Why is mercury necessary for fluorescent lamps?

The answer to this question lies in the working principle of fluorescent lamps. A fluorescent lamp consists of a glass tube, a gas filling, electrodes and a phosphor layer (see figure below). After the necessary operational voltage is applied to the two electrodes of the lamp, a gas discharge between the electrodes is established.

Electrons moving in the field generated by the electrodes collide with atoms of the gas. These collisions excite the atoms onto a higher level of energy. The atoms return to the original level, and give off this difference in energy mostly in the form of UV light.

The efficiency of this gas discharge depends decisively on the combination of the gas filling and the corresponding layer of phosphor on the glass wall. Experiments have shown that the best choice for the gas filling is not a regular gas or a gas mixture, but mercury. The reason for this is based on the correlation between the light which is emitted by the gas discharge and the phosphors available. (Note: mercury ‘gas’ is actually mercury vapour present at the operating temperature of the lamp. The physical properties of mercury are unique in this respect.)

In general, a low pressure gas discharge as found in fluorescent lamps emits most of the light in the ultra-violet (UV) spectrum. This UV-light is not visible, and therefore has to be converted into visible light by a phosphor layer. Both processes, the generation of UV-light by the gas discharge using electrical energy, and the conversion of UV into visible light cannot be realized without energy losses.

After more than 50 years of research and development and different attempts to replace mercury by less toxic substances, the common physical and chemical understanding of all experts is that the most efficient system for the discharge and the subsequent UV conversion is based on mercury. Presently, linear fluorescent lamps operating on an electronic ballast are reaching system efficiency of more than 100 lm/W. Although the efficiency decreases somewhat with shorter lamps, compact fluorescent lamps still reach approximately 60 lm/W.

One of the most recent and ambitious attempts to replace mercury in fluorescent lamps was undertaken by OSRAM. During the last decade, a flat, very thin lamp was developed using the rare gas Xenon instead of mercury. As with other fluorescent lamps, it also made use of a phosphor layer for the conversion of UV into visible light. The efficiency reached by this system was the absolute best reported of a mercury free solution. However, with 35 lm/W, the efficiency reached was still considerably below that achievable by mercury-using fluorescent lamps. It became obvious in the course of experiments that this efficiency would never reach that of mercury-discharge based lamps.