What do astronomers and the military know about blue light (that no one is talking about)?

Here’s a fun fact. I grew up at an observatory. My dad was not an astronomer, he was the Telescope Operation Manager in charge of keeping the telescopes in working order for astronomers. We moved up there when I was 8 years old. It was an interesting childhood for sure.

Photo by Jim Maurer | https://www.flickr.com/photos/schaffner/

I remember one night, some friends and I roaming around the mountain top. We were probably playing some kind of ninja game sneaking around the domes.

Anyways, at some point an astronomer heard us snooping around and stepped out to investigate. He started shining his flashlight around and instead of a white beam of light, it was red. Wow! We thought it was pretty cool and didn’t realize at the time that red flashlights are standard equipment for astronomers.

In fact standard LED flashlights are banned from observatories at night.

Why?

Because of the massive amount of blue light they put out.

When we talk about blue light, we are usually talking about eye health and sleep problems. It's well known that blue light at night shuts down melatonin production and distorts our natural circadian rhythm. But there is a completely different reason that astronomers shun blue light after dark.

Three reasons actually:

1) Blue light interferes with their data

Telescopes need to operate in darkness. Blue light in particular is damaging to data collection. There is a reason that telescopes are often in remote locations. It’s because the darker the environment, the better their data.

2) Blue rich outdoor lighting is accelerating light pollution and rapidly blotting out the night sky

Many observatories are struggling with light pollution. Observatories located anywhere near a city was witnessing a disturbing deterioration of the night sky that is accelerating year after year. This is not just terrible for data collection. It is also a terrible tragedy for future generations. As it is, most people living in cities only see the night sky when vacationing. If the trend continues we will completely loose our connection with the cosmos and further disconnect us from the nature world. This is an important issue that we will be talking about in the future.

3) Blue light destroys night vision (even at low levels)

Moving around in a dark working environment safely requires your night vision to be on point. It takes at least 30 minutes to fully adjust to the darkness. People over the age of 70 need upwards of 40 minutes to adjust. All it takes to ruin that is a brief flash of blue rich white light. The bluer and brighter the light, the stronger the effect. The pupils immediately restrict to block out the offensive light and it will take another 30 minutes to readjust to seeing in a dark environment.

Photo by Daniel Gies | https://www.flickr.com/photos/daniel_gies

Soft red light has virtually no effect on night vision

When you go into a dome at night you will see that pathways and stairs are marked with dim red lights. Astronomers and support staff carry red flashlights to get around. They often have regular lighting in the control room but they keep it contained in there with a double door isolation system.

Out on the telescope floor, only red illumination is allowed. By using red lighting at night, astronomers can keep their pupils dilated and their observation data safe from local light pollution.

Photo by richieb56 | https://www.flickr.com/people/133879356@N08/

The military also makes use of red light to preserve night vision

Pilots often wear red lens goggles prior to taking off at night so that they can see outside their aircraft. The cockpit makes use of soft red illumination and indicators to avoid compromising night vision.

Submarines use red light when coming to periscope depth at nighttime. Their vision needs to be adapted to the dark in order to view their surroundings outside of the boat. I’ve heard that they also use red lights strategically for circadian rhythm management of their crew.

What can we learn from this?

If you need to use a light in the middle of the night make sure that it is a red light.

Imagine this:

You wake up in the night in your dark room. Your night vision is at peak sensitivity.
Any sudden change in lighting level can ruin it. Even a small amount of blue rich light will destroy your night vision.
You will also be more sensitive to the circadian disrupting effects of blue light simply because your pupils are fully dilated and will allow more of it to hit the back of your eye.

Walking into a bathroom lit with a white night light is a bad idea!

By using red light at night you will:

preserve your night vision making it easier to navigate around the house
be protected from the disruptive effects of blue & green light allowing you to naturally drift back to sleep

We have a red night light in our hallway bathroom. If I wake up in the middle of the night, I take off my eye mask. My pupils are fully dilated and I can navigate out of our dark bedroom. Red light spills out from the bathroom into the hallway making it easy to see. The red night light is plenty bright yet does not cause pupil restriction. Upon returning to my room I can still see well enough to get back to bed and fall effortlessly back to sleep.

But what happens if your bedroom is lit up with LED streetlights streaming through the windows?

Your night vision will be compromised. If you go into a dark bathroom lit with the same red night light, it will seem much dimmer. That's because your pupils are not fully dilated. The longer you are in there, the more your pupils will open up as your eyes adapt to the darkness. If you are in there for 10 minutes or so and come out, your light polluted bedroom will look very bright! Your pupils will be more dilated and allow much more disruptive light in.

We recommend using red night lights in strategic areas and getting your room as dark as possible. Using a red night light in a light polluted room is counter productive and the light may not seem like it is bright enough. Blue-rich light pollution can over power a small red night light.

Action Items

Get your bedroom dark! Use blackout curtains and cover any LED indicator lights.

Use only red lights for late night illumination.