Low pressure arc discharge light source unit

A low pressure arc discharge light source unit for unipolar or bipolar operation comprising a vacuum-tight flat essentially two-dimensional glass envelope having two planar areas essentially parallel to each other at a predetermined distance and having two opposing ends, said envelope being translucent at at least one side thereof, a rare fill gas and a quantity of mercury therein, at least three electrodes opposed to each other within the envelope and connected to electrical conductors, at least two parallel separating walls within the envelope located between the planar areas with at least one of the separating walls having a pair of ends with one end thereof extending up to and joining with one of the opposing ends of the envelope and the other end thereof being separated from the other opposing end of the envelope such that at least two discharge spaces are provided with one of the discharge spaces having a multiple arc length, and a fluorescent coating disposed on the inner side of the envelope and including at least two fluorescent phosphors of different spectral power distribution, each of the discharge spaces being coated with a different fluorescent phosphor.