1 00:00:00,070 --> 00:00:04,090 In my last video, we talked about the high pressure sodium lamp and its ubiquitous use 2 00:00:04,090 --> 00:00:05,620 in outdoor lighting. 3 00:00:05,620 --> 00:00:09,509 Already this type of lighting is starting to be phased out with various technologies, 4 00:00:09,509 --> 00:00:12,150 with new LED technology among the most common. 5 00:00:12,150 --> 00:00:16,289 Now, given the robust nature of the sodium lamp and its proven track record in providing 6 00:00:16,289 --> 00:00:20,939 an efficient light source reliably, does it make sense that we switch to newer light sources? 7 00:00:20,939 --> 00:00:24,170 Well, as usual in life, there are pros and cons. 8 00:00:24,170 --> 00:00:28,359 So let’s start with a basic question: why do we use outdoor lighting? 9 00:00:28,359 --> 00:00:32,520 For street lighting, specifically, the basic answer is to improve safety, particularly 10 00:00:32,520 --> 00:00:34,200 pedestrian safety. 11 00:00:34,200 --> 00:00:38,270 The odds of a crash of any kind are greater at night, because it’s harder to see. 12 00:00:38,270 --> 00:00:42,510 With the aid of artificial roadway lighting, a driver can see much farther than their car’s 13 00:00:42,510 --> 00:00:45,110 headlights shine, especially to the sides. 14 00:00:45,110 --> 00:00:48,170 Now of course there are disadvantages to large amounts of street lighting, which we’ll 15 00:00:48,170 --> 00:00:52,330 get to, but assuming safety is the goal, are sodium lamps good? 16 00:00:52,330 --> 00:00:53,630 Well, no. 17 00:00:53,630 --> 00:00:54,630 Not really. 18 00:00:54,630 --> 00:00:58,559 Remember how I said that the Sodium D-line is close to our eyes’ peak sensitivity, 19 00:00:58,559 --> 00:01:01,300 but only under photopic daylight conditions? 20 00:01:01,300 --> 00:01:05,810 Well it turns out that our eyes see quite differently at night than during the day. 21 00:01:05,810 --> 00:01:10,930 Under nighttime, scotopic lighting conditions, our eyes actually see bluish light better. 22 00:01:10,930 --> 00:01:14,780 And that makes sense--after all, moonlight and starlight are pretty bluish, so under 23 00:01:14,780 --> 00:01:19,159 these dimly lit conditions, having a greater sensitivity to blue light would mean we can 24 00:01:19,159 --> 00:01:20,640 see better. 25 00:01:20,640 --> 00:01:25,340 Under scotopic lighting conditions, only the rod cells in our eyes are activated. 26 00:01:25,340 --> 00:01:28,890 Rod cells cannot distinguish color, but they are much more sensitive to light than the 27 00:01:28,890 --> 00:01:30,130 cone cells. 28 00:01:30,130 --> 00:01:36,230 The peak sensitivity of the rod cells is around 498 nanometers, which is a green-blue color. 29 00:01:36,230 --> 00:01:40,259 Now of course under street lighting our cone cells are still active--we can after all see 30 00:01:40,259 --> 00:01:43,259 colors and are not exclusively using the rod cells. 31 00:01:43,259 --> 00:01:48,729 This dim-but-not-quite-dark lighting scenario is often called mesopic vision, a mix of the two. 32 00:01:48,729 --> 00:01:53,090 Still, stimulation to the rod cells will be far more visible and is more important. 33 00:01:53,090 --> 00:01:57,960 So then, how well does the light from sodium vapor lamps line up with our scotopic light sensitivity? 34 00:01:57,960 --> 00:01:58,960 Not well. 35 00:01:59,000 --> 00:02:03,340 This is the CIE 1951 scotopic luminosity function graph. 36 00:02:03,350 --> 00:02:08,190 The X axis is the wavelength of light in nanometers, and the Y axis is the eye’s relative sensitivity 37 00:02:08,190 --> 00:02:11,620 to these wavelengths under scotopic nighttime conditions. 38 00:02:11,620 --> 00:02:15,310 As you can see, peak sensitivity is around the 500 nanometer mark. 39 00:02:15,310 --> 00:02:18,020 And where’s the wavelength produced by a sodium vapor discharge? 40 00:02:18,020 --> 00:02:21,080 It’s about here, 589 nanometers. 41 00:02:21,080 --> 00:02:24,440 The rod cells are barely activated by a sodium discharge. 42 00:02:24,440 --> 00:02:28,500 While the discharge may be extremely efficient at producing visible light, at night time 43 00:02:28,500 --> 00:02:31,820 this light is fundamentally misaligned with our eye’s sensitivity. 44 00:02:31,820 --> 00:02:36,220 See, if you look at the response curves of scotopic and photopic conditions together, 45 00:02:36,220 --> 00:02:40,380 you can see that the sodium discharge lines up great with our photopic vision. 46 00:02:40,380 --> 00:02:44,910 But when our eyes are adjusted to nighttime lighting conditions, it’s actually pretty bad. 47 00:02:44,910 --> 00:02:48,370 This is why focusing on the sodium D-line emission’s seemingly perfect alignment with 48 00:02:48,370 --> 00:02:50,510 our vision is somewhat of a farce. 49 00:02:50,510 --> 00:02:55,270 While it’s true during the day, it’s literally quite far from the truth at night. 50 00:02:55,270 --> 00:02:59,640 Aside from the simple spectral misalignment of the sodium lamp, research shows that people 51 00:02:59,640 --> 00:03:04,060 can indeed see better under light sources with a bluer spectral content. 52 00:03:04,060 --> 00:03:07,990 In Peter Morante’s research for the Lighting Research Center at the Rensselaer Polytechnic 53 00:03:07,990 --> 00:03:13,410 Institute (link below), survey respondents strongly preferred the light from a 6500k 54 00:03:13,410 --> 00:03:17,650 correlated color temperature light source over that of high pressure sodium, with metrics 55 00:03:17,650 --> 00:03:22,540 of visibility, brightness, safety and security, color rendering, and overall preference all 56 00:03:22,540 --> 00:03:28,080 favoring the newer light source, which also used only 55% of the energy of the high pressure 57 00:03:28,080 --> 00:03:30,260 sodium lamps it replaced. 58 00:03:30,260 --> 00:03:35,360 The study, which by the way is really comprehensive and worth taking a look at, was done in 2008, 59 00:03:35,360 --> 00:03:39,180 which was a tad before LED technology became economically viable. 60 00:03:39,180 --> 00:03:43,110 The study compared induction lighting, which is sort-of like fluorescent lighting (and 61 00:03:43,110 --> 00:03:47,310 worth a video on its own one day because it’s pretty neat if not so practical any longer) 62 00:03:47,310 --> 00:03:52,080 as well as metal halide lighting to the sodium lamps for the purposes of creating a recommendation 63 00:03:52,080 --> 00:03:54,300 to a local utility company. 64 00:03:54,300 --> 00:03:58,490 The conclusion was that metal halide didn’t make sense due to higher maintenance costs, 65 00:03:58,490 --> 00:04:00,760 but induction would make tons of sense. 66 00:04:00,760 --> 00:04:03,000 But I also liked this final recommendation: 67 00:04:21,140 --> 00:04:25,480 Well, it’s been about 10 years time since that study, and LEDs are economically viable. 68 00:04:25,490 --> 00:04:29,230 In fact, they’ve become incredibly economically viable. 69 00:04:29,230 --> 00:04:32,930 But before we move on to them, let’s go over the main issue once more. 70 00:04:32,930 --> 00:04:37,140 Because of the spectral misalignment of the sodium vapor lamp with our scotopic (and mesopic) 71 00:04:37,140 --> 00:04:42,330 light sensitivity, it takes more light output from a sodium lamp to produce the same visible 72 00:04:42,330 --> 00:04:44,690 light level from a bluer light source. 73 00:04:44,690 --> 00:04:49,160 In fact, Dr. Alan Lewis of the New England College of Optometry measured individual’s 74 00:04:49,160 --> 00:04:52,980 response time to a hazard approaching from the sides, and found that in well-lit areas 75 00:04:52,980 --> 00:04:57,910 such as a major motorway, a high pressure sodium light system needs to produce 3.9 times 76 00:04:57,910 --> 00:05:02,420 as much light as a metal halide source to achieve the same response time. 77 00:05:02,420 --> 00:05:07,510 The effect is even greater in dimly lit areas, where he found that 7.8 times as much light 78 00:05:07,510 --> 00:05:12,270 from high pressure sodium was required to match response time under cooler, metal halide 79 00:05:12,270 --> 00:05:13,270 light sources. 80 00:05:13,270 --> 00:05:16,890 So it seems as though the high pressure sodium light is less efficient that it appears on 81 00:05:16,890 --> 00:05:21,050 paper, and that roadway safety is greatly increased when light sources are used that 82 00:05:21,050 --> 00:05:24,400 are tuned to our scotopic and mesopic light sensitivity. 83 00:05:24,400 --> 00:05:26,940 It’s looking pretty bad for high pressure sodium. 84 00:05:26,940 --> 00:05:32,060 But there’s one thing that HPS technology doesn’t do that bluer light sources might. 85 00:05:32,060 --> 00:05:35,520 That, my friends, is circadian rhythm disruption. 86 00:05:35,520 --> 00:05:39,340 Research has suggested that the color of light we are exposed to has a great impact on what 87 00:05:39,340 --> 00:05:42,360 time our biological clocks think it is. 88 00:05:42,360 --> 00:05:46,880 The shorter wavelength of daylight sun and blue sky may help keep us awake by suppressing 89 00:05:46,880 --> 00:05:51,610 melatonin production, and the long wavelength light of the sunset may signal our bodies 90 00:05:51,610 --> 00:05:55,550 to sleep by allowing melatonin to seep into our blood streams. 91 00:05:55,550 --> 00:05:59,401 To help us understand the impact different light sources can have on circadian rhythm 92 00:05:59,401 --> 00:06:03,380 disruption, light sources are characterized by their melanopic content. 93 00:06:03,380 --> 00:06:07,230 There’s a great source from the US Department of Energy linked down below which goes over 94 00:06:07,230 --> 00:06:11,950 this in much greater detail, but as a general overview, with high pressure sodium technology 95 00:06:11,950 --> 00:06:17,160 normalized at 1 for both scotopic light content and melanopic light content, a metal halide 96 00:06:17,160 --> 00:06:23,200 lamp with a color temperature of about 4,000K will have about 2.5 to 2.8 times as much scotopic 97 00:06:23,200 --> 00:06:30,050 light content, but also produces 3.16 to 3.75 times as much melanopic light content. 98 00:06:30,050 --> 00:06:34,120 If you look at the various color temperatures of LED light sources, you can see that as 99 00:06:34,120 --> 00:06:39,960 the color temperature increases, both scotopic light content AND melanopic light content increase. 100 00:06:39,960 --> 00:06:44,690 However, melanopic light content increases at a greater rate than scotopic light. 101 00:06:44,690 --> 00:06:46,760 What this means is, there’s a trade-off. 102 00:06:46,760 --> 00:06:48,380 And it’s complicated. 103 00:06:48,380 --> 00:06:52,020 The higher the color temperature, the more scotopic light it produces, which means you 104 00:06:52,020 --> 00:06:57,540 could use less light (and thus less energy) to get the same perceived brightness and safety levels. 105 00:06:57,540 --> 00:07:02,440 But, because melanopic light content increases at a greater rate, although you may need less 106 00:07:02,450 --> 00:07:07,800 light of a higher color temperature, it will disrupt circadian rhythm to a greater extent. 107 00:07:07,800 --> 00:07:11,520 You can see that high pressure sodium has among the lowest melanopic light content of 108 00:07:11,520 --> 00:07:17,240 any light source, with only amber LEDs and low pressure sodium producing less melanopic light. 109 00:07:17,240 --> 00:07:19,480 So then, we’re presented with a choice. 110 00:07:19,480 --> 00:07:23,730 We can clearly use less energy and expect a safer nighttime driving experience with 111 00:07:23,730 --> 00:07:28,500 light sources of higher color temperatures, but this may cause unwanted side-effects that 112 00:07:28,500 --> 00:07:30,950 sodium lighting largely doesn’t. 113 00:07:30,950 --> 00:07:33,740 And this is completely ignoring aesthetic preferences. 114 00:07:33,740 --> 00:07:38,230 I myself generally detest cooler lighting, as I find it harsh and unpleasant. 115 00:07:38,230 --> 00:07:40,810 But I can acknowledge its safety advantages. 116 00:07:40,810 --> 00:07:45,370 In my area on Interstate 88, many of the roadway lights have been changed from high pressure 117 00:07:45,370 --> 00:07:47,050 sodium to LED. 118 00:07:47,050 --> 00:07:52,180 The change is dramatic, with visibility greatly enhanced once under the cooler light. 119 00:07:52,180 --> 00:07:56,550 As much as I don’t like the color, I can tell it’s a lot safer. 120 00:07:56,550 --> 00:08:00,169 Visibility in my periphery is tremendously better, and I’m certain these new lights 121 00:08:00,169 --> 00:08:03,020 are using less energy than those they replaced. 122 00:08:03,020 --> 00:08:06,980 Before I move into the conclusion of this video, let’s discuss the issue of light pollution. 123 00:08:06,980 --> 00:08:10,700 Light pollution is exactly what it sounds like--excess light in our environment that 124 00:08:10,710 --> 00:08:15,500 is irritating, unnecessary, poorly distributed, or in general unwanted. 125 00:08:15,500 --> 00:08:19,730 Outdoor lighting is by far the most prolific source of light pollution, and it has gotten 126 00:08:19,730 --> 00:08:24,240 so bad that people living anywhere near a city can barely see the night sky. 127 00:08:24,240 --> 00:08:29,440 There’s even an anecdote about a widespread blackout in Los Angeles causing many panicked 128 00:08:29,440 --> 00:08:34,039 911 calls from lifelong residents who had never seen the Milky Way before and were a 129 00:08:34,039 --> 00:08:35,339 little scared of it. 130 00:08:35,339 --> 00:08:39,750 Light pollution is a complicated problem, but the LED may actually help to solve it. 131 00:08:39,750 --> 00:08:43,270 Now to be clear, the solutions I’m about to offer aren’t exclusive to LEDs, but the 132 00:08:43,270 --> 00:08:48,020 way light is emitted from an LED chip makes controlling it relatively easy. 133 00:08:48,020 --> 00:08:53,300 So first, let’s discuss one of the biggest causes of light pollution; lights that point up. 134 00:08:53,300 --> 00:08:56,580 This is less obvious than it seems, but it’s incredibly important. 135 00:08:56,590 --> 00:09:01,830 Right outside my apartment are drop-lens cobra luminaries containing high pressure sodium lights. 136 00:09:01,830 --> 00:09:06,450 Because the lens protrudes downward from the fixture, a lot of light escapes to the sides 137 00:09:06,450 --> 00:09:07,640 and indeed upwards. 138 00:09:07,640 --> 00:09:12,270 I’m on the 4th floor of my building, and my eye-level is above these street lights, 139 00:09:12,270 --> 00:09:15,150 but I can still see the source of the light. 140 00:09:15,150 --> 00:09:17,450 This is not ideal for a number of reasons. 141 00:09:17,450 --> 00:09:22,890 First, a lot of light is being wasted by lighting up things that are not the road. 142 00:09:22,890 --> 00:09:24,270 That’s kinda dumb. 143 00:09:24,270 --> 00:09:27,350 But secondly, a lot of this light is going up into the sky. 144 00:09:27,350 --> 00:09:31,380 Granted, this style of fixture isn’t the worst offender, but a better design would 145 00:09:31,380 --> 00:09:36,460 be a flat lens that does not allow light to escape above the horizontal. 146 00:09:36,460 --> 00:09:40,460 This may still throw light farther to the sides than necessary, but none of it will 147 00:09:40,460 --> 00:09:42,560 end up lighting the sky. 148 00:09:42,560 --> 00:09:46,730 The worst offenders for this kind of light pollution are lights that illuminate buildings 149 00:09:46,730 --> 00:09:51,760 by shining upwards, wall-pack lights without shielding, and these decorative fixtures. 150 00:09:51,760 --> 00:09:54,940 I’ll admit they’re pretty, but they’re really wasteful. 151 00:09:54,940 --> 00:09:57,560 Recently I was on an airplane flying into Chicago at night. 152 00:09:57,560 --> 00:10:01,730 I took some video as we landed, and you can see the difference between a well-managed 153 00:10:01,730 --> 00:10:03,870 light and a poorly managed light. 154 00:10:03,870 --> 00:10:05,300 This roadway is lit well. 155 00:10:05,300 --> 00:10:10,570 I cannot see the actual light sources, I can only see the reflected light from the road. 156 00:10:10,570 --> 00:10:12,009 That’s what we want. 157 00:10:12,009 --> 00:10:16,649 As we got closer to the airport, these neighborhoods had tons of lights that were visible from 158 00:10:16,649 --> 00:10:17,870 above. 159 00:10:17,870 --> 00:10:21,970 Much of the light produced by these lamps is shining into the sky and being wasted. 160 00:10:21,970 --> 00:10:27,490 I should not be able to see the actual light source from an airplane, yet I can. 161 00:10:27,490 --> 00:10:29,890 This is contributing to skyglow. 162 00:10:29,890 --> 00:10:33,779 Skyglow is what makes the night sky hard to see when you’re close to a city. 163 00:10:33,779 --> 00:10:37,640 This is probably the most widespread light pollution problem, and while it’s not caused 164 00:10:37,640 --> 00:10:42,260 exclusively by poorly designed fixtures, they are a major component. 165 00:10:42,260 --> 00:10:46,180 But once again, the solutions to skyglow are complicated. 166 00:10:46,180 --> 00:10:51,089 Largely because light sources that cause the least skyglow are high and low pressure sodium. 167 00:10:51,089 --> 00:10:56,029 In fact, low pressure sodium is used widely around large astronomical observatories because 168 00:10:56,029 --> 00:11:01,040 their nearly monochromatic light output can easily be filtered out, eliminating any skyglow 169 00:11:01,040 --> 00:11:02,040 they create. 170 00:11:02,040 --> 00:11:06,330 I saw a large number of people saying that high pressure sodium can also be filtered 171 00:11:06,330 --> 00:11:10,570 out, but I don’t think that’s true due to the pressure broadening and their spikier 172 00:11:10,570 --> 00:11:11,620 output. 173 00:11:11,620 --> 00:11:15,110 Someone correct me if I’m wrong but I could only find references to low pressure sodium 174 00:11:15,110 --> 00:11:16,810 being used around observatories. 175 00:11:16,810 --> 00:11:21,520 Anyway, where it gets tricky is that an LED light source has about three times as much 176 00:11:21,520 --> 00:11:25,020 sky glow impact than a high pressure sodium light. 177 00:11:25,020 --> 00:11:29,899 But also, you need less of it, so perhaps the sky glow impact is similar. 178 00:11:29,899 --> 00:11:35,110 In addition, the sky glow impact of incandescent lighting is barely higher than low pressure sodium. 179 00:11:35,110 --> 00:11:39,870 So it could be that LED street lighting correlated to a 2700K color temperature causes less sky 180 00:11:39,870 --> 00:11:42,120 glow than high pressure sodium. 181 00:11:42,120 --> 00:11:44,330 But I think further research needs to be done there. 182 00:11:44,330 --> 00:11:48,690 In any case, what makes LEDs potentially much better at reducing skyglow is the optical 183 00:11:48,690 --> 00:11:50,770 systems that can be combined with them. 184 00:11:50,770 --> 00:11:56,529 Early LED fixtures may have used a large number of small 1 watt LEDs and tiny lenses to direct 185 00:11:56,529 --> 00:11:57,839 their light. 186 00:11:57,839 --> 00:12:02,440 Some really bad designs may have simply had an exposed chip, prodiving little directional 187 00:12:02,440 --> 00:12:03,440 control. 188 00:12:03,440 --> 00:12:05,839 You still see this a lot in cheap flood light fixtures. 189 00:12:05,839 --> 00:12:10,000 But newer fixtures like these from Cree will use large chip-on-board emitters, like these 190 00:12:10,000 --> 00:12:14,580 10W chips but larger, and because they only emit light in one direction, it’s very easy 191 00:12:14,580 --> 00:12:16,860 to control their output with a lens. 192 00:12:16,860 --> 00:12:20,880 You can see on the spec sheet that there are 5 lenses in total, though larger fixtures 193 00:12:20,880 --> 00:12:22,210 have more. 194 00:12:22,210 --> 00:12:25,130 Most importantly, the optic system is customizable. 195 00:12:25,130 --> 00:12:30,290 Depending on fixture height and spacing, you may need a wider spread of light or a shorter one. 196 00:12:30,290 --> 00:12:33,610 Due to the customizable optics, you can get wonderfully consistent lighting on a road 197 00:12:33,610 --> 00:12:35,910 surface such as this area here. 198 00:12:35,910 --> 00:12:40,160 This may also help to prevent light pollution because less overall light is needed. 199 00:12:40,160 --> 00:12:44,660 The hotspots of light you see from above here mean some areas get too much light, and others 200 00:12:44,660 --> 00:12:45,850 get too little. 201 00:12:45,850 --> 00:12:50,000 A more scotopic light source with better and more consistent control may not only reduce 202 00:12:50,000 --> 00:12:54,020 light pollution, but may use less energy and provide safer driving. 203 00:12:54,020 --> 00:12:58,410 Now many of these developments are rather new, especially the newer optic designs. 204 00:12:58,410 --> 00:13:03,650 But the advantages of LED lighting become confused when drop-in replacement bulbs are used. 205 00:13:03,650 --> 00:13:07,149 I don’t have any major issues about going this route--after all replacing the entire 206 00:13:07,149 --> 00:13:11,170 fixture can be costly, and some drop-in designs are fairly good. 207 00:13:11,170 --> 00:13:15,480 But the optic system of a sodium, mercury vapor, or metal halide fixture is designed 208 00:13:15,480 --> 00:13:21,920 to reflect and project light emanating from a tiny arc tube, which an LED drop-in can’t recreate. 209 00:13:21,920 --> 00:13:25,280 Many of the complaints regarding poor light distribution in LED replacement lamps may 210 00:13:25,290 --> 00:13:27,600 simply be from the use of these replacements. 211 00:13:27,600 --> 00:13:30,490 Then there’s the issue with existing ballasts. 212 00:13:30,490 --> 00:13:34,500 Some drop-in lamps claim to work with existing ballasts, but I have a few misgivings regarding 213 00:13:34,500 --> 00:13:38,470 how well their power supplies deal with the voltage the ballast provides--particularly 214 00:13:38,470 --> 00:13:43,490 if the high voltage ignitor sends some crazy voltage spikes to the LED drop-in. 215 00:13:43,490 --> 00:13:47,110 I’m sure they can be designed to cope, but it still worries me somewhat. 216 00:13:47,110 --> 00:13:50,660 So then, we have a series of complicated choices to make. 217 00:13:50,660 --> 00:13:54,910 Sodium vapor lights are pretty efficient, have only a moderate contribution to sky glow, 218 00:13:54,910 --> 00:13:59,170 cause only minor circadian rhythm disruption if any, and have a proven track record of 219 00:13:59,170 --> 00:14:00,560 reliability. 220 00:14:00,560 --> 00:14:06,260 But their color also makes them far less effective at improving safety, and due to the misalignment 221 00:14:06,260 --> 00:14:10,890 of their output with our scotopic light sensitivity, they require more light (and thus use a lot 222 00:14:10,890 --> 00:14:13,950 more energy) than a whiter light source. 223 00:14:13,950 --> 00:14:18,700 Perhaps less an issue but still important is that they contain both mercury and elemental 224 00:14:18,700 --> 00:14:22,390 sodium, meaning their disposal is far more dangerous and complicated. 225 00:14:22,390 --> 00:14:26,899 If we were to switch to a white LED source with a color temperature of about 5,000 K, 226 00:14:26,899 --> 00:14:29,730 nighttime visibility would be greatly increased. 227 00:14:29,730 --> 00:14:33,399 Studies have shown that people see hazards far sooner under this light, and as the bluer 228 00:14:33,399 --> 00:14:37,870 wavelengths match our scotopic and mesopic color sensitivity more closely, we can use 229 00:14:37,870 --> 00:14:41,220 less of it while also achieving a greater safety benefit. 230 00:14:41,220 --> 00:14:43,090 This reduces the need for energy. 231 00:14:43,090 --> 00:14:47,660 However, this bluer light contributes more to circadian rhythm disruption and skyglow, 232 00:14:47,660 --> 00:14:52,020 but some of that is mitigated by the lower output required by this light source. 233 00:14:52,020 --> 00:14:56,670 Still, many people may not find the aesthetics of this light source pleasing. 234 00:14:56,670 --> 00:15:00,920 One possible compromise would be to use a warmer color temperature LED light source. 235 00:15:00,920 --> 00:15:06,330 Data from the US Department of Energy tells us that a 3000K LED light would produce 1.89 236 00:15:06,330 --> 00:15:11,060 to 2.39 times as much scotopic light as a high pressure sodium lamp, while increasing 237 00:15:11,060 --> 00:15:15,630 melanopic content between 2.1 and 2.99 times. 238 00:15:15,630 --> 00:15:19,720 Because of its greater scotopic output, a 3000K LED replacement should only need to 239 00:15:19,720 --> 00:15:23,550 produce about half as much light as a high pressure sodium lamp. 240 00:15:23,550 --> 00:15:27,580 This effectively cuts the circadian rhythm disruption potential in half, too, placing 241 00:15:27,580 --> 00:15:30,290 it near about the same as high pressure sodium. 242 00:15:30,290 --> 00:15:34,920 The greatest downside to using a warm color temperature LED is in their efficiency. 243 00:15:34,920 --> 00:15:39,360 The efficiency of these lights is very similar to that of an average high pressure sodium 244 00:15:39,360 --> 00:15:40,360 lamp. 245 00:15:40,360 --> 00:15:45,250 At 67 lumens per watt, these 3,000K LEDs are just slightly less efficient than the sodium 246 00:15:45,250 --> 00:15:47,180 lamp featured in my last video. 247 00:15:47,180 --> 00:15:51,350 Although you would need only about half as much light output, there are HPS lamps which 248 00:15:51,350 --> 00:15:53,970 approach 150 lumens per watt. 249 00:15:53,970 --> 00:15:58,529 This would mean that a 3000K LED will use just about as much energy as a very efficient 250 00:15:58,529 --> 00:16:00,060 HPS lamp. 251 00:16:00,060 --> 00:16:03,780 I’d still call that good, but it makes replacement less compelling. 252 00:16:03,790 --> 00:16:07,140 To normalize the effects of scotopic light content, I’ve multiplied the lumens per 253 00:16:07,140 --> 00:16:11,180 watt number by the scotopic light content for the following light sources. 254 00:16:11,180 --> 00:16:15,500 As you can see, the normalized efficiency of the LED goes up considerably as the color 255 00:16:15,500 --> 00:16:20,240 temperature does, due to both luminous efficiency and greater scotopic content. 256 00:16:20,240 --> 00:16:25,540 This is likely why most LED street lamp installations are done with the blueish 5700K and higher 257 00:16:25,540 --> 00:16:27,250 color temperatures. 258 00:16:27,250 --> 00:16:30,730 You can use the least amount of energy to produce the same amount of perceived brightness 259 00:16:30,730 --> 00:16:31,730 and safety. 260 00:16:31,730 --> 00:16:36,690 However, the normalized efficiency of even the 3000K LED is very similar to that of high 261 00:16:36,690 --> 00:16:41,880 pressure sodium, and few HPS lamps actually output 150 lumens per watt. 262 00:16:41,880 --> 00:16:47,230 Also, the calculated lumens per watt of the LED is based on the input power of the fixture, 263 00:16:47,230 --> 00:16:51,320 so the losses in the ballast (which are fairly high for high pressure sodium) aren’t accounted 264 00:16:51,320 --> 00:16:52,320 for here. 265 00:16:52,320 --> 00:16:56,270 To conclude, although the high pressure sodium light is very efficient, its primary output 266 00:16:56,270 --> 00:16:59,400 color is misaligned with our nighttime visibility. 267 00:16:59,400 --> 00:17:03,980 Only about a quarter of its light is actually effective at stimulating the cells in our eyes. 268 00:17:03,980 --> 00:17:08,140 Although the cool color temperature of many LED replacements is harsh and aesthetically 269 00:17:08,140 --> 00:17:12,320 displeasing, studies have shown that it is not only more efficient but also makes driving 270 00:17:12,329 --> 00:17:13,929 at night safer. 271 00:17:13,929 --> 00:17:17,379 There is however the potential for greater circadian rhythm disruption and larger amounts 272 00:17:17,379 --> 00:17:19,809 of skyglow using these bluer light sources. 273 00:17:19,809 --> 00:17:24,640 Still, it seems clear that the high pressure sodium lamp is on its way out. 274 00:17:24,640 --> 00:17:28,179 Advancements in LED technology are happening at a breakneck pace. 275 00:17:28,179 --> 00:17:30,840 Just 10 years ago they weren't seen as viable. 276 00:17:30,840 --> 00:17:35,269 But today, even the least efficient of LED replacements ends up meeting the efficiency 277 00:17:35,269 --> 00:17:39,159 of high pressure sodium when scotopic light output is considered. 278 00:17:39,159 --> 00:17:43,359 As it stands in 2018, we are faced with a choice of efficiency over aesthetics. 279 00:17:43,359 --> 00:17:48,510 I’m pretty sure I’d enjoy roadways lit with the relatively warm 3000K LEDs, and these 280 00:17:48,510 --> 00:17:50,679 also wouldn’t disrupt sleep much. 281 00:17:50,679 --> 00:17:55,690 But you can save a lot more energy (and potentially have safer roadways) with 5700K lighting. 282 00:17:55,690 --> 00:18:00,850 Either way, it seems clear that LED technology will very soon overtake the tried-and-true 283 00:18:00,850 --> 00:18:06,269 high pressure sodium lamp, just as the HPS lamp itself replaced the mercury vapor lamp. 284 00:18:06,269 --> 00:18:11,020 And in 40 or 50 years, who knows what technology might light our roadways. 285 00:18:12,240 --> 00:18:15,490 So I have a couple of things to close out, first you may have noticed in my chart that 286 00:18:15,490 --> 00:18:20,341 the mercury vapor lamp had a normalized efficiency of over 100 lumens per watt, and the 50 watt 287 00:18:20,341 --> 00:18:24,300 sodium lamp in the last video was only 78 lumens per watt. 288 00:18:24,300 --> 00:18:29,580 Mercury vapor bulbs do have considerable operating disadvantages compared to HPS, most notably 289 00:18:29,580 --> 00:18:33,539 their steady decrease in light output as they age, but I think it is somewhat humorous that 290 00:18:33,539 --> 00:18:37,760 our current understanding of the visual system suggests that sodium light may have been a 291 00:18:37,760 --> 00:18:39,710 step backwards in some situations. 292 00:18:39,710 --> 00:18:43,269 You may have noticed that I didn’t talk about the blue light from LEDs and how this 293 00:18:43,269 --> 00:18:48,799 is supposedly ruining our eyes--that’s because the “science” behind this is questionable 294 00:18:48,799 --> 00:18:50,259 at best. 295 00:18:50,259 --> 00:18:54,530 You can clearly see in this chart that there is blue-light content in nearly all light 296 00:18:54,530 --> 00:18:58,619 sources, and lower color temperature LEDs have less blue-light content than their higher 297 00:18:58,619 --> 00:19:00,590 color temperature varieties. 298 00:19:00,590 --> 00:19:04,899 I don’t doubt that blue light can disrupt circadian rhythm--that much seems certain. 299 00:19:04,899 --> 00:19:09,409 But considering that our eyes can withstand the intensity of sunlight, which is far far 300 00:19:09,409 --> 00:19:14,690 greater than any normal artificial light source and also has a lot of blue light (and ultraviolet 301 00:19:14,690 --> 00:19:19,309 which definitely IS harmful), I think the blue light thing is just fear mongering. 302 00:19:19,309 --> 00:19:23,609 If someone can point to some verified, peer-reviewed research supporting this, and not a dodgy 303 00:19:23,609 --> 00:19:25,970 website, I’ll consider changing my stance. 304 00:19:25,970 --> 00:19:30,879 In any case, the high flexibility of LED technology means that it can be tuned in pretty much 305 00:19:30,879 --> 00:19:32,070 any way you like. 306 00:19:32,070 --> 00:19:35,919 I also want to give a shoutout to VWestlife for the suggestion of LED fixtures with both 307 00:19:35,919 --> 00:19:40,289 high and low color temperatures that will switch to the warm light later in the night. 308 00:19:40,289 --> 00:19:44,519 I think that’s a great idea, though obviously it would add expense to any fixture. 309 00:19:44,519 --> 00:19:49,119 However, I was surprised to learn that the Cree LED fixtures I’ve been using as a reference 310 00:19:49,119 --> 00:19:54,639 are all capable of dimming, and they have a 0 to 10V control input to enable this. 311 00:19:54,639 --> 00:19:59,120 Reducing light levels to perhaps 50% of normal after midnight might become a common practice, 312 00:19:59,120 --> 00:20:01,190 and I think that would be pretty wise. 313 00:20:01,190 --> 00:20:04,749 Maybe this will get combined with technology similar to Philip’s warm-glow and we’ll 314 00:20:04,749 --> 00:20:07,370 get incandescent-like lighting at night. 315 00:20:07,370 --> 00:20:11,210 For those worried about light pollution for astronomical observatories, there are amber 316 00:20:11,210 --> 00:20:15,470 LED street lights available designed to replace low pressure sodium lights. 317 00:20:15,470 --> 00:20:20,139 This is also great news for wildlife--many animals cannot see the wavelength of light 318 00:20:20,139 --> 00:20:24,809 produced by low pressure sodium, so this light source is used where lights may be disruptive. 319 00:20:24,809 --> 00:20:28,720 One particular example is near beaches where sea turtles lay their eggs. 320 00:20:28,720 --> 00:20:32,340 After they hatch, baby sea turtles follow moonlight to the ocean, and street lighting 321 00:20:32,340 --> 00:20:35,100 was confusing the poor things and they were travelling inland. 322 00:20:35,100 --> 00:20:39,549 Since they cannot see the wavelength of a low pressure sodium light or its amber LED 323 00:20:39,549 --> 00:20:44,419 equivalent, they aren’t confused and successfully make it to the ocean where they belong. 324 00:20:44,419 --> 00:20:48,979 As a last little tid-bit, the spec sheet from Cree says that their LED cobra head replacements 325 00:20:48,979 --> 00:20:55,440 should produce at least 95% of their original light output after 100,000 hours. 326 00:20:55,440 --> 00:20:59,399 Assuming the driver and heat sink are robust enough, these fixtures should last well beyond 327 00:20:59,399 --> 00:21:01,340 20 years. 328 00:21:01,340 --> 00:21:02,519 That is impressive. 329 00:21:02,519 --> 00:21:04,499 Thanks for watching, I hope you enjoyed the video! 330 00:21:04,499 --> 00:21:08,289 If you haven’t subscribed to Technology Connections yet, and you like my nerdy deep-dives 331 00:21:08,289 --> 00:21:11,149 into whatever floats through my head, what are you waiting for? 332 00:21:11,149 --> 00:21:12,250 Hit the button! 333 00:21:12,250 --> 00:21:13,250 Please? 334 00:21:13,250 --> 00:21:16,909 As always, thanks to everyone who supports this channel on Patreon! 335 00:21:16,909 --> 00:21:20,139 You are all making a big difference and it’s super awesome of you. 336 00:21:20,139 --> 00:21:23,701 If you’re thinking of joining these awesome people and becoming a patron 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