LED Lighting and Dark Skies

Are LEDs good for dark skies?

Unfortunately, no… they could be, but as they are presently being used, they are definitely not. Not even close.
Sky glow increase from a high-pressure sodium to LED conversion.
(For the much more dramatic increase with an LPS to LED conversion, see here)

But how can that be?

In recent years the press and the U.S. Department of Energy are effusive about the “revolution” in lighting being brought about by LEDs. In remarks in a press release announcing the 2014 Nobel Prize in physics (awarded to researchers who invented the blue LED that underlies the ultra-bright white LEDs) Nobelprize.org states:

5mm_blue_LEDNew light to illuminate the world

This year’s Nobel Laureates are rewarded for having invented a new energy-efficient and environment-friendly light source – the blue light-emitting diode (LED). In the spirit of Alfred Nobel the Prize rewards an invention of greatest benefit to mankind; using blue LEDs, white light can be created in a new way. With the advent of LED lamps we now have more long-lasting and more efficient alternatives to older light sources.

And in their announcement of the Prize, the International Dark-Sky Association notes:

If we light properly we can use LEDs to save energy, improve visibility, and lower light pollution levels.

The DOE, power companies and other governmental agencies have provided substantial financial assistance to communities in replacing current roadway lighting with LED. A prime example is Los Angeles, which has replaced over 140,000 high-pressure sodium streetlights with LED. The Los Angeles Bureau of Street Lighting reports that energy use has been reduced over 60%, saving $7.5 million per year; maintenance costs are predicted to fall by $2.5 million per year.

Los Angeles, Ventura Blvd
Los Angeles, Ventura Blvd - before retrofit Los Angeles, Ventura Blvd - after retrofit
Before retrofit After retrofit

So – the buzz is that LEDs promise not only huge energy savings but also reduced light pollution. How can we claim otherwise?

Looking closely at the facts and numbers, a different story emerges. Regarding energy savings the story is not as simple – and not as dramatic – as described (see below). Regarding light pollution the conclusions are clear, and not positive.

LEDs and Light Pollution

For sky glow light pollution, the conclusion is unambiguous: research shows that white light – of any kind (CCT) – has substantially greater impact on sky glow than yellow light (see our page Lamp Spectrum and Light Pollution). Compared to the currently prevalent high-pressure sodium lights, even the lowest-impact white LEDs1 (though higher-impact LEDs are far more widely used) will increase visible sky glow 2.2x – more than double. Compared to the most night-friendly light, low-pressure sodium, the increase is 6x. This is not 6% nor even 60% – it is 6x or 500%! The most optimistic predictions for decreased lighting amounts that might be achieved with LEDs (due to better optical control) cannot compensate for this.

1 A complete analysis of sky glow changes must take into account actual “fixture” lumen totals: as this is determined in our overall analysis we will update this page.

The Los Angeles LED Retrofit and Dark Skies

In the Los Angeles retrofit, the LEDs used (4000K CCT) cause, lumen-for-lumen, about 2.7x (or 170%) more sky glow than the HPS lighting they replaced. Even when total lumen amounts are reduced proportional to the illuminance reduction seen on Ventura Blvd (41%2 – see figures on the images above), the sky glow impact of the streetlighting has increased by about 60%. And in a fair equal-illuminance comparison, the LED system has about 170% greater impact. Again, more than double.

2 Other colors of LED (PC amber and narrow-spectrum amber) have lower impact than white (comparable to high-pressure and low-pressure sodium, respectively), and offer many of the advantages of LED without further degrading dark skies. But these are rarely being used in outdoor lighting. The industry is claiming, and too many dark sky supporters are accepting, that we must have white light.

LEDs and energy savings

Though LEDs offer opportunities for modest energy savings, it appears in many of the most-publicized examples energy savings arise primarily because lighting levels are being reduced. In the example above on LA’s Ventura Blvd, average illuminance was reduced from 2.99 to 1.76 fc – a 41% reduction3. That’s a real energy benefit, but most of it comes from  reduced lighting levels (available using any light type!), not from increased energy efficiency of LEDs. If LA had reduced light levels 41% while retaining HPS lamps (or PC amber LEDs with an HPS-like spectrum), they would have reduced energy use but also reduced light pollution by 41%, instead of increasing it 60%.

This is not a characteristic of just the LA retrofit, either. An analysis by Kostic et al. and published 2013 in Lighting Research and Technology concludes:

[T]he average energy savings when using LED instead of high-pressure sodium (HPS) luminaires amount to 19–26% for single-sided, staggered and opposite layouts, although they are frequently negligible if mesopic effects are not included. The total costs of the LED lighting solutions, even including mesopic effects, are 1.36 to 6.44 times higher than those of the comparable HPS lighting solutions. Therefore, LEDs are questionable for street and roadway lighting.

So the conclusions are clear – switching to white LEDs from the current yellow light technologies using sodium lamps saves only a little energy, much less than is being touted, unless lighting is reduced. But sky glow light pollution will increase. Dramatically.

3 The needed lighting level, and no more, should always be used – with any lighting technology. It is hard to understand why LA had Ventura Blvd illuminated to 3.99 fc, as that level is higher than any professional lighting recommendation.

How could LEDs help dark skies?

  • First and foremost, use yellow LEDs (not low-CCT white LEDs)! Narrow-spectrum amber with a spectrum similar to LPS (0.4x the sky glow impact of HPS), or PC-amber with a spectrum similar to HPS (1x the impact of HPS) should be used, or at worst filtered warm-white LEDs (1.5x the impact of HPS). These are not the most efficient LEDs at present, but can be a good tradeoff to balance energy and dark sky impacts.
  • Take advantage of LED dimming technologies to dim or turn off lights when activity or traffic levels justify it.

…And for any light type:

  • Only light roadways when a full assessment of costs (both fiscal and environmental) and benefits, including fiscal, environmental, and safety, shows overall benefits.
  • Always use fully shielded fixtures.
  • Use the lowest illumination level possible.



We are completing an overall analysis of light pollution impacts and energy claims of prominent U.S. LED retrofits, which we will post here soon.

To celebrate, promote, and protect the glorious dark skies of Flagstaff and northern Arizona.