In my last video, we talked about the high
pressure sodium lamp and its ubiquitous use in outdoor lighting. Already this type of lighting is starting
to be phased out with various technologies, with new LED technology among the most common. Now, given the robust nature of the sodium
lamp and its proven track record in providing an efficient light source reliably, does it make sense
that we switch to newer light sources? Well, as usual in life, there are pros and
cons. So let’s start with a basic question: why
do we use outdoor lighting? For street lighting, specifically, the basic
answer is to improve safety, particularly pedestrian safety. The odds of a crash of any kind are greater
at night, because it’s harder to see. With the aid of artificial roadway lighting,
a driver can see much farther than their car’s headlights shine, especially to the sides. Now of course there are disadvantages to large
amounts of street lighting, which we’ll get to, but assuming safety is the goal, are
sodium lamps good? Well, no. Not really. Remember how I said that the Sodium D-line
is close to our eyes’ peak sensitivity, but only under photopic daylight conditions? Well it turns out that our eyes see quite
differently at night than during the day. Under nighttime, scotopic lighting conditions,
our eyes actually see bluish light better. And that makes sense--after all, moonlight
and starlight are pretty bluish, so under these dimly lit conditions, having a greater
sensitivity to blue light would mean we can see better. Under scotopic lighting conditions, only the
rod cells in our eyes are activated. Rod cells cannot distinguish color, but they
are much more sensitive to light than the cone cells. The peak sensitivity of the rod cells is around
498 nanometers, which is a green-blue color. Now of course under street lighting our cone
cells are still active--we can after all see colors and are not exclusively using the rod
cells. This dim-but-not-quite-dark lighting scenario
is often called mesopic vision, a mix of the two. Still, stimulation to the rod cells will be
far more visible and is more important. So then, how well does the light from sodium
vapor lamps line up with our scotopic light sensitivity? Not well. This is the CIE 1951 scotopic luminosity function
graph. The X axis is the wavelength of light in nanometers,
and the Y axis is the eye’s relative sensitivity to these wavelengths under scotopic nighttime
conditions. As you can see, peak sensitivity is around
the 500 nanometer mark. And where’s the wavelength produced by a
sodium vapor discharge? It’s about here, 589 nanometers. The rod cells are barely activated by a sodium
discharge. While the discharge may be extremely efficient
at producing visible light, at night time this light is fundamentally misaligned with
our eye’s sensitivity. See, if you look at the response curves of
scotopic and photopic conditions together, you can see that the sodium discharge lines
up great with our photopic vision. But when our eyes are adjusted to nighttime
lighting conditions, it’s actually pretty bad. This is why focusing on the sodium D-line
emission’s seemingly perfect alignment with our vision is somewhat of a farce. While it’s true during the day, it’s literally
quite far from the truth at night. Aside from the simple spectral misalignment
of the sodium lamp, research shows that people can indeed see better under light sources
with a bluer spectral content. In Peter Morante’s research for the Lighting
Research Center at the Rensselaer Polytechnic Institute (link below), survey respondents
strongly preferred the light from a 6500k correlated color temperature light source
over that of high pressure sodium, with metrics of visibility, brightness, safety and security,
color rendering, and overall preference all favoring the newer light source, which also
used only 55% of the energy of the high pressure sodium lamps it replaced. The study, which by the way is really comprehensive
and worth taking a look at, was done in 2008, which was a tad before LED technology became
economically viable. The study compared induction lighting, which
is sort-of like fluorescent lighting (and worth a video on its own one day because it’s
pretty neat if not so practical any longer) as well as metal halide lighting to the sodium
lamps for the purposes of creating a recommendation to a local utility company. The conclusion was that metal halide didn’t
make sense due to higher maintenance costs, but induction would make tons of sense. But I also liked this final recommendation: Well, it’s been about 10 years time since
that study, and LEDs are economically viable. In fact, they’ve become incredibly economically
viable. But before we move on to them, let’s go
over the main issue once more. Because of the spectral misalignment of the
sodium vapor lamp with our scotopic (and mesopic) light sensitivity, it takes more light output
from a sodium lamp to produce the same visible light level from a bluer light source. In fact, Dr. Alan Lewis of the New England
College of Optometry measured individual’s response time to a hazard approaching from
the sides, and found that in well-lit areas such as a major motorway, a high pressure
sodium light system needs to produce 3.9 times as much light as a metal halide source to
achieve the same response time. The effect is even greater in dimly lit areas,
where he found that 7.8 times as much light from high pressure sodium was required to
match response time under cooler, metal halide light sources. So it seems as though the high pressure sodium
light is less efficient that it appears on paper, and that roadway safety is greatly
increased when light sources are used that are tuned to our scotopic and mesopic light
sensitivity. It’s looking pretty bad for high pressure
sodium. But there’s one thing that HPS technology
doesn’t do that bluer light sources might. That, my friends, is circadian rhythm disruption. Research has suggested that the color of light
we are exposed to has a great impact on what time our biological clocks think it is. The shorter wavelength of daylight sun and
blue sky may help keep us awake by suppressing melatonin production, and the long wavelength
light of the sunset may signal our bodies to sleep by allowing melatonin to seep into
our blood streams. To help us understand the impact different
light sources can have on circadian rhythm disruption, light sources are characterized
by their melanopic content. There’s a great source from the US Department
of Energy linked down below which goes over this in much greater detail, but as a general
overview, with high pressure sodium technology normalized at 1 for both scotopic light content
and melanopic light content, a metal halide lamp with a color temperature of about 4,000K
will have about 2.5 to 2.8 times as much scotopic light content, but also produces 3.16 to 3.75
times as much melanopic light content. If you look at the various color temperatures
of LED light sources, you can see that as the color temperature increases, both scotopic
light content AND melanopic light content increase. However, melanopic light content increases
at a greater rate than scotopic light. What this means is, there’s a trade-off. And it’s complicated. The higher the color temperature, the more
scotopic light it produces, which means you could use less light (and thus less energy)
to get the same perceived brightness and safety levels. But, because melanopic light content increases
at a greater rate, although you may need less light of a higher color temperature, it will
disrupt circadian rhythm to a greater extent. You can see that high pressure sodium has
among the lowest melanopic light content of any light source, with only amber LEDs and
low pressure sodium producing less melanopic light. So then, we’re presented with a choice. We can clearly use less energy and expect
a safer nighttime driving experience with light sources of higher color temperatures,
but this may cause unwanted side-effects that sodium lighting largely doesn’t. And this is completely ignoring aesthetic
preferences. I myself generally detest cooler lighting,
as I find it harsh and unpleasant. But I can acknowledge its safety advantages. In my area on Interstate 88, many of the roadway
lights have been changed from high pressure sodium to LED. The change is dramatic, with visibility greatly
enhanced once under the cooler light. As much as I don’t like the color, I can
tell it’s a lot safer. Visibility in my periphery is tremendously
better, and I’m certain these new lights are using less energy than those they replaced. Before I move into the conclusion of this
video, let’s discuss the issue of light pollution. Light pollution is exactly what it sounds
like--excess light in our environment that is irritating, unnecessary, poorly distributed,
or in general unwanted. Outdoor lighting is by far the most prolific
source of light pollution, and it has gotten so bad that people living anywhere near a
city can barely see the night sky. There’s even an anecdote about a widespread
blackout in Los Angeles causing many panicked 911 calls from lifelong residents who had
never seen the Milky Way before and were a little scared of it. Light pollution is a complicated problem,
but the LED may actually help to solve it. Now to be clear, the solutions I’m about
to offer aren’t exclusive to LEDs, but the way light is emitted from an LED chip makes
controlling it relatively easy. So first, let’s discuss one of the biggest
causes of light pollution; lights that point up. This is less obvious than it seems, but it’s
incredibly important. Right outside my apartment are drop-lens cobra
luminaries containing high pressure sodium lights. Because the lens protrudes downward from the
fixture, a lot of light escapes to the sides and indeed upwards. I’m on the 4th floor of my building, and
my eye-level is above these street lights, but I can still see the source of the light. This is not ideal for a number of reasons. First, a lot of light is being wasted by lighting
up things that are not the road. That’s kinda dumb. But secondly, a lot of this light is going
up into the sky. Granted, this style of fixture isn’t the
worst offender, but a better design would be a flat lens that does not allow light to
escape above the horizontal. This may still throw light farther to the
sides than necessary, but none of it will end up lighting the sky. The worst offenders for this kind of light
pollution are lights that illuminate buildings by shining upwards, wall-pack lights without
shielding, and these decorative fixtures. I’ll admit they’re pretty, but they’re
really wasteful. Recently I was on an airplane flying into
Chicago at night. I took some video as we landed, and you can
see the difference between a well-managed light and a poorly managed light. This roadway is lit well. I cannot see the actual light sources, I can
only see the reflected light from the road. That’s what we want. As we got closer to the airport, these neighborhoods
had tons of lights that were visible from above. Much of the light produced by these lamps
is shining into the sky and being wasted. I should not be able to see the actual light
source from an airplane, yet I can. This is contributing to skyglow. Skyglow is what makes the night sky hard to
see when you’re close to a city. This is probably the most widespread light
pollution problem, and while it’s not caused exclusively by poorly designed fixtures, they
are a major component. But once again, the solutions to skyglow are
complicated. Largely because light sources that cause the
least skyglow are high and low pressure sodium. In fact, low pressure sodium is used widely
around large astronomical observatories because their nearly monochromatic light output can
easily be filtered out, eliminating any skyglow they create. I saw a large number of people saying that
high pressure sodium can also be filtered out, but I don’t think that’s true due
to the pressure broadening and their spikier output. Someone correct me if I’m wrong but I could
only find references to low pressure sodium being used around observatories. Anyway, where it gets tricky is that an LED
light source has about three times as much sky glow impact than a high pressure sodium
light. But also, you need less of it, so perhaps
the sky glow impact is similar. In addition, the sky glow impact of incandescent
lighting is barely higher than low pressure sodium. So it could be that LED street lighting correlated
to a 2700K color temperature causes less sky glow than high pressure sodium. But I think further research needs to be done
there. In any case, what makes LEDs potentially much
better at reducing skyglow is the optical systems that can be combined with them. Early LED fixtures may have used a large number
of small 1 watt LEDs and tiny lenses to direct their light. Some really bad designs may have simply had
an exposed chip, prodiving little directional control. You still see this a lot in cheap flood light
fixtures. But newer fixtures like these from Cree will
use large chip-on-board emitters, like these 10W chips but larger, and because they only
emit light in one direction, it’s very easy to control their output with a lens. You can see on the spec sheet that there are
5 lenses in total, though larger fixtures have more. Most importantly, the optic system is
customizable. Depending on fixture height and spacing, you
may need a wider spread of light or a shorter one. Due to the customizable optics, you can get
wonderfully consistent lighting on a road surface such as this area here. This may also help to prevent light pollution
because less overall light is needed. The hotspots of light you see from above here
mean some areas get too much light, and others get too little. A more scotopic light source with better and
more consistent control may not only reduce light pollution, but may use less energy and
provide safer driving. Now many of these developments are rather
new, especially the newer optic designs. But the advantages of LED lighting become
confused when drop-in replacement bulbs are used. I don’t have any major issues about going
this route--after all replacing the entire fixture can be costly, and some drop-in designs
are fairly good. But the optic system of a sodium, mercury
vapor, or metal halide fixture is designed to reflect and project light emanating from
a tiny arc tube, which an LED drop-in can’t recreate. Many of the complaints regarding poor light
distribution in LED replacement lamps may simply be from the use of these replacements. Then there’s the issue with existing ballasts. Some drop-in lamps claim to work with existing
ballasts, but I have a few misgivings regarding how well their power supplies deal with the
voltage the ballast provides--particularly if the high voltage ignitor sends some crazy
voltage spikes to the LED drop-in. I’m sure they can be designed to cope, but
it still worries me somewhat. So then, we have a series of complicated choices
to make. Sodium vapor lights are pretty efficient,
have only a moderate contribution to sky glow, cause only minor circadian rhythm disruption
if any, and have a proven track record of reliability. But their color also makes them far less effective
at improving safety, and due to the misalignment of their output with our scotopic light sensitivity,
they require more light (and thus use a lot more energy) than a whiter light source. Perhaps less an issue but still important
is that they contain both mercury and elemental sodium, meaning their disposal is far more
dangerous and complicated. If we were to switch to a white LED source
with a color temperature of about 5,000 K, nighttime visibility would be greatly increased. Studies have shown that people see hazards
far sooner under this light, and as the bluer wavelengths match our scotopic and mesopic
color sensitivity more closely, we can use less of it while also achieving a greater
safety benefit. This reduces the need for energy. However, this bluer light contributes more
to circadian rhythm disruption and skyglow, but some of that is mitigated by the lower
output required by this light source. Still, many people may not find the aesthetics
of this light source pleasing. One possible compromise would be to use a
warmer color temperature LED light source. Data from the US Department of Energy tells
us that a 3000K LED light would produce 1.89 to 2.39 times as much scotopic light as a
high pressure sodium lamp, while increasing melanopic content between 2.1 and 2.99 times. Because of its greater scotopic output, a
3000K LED replacement should only need to produce about half as much light as a high
pressure sodium lamp. This effectively cuts the circadian rhythm
disruption potential in half, too, placing it near about the same as high pressure
sodium. The greatest downside to using a warm color
temperature LED is in their efficiency. The efficiency of these lights is very similar
to that of an average high pressure sodium lamp. At 67 lumens per watt, these 3,000K LEDs are
just slightly less efficient than the sodium lamp featured in my last video. Although you would need only about half as
much light output, there are HPS lamps which approach 150 lumens per watt. This would mean that a 3000K LED will use
just about as much energy as a very efficient HPS lamp. I’d still call that good, but it makes replacement
less compelling. To normalize the effects of scotopic light
content, I’ve multiplied the lumens per watt number by the scotopic light content
for the following light sources. As you can see, the normalized efficiency
of the LED goes up considerably as the color temperature does, due to both luminous efficiency
and greater scotopic content. This is likely why most LED street lamp installations
are done with the blueish 5700K and higher color temperatures. You can use the least amount of energy to
produce the same amount of perceived brightness and safety. However, the normalized efficiency of even
the 3000K LED is very similar to that of high pressure sodium, and few HPS lamps actually
output 150 lumens per watt. Also, the calculated lumens per watt of the
LED is based on the input power of the fixture, so the losses in the ballast (which are fairly
high for high pressure sodium) aren’t accounted for here. To conclude, although the high pressure sodium
light is very efficient, its primary output color is misaligned with our nighttime visibility. Only about a quarter of its light is actually
effective at stimulating the cells in our eyes. Although the cool color temperature of many
LED replacements is harsh and aesthetically displeasing, studies have shown that it is
not only more efficient but also makes driving at night safer. There is however the potential for greater
circadian rhythm disruption and larger amounts of skyglow using these bluer light sources. Still, it seems clear that the high pressure
sodium lamp is on its way out. Advancements in LED technology are happening
at a breakneck pace. Just 10 years ago they weren't seen as viable. But today, even the least efficient of LED
replacements ends up meeting the efficiency of high pressure sodium when scotopic light
output is considered. As it stands in 2018, we are faced with a
choice of efficiency over aesthetics. I’m pretty sure I’d enjoy roadways lit
with the relatively warm 3000K LEDs, and these also wouldn’t disrupt sleep much. But you can save a lot more energy (and potentially
have safer roadways) with 5700K lighting. Either way, it seems clear that LED technology
will very soon overtake the tried-and-true high pressure sodium lamp, just as the HPS
lamp itself replaced the mercury vapor lamp. And in 40 or 50 years, who knows what technology
might light our roadways. So I have a couple of things to close out,
first you may have noticed in my chart that the mercury vapor lamp had a normalized efficiency
of over 100 lumens per watt, and the 50 watt sodium lamp in the last video was only 78
lumens per watt. Mercury vapor bulbs do have considerable operating
disadvantages compared to HPS, most notably their steady decrease in light output as they
age, but I think it is somewhat humorous that our current understanding of the visual system
suggests that sodium light may have been a step backwards in some situations. You may have noticed that I didn’t talk
about the blue light from LEDs and how this is supposedly ruining our eyes--that’s because
the “science” behind this is questionable at best. You can clearly see in this chart that there
is blue-light content in nearly all light sources, and lower color temperature LEDs
have less blue-light content than their higher color temperature varieties. I don’t doubt that blue light can disrupt
circadian rhythm--that much seems certain. But considering that our eyes can withstand
the intensity of sunlight, which is far far greater than any normal artificial light source
and also has a lot of blue light (and ultraviolet which definitely IS harmful), I think the
blue light thing is just fear mongering. If someone can point to some verified, peer-reviewed
research supporting this, and not a dodgy website, I’ll consider changing my stance. In any case, the high flexibility of LED technology
means that it can be tuned in pretty much any way you like. I also want to give a shoutout to VWestlife
for the suggestion of LED fixtures with both high and low color temperatures that will
switch to the warm light later in the night. I think that’s a great idea, though obviously
it would add expense to any fixture. However, I was surprised to learn that the
Cree LED fixtures I’ve been using as a reference are all capable of dimming, and they have
a 0 to 10V control input to enable this. Reducing light levels to perhaps 50% of normal
after midnight might become a common practice, and I think that would be pretty wise. Maybe this will get combined with technology
similar to Philip’s warm-glow and we’ll get incandescent-like lighting at night. For those worried about light pollution for
astronomical observatories, there are amber LED street lights available designed to replace
low pressure sodium lights. This is also great news for wildlife--many
animals cannot see the wavelength of light produced by low pressure sodium, so this light
source is used where lights may be disruptive. One particular example is near beaches where
sea turtles lay their eggs. After they hatch, baby sea turtles follow
moonlight to the ocean, and street lighting was confusing the poor things and they were
travelling inland. Since they cannot see the wavelength of a
low pressure sodium light or its amber LED equivalent, they aren’t confused and successfully
make it to the ocean where they belong. As a last little tid-bit, the spec sheet from
Cree says that their LED cobra head replacements should produce at least 95% of their original
light output after 100,000 hours. Assuming the driver and heat sink are robust
enough, these fixtures should last well beyond 20 years. That is impressive. Thanks for watching, I hope you enjoyed the
video! If you haven’t subscribed to Technology
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I really liked the discussion of human sensitivity to light spectra and its affects on safety and circadian rhythms.
One issue is that he doesn't compare spectra so much as light frequencies in nm. Also, there isn't any discussion of CRI :/
LEDs could light the way, but only in warm white.
also led lights can have compact optics like tir lenses that will reduce wasted light.
I love the low pressure's yellow light, it is kind of nostalgic and warmto me, especially when it's raining. Whenever i walking under the cold light of LED. I feel loneliness, cold, but kind of exciting In sumary. Low pressure light makes me think about past, and Led make me more about future
He cites that sunlight has blue light and UV light, known to be harmful. But he does not mention the intensity or the situations it can be harmful.
Lets say in the dark you are using a UV light. Your pupils will be larger than in the sunlight and you absorb more of the UV light, damaging your eyes more. That much seems obvious.
If you stare at the sun I am sure your eyes would be damaged. By which light I do not know. But to equate sunlight meaning that blue light is not harmful is not a good way to respond to it.
I'm actually subscribed to this channel. He has a lot of really in-depth, well researched, and just interesting videos. He did one a little bit ago about LED traffic lights too, would recommend!
Important factor ignored for road safety issues : weather.
Many years ago before LED lighting, where I lived there was a "ring" interstate with two very similar interchanges at the North and South connections, both pretty far out from the city center. They were built a few years apart, and for whatever reason one used mercury lighting while the other used sodium. (the one end was near the airport, which may have been a factor)
In normal nighttime conditions the mercury lighting made the surrounding area feel subjectively easier to view, so it at least felt safer even if it actually wasn't. But in rain, mist, or fog the glare from the mercury lights bounced light all over the place and it was almost visually painful to be there. I always tried to avoid that interchange in that type of bad weather.
Even if those conditions exist only a few times per year, I think it's worth planning for to prevent gridlock or catastrophe.
My personal belief is that more advanced onboard vehicle lighting systems (auto-dim, beam shaping, and overall much brighter) could mostly remove the need for massive roadway lighting systems. City lighting could then be adapted more for pedestrians, or apply the same philosophy there and make everyone carry their own light to see / be seen by.
Random related fun story ... Back in the 1960s my father lived in a pretty rural area of Ohio, and he installed what were then super bright high beams on his '68 Dodge Charger. The package (he ordered them from J.C. Whitney) they came in actually said "aircraft landing lights" on it. When he moved to Florida in 1969 with my mom, he had her take the car through vehicle inspection which at the time had a measurement of high and low beam headlight brightness. (I remember the process from when I was maybe 3 or 4 years old – they rolled out a meter and lined it up with the lights at a set distance away from the front of the car) They had her flick the brights on and the guy running the test freaked out because the meter pegged, so my dad had to replace those lights before the car could be re-registered.