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Streetlights

Published : 4th December 2006

Hello all.

Tonight my wife and I were having a walk, when she posed the question :

"Why are street lamps orange?"

It took me a while to think of it, but I think I remember that the reason the moon looks blue to us is that the blue receptors in our eyes are better at night than the red and green ones, so in the dark, white light will appear blue to us.

So it strikes me that by having a streetlight at night which is a colour far from blue, perhaps this means the street is illuminated for our red and green receptors, without effecting our ability to see in the unlit areas as the blue area of our vision doesn't come in to play - or at least allowing our eyes to adjust faster when we leave lit areas.

Any mileage in that suggestion, or is orange just a really cheap colour for bulbs?

Owen Tooth
Digi-DP
Derby, UK


They are low Pressure Sodium (distinct orange colour) and are near, No I take that back, impossible to colour correct as far as I know.

Gerard Brigante
BL 4s owner / Op


class="style15">>>reason the moon looks blue to us is that the blue receptors in our >>eyes are better at night than the red and green ones

Actually your eyes have 20 times more red and 16 time more green cones that blue. Blue is a colour we don't see well at all. WE never needed to because no food we hunted was blue nor were any enemies so our eyes never developed the need to see much of it. Sunlight is blue (5500k) and so is the reflection of moonlight and that's why it looks blue. In the dark your rods play a bigger role than your cones.

class="style15">>>Any mileage in that suggestion, or is orange just a really cheap colour >>for bulbs?

Sodium vapour lamps are easy to make as the gases used are cheap, easily obtainable, plus the fixture runs very efficiently (up to 80% of the electricity is converted to light) and it is relatively stable over time.

Since we see orange better than blue, it is a popular colour over mercury which is not as stable and one of the reasons why many night lights are sodium as you hinted. Of course we give up colour reproduction with them but it lets you see at night and that is all that matters.

--
Disclaimer :

My opinions, thoughts, and beliefs are my own and may not reflect yours. The use of the pronouns "you, "some", and "many" to name a few are generalizations and without a proper name attached to them are not references to anyone reading my posts.

Walter Graff
Director
BlueSky Media, Inc.


class="style17">>>Any mileage in that suggestion, or is orange just a really cheap colour >>for bulbs?

class="style17">>>They are low Pressure Sodium (distinct orange colour) and are >>near, No I take that back, impossible to colour correct as far as I know

These lamps are (were) chosen for maximum light output per watt (cheaper to run). Shame it makes everything look sh*t brown.

Astronomers use a filter to cut out this wavelength by the way


Andy Taylor
Camera Engineer
Arri Media
3 Highbridge
Oxford Road
Uxbridge
Middlesex UB8 1LX


Owen Tooth wrote:

class="style17">>>It took me a while to think of it, but I think I remember that the reason >>the moon looks blue to us is that the blue receptors in our eyes are >>better at night than the red and green ones, so in the dark, white light >>will appear blue to us.

When I look carefully at moonlight cast upon an open field or a person, with no other light sources, moonlight is monochromatic. It doesn't really have a colour - for me. I believe the colour receptors are not as sensitive as the monochrome receptors in our eyes, which is why the image is monochrome.

However Moonlight is just the suns rays bounced off the moon, so the "colour" is probably around 5600K, when you have another source say a flashlight, or fire with a much lower colour temp, which is brighter than the scant foot candles the moon is reflecting, then your brain/eye combination adjust for the flashlight or fire, and the "moonlight" looks blue.

This plus years of having moonlight being represented as blue have worked their secret psychological agenda.

As far as the streetlights, Ugh- I think Caucasian (Sp) skin tones look absolutely grey under them. Probably they are the design and colour having to do with cost of purchase, and cost of producing some visible light per kilowatt hour.

Steven Gladstone
New York Based Cinematographer
Gladstone Films
www.gladstonefilms.com


class="style17">>Any mileage in that suggestion, or is orange just a really cheap colour >for bulbs?

Owen,

I think the lamps you're referring to are "low pressure sodium". They emit a light that is truly monochromatic - that is, they produce orange and ONLY orange light. No amount of filtration will fix monochromatic sources.

The reason of course is efficiency. Very high lumen output to power input ratio. Perfect for street lighting... Er, wait a minute... Turns out they are dangerous in that green cars can't be seen. Hence I thought LPS bulbs had been phased out of street lighting duties years ago? High pressure sodium is currently the fashion as the spectral distribution is just that little bit broader.

And seeing this is CML-Chat :

LPS is always a good choice for pub car-park lighting. Stand back and watch with amusement the tanked patrons who own green cars. At closing time they're wandering around trying to find their cars.

Clive Woodward,
the moon's only blue in film and TV!
Perth, Western Australia.


class="style17">> and cost of producing some visible light per kilowatt hour.

Well they're efficient like other discharge lamps, mercury vapour, or HMI's which are a kind of mercury vapour lamp.

IOW not putting out large amounts of IR which tungsten lamps do...

The older sodium lamps are very yellow looking (there were some on the West Side Highway, I don't know now).

Really they are radiating in the spectrum of sodium plus whatever else they throw in there...

Sam Wells


class="style17">>They are low Pressure Sodium (distinct orange colour) and are near, >No I take that back, impossible to colour correct as far as I know.

From the WikiPedia article on "Sodium Vapour":

"These lamps produce a virtually monochromatic light in the 589 nm wavelength. As a result, objects have no colour rendition under a [Low Pressure Sodium] light, only the reflection of the 589 nm light."

Our colour visual system works largely by comparing the ratios of wavelengths emitted from a given object with the ratios emitted from neighbouring objects. This is known as colour constancy.

Another quote from WikiPedia, this time from the "colour constancy" article :

"Colour constancy works only if the incident illumination contains a range of wavelengths. The different cone cells of the eye register different ranges of wavelengths of the light reflected by every object in the scene. From this information, the visual system attempts to determine the approximate composition of the illuminating light. This illumination is then discounted in order to obtain the object's "true colour" or reflectance: the wavelengths of light the object reflects. This reflectance then largely determines the perceived colour. The precise algorithm used for this process is not known."

Yes, our eyes loose much of their colour sensitivity in low light because our colour receptors are less sensitive than our monochromatic sensors.

Jack Kelly
Dir / Prod / Camera
London


Jack Kelly (AKA Daniel) wrote :

class="style17">>> "These lamps produce a virtually monochromatic light in the 589 nm >>wavelength.

IIRC, part of the reason for using low sodium lights is that, due to the single line nature of the light output, it is easier for observatories and others who care to filter out the "light pollution" caused by these lights.

John Gilman - Zoundz Audio
Vidiot, Sound slut, rabble rouser
Beautiful Downtown Burbank


class="style17">>>Any mileage in that suggestion, or is orange just a really cheap colour >>for bulbs?

The use of the lurid yellow low pressure sodium (SOX) street lamps isn't global. The UK is one of the more prominent users of these garish lamps.

The reason? They are the single most efficient light source available.

So yes, it's a really cheap colour for bulbs from a power perspective.

class="style17">>>Sodium vapour lamps are easy to make as the gases used are >>cheap, easily obtainable, plus the fixture runs very efficiently

I wouldn't say they're EASY to make. The glass has to be coated inside to prevent it being attacked by the sodium.

Incidentally, the reason they start off pink when lighting is because a neon discharge is used to vaporise the sodium.

Her in the murder capital of Europe (Glasgow, Scotland) they are completely useless at helping people identify their attackers due to the dreadful colour rendering.

Clive Mitchell
http:/www.bigclive.com


class="style17">>I wouldn't say they're EASY to make. The glass has to be coated inside >to prevent it being attacked by the sodium.

It was expensive when they started but with today's indium oxide films it's easy and cheap too. Lumen for lumen it's the best lamp out there with a three year service schedule and a efficiency that is far higher than high pressure sodium and double metal halide retaining 90% of it's original output after three years. Nothing else made comes close.

Disclaimer :

My opinions, thoughts, and beliefs are my own and may not reflect yours. The use of the pronouns "you, "some", and "many" to name a few are generalizations and without a proper name attached to them are not references to anyone reading my posts.

Walter Graff
Director
BlueSky Media, Inc.
www.bluesky-web.com


I believe they're also popular because the bulbs last for a very long time. Kind of a pain to change those bulbs.

Phil Badger
LA


 

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