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!
