Red Screen
Fills the whole screen with red, #FF0000, and opens with the dimmer already turned well down - a bright red screen defeats the point of a red screen. Turn it lower still with the slider until it is only just readable in the dark you are sitting in.
Press Esc at any point to come straight back to this page.
Where a dim red screen earns its place
- At a telescope. Star hopping, focusing and averted vision all depend on eyes that are fully adapted to the dark, and one glance at a white phone screen costs you most of that adaptation.
- On a boat or in a cockpit at night, where the instruments and charts are traditionally lit red for exactly this reason: you have to be able to read them and still see out.
- Anywhere you need a little light without waking anyone. A dim red tablet on a bedside table or in a hallway is enough to move around by and far less jarring at three in the morning than a white one.
- In a darkroom, as ambient color while you work with paper that is not sensitive to red. Treat it as ambience, not as a safelight - see below.
Why red preserves dark adaptation
Your eyes have two kinds of light receptor. Cones handle color and detail and need a reasonable amount of light. Rods handle low light, need roughly twenty minutes in the dark to recover most of their sensitivity and about forty to reach all of it, and hold almost no advantage over cones at the deep red end of the spectrum. Dim red light is therefore bright enough for cones to read by while leaving the rods largely undisturbed, so you can look at a chart or a screen and still see the faint things afterwards. The two words doing the work there are dim and red. A red screen at full brightness floods the rods anyway and undoes the adaptation you were protecting, which is why this page starts dark and why the dimmer matters more than the color does.
What the dimmer actually is
It is a black layer over the red, at whatever opacity you choose. No web page can reach your hardware brightness, so this is the only kind of dimming a browser can do - and for this use it is the useful kind, because it takes the red down without touching anything else on the machine. Turn your operating system brightness down as well before a real night session: the two together get far darker than either alone, and on an OLED screen dimming at the hardware level is what actually reduces the light coming out of the panel.
A note for darkroom use
A proper safelight is a filtered lamp with a known spectrum, chosen for the paper in front of it, and a screen is not that. A display makes red by driving its red subpixels, and depending on the panel it can leak enough elsewhere in the spectrum to fog sensitive paper over a long exposure. Use this for ambience, for finding your way around the room, or with paper you have already tested it against - and run a fog test with a coin on a sheet before you trust it with anything you care about.
Sources
Frequently asked questions
Does a red screen really protect night vision?
Dim red light does, and it is a well established effect: the rod cells your night vision depends on are least sensitive at the red end of the spectrum, so red light at low intensity interferes with them least. But intensity matters more than color. A phone showing full brightness red is still a lot of light, and it will cost you adaptation. Get the brightness down first, then make it red.
Why does this page open already dimmed?
Because on every other color page a full brightness start is what you want, and on this one it is the opposite. If you are opening a red screen you are almost certainly in the dark already, and a sudden full red flash is exactly what you came here to avoid. The slider is there to take it lower still, or to bring it back up if you are using this in daylight.
Is this the same as night mode or a blue light filter?
No. Night shift, night light and similar features warm the colors of everything on your screen while you keep working. This replaces the screen with a single flat red field and gives you nothing to work with, which is the point: it is a light source and a background, not a reading mode.
Can I use this on a phone at the telescope?
Yes, and it is probably the most common way it gets used. Turn the phone brightness right down before you open it, and if you are on an iPhone remember that Safari cannot hand a page the whole screen - you will still have browser chrome at the edges. Adding the page to your home screen gets you closer to a full screen, and putting the phone face down between looks costs nothing.