Archive for November 16, 2019

Sodium Vapor Lamps are Better than LED Lamps for Outdoor Lighting: 1

November 16, 2019 3 comments

This is a post about another topic which I had originally meant to write about two years ago. In 2019, many of you either live in a city that uses LEDs lamps for street-lighting or is transitioning to them. In my opinion, changing over from High Pressure Sodium (HPS) lamps to LEDs of any generation or color temperature is one of more stupid public decisions made in the past decade. Here is a list of reasons for why this is the case.

1] LEDs are not significantly more efficient than HPS lamps. Contrary to the lies pushed by supporters of LED street-lighting, HPS and LED lamps convert electricity to light with about the same efficiency. The typical HPS lamp gives off about 100-150 lumens per watt, while available LED equivalents give off about 130-150 lumens per watt. Some of you might argue that even a 20-30% decrease in the use of electricity is a big saving, but if you actually look at the cost of electricity and the already high efficiency of both light sources, the savings per year for lighting a small city are no more than the salary of a few useless paper-pushers in sinecured posts within the local municipality. In other words, you cannot justify the decision based on conventional actuarial calculations. So ya.. you have to lie and bullshit to justify such a change.

2] The allegedly longer life-span of LED street-lighting has not been verified under real-life conditions. Although LED street-lamps are supposed to last between 50k-100k hours, which is about 2-4 times the 24k hours for HPS lamps, that claim has yet to be properly verified under real life conditions. See.. because HPS lamps have been around for decades, we have a far better understanding of their exact lifespan, reliability and most importantly- modes of failure. We can predict, with great certainty, the duration any given HPS lamp will keep working properly and how much its light output will decrease over time. Also, HPS lamps unlike their LED counterparts fail slowly and gracefully. This is especially relevant since the technology for making HPS is far more standardized than the ones used for making LED lamps.

3] Color Rendering Index (CRI) is largely irrelevant for nighttime street-lighting. One of the popular bullshit lies used to promote LED street-lighting is that they have a better CRI index than HPS. While it technically true that LEDs can have better CRI than HPS, this is almost totally irrelevant for nighttime street-lighting. Here is why. See.. color recognition by the human eye works best under conditions of relatively high light intensity, the unit for which is known as ‘lux‘.It is pretty hard to discriminate color properly well below illumination levels of 50 lux and basically a fools errand to do it below 5 lux. While you can still distinguish a bright red car from a blue one at.. say..10 lux, discriminating between shades of red and blue is really hard at illumination levels at or under 20 lux. Even the most important roads are typically lit to a bit under 10 lux and major intersections always remain under 40 lux.

In most parts of this country and world, you would be lucky to reach a third of those values. To make a long story short, CRI of a light source is largely irrelevant for street-lighting. Artificial lighting powerful enough to restore human color perception to daylight levels requires illumination levels of about 1000-2000 lux. Human Mesopic and Scotopic vision (twilight and low-light vision) under lighting levels created by street-lighting is far more dependent on perception of edges and variations in albedo than color. Which leads us to other problems seen with LED street-lighting.

4] LED lamps, unlike HPS, are highly directional light sources. While the directionality of LEDs makes them useful in certain applications, it makes them inferior to HPS lamps for street-lighting. I am sure that many of you must have noticed that there is often a dark zone between two consecutive streetlights. The thing is.. this effect is far milder with the diffuse and bright light produced by HPS. In contrast, LED streetlights are notorious for large and unpleasant dark zones between them as well poor illumination of adjacent sidewalks, trees, buildings and other visual landmarks. There is a reason why it is easier to quickly visualize everything occurring on or around a HPS illuminated street compared to one lit with LEDs.

Then, there is the issue of glare. LED light sources, by their very mode of operation, have high surface luminosities. While this is not a big issue if they are properly shielded and diffused, the majority of LED street-lights don’t seem to care about such basic design features. Consequently, looking at an LED streetlight (even momentarily) by accident will compromise your low-light vision adaptation to a significantly greater extent than an HPS lamp. All of these issues, and more, make LED street-lighting look and feel inferior to its HPS equivalent. Notice that we haven’t even touched on issues such as color temperature of LED street-lights as well as the effect of bluer night-time light sources on sleep and circadian rhythms etc.

So what is behind the bullshit drive to convert cities to LED street-lighting. Well.. in my opinion, it is a convergence of a few factors. Firstly, it is about money- specifically the profits to be made by making such a switch. Secondly, it is about conformity and group-think, something at which the supposedly “individualistic” west excels. A third, and more cultural reason, is that the west has experienced technological stagnation for past three decades and every small sign of apparent “progress” is pushed hard as proof that the moribund religion of “secular technological progress” is alive and well. I am sure that comments will provide material for a second post on this topic.

What do you think? Comments?