power is applied to lamp 30 transistor 42 will continue to conduct until the bias voltage to amplifier 65 drops below the preset value at a 39 at which point transistor 42 is turned off lamp 30 has therefore been energized for some period t 1 and has produced a light component which is detected by photodiode 52 the amplified signal will be present as signal v c at the negative input of amplifier 65 amplifier 65 will therefore not go positive until it sees a greater line voltage input b at the positive terminal transistor 42 will then be turned on and conduct during a shorter time period t 2 terminated at point b 39 where the line voltage drops below the value of v c again the photosensor signal generates a signal proportional to the increased light output of lamp 30 it can then be appreciated that with the application of each successive line voltage waveform up to the point where equilibrium conditions are achieved v c will continue to increase causing v o to go positive at greater values of the applied line voltage waveform c illustrates a typical waveform for normal lamp operating conditions for the circuit shown in fig2 and with a current gain of 200 for transistor 42 lamp current will increase from 0 to 1 ampere when the line voltage has a net change of approximately 10 volts the time required for the line voltage to change by 10 volts.