David Madore's WebLog: On urban lighting

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(Monday) · Last Quarter

On urban lighting

This is one of my recurring rants, so I'm surprised to find that I still didn't mention it on this 'blog. In short, I'm a frequent night-walker, and I'm sick of sodium vapor lamps.

This interesting page on types of lamps (in French) compares the following kinds: ordinary incandescent, halogen (incandescent), fluorescent (sometimes improperly called “neon”: true neon tubes emit a red hue and require very high voltage), mercury vapor, (low- and high-pressure) sodium vapor, and metal halide. Incandescent lights (essentially) emit a blackbody radiation, whereas the others have specific emission line corresponding to specific electronic transitions.

The characteristic orange emission line of excited sodium, and consequently the orange color of sodium vapor lamps, is very easily recognizable. Now a specific (visible) narrow emission line means that the lamp has a high luminance-per-unit-power efficiency (over one hundred lumens per watt, if I believe the page I just cited), and since sodium vapor lamps are relatively cheap to produce and last a long time, they are economically viable and have been used extensively in urban lighting. I don't know what it is in other parts of the world, but in Paris many places are lit almost exclusively with sodium vapor lamps. Unfortunately, their specific orange light greatly distorts (or even totally disables, in the case of low-pressure sodium vapor) normal color vision; furthermore, I find it terribly gloomy. It's all right to light highways with this sick orange light, but a city is a different matter. Astronomers may prefer the orange light of sodium vapor (because it's easy to filter out), but when I look at a moderately clouded sky I don't like seeing it pinkish: the sky was not meant to be pinkish, or orange, except perhaps at sunset.

Mercury vapor is hardly better: it is “white”, but it is some kind of sick, blueish-greenish white, and though this may not be very scientific, I find mercury vapor lamps just about as depressing as sodium vapor lamps. I think mercury vapor was used prior to sodium vapor, and is now being phased out. Fluorescent lights are a bit better, but they are not very well suited for urban lighting because their natural shape is in tubes (not easy to put in lampposts) and it costs more to have it in bulb-like shapes. Ordinary incandescent lights are all right in hue (and are often used within houses), but they don't give enough light to be used alone in city lighting (by modern standards, one lux or two is not enough even as lower bound); besides, they don't last nearly long enough.

Halogen lamps and metal halide lamps give a very “good” white (the closer it is to the daylight sun's stimulus the better the white; halogen is not quite as white as metal halide, but it gives a better rendering of colors since it has a continuous spectrum). In places where a good white illumination not distorting colors is a premium, these lamps are used. For example, in Paris, the Louvre, the Hôtel de Ville (City Hall) or Notre Dame are lit at night with such white hues (in the case of Notre Dame, ordinary incandescent lights have been used on the square in front of the church, and the façade is lit with a very subtly chosen palette of whitish or “cream” tones—I don't know exactly what kinds of lamps are used).

But cities would be so much more beautiful at night if we were willing to pay the money to have them lit with a combination of halogen and metal halide lamps (plus some incandescent and fluorescent lamps in specific places, and, of course, specially colored lamps for various artistic effects where desired) and greatly reduced the use of sodium vapor. (In some cases, sodium vapor is all right: for example, the Eiffel tower is lit with sodium vapor at night, and that sort of goes with the style; these days, for the first ten minutes of every hour of the night they have a special “flickering” display of white light on the tower, and the contrast between orange and white is rather nice.) As for mercury vapor, it should be banned altogether.

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