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PhYSICS IN dAILY LIfE: NEW

III L.J.F. (Jo) Hermans, Leiden University • The Netherlands Email: [email protected] III DOI: 10.1051/ epn /2009404 o t o h p k c o t s i ©

he old-fashioned incandescent light bulb with decided to ban the bulb. ey have a point. ere is no its filament is a marvellous piece of way we can ever make a glowing piece of Tungsten into T . If we it on, it needs only a an efficient light source. For one thing: the emission split second to light up our office, our home or our peak at 3000 K is around 1 μm wavelength, as follows fridge. Sure, this instant reaction is largely due to the directly from Wien’s law. e corresponding emission low heat capacity of the filament. But there is more to it curve has only a small overlap with our eye’s narrow than most of us realize: Tungsten, like all ordinary sensitivity curve at around 0,5 μm. And if we go much metals, has a positive coefficient. Indeed, if higher than 3000 K, the filament won’t last very long. By we calculate the resistance from the bulb’s power and invoking vapour to redeposit evaporated the grid , and compare it with a direct measure - Tungsten back onto the filament, we may get a bit closer ment at ambient temperature, we find that the hot to the of 3700 K. But even if we were able filament has a larger resistance by a factor of 20 or so. to find a high-melting-point metal which could be is means that, if we switch the bulb on, the initial heated to 6000 K (roughly the effective solar tempera - power is very high, making the bulb rush to its opera - ture, with an emission peak that nicely fits our eye ting temperature in no time at all. And the other nice sensitivity), its black-body radiation curve would still be thing is: should the voltage go up for some reason much broader than the eye sensitivity curve, with a lot (which it did by the way, from 220 to 230 V over the of energy wasted. past few decades) the voltage surge will be counterac - What we need is a smart light source which selectively ted by the increased resistance. is dampens the emits radiation that our eyes can see. And which has power increase, and allows the no filament that slowly but surely evaporates. bulb to withstand the So we turned to gas discharge and invented fluorescent surge. Bulbs from the TL long ago, with an efficiency of 100 good old ‘70s or per watt, and – more recently – its folded version ‘80s should have no known as the energy saving , reaching 50 lm/W. problem adapting And, of course, the Light Emitting Diode as its solid- to the 21 st century. state counterpart, with a similar efficiency, depending Alas! e efficiency on the type. Compare this to a poor 12 lm/W for the of the incandescent good old incandescent bulb! light bulb is downright We may not always be happy with politics: when it comes lousy. It is so poor, that to lighting, however, we have to admit that‘Brussels’ has a the members of point. Incandescent light bulbs may be fast and conve - the European nient. eir emission spectrum may be nice and Parliament continuous. But in terms of efficiency, they are beyond recently hope. It’s about time to kiss that bulb goodbye. I

30 EPN 40/2 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn/2009404