Last week I received my (dare I say annual?) invitation to be a judge at the Waikato Science Fair. This is a great event where school children get to show off some of the science projects they have been working on. It fills the pavilion at Hamilton Gardens and is a great exhibition to look at.
It covers all areas of science, though of course the projects that most interest me are the physics ones. There are always some really great, novel projects, like one on range of wireless internet, or measuring the speed of an arrow, but alongside those there are a few experiments (usually comparisions of one thing against another) that crop up again and again every year, and begin to be a little tedious.
One is measuring the light output from energy saving light bulbs (e.g. compare the fluorescent ones with the incandescent ones.) Now, it is interesting to do this comparison, especially if you are wondering how on earth you can get the same amount of light with so much less power input, but it’s not very novel, so to save you the trouble, here is my explanation.
Now, light is a form of energy, and to generate it you need to be supplying power to the bulb. There is no way around that. But in the case of incandescent light bulbs, only a small fraction of the power you supply actually gets turned to light. The rest goes to heat. (Not that that is necessarily bad in itself; in winter that heat is welcome – but unless you’ve got good ceiling insulation most of it probably escapes straight out of your roof)
This is because the incandescent bulb gives off energy at a range of different frequencies. Much of it is in the infrared region of the electromagnetic spectrum (the bit beyond red) which we simply can’t see (but experience it as heat). However, the fluorescent tube has its emissions more tightly in the visible region of the electromagnetic spectrum – where we can see it.
We measure light output in lumens – crudely speaking this is a measure of the amount of power coming off a light source that we can see. An incandescent bulb will supply about 15 lumens for each watt of electrical power input; a fluorescent bulb more like 50 – 100 lumens for every watt. You should find this value written on the packet the bulbs come in. A sodium street lamp will give you about 200 lumens for every watt (these are pretty efficient things so long as they’ve got reflectors on the top so they don’t chuck half their light into the sky where it’s no good to anyone except airline pilots). The theoretical maximum is 683 lumens per watt. This is where the light source emits light at exactly 540 times ten to the power 12 (540 trillion) hertz, which is where the human eye is most sensitive.
Illuminance levels (‘light levels’) are measured in lux – one lux means one lumen falling on one metre squared of surface. This is what your lux meter (light meter) measures. But please do something more useful with it than point it at a light bulb – what about measuring the extent to which sunlight penetrates through different kinds of cloud?
This year the science fair runs from Thursday 20 – Saturday 22 August – if you are in Hamilton go along to the gardens and have a look.