Fluorescent light unit with dimmable light level

A multiple light level gas discharge light unit, such as a fluorescent light unit, for operation from a-c electrical power, comprising a gas discharge light bulb having a pair of cathodes therein, a three-terminal base, means connecting a first end of one of said cathodes to a first terminal of said base, a ballast reactor connected between a first end of the other of said cathodes and a second terminal of said base and adapted to ballast said light bulb at a relatively high light level, and ballast circuit means connected between said second terminal and a third terminal of said base and adapted to ballast said light bulb at a relatively low light level in cooperation with said ballast reactor, said ballast circuit means comprising an impedance connected between said second and third base terminals, and bilateral switch means connected in parallel with said impedance, and said light unit including a starter switch connected between the remaining ends of said cathodes and adapted to close for a period of time when electrical power is applied to said base, in order to cause preheat current to flow through said cathodes, and thereafter be in open condition when said light bulb is operating, said bilateral switch means being adapted to be conductive during at least a portion of each electrical power half-cycle during said preheating of the cathodes for said relatively low light level operation, and control means connected to control the conduction phase angle of said bilateral switch means to vary the light level of said relatively low light level operation.