A Light Bulb Moment

I couldn't believe my eyes. I stood up, looked around furtively, and then crouched down again to peer into the little display bucket near the floor. I was at the back of a long, narrow and rather seedy shop near to where I work. There were only a couple of other customers in the place, both middle-aged geezers like me in old, shabby coats. Their tastes were legitimate, though: chains, tape, lubricant, that sort of thing; whereas mine was sordid, polluting and illegal. I had made no mistake. Right in front of me was a bevy of curvy little beauties -- 100 watt light bulbs with a standard bayonet fitting, each trying to burst out of their flimsy, tight fitting cardboard boxes and each available to enjoy for just 35p a go. So, while no one was looking I grabbed a couple and took them to the counter with an insouciant air. The guy at the till said nothing, but allowed himself a half smile as he put them in little plain bag and took my money. I made my way back to the office with a slightly paranoid feeling that everyone who greeted me knew what incandescent porn I had in that little bag. It was only when I got home and added the two lovelies to my existing harem of six that I felt fully relaxed.

I jest, of course, for incandescent 100 watt GLS (General Lighting Service) lightbulbs are still not difficult to find, especially on the Internet, but they are likely to become so. Nor is buying and selling them actually illegal, although manufacture and importation is now prohibited in the EU. However, any bought from September 2009 onwards are from existing warehouse stocks and this year 60 watt bulbs will go the same way with a total phase out next year. You can get the official story here.

Does this matter? Not much, perhaps, although plenty of people think otherwise and I do have some sympathy for that point of view. As a household, we use GLS bulbs mostly in overhead room lights (100 watts, but not on for long periods), a few table lamps (mostly 60 watts and in fairly frequent use) and inside cupboards, loft etc. We use halogen lamps (a type of incandescent) in the kitchen and bathroom, but elsewhere CFLs (Compact Fluorescent Light) predominate in terms of use. I took a census, in fact, of individual bulbs:  CFL 20,  halogen 21,  incandescent 39.

That might not look very good from an environmental perspective but I think we have the balance right. CFLs are used where light is desired for long periods such as passages and stairs and some standard / table lamps. We don't light up the place like a department store, but equally, we don't like creeping about in the dark. Lights are generally switched off in unoccupied rooms. GLS bulbs are preferred for briefer periods of illumination and halogens for the best quality. I can't demonstrate the usage but my feeling is that about 75% of our illumination over the year is met by CFL.

Critics of CFLs say that the light quality is poor and enthusiasts say that they have improved a lot over the last decade. Both are true. One of the problems with CFLs is that they last so long. I have one or two that must be 15 years old and they are still okay although, since they degrade over time, they are now easily outclassed by newer ones. The fact that this and other alternative lighting technologies are improving is a bit of a disincentive to buy into them too soon.

Human beings evolved under the continous, virtually flat spectrum of the Sun and that is the light that our brains and bodies are happiest with. Incandescent lamps also have a continuous spectrum, albeit tilted towards the red end of the spectrum, but I think we actually appreciate that nightly spectral shift, having evolved not only with sunrises and sunsets, but camp fires too. Halogen lamps are improved incandescents. They don't suffer from filament evaporation and have higher output. They offer the most even light of all commonly available lamps and it is no wonder that they are used so extensively for display purposes. Fluorescent lighting, on the other hand, produces a line spectrum from its phosphor coating and however many different layers are deposited on the inside of the glass in order to fill in the frequency gaps or shift the spectrum, the brain is never completely fooled. However, I do find that I am perfectly happy to read by the light of the best and brightest CFLs, although some people I talk to are not, while others claim to be made ill by them. Maybe I'm just insensitive.

So what of the environmental benefits? Looking first at electricity consumption, these are clearly substantial for an individual bulb. A standard, clear 100 watt GLS bulb has a light output (technically, luminous flux), when new, of about 1700 lumens or 17 lumens per watt. The theoretical maximum output of any lamp is 683 lumens per watt (albeit at a single frequency), so the energy efficiency of such a bulb is 17/683 ≈ 2.5% and the rest of the electricity (97.5%) is converted to heat. Equivalent halogen lamps are a bit better and can manage 3-4% efficiency. By contrast, fires, candles and oil lamps, our only sources of artificial light for thousands of years, are below 0.1% efficient. A good 23 watt CFL, on the other hand, should have roughly the same light output (1700 lumens) at 74 lumens per watt as the GLS lamp. This gives an efficiency of 11% which, curiously is about the same as the Sun in the visible range, although the spectrum itself is so different. Thus, CFLs get hot, but not nearly as much as incandescents. In other words, CFLs are over four times as efficient and save, individually, a lot of electricity.

By the way, get used to lumens, since new lightbulbs in the EU must now be marked principally with their light output. This makes sense, but it helps to read and understand the labels. The EU seems to have adopted a standard of 1400 lumens as the equivalent of a 100 watt GLS (and 750 lumens for a 60 watt). I find this on the low side, but it is justified probably by the fact that pearl and opal type bulbs are generally less bright than clear ones. CFLs of 20 watts should be able to meet this standard when new, but I have an 18 watt one which claims to be a 100 watt GLS equivalent. Since it boasts only 1100 lumens output, the manufacturer, Philips, really should know better. Subjectively, I find that an output ratio of 4:1 gives a truer measure of CFL:GLS output that the official 5:1.

However, the amount of electricity that you save will depend on how much you actually use for lighting. Typically, that will only be around 15-20% of the total consumption, so even with all CFLs in your home you will save a maximum of about 15% on your electricity bill and typically it will be under 10% if you mix and match. Worth doing, but only if it works for you and your fittings can accomodate them comfortably. A better qualified man than me made this point a little while ago. There is not much point in saving energy or money if you have to buy new, possibly imported, light fittings. The sight of a CFL poking its tubes above a small lampshade, for instance, I find quite displeasing.

Another factor to consider is colour temperature, a somewhat complicated subject which revolves around the notion of a black body radiator, an idealised object which is perfectly black at a temperature of absolute zero. The upshot of this is that colour balance of a light source can be described by the temperature (in degrees Kelvin, K) that a black body would need to have in order to emit light with a given frequency distribution. The quality of any light we see can be described partly in terms of its colour temperature. Think of glowing coals (red) and a lightbulb filament (white). Paradoxically, the subjective "warmth" or "coolness" ascribed to light is inversely related to its colour temperature. The warm glow of a 100 watt GLS bulb has a colour temperature of 2700 degrees K, whereas the proverbial cold light of day (an overcast sky at noon) is about 6000 degrees K.

Modern CFLs are often marked with their colour temperature and their advocates assert, rightly, that they can be produced in a variety of types. However, if you go looking for the colour temperature, you probably won't find it, because of course it would have been just too easy to mark "warm" lamps with, say "2700K". No, you will often find it indicated as "827". Similarly, "cool" lamps with a colour temperature of, say, 4100 degrees K are marked as "841" or even just "84" etc. Well, this certainly gives us lots of choice and room for puzzlement, but I think the benefits are rather oversold and I wonder how many people really care about having different types of "white balance" in their home. In fact, the availability of the cooler, but often brighter lamps, unknowingly bought, may have contributed to their poor reputation.

Artists and craftworkers know that it is no use trying match colours in standard artificial light, since daylight will always destroy the match. It might be tempting, therefore, to use "cool" rated CFLs. They may even be labelled "daylight". Don't bother, it won't work. The spiky nature of the spectrum never gets close to sunlight and the complex subject of metamerism comes into play. As an occasional amateur dauber myself, I know from personal experience here. Look down your street at night and it's obvious which are the white vehicles, even though the light they are reflecting from sodium streetlamps is anything but white. Such is the visual reality that our brains construct for us. This phenomenon, of "white" as a subjective experience under varying light conditions, has often been explored by painters with interesting results. The best way to obtain a cheap approximation to sunlight is to take a standard incandescent bulb and smear a thin layer of phthalocyanine blue over it, although you can also buy the pricey, blue GLS lamps manufactured for the purpose. But I digress. . .

After all that technical stuff, here's a little story. In my workplace there used to be a certain head of building maintenance, let's call him Al. Whenever and wherever he was walking -- down corridors, offices, labs, meeting rooms, public areas, the canteen -- if he saw lights that were had been switched on unnecessarily (almost all GLS types) he would turn them off. Al left many years ago and the almost every light has now been replaced by fluorescent types, short multi-tube fittings or CFLs. These are now left on virtually 24x7 since, being so energy efficient, the more they are left on, the more is saved, obviously. Modern technology, you see, makes life so simple. You don't even have to turn off the lights.

Besides declining light output, another problem with CFLs is their poor duty cycle. Under ideal conditions of use, where they are left on for long periods, they can last ten times longer than incandescents, but if you switch them on and off a lot their lifetime falls markedly. If they are put through a continuous five minute on/off cycle they last no longer than incandescents. So here we have another reason for leaving them on. People have also raised the issue of their mercury content and, indeed, it is best if they are disposed of, unbroken, through a proper recycling facility. However, in countries like the UK, where a significant proportion of electricity is generated from coal, far more mercury escapes from that process than could do so from CFLs and their use will actually decrease the amount of mercury released into the environment through reduction in overall demand.

Are there any other types of lightbulb that we could use? Possibly, but I am not convinced as yet. The most efficient of all is the low pressure sodium light, familiar from street lamps. These are typically 25% efficient, but the virtually monochromatic orange light is hardly one that we would entertain in our homes (cf. Victor Meldrew), even if, remarkably, some people were happy to persuade themselves a few years ago that it could be like sunlight (really, nothing could have less so, but willing self-delusion, I assume, was the point of the installation). For mass public lighting, however, low pressure sodium lamps are the best in terms of cost. As an aside, they are also the best for astronomers since the narrow spectrum light pollution at 589.0 - 589.6 nm is easy to suppress with a neodymium filter whereas, given the poor design of much public lighting, the gradual encroachment of the broader spectrum peachy-pink light from high pressure sodium lighting (typically 20% efficient) and especially the harsh white light of metal halide lamps (around 15% efficiency) is, from a skywatcher's point of view, a disaster. Oh dear, another digression . . .

In the home, the only real alternative to GLS and CFL is LED lighting, since high pressure metal halide lamps are deemed unsafe for domestic use. LED bulbs have efficiencies similar to CFLs, last even longer and do not suffer from duty cycle problems. Also, they are easier to mount in many conventional glass envelopes, such as GU10, than fluorescent devices. However, the white light is typically composed of blended line spectra and I have yet to see one that gives a really good subjective quality of white light. In my experience they seem to have a colour fringe which varies according to the angle of vision and which I find distracting, although I admit that I have not checked this within the last couple of years. For specialist colour lighting, however, they are often the best choice. The technology is no doubt developing fast and very soon there may be excellent products available for white bulbs too, but whether they will be cheap enough to tempt consumers en masse remains to be seen.

I am not against new lighting technologies -- far from it. CFLs and soon, probably, LEDs do have a place in our homes, but they are not yet ready to take over completely. All of this means that I still covet my cheap incandescent bulbs, with their instant turn on, and I'll be making sure that, as the various types are progressively withdrawn, I'll be building up modest stocks by furtive visits to little hardware shops.