Why do low energy lightbulbs take so long to reach full brightness?

01 April 2012

Question

Hi Chris,

(Great Show)

Why do energy saving lightbulbs take so long to get to a bright light and why is this level still dimmer?

Oliver Clark (Aged 12)

Answer

Dave - This is all to do with how an energy saving light bulb works. Conventional old-fashioned light bulbs basically work by putting a lot of electricity through a very thin wire, which gets very, very hot. When something is hot, it glows. At about 2000 degrees centigrade, which is the temperature of the filament in a normal light bulb, it glows close to white; a slightly yellowish white. The problem is that about 90% of the light given out at that temperature is actually the in the infrared region. So it's a hideously inefficient way to make light.

Chris - It's just giving you heat which you can't see.

Dave - Yes, you can't see it, so it's entirely wasted. Energy saving light bulbs create light in a very different way. You give energy to a mercury gas, by passing a spark through it. That then gives the atoms extra energy, which they then release in the form of ultraviolet and very, very little infrared. The ultraviolet light then gets converted into visible light by some phosphors on the inside of the tube. These are the things which are added to your clothes to make them appear brighter in the sunshine and the reason why your clothes glow if you go into a nightclub. The problem is that mercury is normally a liquid at room temperature, so if it is cold, the mercury isn't much of a gas. So the light has got to warm up a bit until there's enough gas in there to produce full brightness.

Why aren't they as bright as they should be, compared to a normal light bulb? Well that's probably mostly to do with manufacturers being slightly optimistic about how bright they actually are!

Chris - So, if you put more of the ingredients in and a bit more energy, you should be able to get the same amount of light. They're just being a bit over ambitious.

Dave - Yes, I think they used to quote it as being 7 times more efficient than a conventional light bulb. These days they're supposed to use 5. I think with 5 then they should be equivalently bright.

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