Key takeaways
- Your retina has cones for daylight and colour and rods for near darkness. Rods are bleached by bright light and need 20 to 30 minutes to recover.
- Rods are most sensitive to blue-green light and almost blind to deep red, so a dim red light lets you read without bleaching them.
- Turn your phone and camera screens down, use a red mode, and say "light on" before you switch on a white torch.
- Look slightly to one side of a faint object. This averted vision places its image on a part of the retina that is rich in rods.
A red light protects your night vision because the rods of your retina, which do the seeing in near darkness, are almost blind to deep red. A dim red torch lets you read your settings without undoing the 20 to 30 minutes your eyes need to adapt.
Step outside on a dark night, and at first you see almost nothing. Give it a quarter of an hour, and the sky fills with stars you did not notice before. Nothing in the sky has changed. Your eyes have.
Two kinds of light sensors
The retina contains two kinds of cells. Cones work in daylight and see colour. Rods are far more sensitive and work in near darkness, but they see no colour. In bright light the rods are bleached by the light and effectively switched off. When you move into the dark, they gradually recover their sensitivity.
| Feature | Cones | Rods |
|---|---|---|
| Work best in | Daylight and bright light | Dim light and near darkness |
| Colour | Yes | No |
| Number in each eye | About 6 million | About 120 million |
| Most sensitive to | Yellow-green light, around 555 nm | Blue-green light, around 500 nm |
| Where they are | Concentrated in the centre of the retina | Everywhere except the very centre |
| Recovery in the dark | A few minutes | 20 to 30 minutes, and slowly more after that |
How long does dark adaptation take?
| Time in the dark | What your eyes are doing |
|---|---|
| 0 to 5 minutes | The cones adapt first, and you still see very little |
| 5 to 10 minutes | The rods take over, and the first faint stars appear |
| 20 to 30 minutes | Most of the improvement has happened |
| 40 minutes to an hour | A slow further gain towards full sensitivity |
Most of the change happens in the first 20 to 30 minutes, and it keeps improving slowly after that, so you can gain a little more over an hour.
Why red light is different
Rods are most sensitive to blue-green light and are almost blind to deep red. That is why a dim red torch lets you read a map or check your settings without bleaching the rods, while a white light does the same damage as a flash of daylight. It is the reason astronomers, sailors and pilots have used red lights for generations. The red toggle at the top of every page of this website uses the same idea. It turns the whole page red so that you can read it outside without spoiling your eyes, and AstroNight offers a red interface in the same spirit.
There are two cautions. Red light is not invisible to the rods, so a bright red light still bleaches them, and the light must stay dim. And in dim light the eye shifts towards blue, a change called the Purkinje effect, so red things look darker than they do by day, and a chart printed in red ink can vanish under a red light.
Which lights help and which harm?
| Light source | Effect on dark adaptation | What to do |
|---|---|---|
| A white torch, or a phone at full brightness | Undoes much of your adaptation in seconds | Avoid it |
| A dim white phone screen | Still harmful | Switch to a red mode |
| A dim red torch | Very little harm | Best choice, kept low and pointed at the ground |
| A camera screen | Brighter than it looks in the dark | Turn the brightness down, or cover it |
| Car headlights | A strong flash for everyone nearby | Park away from the observing area |
| The red light switch on this website | Shows the page in red only | Use it when you read outside |
Habits that keep your night vision
- Turn the brightness of your phone as low as it will go. A dim red screen is far better than a dim white one.
- Cover the screen of your camera, or turn its brightness down, because the small screen is brighter than you would think.
- Choose a torch with a red mode, and keep it pointed at the ground and never at another person's face.
- Warn your friends before you use a white light. A courteous word, such as "light on", is the custom among stargazers.
- Avoid car headlights if you can, and park so that they do not point at the observing area.
A well adapted eye is the cheapest and most sensitive instrument you will ever own.
Use the side of your eye
The centre of the retina has no rods at all, which is why faint objects seem to vanish when you look straight at them. Looking slightly to one side, a technique known as averted vision, places the image on a part of the retina with many rods, and the object brightens. Try it on the Andromeda Galaxy or on the Pleiades, and the difference is startling.
A note for photographers
Your camera does not care about your eyes, but you do. A well adapted eye lets you see the real sky, check your framing, notice a drifting cloud and enjoy the night itself, which is why many of us are out there in the first place. It is just as useful on a meteor shower night, when you may spend hours looking up.
Frequently asked questions
How long does it take for your eyes to adjust to the dark?
Most of the adjustment happens in 20 to 30 minutes, and your sensitivity keeps improving slowly for up to an hour. A bright white light can undo much of that progress in moments, so protect your night vision once you have it.
Why do astronomers use red lights?
The rods in the retina that work in dim light are almost blind to deep red. A dim red light lets you read charts and camera settings without bleaching the rods, so your eyes stay adapted to the dark.
Does red light ruin night vision?
Much less than white light, but not never. Red light is still visible to the rods, so a bright red light will reduce your adaptation. Keep it dim and pointed at the ground.
Is a red filter on my phone screen enough?
It helps a great deal, as long as the brightness is also turned down as far as it will go. A red screen at full brightness is still far brighter than the dark-adapted eye is used to.
What is averted vision?
It is the habit of looking slightly to one side of a faint object. The centre of the retina has no rods, so the object disappears when you look straight at it, but an off-centre glance places its image where there are many rods, and it brightens.
Sources and further reading
AstroNight is now on the App Store. The free planner and coach for astrophotography works on iPhone and iPad with iOS 17 or later, offline, with no account. Get it on the App Store.
