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.