A lamp socket has a housing forming a cavity centered on an axis and having a generally cylindrical side wall and a floor, an internally threaded sleeve coaxially received in the cavity, a central socket contact on the floor at the axis, and an annular disk separate from the housing, surrounding the central contact. The disk is formed on the axis with a central aperture and is spaced at the aperture substantially further from the floor than the central socket contact. Thus only a lamp base with a central lamp contact on an extension stem can fit in the socket and engage through the aperture and past the disk to make contact with the central socket contact.