Red light and dark adaptation

Full dark adaptation takes twenty to forty minutes and one glance at a white phone screen resets most of it. Use deep red light, as dim as you can work with.

Two mechanisms are at work, on very different timescales. Your pupil opens within seconds, which is the part people notice. Underneath it, the light-sensitive pigment in your rod cells regenerates far more slowly, and that is the part that matters for faint detail.

The vision literature puts numbers on it. Cones adapt in roughly five to seven minutes but are not much use in very dim light. At around ten minutes there is a "rod–cone break" where the rods take over, and peak sensitivity arrives somewhere between twenty and thirty minutes. Rhodopsin rebuilds to about half in five minutes but needs close to half an hour to saturate. Bright light bleaches it essentially instantly, which is why one careless glance costs you so much.

Red light works because rhodopsin is relatively insensitive to long wavelengths, above roughly 650 nm. That lets your cones do the reading while the rods stay dark-adapted, which is why observatories, aircrews and submariners all standardised on it.

Brightness matters as much as colour. A bright red torch is still bright. The goal is the dimmest light that lets you do the task, which is usually far dimmer than people expect.

Before observing, choose Observing in the GoodSeeing header. On compact screens, use the theme button to cycle through Dark, Light and Observing. Observing applies a red-spectrum treatment to the page and reveals a Display dimming slider; your theme and dimming preference are saved in this browser when local storage is available. Lower the device brightness as well, and use the lowest setting that still lets you read comfortably.

The webpage cannot dim your browser controls, notifications, lock screen or other apps. Display hardware and brightness settings still matter, so this is not a guarantee of preserved dark adaptation or a claim of zero blue-light emission. Set up the display before going outside, keep bright notifications out of the session, and use enough light to move safely.