During the 14th presentation of Protons for Breakfast, someone asked me why compact fluorescent (CF) light bulbs are so dim when they are first switched on. I explained that I thought it was due to the temperature dependence of the vapour pressure of the mercury within the lamp. This evening I got a chance do an experiment with my 4-in-1 multi purpose measuring device, and removed a CF bulb from my freezer where I had stored it for a couple of days at a temperature of around -12 °C. I then plugged it into a light fitting as quickly as possible, and measured the light output as a function of time using my 4-in-1 multi purpose measuring device.
The variation of intensity is dramatic. The lamp became over 34 times brighter during the first 30 minutes. After 90 minutes the bulb reached a temperature of 85 °C and was more than 40 times brighter than it had been initially.
This result looks pleasing, but is actually quite confusing when looked at in detail. The problem is that the vapour pressure of mercury increases by a factor of roughly 1000 as one warms up from -12 °C to 85 °C. So at first sight one might expect that, since there are 1000 times more atoms of mercury, there ought to be 1000 times as much light. However, I suspect that three other factors are relevant:
- Firstly the increase in the density of mercury atoms affects the electrical resistance of the plasma, and in particular the energy with electrons collide with mercury atoms.
- Secondly, as the density of mercury atoms increases, the some parts of the plasma begin to absorb light (probably UV light) emitted by other parts of the plasma.
- Finally, the efficiency of the phosphor coating on the tube almost certainly increases with increasing temperature.
So I suspect that some combination of these three factors results in the curve seen above. Wikipedia covers this issue here – but it looks even more complicated than I thought!