Science Questions

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

Sun, 1st Apr 2012

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Oliver Clark asked:

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)





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 itís a hideously inefficient way to make light.

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

Dave -   Yes, you can't see it, so itís 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 arenít 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.  Theyíre 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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Presumably you mean the ones based on small fluorescent tubes wrapped up.  There are two reasons for this.  Firstly the tubes depend on mercury vapour to run properly and need to warm up a bit to work properly.  Secondly to avoid excessive stress on the electronic driver circuits.  While the tubes are warming up they tend to need a bit of extra power to get them going.  with non electronic tubes this is easy and they warm up much quicker but with the electronic circuits you would need much more robust bulky and expensive components to allow for this quick warmup and they would be under utilised for the main part of the life of the bulb so it is a balance between reliability and warmup speed and the compact tubes compromise by having a slowish warmup of a minute or two. Soul Surfer, Mon, 2nd Apr 2012

Can you tell me what type of bulb I should use that would be dim when I turn it on that eventually gets brighter? I can't stand the shocking brightness when I use any other led bulbs and have nearly gone mad trying to find a bulb that doesn't force me to put on my sunglasses in the bathroom when turning on the light. Would love to hear what your recommendation is. Thank you in advance! Monique Guest Monique, Sun, 27th Nov 2016

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