An electrodeless fluorescent lamp adapted for economical substitution in place of an existing incandescent lamp comprises an ionizable medium contained within a phosphor coated globular envelope. A closed loop magnetic core transformer, contained within a reentrant cavity in the base of the envelope, induces a radio frequency electric field which ionizes the medium to stimulate production of visible light output from the phosphor. A tubular extension of the envelope traverses the cavity, linking the core with a column of ionizable medium, to form a "secondary winding" on the transformer. A base assembly, attached to the envelope, supports a lamp plug for receipt of input energy. A radio frequency power supply, contained within the base assembly, provides electric current to a primary winding on the core.
A method for producing lamps in accordance with the invention comprises preforming the reentrant cavity, threading the magnetic core and winding structure onto a tubular member; fusing that member to the cavity; and assembling the cavity structure to a phosphor coated, globular shell.