A recent dark sky project has had me thinking and researching light pollution, sky glow, and by extension an optical phenomenon that we can experience on most clear days or nights. Rayleigh scattering is an interaction of light with Earth’s atmosphere. It’s most notable characteristic is it scatters lower wavelength light colors (violet and blue) much more than higher wavelength light (green, yellow, orange, and red). The effect is dramatic and dependent on the amount of atmosphere the light attempts to cross, and the spectral content of the light.
The scattering dependence on wavelength is very pronounced. The formula to calculate Rayleigh scattering is inversely proportional to the fourth power of the wavelength or 1/λ4 . One source stated that blue light is scattered 9 times more than red light.
Here are some familiar examples of Rayleigh scattering that happen to be some of the most beautiful sights in nature:
Blue sky
Sunlight on a clear day shines through the atmosphere and the blue light is scattered significantly through the sky, continuously bouncing off of particles in the atmosphere making the sky glow blue all across the sky dome. In reality, violet light is scattered more than blue, but the sun creates less violet light than blue, and the human eye is weak at perceiving violet light, while very good at seeing blue light. Green, yellow, orange, and red light scatter less than blue. These effects combine on a clear day to create the radiant blue sky that most people love. The image above is Rayleigh scatter as a function of wavelength. As the following examples demonstrate the atmosphere can act as a blue blocking filter in certain situations.
Sunset and sunrise
When the sun sets on a clear evening, another spectacular display of Rayleigh scattering gradually unfolds. The sunlight appears white when the sun is overhead and traversing a relatively short length of atmosphere. Even then enough blue light scatters to create the blue sky. But as the sun sets and approaches the western horizon, sunlight has to cross through an increasing amount of atmosphere as the sun starts grazing the atmosphere “edge on.” The increasing distance through the atmosphere gradually scatters more and more of the blue, then green light. At this point the sunset is yellow / golden. As it sets further, yellow light is scattered / filtered, and the remaining light appears orange. In it’s final slip over the horizon, even the orange light is scattered, and we’re left with the deepest red of the sunset. The sunrise is the exact same phenomenon in reverse sequence: red to orange to yellow to white.
Image: A recent sunset, in Central Pennsylvania.
Moon rise and moon set
You’ve likely noticed that a large moon on a clear night appears golden on the horizon when rising, and can look the same when setting. It is the same Rayleigh scattering, but the color range is truncated because it is not direct sunlight, but rather indirect sunlight reflected off of the lunar surface. This moon light has less red and orange content, and so they don’t feature as prominently when the moon is on the horizon. But the “yellow moon” is a sight.
Blood moon during some eclipses
When the sun, Earth, and moon align, with the Earth in between the sun and moon, another Rayleigh scattering occurs. Specifically, when the alignment isn’t perfect, a significant amount of sunlight grazes through the Earth’s atmosphere “edge on” and then hits the moon “behind” the Earth. In some cases, this can happen on both sides of the Earth, doubling the effect. The long length of “edge on” atmosphere filters out all but the red light, which passes through the atmosphere striking the moon. This is the blood moon during some eclipses.
The disruptive side of Rayleigh scattering
Unfortunately, all of the beautiful phenomenon above are not the only times Rayleigh scattering is visible. If significant blue light is introduced to the sky at night, the scattering also occurs. Unfortunately, in our rush to save energy (a good thing) and convert outdoor lighting to LED, we have also dramatically increased the blue light entering the sky, especially in more densely populated urban areas. Our phosphor-converted blue pump LEDs are extremely efficient, but create a lot of blue light especially at cooler/higher CCTs. This rapid LED adoption is a large reason why light pollution, as measured by satellites, is increases roughly 10% per year, globally. The blue from many LED lights is scattered by the atmosphere at night and creates a glow that we see as sky glow. For those of us in rural areas, this is much more prominent in the directions of cities on the horizon.
As most people in the lighting industry know, sky glow has many negative effects on plants, animals, and humans. It’s the blue content in LEDs that contributes most to sky glow due to Rayleigh scattering. There are a number of simple ways to reduce this blue light from LED lighting at night:
- Lower the light output. Less brightness is less blue light.
- Lower the CCT to reduce the blue light content in outdoor lighting. The lower the CCT, the more the blue light content is reduced, and the smaller the contribution to sky glow.
- Reduce light directed into the atmosphere using full cut off luminaires.
- Reduce the hours that outdoor LED lighting is on at full output, through the use of controls like dimmers, time clocks, or IoT automation that turns off lights after a certain time in the evening.
These four strategies or various combinations of these strategies dramatically reduce blue light scattered at night, creating less sky glow.
The next time you see a gorgeous blue sky, sunset, yellow moon, or blood moon, think about Rayleigh scattering and remember that blue LED light at night is the largest contributor to sky glow and light pollution.
You must be logged in to post a comment.