It is used to activate the phosphor mass of the instrument scales and the inscriptions on the panels. In the front part of the body there is a UV filter; when you turn it, the filter opens slightly to illuminate the visible part of the lamp spectrum and start it. For starting and changing the brightness, it has a rheostat with a switch with a resistance of 100 ohms. To start, it is necessary to smoothly reduce the resistance of the rheostat before ignition, then be sure to reduce the brightness (operating at maximum brightness will disable the lamp). Lamp type LUF4 24-28V 4W.