For an entry door, a doorknob lock set includes an elongated stem which extends through a transverse opening in the door and is rotatable about its longitudinal axis to actuate the door latch. Doorknobs are provided at both ends of the stem to rotate the stem; and the stem includes a key lock mechanism including a key slot at the outer end of the stem. To illuminate the key lock slot, the outer doorknob is replaced by a knob fabricated from a translucent material. The translucent knob is provided with a central bore to receive the outer end of the stem; and the knob is mounted on the stem to frame the key slot. The knob is provided with at least one cavity opening from the inner face of the knob and extending toward the outer face. An illuminating device such as an incandescent bulb is placed in that cavity. Electric conductors connected to the illuminating device extend out of the cavity for connection to a source of electric energy.