Suppose the mercury were taken out of CFL bulbs. Would the light shine dimmer, or not at all?
Not at all.
Fluorescent lights work by passing electricity through a mercury vapor gas, which causes the mercury gas to produce ultraviolet light. This ultraviolet light then strikes a phosphor coating on the inside of the glass tube, causing the phosphor to glow.
No mercury, no light.
Over the years, engineers have tried many different combinations but nothing works as well as the mercury vapor + phosphor combo. Xenon + phosphor bronze also works but at a very low efficiency.
You could take out the mercury if you replace it with something else – several gases and vapors give of light when excited in a gas discharge tube. Mercury has the advantage of being pretty efficient, however. (Neon is another gas that lights up, pretty famously. But most people don’t want to read under a neon lamp).
This was true bacxk when the mercury lamp was first invented, when it had no phosphor inside. It put out an oddly colored glow. When they replaced the glass envelope with a quartz one (which took the heat better), they found that people complained about eye problems – it turned out that the strong 254 nm line wa putting ouyt a LOT of ultraviolet light and hurting people’s eyes. Later they learned that they could line the interior of a flass tube with a phosphor and convert that ultraviolet light to useful visible light, and the fluorescent tube was born.