|When the temperature is sub-zero (e.g., -40°), is it necessary to heat the electrodes or the gas or both for the tube to light? What is the optimum tube temperature with respect to efficiency?
Fluorescent lamps do not operate well in extreme cold. Below about 15° C (59° F), the density of mercury atoms in the tube's vapor is too low to produce efficient light. While the tube also contains inert gases that allow it to start at almost any temperature, the scarcity of mercury atoms leads to a reduced light output. In any case, the electrodes must be heated to make them emit electrons to sustain the discharge.
The optimal internal temperature for a fluorescent lamp is about 60° C (140° F). The tube reaches this internal temperature when its outside is about 40° C (104° F). When the surrounding temperature exceeds 40° C, the tube begins to waste energy again because the density of mercury atoms in the vapor becomes too large.