Honeywell UC18W1004 Enviracaire 18 watt SnapLamp bulb. Genuine Honeywell. This bulb snaps in to your existing knob. Note running a burnt-out bulb may burn-out the ballast.
Overall bulb length is 8½ inches, glass length measures 7½ inches. The bulb has has two glass tubes fused together and two filaments, each tube is ¾" in diameter.
The Honeywell UC18W1004 is a customized 18 watt Philips Hot Cathode Germicidal Sterilamp Tube. Bulbs effective life is 8000 hours.
This bulb is used in the UV100E1043 and the earlier UV100A1000.
The UC18W1004 snaps in to the knob - KNOB NOT INCLUDED - and replaces the bulb in the knob-bulb assembly UC100A1005 or UC100E1006. The UC100A1005 was the earlier slate color. Knobs earlier than 2005 did not have a detachable bulb, see section below.
The knobs for the 18 watt units will not fit in the 36 watt units (and visa-versa), the knobs are keyed.