Science Questions

Why are streetlights orange?

Tue, 3rd Apr 2012

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Karina, Bishops Stortford asked:

Why are streetlights orange?


The reason that most streetlights are orange is because they contain the chemical sodium.  Some electricity is passed into the lightbulb and this gives energy to the sodium.  Sodium, when it gets excited by the energy, gives out a lot of orange light.  Making it a very cheap, and efficient way to illuminate a very large area.

But to produce white light is very complicated.  To make white light you have to mix lots of different colours of light together and itís the mixture that looks white.  The way in which lamp designers make this happen is to use a mixture of different chemicals. When each of those chemicals gets excited they then produce different colours of light which you then see mixed together so it appears to be white.

Because itís more complicated and involves more chemicals itís more difficult to do and itís more expensive and because orange light does a good enough job, most of the time we use orange sodium lights rather than white ones.


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I do not think that I like this answer very much. It is not really more complicated to produce white light than orange, and white lighting is mass produced for house lighting and building lighting anyway. Sodium lamps are no less expensive than various types of white lamps.

In this part of the world white street lighting is normally used in neighbourhoods and along fairly minor roads, with sodium lighting on freeways and at major intersections on other roads. This suggests to me

(1) that sodium lighting is, at least in this country, more expensive or less available than white lighting of various sorts, and

(2) that there is some visual advantage in the use of sodium lighting for road illumination.

In terms of the latter point, it has been my experience in night driving that sodium lighting is much more restful on the eyes, with much less glare than white lighting, and that it provides a better illumination at street level (except that it provides essentially monotone illumination -- there is no sense of colour in the reflected light).

I wonder if there is an expert on visual perception or road safety about who could revisit this question.
damocles, Wed, 4th Apr 2012

I believe the main reason they were originally adopted in a lot of places was simply because they put out more light for a given amount of power input - in other words, they were more efficient than the alternatives available at the time.

However, things have changed a bit since then, so there may be more efficient alternatives now. Geezer, Wed, 4th Apr 2012

I would agree, it is easy enough to produce white light.

Simply produce a hot filament, such as tungsten, and it produces a broad spectrum of light according to Planck's Law.

One of the problems is that one ends up producing light that is not in fact visible to the human eye in the UV and IR spectrums.

The low pressure sodium vapor lamps produce a much tighter spectrum of light, in a part of the spectrum that human eyes are quite sensitive to.

So, less energy is wasted in light that is not visible to human eyes, and thus they require less power.  Presumably the bulbs also last longer than the typical tungsten bulbs.  Cities, of course, like low maintenance.

I would also wonder if bugs would be less sensitive to the Sodium vapor lamps which might be a distinct advantage.
CliffordK, Wed, 4th Apr 2012

It must be, because a lot of bugs respond to specific wavelengths, and as your graphs show, the sodium lights only have a narrow output spectrum, so a lot less bugs will be affected.. Nizzle, Thu, 5th Apr 2012

Why and how? Bored chemist, Thu, 5th Apr 2012

Why and how?
dunno but do know bug porch lites are yellow so to detract bugs CZARCAR, Thu, 5th Apr 2012

Why and how?

Possibly infrared communication ... RD, Thu, 5th Apr 2012

And, since it's documented that bees can see into the UV it's fair to say that insects can see and respond to a range of wavelengths which rather brings that assertion "because a lot of bugs respond to specific wavelengths" into question. They can see a broader range of the em spectrum than we can. Bored chemist, Thu, 5th Apr 2012

However...  whether or not the bugs can see different frequencies than humans,
Bugs are not attracted to yellow bug lights like they are attracted to white lamps.

Many bug zappers actually use light in the UV spectrum.

There are several theories why bugs are attracted to white lights.  One is that they use the moon or sun as part of their navigation.  One can always fly towards the moon, but never reach it.  On the other hand, if one flies towards a white lightbulb, keeping it in the corner of the eye, one keeps flying towards it from any angle.

Anyway, yellow light is less attractive to bugs.  Perhaps because it is a component of flames, or perhaps because it is often a component of reflected light, like that from grass. CliffordK, Thu, 5th Apr 2012

The "yellow" of the light that we see in a bug light is not yellow light really in the same way that sodium light is -- it is essentially white light with all of the UV and visible violet filtered out: the mixture of ROYGB that remains is perceived by us as the "average" -- yellow. Yellow light like this can be obtained in two ways: a filament operating at "yellow heat", that is, somewhat cooler than the usual white hot filament, or by using an actual "sunscreen" type filter.

Certainly bug vision operates well into the UV in most cases. I am not sure that it extends as far towards the orange-red end of things as ours does.

It is also the case that the ability to see a broad spectrum of light need not mean an inability to discriminate the colours in that spectrum. Modern human beings react very differently to red and green lights, which is just as well, or our traffic would be much more dangerous and chaotic than it is at present. Bugs are probably not as clever as we are, but red/orange/yellow bad, green/blue/violet good might not be a bad life program for a bug in some cases. damocles, Thu, 5th Apr 2012

Except that Red/Green color blindness is the most common type color blind defect. CliffordK, Thu, 5th Apr 2012

Except that Red/Green color blindness is the most common type color blind defect.

....and in Boston, the lights are sometimes horizontal. Arggh! (Mind you, it doesn't make much difference because nobody in Boston pays any attention to traffic signals.) Geezer, Fri, 6th Apr 2012

There's a 3rd way to get yellow light. Look carefully at this :)  and you will see that the yellow isn't yellow at all, it's red and green.
Also re "It is also the case that the ability to see a broad spectrum of light need not mean an inability to discriminate the colours in that spectrum." True enough, but it also doesn't mean an ability to do so. Humans have quite complicated eyes with three different receptor types (typically).
Do bugs?
That was my original question. Do they react differently to different wavelengths and how, and why do they do so? Bored chemist, Fri, 6th Apr 2012

Hmm ...

I might try to google bug eyes, but that sounds like it means something quite different damocles, Fri, 6th Apr 2012

There's a difference between "seeing" and "responding to" BC..

As to the How and Why?
Different pollinators (butterflies, bees, hummingbirds, etc...) typically favor different plants in a highly diverse flower garden because they "respond to" them differently (less or more attracted). This brings mutual benefits for plant and pollinator alike. For the pollinator, there's less competition for their favourite flower in a mixed garden. And for the plant, there's a higher chance of pollination if they are the 'favourite kind' of any given pollinator.

Therefore, a sodium light with a narrow spectrum, will still be seen by the animals, but they will not react to them compared to a street light that would emit over a broader spectrum.
Nizzle, Tue, 10th Apr 2012

Could it be something to do with the fact white light is easily reflected in fog, whereas yellow light is not - like the fog lights in cars. Teresa, Tue, 10th Apr 2012

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