Power in Watts: Power in Watts; Output in Lumens: Luminous Efficacy in lm/W
40 410 10.25
60 700 11.66
75 1,160 15.46
100 1,400 14
150 2,100 14

Based on the Phillips range- other makes may be slightly different.

Which goes to show that they are sadly not very efficient as on average some 90% of the power you put into an incandescent bulb goes to heat rather than light. They are however popular for the quality of light they put out. This is a very subjective topic- it's to do with the percieved warmth of light and how it looks. Many people say that CFL lighting is too harsh and white.
What's not in doubt is the quickness of the light. The filament has a low resistance when cold and raises when hot (typically the range is 10 - 150 ohms) and so the light can be at full brightness in approximately 0.1 seconds.

In the drawing above I've shown a GLS (General Lighting Service) bulb but this is not the only shape incandescents come in. Nor is it the only cap size, and indeed nor is it the only cap fitting. An Edison screw is not uncommon around the world but in the UK if you look at your ceiling fittings you are almost certain to find that they are not Edison Screws but bayonet fittings, such as shown below:

bayonet fitting

Bayonet fittings are not as secure as screw fittings but they are cheaper to make and the contact is more certain. An Edison screw has a contact at the centre of the screw tip and another isolated contact is the screw itself. This means that if you don't screw it down far enough the centre contact will not make contact and the bulb won't shine. Bayonets come in double contact at the tip normally (although the metal cap can be a contact too) and so this problem is eradicated as if you haven't put the bayonet in properly no contact is made and if you have contact must be made- there's no halfway-house.
Confusingly bayonets and Edisons come in a range of sizes and the sizes have different names. Here they are:

And even though the European - Bloody - Union has managed to muck up the type of bulb we can use it hasn't got round to messing about with the caps. Yet. So the practical upshot is that the caps that we used for the old incandescents are the same as the new types. Speaking of which, here's the first one.

Incandescent CFL
40 ~11
60 ~14
75 ~23
100 ~26
150 ~40

Which brings me to my next disadvantage about CFLs quite neatly. If you switch them on and then off again in quite a short time the lifespan is reduced significantly. In fact if the cycle is five minutes they aren't that much better than an incandescent. Now think about a family of four and toilets...........yes, they aren't much good there. I don't know about you, but in the bedroom I like to switch the big light on for a couple of minutes and then switch the table lamp on. CFLs wouldn't be much good there either. They're also susceptible to voltage and current spikes which will shorten the life. In fact advice in the United States is that if you are leaving a room for less than fifteen minutes you should leave them on. Hardly saving energy there.
Now let's rip into the starting time. Incandescents reach full brightness at 0.1 seconds or less. Although CFLs do turn on inside a second it takes a lot longer to reach full brightness unless it's marked as an "instant on" in which case it will cost even more. Some mercury amalgams can take up to three minutes, and the one in our house I timed as being 2 mins 17 secs before you can actually see the details in quite a small room.

Health issues are currently being debated as these things emit ultra-violet light. UV light is bad for you if you have very sensitive skin because it can cause irritations. Manufacturers argue that double wall insulations mitigate this problem, but the debate continues.

Then there's mercury. Mercury is poisonous, but to be fair RoHS laws and NEMA in North America have capped these to an acceptable level, and in fact the energy saving means that where coal would be burnt, less mercury is in the bulbs as a vapour than would be sent into the atmosphere as a release from the coal. That doesn't help when one breaks though, because it leaves a mercury "stain" for weeks afterwards particularly in carpet, and this "stain" though not visible comes close to breaching safe levels of exposure for adults and may exceed that for children if they are playing in the localised area.

Power factor is another issue. In incandescent lamps, all you have is a resistive element and so there's no reactance to consider. This is not true of CFL types as they do not work on low frequency alternating currents and so the first part of the input stage has to be a rectifier and then power smoother. This introduces reactance to the equation and so alters the power factor, which in turn alters the apparent power. This should not actually be a large problem in domestic situations unless of you course you live in Chatsworth House, but in industrial or office complexes you could end up paying more on your bill than is necessary unless you have the appropriate correction on your distribution system.

Let's look at the other circuitry that comes with CFLs. It's an electrical ballast, and for those who don't know a ballast is a design to limit current flowing through a circuit. You can do this with a resistance or (if it's AC) a reactance or a combination of both. These are known as electronic ballasts. There's also a magnetic ballast but you won't find many of those around now as it's become an obsolete technology.

There are several kinds of ballast. Here are the three most common:

The rapid start ballast applies a voltage to the tube and heats the cathode simultaneously These use more energy than other types as heating continues, but the cycle life is better than most other types of ballast. If the ballast is a dimming type it can be used with dimming triacs but if not, it can't be dimmed- look for the phrase "dimmable" on the box or if the lamp's come from Germany (they make loads) "dimmbar" means it can be dimmed and "nicht dimmbar" means it can't.

Programmed start is preferable for short duration lighting since it allows the cathodes to heat first and then applies a voltage to strike an arc. Otherwise it works like a rapid start.

Instant start bulbs are the most energy-efficient types since an instant start ballast uses a high voltage (about 600V) to create a discharge arc to light the tube. The trade-off is that some of the material on the cold electrodes is destroyed each time the lamp is turned on and so the life of the bulb is severely curtailed if it's on and off frequently. Consequently these bulbs are used where a long duration of lighting is required, round-the-clock warehouses and shops being examples.

The main crux of a lamp ballast whatever the type is to rectify incoming AC to DC, smooth it, convert it by a pair of power transistors and some capacitors to a higher frequency AC (an inverter or oscillator) and apply it to the lamp tube. This means that you may get electro-magnetic interference. You may also get a flicker. Personally I can't see the flicker but I have met people who claim they can and it gives them headaches and I've no reason to disbelieve them. It also means that there are more bits to break down.

What's in the tube (also known as a bulb or burner)? There are commonly three chemical constituents inside the glass, mercury vapour (dangerous) phosphor (potentially dangerous) for the coating and argon (not dangerous at all). Shoving a current through the mercury vapour and argon causes it to glow, and the phosphor diffuses the light. This can be a problem too, because some IR at 38 kHz might be present Why is this a problem? Because your television set has an infra-red remote control. IR from the CFL might be interpreted as a signal. Nothing worse than switching the light on when your wife's watching some sort of girly film and suddenly the telly flicks over to West Ham versus Arsenal. They just don't believe that you haven't done anything.

As a final note on CFLs let's look at temperature. Most are not designed for outdoor use and the ballast won't work in cold temperatures although some are rated down to -23 degrees centigrade. The trade off for the latter is that there won't be a full light output for a few minutes.

In the course of this article I may hinted subtly (!) that I'm not a big fan of these bulbs; I think they unnecessarily complicate what should be a simple matter and that use of them should be a personal choice and not forced upon us. What's never in doubt is that they do use a lot less electricity than incandescents and so they can save you money if you can put up with the drawbacks.

There's more than one kind of fluorescent, and so over the page we'll have a look at another.