Cribsheet for Kinds of Lightbulbs

Go Back

Fluorescent Lamps


Below is probably the best graphic I've ever seen for this type of lamp and it shows the type of tubular fluorescent with which everybody's familiar, the big strip bulbs that come in lengths from about four inches to eight feet. The graphic comes from the Lighting Industry Federation of the UK and their website is magnificent. Here's a link to it:


This graphic is so good it is almost self-explanatory but just in case here's a banging-on at length about this sort of lighting and the principles of its operation.


First off, we need to talk about the inelastic scattering of electrons. It's called inelastic because a loss of kinetic energy occurs and you'll see in a moment where this bit comes in. Alright then, in the graphic above, you can see an electrode. There's one at the other end of the tube as well. They create a charge, exciting ions, and from that you get incident electrons. Incident electrons collide with atoms in the gas and cause a transfer of energy to the outer electron of  that atom (which is where the inelastic bit comes in).

The outer electron jumps to a higher energy level, but that's unstable and a photon gets emitted as the outer electron sinks to a more stable lower level. The trouble is that photon is ultraviolet, so we can't see it. Never mind, because what happens next is that the UV photon smacks into an atom of the phosphor coating, which causes a similar energy jump and sink, and the phosphor emits a photon that has a lower energy and is within the visible spectrum- in fact the light is a brilliant white. You'll notice up there that the gas mixture is argon and krypton, and this is the most common by far, but another alternative is argon and neon.


How much light do you get? It depends on many factors, not the least of which is the age of the bulb since it performs less well as it gets older. A new bulb measured after 100 hours should put out somewhere round 60 lumens per watt but in fact this varies and ~50 to ~70 is observable. That is miles better than an incandescent, however the light output is very harsh. Fine if you are in a lab but not so good in the house where people want a warmer bulb. Speaking of which, here's another fact.


A fluorescent tube runs cool, which is good in summer but in winter means it's not contributing to heating the environment of the room it's in. If it's a big room, this means you need more heating from a heating source and in fact some studies have found that the costs saved by using this kind of lighting are reduced by the heating increase needed so much that the benefit is questionable. In the UK it can be cold from September to May, and there are bits of Canada where it's all ice all year.


The tubes are marked not only as length but also as diameter and in the UK this is stated by a T number. The number is how many eighths of an inch the diameter is, and there are T5, T8 and T12 lamps, yet once again the European-Bloody-Union is messing about and has decreed that T5 should become standard and gradually T8 and T12 are being phased out.


The ballast used is the same as for CFLs except that a lot of them use an electro-magnetic ballast consisting of large coils. These are the least efficient and cause a distortion on the mains frequency and sometimes you can hear them hum. They need a starting capacitor.


Bulbs should last for 23,000 hours (T5) under a twelve-hour switching cycle but in homes a twelve hour switching cycle is not commonplace. Expect 6,000 hours as the worst-case scenario. The biggest disadvantage with these is fragility. In food prep areas it is mandatory to have a shatterproof tube but these cost slightly more. I once broke one of these whilst fitting it, and got some ribbing from my work colleagues. So I went and got another one, lifted it up and promptly dropped it whereupon it shattered into a jillion pieces, much to the merriment of the people around. I managed to fit the third one (after sweeping up the mess)  despite the betting going on around me as to how I was going to destroy it this time, and let's just say I was not allowed to forget the incident in a hurry and nor was I employee of the month that month either.


Like CFLs fluorescent tubes take a while to light. If they've got magnetic ballasts they take a few seconds to switch on and in any case don't achieve full brightness for a couple of minutes, though the difference is much less appreciable then CFLs. The ambient temperature is important too. Full efficiency of a T8 is at about 25 degrees centigrade, whereas for a T5 it's more like 35 degrees.


Nevertheless, I prefer strip bulbs to CFLs. Both of those belong to the fluorescent family, but in recent years a new technology has come on to the market. It is LED or Light Emitting Diode.


LED Lamps


An LED is a semiconductor which allows current to jump from an anode to a cathode via a small bonding wire and in the jump over the junction(at the end of which is a semiconducting crystal) a form of electroluminescence is observed. The usual output for a GLS bulb  is to have one short wavelength of light (either blue or ultraviolet) emitted and have the photon collide with a phosphor coating to produce a white light. A single white LED can demand 4V and a small amount of current and the current has to be limited by a ballast otherwise the LED will destroy itself. For a single LED the ballast usually consists of a forward resistor of the appropriate value, however in a lightbulb you need many LEDs all in close proximity. There are many ways of arranging this, but in practice three or more strings of series LEDs are paralleled together to provide redundancy if one string fails; it only needs one LED in a string to go and the whole string becomes defunct.

A premium Philips LED bulb can put out a staggering 94 lm/W. It is however very costly; in excess of £20. However it should last at least 20,000 hours as the worst-case. Here's the major problem: white LEDS are very difficult and costly to produce. LEDs that are not quite as good cost less, and cheap LED bulbs are truly awful; you may as well hang a candle on your light fittings.


LEDs can in fact be made in a range of colours ( red, orange, yellow, green and blue) and the colour depends on the materials chosen in the manufacture of the LED. Aluminium-Galium-Arsenide (AlGaAs) produces red, Aluminium-Indium-Gallium-Phosphide (AlInGaP) produces red, orange and yellow depending on the proportions of each material, as does the more commonly found Gallium-Arsenide-Phosphide (GaAsP). Indium-Gallium-Nitride (InGaN) produces green and blue. Mixing the colours in the right proportions gives a white output and this would be a pure LED, but as already stated, a common method is to put out a blue or UV photon and have it hit some phosphor.


Now here's a problem. Because of the nature of the build, LEDs tend to produce light not omnidirectionally but in fact in a cone. You can combat this by offsetting the individual LEDs in a bulb by a few degrees from each other, but LED bulbs still tend to be a bit bright at the centre and dim at the edges. This means you may need to have two lamps where before only one was needed. LEDs are however much better for spotlighting as they can shine a high intensity beam at a point.  The practical upshot is that LED lamps in domestic use tend to be used in spotlights and directional lamps. They also find a natural home in galleries, museums and shop windows, where light is needed at a point to emphasise particular features of an object.



led bulb

On the right there is one with what looks like a GU10 fitting, and I'm

surmising that because the CE mark indicates Europe and a GU24 is

used in North America rather than Europe. You'll notice that the LEDs

are not the ones familiar to us as indicators but the square surface

mounted type, and you'll notice that there's a lot of them. Something

else you'll notice is that they are arranged in a cone and so the

directional light I was banging on about earlier can now clearly be

imagined. This is why you can't seriously use LED lighting where

an omnidirectional source is necessary. However the Germans,

inventive as ever, have managed to make street lamps that

use LED bulbs instead of the sodium ones more familiar to

late-night revellers.

There are in fact five broad families of lamps:  the Incandescent, which consists of tungsten filament bulbs and tungsten halogen, the Light Emitting Diode, the Fluorescent, the High Intensity Discharge and the Induction.High intensity Discharge are very high voltage and the most striking example is street lights which use a sodium bulb. I don't intend to do a discourse on these for the simple reason that if you are working with sodium bulbs you'll have passed an exam to do so and so will probably know more about them than I do.

Induction lamps are electrodeless and it's not very likely you'll see this technology as it's something of a backwater in the lighting world, but basically it means there is no physical connection between the bulb and the passing of power is done by transformers in the bulb and fitting. So underdeveloped is this technology that no reliable data exists on them; various manufacturers have gone different ways with it and thus the means to buy the technology is both scarce and expensive. What is clear is that since the electrodes causes of failure in the bulb, an induction bulb promises a longer life than other types.

On the last page of this article, we look at some common fittings for light bulbs.

Common Fittings