Lightbulbs are Really Interesting

If I was asked to imagine a title for the most boring talk ever, I might possibly suggest a talk about lightbulbs.
This was the subject of this month's Science Café - and it was actually fascinating.
There was a lot of interest in the subject - we only just fitted into the room next to the front door of the Swan, and we may re-locate to the bigger room at the back if there's as much interest in future talks.
The speaker was Barry, who is an electrical engineer, and his knowledge was broad. He started off with some background information before we got anywhere near lightbulbs, on the visible spectrum of light, and the wavelengths to either side of what we can see - down into infra-red and radio waves, and up through ultraviolet to X-rays and gamma rays.
There's a mast in Droitwich that transmits the BBC World Service all around the world! Mobile phones are higher up the wave-lengths, and typically need to be within 3 to 5 miles of a mast, and 5G will mean a phone has to be even closer to a mast, because of all the extra information the signals are carrying. There was enough interest, and enough information, for a whole talk on mobile phones, but after a short digression we were back on the track of light bulbs.
Incandescent bulbs have a tungsten filament inside, and at the moment there's a fashion for these to look quite "retro" - like the new lights in Kilverts that hang from little pulleys.
Eventually, though, the filaments evaporate, and a lightbulb was passed around that had black marks on the inside of the bulb where this had happened.
To increase efficiency, halogen bulbs were invented - the bulb is filled with a halogen gas, (the halogen family of elements is Fluorine, Chlorine, Bromine, Iodine). These re-deposit the tungsten on the filament, thus extending its life. Halogen bulbs are used for all sorts of purposes because they are cheap and reliable. They are also good because they provide a broad spectrum of light. The incandescent bulbs and LEDs fool our eyes into thinking they are shining with a white light, but its usually really blue and yellow with gaps in the spectrum.
Neither of these types of lightbulb are very efficient at producing light - it's about 2% light and 98% heat, which is why you get lightbulbs to keep eggs and chicks warm in poultry farming. There was also some discussion of car headlights and the shape of the beams - and why LED headlights can be so blinding!
LEDs are a completely different form of light source. They are made from a very pure crystal that does not occur in nature, and this is where we get into semi-conductor territory. The machines that make these crystals are immensely expensive, and only about half a dozen companies in the world can make them - then sending the crystals on to China to be assembled into the actual lights. Each light, though, only needs a tiny crystal, which needs to be embedded in plastic with wires added to it for the electrical current. He passed a box around with examples, some of which were like dust particles. He also passed around a long strip with LED lights every inch or so along it - each being a cluster of red, green and blue lights, so when they were all on together they produced white light, and then each colour could be switched on individually to make red, green or blue light. I liked that bit - the colours were pretty.
The name stands for Light Emitting Diodes, and they are very energy efficient and very long lasting. However, Barry said that when councils started replacing their street lights with LEDs en masse, this was a mistake, because they were not quite as efficient as the manufacturers claimed - they are improving all the time, though.
One chap in the audience was particularly interested in the best light for use in photography - and such a light does not exist commercially, though Barry brought along one he had made himself. He also demonstrated, with some clever equipment, how lights switch themselves on and off too fast for the eye to see, because of Alternating Current in the electricity supply. However, we are subconsciously aware of it, which is why some people are sensitive to fluorescent lights and get headaches.
It was an excellent talk, and I really enjoyed it - and I learned a lot!

The next Science Café is on Monday 24th February, and is about Refrigeration and the Quest for Absolute Zero.