input terminal of detector 18 which may be constructed for example with a motorola m1514 integrated circuit where it is compared with a reference potential established as illustrated in waveform 2a as just exceeding the maximum white level of the output video signal the output of detector 18 is a constant amplitude pulse having a leading and trailing edge as illustrated in waveform 2b the amplitude of the pulse from detector 18 is a fixed level determined by the saturation limit of detector 18 and having a duration equal to highlight period t 3 t 4 the constant amplitude pulse from detector 18 is coupled to one input of amplifier 100 where it is utilized to increase the beam current established by the normal g 1 bias for image pickup tube 10 as illustrated by waveform 2c waveform 2c illustrates the normal g 1 bias level which establishes the beam current in tube 10 being increased to a preset level by the effect of the constant amplitude pulse from detector 18 the beam current increase as shown in waveform 2c is made sufficiently high in amplitude that is in excess of the normal beam reserve current of two times normal beam.