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A PHYSICIST WRITES . . .
(May 2007)
After my last couple of columns both on the subject of light, I’m still thinking about this topic. I’ve pointed out before that the filament bulbs on your car convert electrical energy into light very inefficiently. When you also take into account the poor efficiency of generating the electricity under the bonnet, you discover that less than a hundredth of the energy in the extra petrol you burn (when your lights are switched on) is sent out in the form of light. All the rest is simply lost as heat.
But why stop there? You may think this is absurd, but it seems logical to me to go on and try to estimate how much of the light from a headlight, say, does anything useful by entering people’s eyes (including reflecting back into your own) ... I would guess that it’s less than one-millionth, on average.
And what does this mean? It tells you that with your headlights on, only a few billionths of the additional petrol-energy you are using to power them actually gets into people’s eyes (or comes back to yours), in the form of light, to warn them (or you) of approaching danger! Are the car manufacturers aware of this colossal wastage of energy, I’d like to know? And if so, what are they doing to reduce it?
Not much, apparently, though some models are being fitted with gas-discharge headlights. These are at least twice as efficient as filament bulbs — so, are they designed to consume half the power? Of course they are not. Who would pay the extra for them if they were no brighter than ordinary headlamps? Also, their brighter light emerges from a smaller lamp-glass, with the result that oncoming drivers are either dazzled or (if the lamps are at all misaligned) blinded. And the beam from these headlamps tends to be more directional too, hence they don’t light up the sides of the road ahead so well. Such is progress.
What about two other well-established methods of generating light: fluorescent bulbs and tubes, and light-emitting diodes (LEDs)? Both types have advanced a lot in recent years, but neither of them comes near to producing the concentrated light needed for a vehicle headlamp. Another problem with LEDs is that they emit one colour, and combining them (in different colours) so as to produce white light is not easy to do. Nor are LEDs as efficient as fluorescents.
As you probably know, fluorescent bulbs are about five times better than ordinary incandescent bulbs at converting electricity into light. Why then did a national newspaper tell us last month that fluorescents are not even twice as (ie, 100% more) efficient? Here’s what it printed: “Low-energy bulbs are up to 80% more efficient than tungsten-filament versions.” I’m afraid this was another case of journalistic percentage confusion (to add to the one I reported last time).
Let me explain — if only to help you avoid a similar trap yourself, perhaps. A fluorescent bulb emits five times as much light as a filament bulb that’s consuming the same power. Thinking of what’s being added on, you could describe this as “400% more light” or “400% more efficient”, if you wanted the difference to look misleadingly big.
In your home, however, you are probably replacing your old bulbs with low-energy ones having the same light output. These of course use one-fifth of the electricity. Can you translate this drop into a percentage change? Thinking of the four-fifths reduction, it’s “80% less electricity”. Now picture that journalist picking up this fact from somewhere, but wanting to rephrase it more positively. How easy it must have been to type “80% more efficient” (instead of 400%). But large changes in opposite directions simply don’t equalize like that as percentages, though you might expect them to.
Sorry — this has become a maths lesson! Let’s get back to lights, specifically lights on Volvos. A warning and a helpful hint have both come my way. The warning concerns the V50 D5 Sport (and other models too?). If you press the indicator stalk just half-way, the indicators automatically flash just three times.
My owner-informant says that this seemed a great idea at first, but when the novelty wore off he realized how easy it was to flick the stalk lazily for three flashes even though five or six would be more appropriate, when changing lanes on a motorway for example. Also, if he tries to give a quick left-right flash as a thank-you signal to someone behind, the indicators cause confusion in all directions. It doesn’t seem possible to disable this three-flash mode.
The other item may be of interest to owners of Volvos whose dashboards show a reminder light when the next service is due. Suppose the service has been performed early, or you’ve carried it out yourself, or there’s some other reason why you want to extinguish the light. How to do it? Just press the mileage trip-button for a few seconds. That should save you at least one drop of petrol!
Peter Soul
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