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Author Topic: Do Low Energy Bulbs get brighter?  (Read 5696 times)

Offline rhade

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Do Low Energy Bulbs get brighter?
« on: 15/07/2009 17:13:29 »
Now that the UK government is trying to phase out traditional light bulbs in favour of the so-called low energy bulbs, or coiled up fluorescent tubes, there are a few things I was wondering.

Specifically, a friend of mine says his low energy bulb gets brighter the longer he leaves it on. Is this true, or do you reckon his eyes are just adjusting to it?

How do they work exactly ( in laymans terms, please) ?

And, perhaps more importantly, why are some of them (like the one I'm currently using) quite good, while others are a bit crap?


 

Offline techmind

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Do Low Energy Bulbs get brighter?
« Reply #1 on: 15/07/2009 18:28:31 »
All pearl ("frosted") tungsten/incandescent bulbs will be removed from the shops in the UK on 31st August this year as a result of an EU directive ratified this April.
"Clear" bulbs will have each wattage phased out each year (starting with the highest) over the next few years.


CFLs will tend to come on dim, but brighten over the first 2-3 minutes that they are switched on. They may come on at as little as 50% of full brightness. Basically the discharge in the fluorescent tube doesn't work at full power until the tube has reached normal operating temperature. You can easily measure this using a photographers' exposure meter, or the light-meter built in to a better camera (keep the camera/meter fixed in one place during the warm-up time).

Over time (weeks/months/years) the brightness drops with something like exponential decay as the phosphors deteriorate.



A low-energy bulb, also known as CFL "compact fluorescent" is just a small fluorescent tube folded into a funny shape, and fitted to a base which contains the driver circuitry that you'd normally have in the light-fitting of a larger/conventional fluorescent tube. The driver circuit generates a high voltage to allow the tube to strike when you first switch it on, and uses one or more transistors to switch it at "high frequency" (10's kHz) so it doesn't flicker. This high frequency switching may cause whistles and whines on a mediumwave/longwave radio nearby.
The bulb may still flicker as the high-frequnecy drive still contains 100Hz ripple from the mains.


Being fluorescent, the light colour is determined by the phosphor, and may come out too "cold" - bluey. It could also come out too green or too magenta which is unnatural as we're used to light from the sun and tungsten bulbs which have blackbody colours (red through orange through pale yellow through to bluey-white) ... which our brains are reasonbly happy to map to "white".

A more subtle problem with fluorescents is that the phosphors (mostly for efficiency) only emit about 5 colours to make "white" whereas conventional tungsten bulbs (and essentially the sun) emit a continuous rainbow spectrum of colours. The gappy spectrum of fluorescents causes the colours of things illuminated by the bulbs to be warped - desaturating many colours except for lurid oranges and yellows.
CFLs may also emit some ultraviolet, which causes dust and whiteners to fluoresce, adding a purpleish tinge and/or further desaturating colours.

This explains why the light from fluorescents often looks "crap" as you put it.
« Last Edit: 15/07/2009 18:33:24 by techmind »
 

Offline rhade

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Do Low Energy Bulbs get brighter?
« Reply #2 on: 03/08/2009 12:03:52 »
Thanks, Techmind!

The friend I mentioned in the question reckons his CFL takes two hours to get bright, so I don't know what kind he bought, but it sounds seriously duff!

I reckon the CFL I've been using ties in with your 2-3 minutes to hit full intensity, from my own observation. After that, it's pretty good, but it takes a while longer for my solar powered calculator to come on than with a "proper" bulb (proper if only in the sense of being an actual bulb shape- how can a coiled up fluorescent tube be a bulb?)

I had a look at your profile on this thing, so I can see that you are well versed in the subject. Thanks again!
 

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Do Low Energy Bulbs get brighter?
« Reply #2 on: 03/08/2009 12:03:52 »

 

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