When observed from space at night, most cities look very similar.
But Tokyo looks very different.
Unlike most major cities, Tokyo still uses mercury-vapour lamps (which were invented in 1901) rather than sodium-vapour lamps (which were invented in 1920) for its street lighting. The spectra of light emitted by mercury- and sodium-vapour lamps are very different:
Above: the sodium spectrum; Below: the mercury spectrum.
The overall colour of light produced by a sodium-vapour lamp is a bright yellow,* whereas the colour of light produced by a mercury-vapour lamp is a bright turquoise-white.
In the photographs above, Helsinki (top) is using sodium-vapour bulbs for its street lighting (though it still has some mercury-vapour lamps it is replacing those), and Tokyo (bottom) is using mercury-vapour bulbs. In Berlin, the division between the old East German and West German parts is still visible from space due to the different types of bulbs used in their streetlamps.
West Germany (on the left of the image) uses mercury-vapour bulbs, and East Germany (on the right) uses sodium-vapour bulbs.
* Light from a sodium-vapour lamp is almost monochromatic, at 589.3?nm. Optical telescope users prefer sodium-vapour light pollution because it is easier to filter out.
You might not have noticed, but the sale of standard filament lightbulbs has been banned since last September.*
Current proposals are to replace standard incandescent bulbs with more efficient compact fluorescent bulbs. Fluorescent lights work by passing a current through a tube containing mercury vapour. The excited mercury atoms emit ultraviolet photons which then collide with the phosphor coating on the inside of the tube, causing it to emit light (to “fluoresce”) in the visible part of the spectrum.
The problem with this is that some of the ultraviolet light is still emitted (in fact germicidal lamps are basically fluorescent bulbs without the phosphor coating). Many people are sensitive to ultraviolet light and cannot enter rooms lit by flourescent lamps.
People with conditions such as photodermatosis (where exposure to ultraviolet causes swelling, rashes and blistering) and people with photosensitive epilepsy (the 50Hz mains supply flicker is visible from fluroescent bulbs but not from incandescent ones) have been stockpiling bulbs since the ban was announced.
I’m not against the sale of compact fluorescent lightbulbs; in fact I’d like to see them more widely used (especially in place of the inefficient halogen spotlights that seem so popular). What I’m against is removing the consumer’s right to choose.
(If you’re desperate for lightbulbs and you’re wondering, I took the photo above outside of Rugby Electrical, in Rugby.)
* Technically, it’s only the sale of bulbs for household use that’s banned. “Industrial use” is still okay.