1. A matrix display board comprising a plurality of light emitting elements each comprising at least three discharge tubes containing mercury vapor at low pressure, the internal wall of each tube being coated with a fluorescent substance chosen to respond respectively to either the red, the green or the blue portion of the spectrum, and separate energizing means associated with each tube for independently controlling the variation of the light intensity emitted thereby so as to obtain light at the element output the resultant wavelength of which may extend over the entire visible spectrum, said separate energizing means comprising generator means for generating a high frequency current during a sequence of reference periods, each said reference period including a like plurality of cycles; and a plurality of gating means, each responsive to said high frequency current for selectively inhibiting the number of cycles of said current applied to a corresponding one of said tubes in a given reference period whereby the intensity of light emitted by one of said elements is varied by selectively varying the number of cycles of said current which are inhibited during a reference period from reaching the tubes comprising said one element.