The tubular envelope of a double-ended fluorescent lamp is provided with a partition assembly of sheet metal or other rigid material that is inserted into the envelope in the space between the electrodes and defines a single discharge channel which forces the arc to traverse the envelope an odd number of times in retroverted fashion, thus reducing the size of the lamp and providing a concentrated light source of high brightness. The partition assembly consists of a number of joined panels that radially extend from a common axis to the envelope walls and divide the interior of the envelope into an odd number of sectors that are interconnected and form a continuous discharge channel. Diaphragm components are fastened to each end of the partition assembly and laterally extend across the interior of the envelope to prevent the arc from bypassing the partition assembly or any of its planar segments.