Conflicted by Light

Posted By Pamela on Apr 3, 2007 | 7 comments

As an Astronomer, I am very pro-dark sky. As a person in favor of migrating birds, baby sea turtles, and general good health, I’m anti-light pollution. As a human who wants to see our planet’s environmental crash slow down and reverse, I’m in favor of energy conservation. Generally, these three sets of opinionated voices in my head work in tandem to encourage people to use down-pointed lights that contain orange-ish low-pressure sodium lamps. When those don’t work, I turn to the web site of the International Dark-Sky Association (IDA) where they list lighting, even Antique Street Lamps that would match my historic neighborhood’s idea of good landscaping. Unfortunately, a new light on the block is going to bring conflict to the normally collaborative voices. That new light is the ulta-luminous white LED. Taking 63 watts to produce 8500 lux, and lasting roughly 30 years, these brand new lights are every energy savers dream come true. Municipalities are considering switching to this new new tech toy to save city resources. Unfortunately, white LEDs (see image left, credit: CREE lighting) are every dark sky dreamer’s worst nightmare.

White light, by definition, is a combination of all the different colors of the spectrum. White LEDs, aren’t a perfectly flat distribution of energy as a function of color. They have a very distinct peak in the blue. It’s this peak that causes problems.

Different colors of light reflect and scatter in different ways. In general, red light can make it though a cloud of fog or dust, while blue light is scattered hither and yon. This is actually how we end up with blue and red nebulae. The red light in red nebula comes directly from background stars and is transmitted through the cloud to our eyes, telescope, or binoculars. Blue nebulae are the result of looking at a cloud of gas that has star light hitting it from the side, top, or bottom. The blue light that was scattered comes out in all directions, and is thus visible indirectly. You can demonstrate this by taking an empty 2 liter, filling it with water and a dash of dried milk powder, and then shinning a low power white flash light through the water from different angles.

Today, our low-pressure sodium light bulbs cast an orange glow on everything, but that orange glow tends not to back scatter too badly. The light passes through dusty air to illuminate parking lots and walk ways, and doesn’t reflect off of typical asphalt walkways with too much enthusiasm. Blue light, with its shorter wavelength and higher energy, won’t be so polite, and will eagerly scatter off of anything and everything: dust, fog, the ground, you, the grass, etc. Scattered light can defeat careful attempts to prevent light pollution that rely on downward facing lights.

When people try and illuminate objects, their goal should be to have the photons travel from the light source to the object where they are reflected only in directions where onlookers may be looking from. In theory, onlookers should not include people in planes or in space. In this perfect ideal situation, night time satellite images would show a darkened planet Earth instead of the current trace of major cities and sprawling suburbs (and the occasional shrimp fleet). Because we like to illuminate parking lots and walkways (and the ground in general), this perfect dream will never happen, but at least the red light is unenthusiastic about going to outer space.

If we replace our current low-pressure sodium lights with white LEDs, we will save energy. Cities and individuals will save time (and money) by not having to replace bulbs as often. As humans, we also psychologically will be happier with the white light. Win. Win. Win. Except for the scattering. And there is another problem – It’s easy to make these things BRIGHT. Car dealerships will be able to afford to have their lots as bright as the noonday sun at midnight. High Schools won’t sweat the costs of lighting up their football fields, and marching bands won’t be restricted to practicing in daylight. Everything can be made bright bright bright for low low costs.

And bright at night is bad. You can find links to papers discussing effects on human health, animals, plants, and nighttime resources like stars on the IDA website.
So, these white light LEDs are leaving me conflicted. They save power, effort, and make things look better at night. But they ruin the night. They steal the stars and have the potential to harm many critters that matter.

I never thought I’d be upset to see a break through in energy saving light sources.


  1. Looks like the IDA’s mission has again expanded.

    I was thinking that having a bright, low cost light source might even make cities and people want to save more money by not using as many of them, but your analogy with car dealerships shoots that down rather effectively.

    Unfortunately, I also see that this may be even a worse situation since, with the energy costs of these lights being as low as they are, the power companies are going to encourage more of their use.

    Bob Gent, the IDA President, who I have the good fortune of knowing, has never quite been able to effectively explain to me why power companies would go for lower lighting, although he does have specific and effective examples that this is the case. These lights may increase the need for that type of education.

    Unfortunately, the only effective solution I see in this case is legislation, and that can be complex and expensive in terms of money and time since it tends to be an individual patchwork from community to community.

    We’ll have to see how this begins to play out.

  2. What ever happens, it is going to be an ugly fight.

  3. Off-gridders in dark, dark places will still rejoice! 🙂

  4. Hi Pamela

    First off, thank you so very much for astronomy cast, it is wonderful to listen to up to date information and wonderful that there are still people like you who just want to share how wonderful the universe is. So many just don’t get it. I will need to pull out my old “Cosmos” vhs tapes and give them another listen to. I have kept them to show my children (one day when I have some) what science believed when I was younger. Should be quite interesting to see what’s changed and what hasn’t.

    I live in Johannesburg in South Africa. That little blotch of light on the Southern portion of Africa more or less in line with Madagascar – thats us. Lucky me, I don’t have to travel too far to have dark skies and the viewing in winter away from the cities is generally very good with us being 1650m above sea level and very dry.

    However even here the light pollution has grown exponentially over the last few years especially as the government has finally got its butt into gear and is trying to eradicate one of the vestiges of apartheid and electrifying the townships where unfortunately most folk still live mostly due to economic constraints rather than misguided political ones.

    The double edged sword grows another edge as the number of poor that are using coal fired stoves has also decreased due to the electrification of squatter areas reducing particulate pollution even further, so one can hardly complain about it.

    More to the point. LED’s generally have much lower viewing angles (the angle or cone that light is emitted) than for example a low pressure sodium bulb so, would this not reduce the amount of light that wasted, i.e. sent to far flung worlds instead of the car dealership?

    In addition would the fact that the light has a higher frequency causing it to scatter more easily not mean that it is more likely to be confined to the city areas where there are lots of particulates in the air from gas and diesel combustion that will attenuate it and not pollute the outlying areas and higher atmospheric layers?

    With this in mind would it not make sense for emerging economies such as ours to employ these new technologies as the light would hopefully be used more efficiently resulting in less fossil fuel power stations and less pollution to boot?

  5. Hi Jeremy,

    Thank you so much for you thoughtful response. South Africa is a beautiful nation. I had a dear friend for a while in graduate school from Cape Town and he shared some beautiful verbal and photographic images of your country with me. It is on my list of places I must eventually travel to.

    If you were talking about Red or Orange LED’s, you’d be entirely right. With white LEDs… I have to admit that in you situation I really don’t know what is best. The problem is the blue (in the white light) scatters so much that it will cause a city to be a scattered light monster sitting on a horizon. All the pollution, haze, and everything else will scatter the light in all directions, and it also scatters more readily off the ground. The LED lights are better then completely unshielded lights, but they are worse than downward focused sodium vapor. But, even unshielded lights are better for our planet then burning coal.

    It is always hard to balance the cost benefits to a society where such rapid development is going on. Small parts of your country are as developed as the cities I have lived in, but other (large) parts are still struggling with the basic needs of clean water.

    I wish I had a perfect answer.

  6. Dr. Gay,
    Thanks for your wonderful AstronomyCasts with Fraser. I look forward to each week’s new show.

    I have an observing question: would it be practical for coastal astronomers to escape light pollution by setting up floating observatories offshore? I know this wouldn’t put them high in the atmosphere but it could take them far away from city lights. I don’t know how the problem of pitching and rolling would be solved unless the boat only went out in calm waters, but does the idea have any merit?

  7. Most LEDs don’t necessarily generate white light directly. They contain phosphorus and other chemical compounds that are excited by a specific wavelength generated in the diode which may not even be visible. If it weren’t a matter of ergonomics you could also utilize high intensity red, yellow, green, or orange LEDS but generally this would cast an eerie glow and also might be a problem for people with certain retinal illnesses or damage. There is also the potential to skew emitted wavelengths (when using phosphors) to the red side of the spectrum, but again it’s all a matter of ergonomics – and ultimately legal issues should such illumination be considered to be “defective” in some way. The liability monster is always lurking which would be another reason for municipalities and businesses to go the route of increased brightness in night time street lighting or other safety related uses were people might pass.

    The one saving grace of LEDs is that they can more easily be turned on instantly rather than having a warm-up period like many sodium street lamps. This opens the opportunity for many lamps to contain motion sensing equipment or otherwise be controlled by sensors located in buildings, roads, or other structures. The energy savings resulting from “in-use” lighting can be phenomenal, particularly in more rural environments (and it also multiplies the useful lifetime of your LED). It might be a bit disconcerting having lights flashing on and off all over the place but smart electronics could minimize the impact, or at least reduce the amount of flashing to acceptable values. Come to think of it there’s no need to turn the LED on or off abruptly either. The same activation circuits that control the lamps could also be used to gradually ramp lights on and off at a rate that’s more acceptable to the human nervous system, say over a span of 2 to 10 seconds or so, depending on the application.

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