Honeywell UC100E1006 Enviracaire 18 watt SnapLamp™ knob and bulb assembly. Genuine Honeywell. This assembly is installed with a simple twist. Future bulb replacement will only require replacing the bulb with a UC18W1004. 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 bulb is snapped in to the knob. The knob has a 3" diameter with an overall length of 4". The knob has a built in light pipe that allows a safe inspection view of the UV light.This UC100E1006 knob and bulb assembly is used in the UV100E1043 and earlier UV100A1000.The UC100E1006 replaces the UC100A1005. The UC100A1005 was earlier slate color.The knobs for the 18 watt units will not fit in the 36 watt units (and visa-versa), the knobs are keyed.