A lamp adapter can adapter a mogul base lamp holder to accept a medium base lamp. The lamp adapter can accommodate only open fixture rated lamps. An open end of the lamp adapter into which a lamp is inserted has an elongated flange which allows only an open fixture rated lamp having a medium base to be completely threaded into the adapter and make electrical contact with a threaded metal sleeve and central button contact of the lamp holder. The geometry of the elongated flange prevents the threading of a closed fixture rated lamp into the adapter by taking advantage of the physical differences between the open and closed fixture rated lamps.