There must be a visual indication located on or near the tube head to indicate when X rays are being produced. This is usually an assembly consisting of two red bulbs, wired in parallel and labelled X RAYS ON. If one of the bulbs is burned out, the operator should either replace it before leaving the room, or leave a note on the light assembly indicating that the bulb is burned out. An unlit warning bulb does not necessarily mean that X rays are not produced. Never trust a bulb, unless it is illuminated ON! Always check the control panel when the bulb is off.