In many parts of the world, if you’re taking a stroll outside or driving your car around
at night time, you’re likely to be basked in the orange-peach-gold-yellow whatever you
want to call it glow common in street lighting.
This particular hue of light has been a staple of outdoor lighting for decades, and it continues
to be used widely.
But, why that color?
Well, it’s special kind of light bulb.
Thanks for watching, I hope you enjoyed the video.
No, of course we’re gonna go into more detail than that!
Much of the world’s street lighting (and indeed more broadly outdoor lighting) uses
sodium vapor lamp technology.
The most common is high pressure sodium.
This artificial lighting technology is a type of discharge lamp.
Discharge lamps are very common in many applications because they are a very efficient way of creating
light from electricity.
Among the earliest sources of artificial light is the incandescent light bulb.
Many, many people were working on its invention, so to say one person invented it is disingenuous
at best, so I’m just not gonna say anything because, well, no matter what I say I’ll
be wrong.
But its principle of operation is really simple.
Hot things glow.
Make thing hot.
Make thing hot without burning.
Put hot thing in vacuum.
Now that thing can’t burn away.
Light!
By running electric current through a thin wire, that wire will get hot because of resistance
and it will glow.
Tada!
The earliest light bulbs used a carbon filament, but shortly thereafter improvements in the
ability to make tungsten filaments allowed for a brighter and longer-lasting bulb due
to tungsten’s much higher melting point.
I must recommend this video by the Engineer Guy about the tungsten filament lamp.
It’s great.
I’ve put a link down below.
But anyway, incandescent lights aren’t very efficient.
Creating light through incandescence wastes the vast majority of electrical energy on
radiation that isn’t visible light--simply infrared light and heat.
Among the most common ways to measure efficiency is in lumens per watt, and incandescent bulbs
are typically around 10 lumens per watt.
Really efficient halogens and high-powered incandescents can reach 20 lumens per watt,
but it’s still not great.
But a discharge lamp, well that can be very efficient.
When you send an electric discharge--basically a small electric arc--through an ionized gas,
you get light!
I wasn’t very good at Chemistry in high school, so I’m just gonna read the passage
from Wikipedia which explains why.
Ehem.
In layman’s terms, if you create an arc discharge through some gases, then because
of ions and stuff bouncing around, you’ll end up with photons and depending on what
the gas is, these photons will be a specific frequency and thus will produce a specific
color of light.
I’ve put a link to the Wikipedia article down below because it has some great images
of the colors produced by specific gases.
The most common type of discharge lamp is actually the fluorescent lamp.
The glass tubes in fluorescent lamps are filled with an extremely low pressure gas mixture
made mostly of argon or neon used as a starter gas, and a wee bit of mercury which produces
the main discharge.
Mercury vapor on its own produces a very cyanish blue light, but it also produces a TON of
ultraviolet light, which the phosphors that coat the glass will convert into visible light,
and depending on the phosphors used you can get a very pleasant or very ghastly white
light of various color temperatures.
You can even use colored phosphors to make any color of light you want.
Sodium vapor lamps rather than using mercury use...sodium.
You guessed it!
When sodium is ionized, its discharge is a very distinctive yellow hue with a wavelength
of 589 nanometers.
Low pressure sodium lamps emit light of pretty much just that wavelength.
While useful, and actually the most efficient discharge lamp available, this light is monochromatic
with a color rendering index of zero, so it is only useful as street lighting, and even
then it's not great.
Low pressure sodium lights are going to appear on this channel in the not-too distant future,
but for now we’re sticking with high pressure sodium which is far more common these days.
This is a 50 watt high pressure sodium lamp.
This is the second smallest common size, with 35 watts occupying the smallest spot, but
they go all the way up to 1,000 watts.
In the center of the bulb is a small tube made of aluminum oxide--which for those that
don’t know is what rubies are.
Of course this is synthetic, but due to the crazy reactivity of elemental sodium, it needs
to be contained in something that will A) contain it without reacting with it and B)
is quite strong and can withstand a lot of heat.
Aluminum oxide fits the bill perfectly.
Inside the tube there’s a bit of xenon, as well as an amalgam of mercury and sodium.
The mercury is added in high pressure sodium lamps to control the rate at which the sodium
vaporizes.
It also helps to improve the color rendering of the lamps by adding some blue light to
their output.
Just like any discharge lamp, a ballast is needed to limit the current the bulb can consume.
Due to an arc discharge having negative resistance, as current goes up, resistance goes down,
and power consumed will just skyrocket without a ballast or choke to stop it from destroying
itself.
The ballast will have either two or three components.
First is the actual ballast itself which is similar in construction to a transformer,
then there is the ignitor which is needed to start the lamp, and some ballasts including
this one will place a large capacitor across the leads to help correct the poor power factor
brought about by the inductive nature of the ballast.
As with many discharge lamps, high pressure sodium lamps generally go through a distinct
warmup routine.
When first powered on, the ignitor is working to strike the arc.
Once it’s been struck, it briefly glows a pinky-blue color as the xenon is ionized.
Quickly the mercury starts to vaporize, and as it does so the ionization of the mercury
vapor releases a pale blue color, often appearing as grey.
But then the distinctive yellow of the sodium discharge takes over.
As the sodium vaporizes, the lamp emits a very pure yellow color, which is not at all white.
This is what the light from low pressure sodium lamps looks like.
But then the high pressure in high pressure sodium does its thing.
The arc tube is very small, and the gases inside it become quite hot.
In a space of given volume, with increased temperature comes increased pressure.
This results in a phenomenon called pressure broadening, which causes ordinarily weak spectral
emissions to become stronger, and thus the lamp emits more wavelengths of light.
Once completely warmed up, the pressure broadening causes the light to appear less yellow and
more white, though still with a prominent yellow cast.
Color perception is subjective, but I’d call this a orangey-peachy-gold color with
a hint of pink.
You are… you,
yes?
You are warming up right,
you’re on?
OK Good.
This is the most exciting part of the video, I guarantee it.
I bet you’ve never had a video more exciting than this one.
I don’t even know what this looks like on camera.
So we’ll, we’ll just.
I should--I should have looked into that!
Yeah.
That was good.
This light is extremely efficient, and the output it makes with only 50 watts is pretty striking,
just like its arc.
Ha!
If I put it side-by-side with a 100 watt equivalent LED bulb (in a lamp that would never, EVER
have an HPS bulb in any ordinary setting), you can tell that it’s much brighter.
For half the energy of its incandescent equivalent, it’s producing about triple the light, going
off lumens.
This particular lamp produces 78 lumens per watt, which is 4 to 8 times greater than an
incandescent.
Some high pressure sodiums lamps are nearly twice as efficient as this one, producing
150 lumens per watt.
Also, and this is hard to demonstrate on video, but the light from the sodium lamp appears
to travel farther than that of the “incandescent”.
The walls on the opposite side of the room seem much, much brighter than they do with
a standard white light.
Some of that has to do with the sodium D-line, that’s the main yellow spectral emission,
closely matching the peak sensitivity of the cells in our eyes.
But only under photopic, daylight conditions.
This gets kinda complicated and we’ll get into it, but the peak sensitivity of the average
human eye is 555 nanometers, and the HPS lamp’s peak output of 589 isn’t far off.
But don’t fixate on that too much because, spoiler alert, this turns out to be a bad thing.
Before we move on to their advantages, let’s quickly discuss why we don’t use this light
source for general household illumination.
Every artificial light source has what’s called a CRI, or color rendering index.
A CRI of 0 means it’s impossible to distinguish color, and a CRI of 100 is a perfect score,
which the sun has.
Incandescent lights has a CRI of 99, but most other light sources aren’t so high.
Poor CRI plagued many fluorescent light sources, particularly early ones.
Though the light of a CFL might appear perfectly white, the colors of objects underneath it
might seem a little off.
A cheap CFL might have a CRI as bad as 70, which will be generally OK but which can cause
some colors to appear oddly.
The average high pressure sodium lamps has a color rendering index of about, drumroll please.
21.
It’s pretty bad.
Here’s an assortment of colorful objects as lit by normal white light.
And now, observe how they look under high pressure sodium.
I’ll show them as it warms up, because it demonstrates how when the sodium emission
first comes into play, it is almost monochromatic.
See how you can barely tell what color things are supposed to be?
As the pressure broadening occurs, you can start to see color, but it is still just odd.
The strangest-looking object I discovered was this can of La Croix.
Take a look at this side by side.
The sodium light mutes all of the color differencesin the background, and the text becomes bizzare looking.
Food in general looks…
unappetizing under high pressure sodium lighting.
Pasta and red sauce?
The sauce will look more of a pukey-brown.
Having some yellow tortilla chips?
Yeah they’re more of a chartreuse now.
Of course, their slow warm up time would be inconvenient for home use, so they’re really
suited for general illumination where they’ll run all night.
So, street lights, parking lots, security lighting, and other dusk-to-dawn applications
are where these lamps really shine.
I came up with that all on my own.
For these applications, they’re really great!
They are very reliable, are long-lasting (24,000 hours is typical), usually have no trouble
starting in the coldest of weather conditions, and also they are very color and brightness
stable over their life.
They will usually retain 80% of their original brightness by the time they go out.
And, their color is very consistent.
You’ve probably seen a parking lot with metal halide lighting, another type of discharge
lamp, with each fixture a different shade of purple, green, or bluish light.
Sodium lights generally are all the same looking with little to no variation among them.
They also benefit from being able to perform a hot restrike.
See a metal halide lamp cannot be restarted until it cools down nearly completely.
If there’s a momentary power interruption, it may be 3 to 5 minutes before these lights
can reignite, and then it will be another minute or two until the light is up to full
brightness again.
But high pressure sodium lamps are able to re-strike the arc just a few seconds after
a brief power interruption, and they come back with nearly their peak intensity.
It’s actually kind of neat to see the arc form in the arc tube when a hot restrike occurs.
That said, their end-of-life failure mode is kind of odd.
Over time, the sodium does react with the aluminum oxide, slowly.
This causes the voltage required to maintain the arc to rise as they age.
At a certain point, the arc-sustaining voltage will exceed the voltage the ballast can provide,
and the light goes out.
But, once it cools, it can be reignited.
This process is called cycling.
A cycling HPS lamp will appear to start normally, but once it reaches full brightness, it goes out.
After it cools, it fires up again, and then when it reaches full brightness, it goes out.
This happens over and over again until the lamp is replaced.
So remember, if you see a sodium light going on and off and on and off, it’s not the
fixture at fault, it needs a new bulb.
But in the end, the high pressure sodium lamp is a very efficient, very robust, and very
effective light source for outdoor applications.
It is also very low maintenance, with the lamps lasting about 5 years assuming an average
of 12 hours daily operation.
So even though their color is… odd and they are slow to warm up, they still make a lot
of economic sense.
In recent years, the sodium vapor lamp is starting to be replaced with new LED lamps.
But should they be?
(yeah) Current LED technology is only about the same efficiency of old fashioned high
pressure sodium.
And with a commonly rated life of 50,000 hours, a drop-in replacement may only last twice
as long.
And with a faulty driver it may fail sooner.
Well, the answer is surprisingly complicated.
It turns out that lumens aren’t quite the objective measurement they seem to be.
While the sodium light may have an efficiency of 150 lumens per watt, it might be that in
nighttime conditions, only a quarter of those lumens actually mean anything to our eyes.
In my next video, we’ll talk about current research that suggests our knowledge of light
sensitivity is flawed.
We’ll also discuss the problems of light pollution and circadian rhythm disruption,
and how high pressure sodium and new LED lighting solutions are both double edged swords.
Thanks for watching, I hope you enjoyed the video!
If this is your first time coming across the channel and you liked what you saw, please
consider subscribing!
I’ve put some great links down below that go into the history of discharge lighting,
along with some other great stuff for you light bulb nerds out there.
Of course, thank you to everyone who supports this channel on Patreon!
Patrons of the channel are who keep these videos coming.
If you’d like to join these amazing folks that support what I do, why not take a peek
at my Patreon page.
Thanks for your consideration, and I’ll see you next time!
For the thousands of people, I’m sure, that were wondering how I got the sodium light
to work in a table lamp--this ballast is wired into an extension cord.
As far as it knows, this plug is the lamp, this plug is its power supply.
Not exactly the safest thing in the world but, it does work!