The invention refers to a fluorescent tube with a luminous efficiency higher than 120 lm 1 W, with mercury vapors at low pressure and noble gases, such as: neon, argon, krypton, xenon, with electrical powers reduced on the average with 50 - 80 % for the same lighting offered by a fluorescent tube, in itself known and equivalent as geometrical dimensions, with heat or cold cathode, that utilizes the volume of a gaseous medium contained between an exterior body (1) which is transparent and endowed on the intrados with a layer ( a ) of luminophore, and a coaxial, interior body (2); the entire volume of the interior body (2) is got out from the electrical discharge regime; there are presented constructive solutions with ceramic pieces of enclosing of the exterior body (1) and the interior body (2), the utilization of some circular cathodes and of a gaseous medium with 1 p.p.b. of mercury.