A converter circuit for addition to the existing lamp operating circuitry of a high pressure mercury lamp consisting of an existing series ballast inductor (L) and a power factor correcting capacitor (C) connected across the input terminals of the existing lamp operating circuit includes inductance (L,) arranged to be connected to the ballast inductor (L) to as to adjust the resultant series inductance to a value suitable for a sodium lamp (SON), and an igniter portion (A) coupled to the converter inductance (L 1 ). The igniter portion (A) operates to apply ignition voltage to the sodium lamp (SON) during at least part of the time that the converter circuit is aupplied with power. Protective circuitry (B) is coupled to the converter inductance (L,) to block or limit high frequency voltage pulses which could otherwise reach the ballast inductor (L). The circuit can be used to convert high pressure mercury lamp operating circuits into a circuit suitable for operating either a high pressure or a low pressure sodium lamp by appropriate choice of the converter inductance (L,) and other converter circuit components.