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The Facts About Light Bulbs

By: Dr Gareth Evans - Updated: 2 Sep 2012 | comments*Discuss
 
Light Bulbs Low Energy Light Bulbs

With everyone advising you to replace your existing light bulbs with low energy ones – and supermarkets regularly offering to sell them to you at mega-cut prices – these unlikely household items have found themselves at the forefront of the energy-saving revolution.

While we all know we should be using the latest generation of energy saving bulbs, it’s even more interesting to know why – so here’s just a few of the fascinating facts behind the hype.

So What’s the Problem?

Unless you’ve been living in a cave for the past decade or so, you can’t have failed to get the message that 'traditional' light bulbs are not energy efficient – but you don’t often get told why.

As anyone who has ever tried to change a recently-blown light bulb knows, they get hot – very hot; in fact, a staggering 98 per cent of the energy used by a conventional incandescent bulb gets turned into heat! The reason for this lies in a little physics – and the impressive-sounding 'First, Second and Third Laws of Thermodynamics' – but don’t panic, it’s really very simple!

Light Bulbs and the Law

Forget your fears of science – the idea’s very easy to grasp:
  • The First Law basically says that there’s no such thing as a free lunch. You can’t create energy from nothing, all you can do is swap one kind for another – in this case electrical energy into light.
  • The Second Law tell us that when you do that swap, it’s never 100 per cent efficient. It’s a bit like changing your money when you go abroad, you never get the exact equivalent and you always lose something on the deal. In the same way, you don’t get all of the electrical energy you put in to a bulb back out as light – only around two percent, with the rest just going to make the glass hot!
  • The Third Law is even simpler – you can’t get round the First or Second Laws!
What this all means in practical terms is that if all you’re trying to do is read a book after it’s gone dark, a traditional light bulb is a pretty energy inefficient way of doing it.

Working to Save Energy

Energy saving bulbs – or Compact Fluorescent Lamps (CFLs) to give them their proper title – get around the problem by changing electrical energy into light in a different way. Instead of passing current through a filament which then heats up and glows, literally white-hot (hence 'incandescent') the CFL works by sending the flow of electricity through a cloud of gas inside the glass. This makes the gas give out ultra-violet light, which is then turned into visible light when it passes through the special coating on the inside of the bulb – which explains the slight time-lag effect when you switch one on.

The bottom line is, without a filament to heat up, a CFL wastes far less energy as heat – providing the same amount of illumination for around 20 per cent of the electricity used by a conventional bulb.

Living Longer

The other major selling point for energy-saving bulbs, of course, is their lifespan – which is very much longer than conventional equivalents. As a general rule, most varieties of CFLs can be expected to last for between 6,000 – 15,000 hours, compared with only around 1,000 hours for their incandescent counterparts, which is obviously a big difference.

However, CFLs don’t have it all their own way, since rapid on-off-on cycling can cut their life expectancy significantly and in some circumstances by as much as 85 per cent – which means they only live about as long as a conventional bulb. Fortunately, it’s an easy thing to avoid; provided you leave them on for about 15 minutes at a time, they should achieve their full potential.

Of course this does mean that if you only wanted five minutes of light, you’ll have to leave it on for ten minutes longer than you actually needed, but even so, it saves 40 per cent of the energy that an incandescent would have used in those five minutes – so it’s still worth fitting those CFLs.

Light Output

One of the big criticisms of energy saving bulbs when they were first invented back in the 1980s was that the quality of light they produced was actually quite unpleasant and many people found that they just couldn’t get used to it. A quarter of a century – and a lot of work by the bulb manufacturers – later and today’s bulbs give out a much more 'normal' light, and in a very short time, very few people are aware of much of a difference.

One fact about CFLs that isn’t widely appreciated is that the actual output of light itself falls over the lifetime of the bulb, but again it’s something that no one really notices. The biggest drop takes place over the first year and then gradually falls by 20 or 30 per cent over the rest of the bulb’s life, but in practice, it happens so slowly that we aren’t aware of it.

Light bulbs – energy saving or incandescent – may be rather humble, everyday objects, but there’s definitely more to them than meets the eye!

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