Multi-digit fluorescent display tube with inclined filament cathode

A multi-digit fluorescent display tube comprising:

• a planar insulating substrate;
• a plurality of digit fluorescent displays arranged in a row in spaced side-by-side relationship along the longitudinal width of the planar substrate to form a plurality of planar digit fluorescent displays;
• each digit fluorescent display comprising a plurality of segment anodes disposed to selectively form one of a plurality of digits and deposited phosphor layers thereon;
• at least one filament cathode stretched along the longitudinal width of the planar insulating substrate adjacent to the plurality of digit fluorescent displays but separated therefrom;
• the filament cathode being disposed at an acute angle with respect to the planar insulating substrate so that distances between the filament cathode and each of the plurality of digit fluorescent displays are linearly progressively shorter so that the perveance of each of the plurality of digit fluorescent displays is adjusted so as to illuminate each of the plurality of digit fluorescent displays in uniform brightness when the fluorescent display tube is operated by heating the filament cathode with a direct current power source;
• and
• a light transparent evacuated sealed envelope in which the plurality of digit fluorescent displays and the filament cathode are disposed and through which the plurality of digit fluorescent displays may be viewed externally when the fluorescent display tube is in operation.