Brightness, room light and tired eyes
There is no correct brightness setting. There is only a setting that matches the room you are sitting in, and that target moves several times a day.
Advice about screen brightness usually arrives as a number: set it to this percentage, or this many candela per square metre. Those numbers are close to useless on their own, because comfort depends far less on how bright the screen is than on how bright it is relative to everything around it.
Why relative brightness rules
Your pupils respond to the overall light level in your field of view, not to the screen alone. In a bright office they close down, and a dim screen becomes hard to read. In a dark room they open wide, and the same screen becomes a glaring rectangle. The panel has not changed. Your eyes have.
Repeatedly adapting between a bright screen and a dark surround is what produces the ache people describe as eye strain: the muscles controlling the pupil and the lens work continuously instead of settling.
The paper test
The most reliable calibration available to anyone costs nothing.
- Open a blank white page — a new document, or any page that is mostly white.
- Hold a sheet of ordinary white printer paper next to the screen, lit by the room you are actually working in.
- Adjust brightness until the white on screen and the white on paper look about the same.
If the screen looks like a light source, it is too bright. If it looks grey and lifeless next to the paper, it is too dim. When they match, the display is behaving like a well-lit page rather than a lamp, and your eyes stop working to reconcile them.
Under office lighting the test usually settles somewhere between forty and sixty per cent. Near a sunlit window it may need eighty or more. In a dark room at night it often falls below twenty, and sometimes below the panel's own minimum — which is the subject of a separate article.
Fix the room before the screen
Several room problems masquerade as brightness problems:
- A window behind the screen. You are looking straight into a bright background. No setting fixes this; move the desk or fit a blind.
- A window behind you. Reflections land on the panel and force brightness up to compete. Angle the screen away.
- Overhead lights reflecting off a glossy panel. Tilting the screen a few degrees usually clears it.
- A dark room with one bright rectangle. Add a low, warm lamp aimed at the wall behind the display.
Things that help beyond brightness
Text size matters more than most people admit. Leaning towards a screen because the type is small produces neck and eye fatigue that no display setting addresses. Windows scales the entire interface at Settings › System › Display › Scale, and using it is not an admission of anything.
Distance helps too: an arm's length from a desktop monitor, with the top of the screen at or slightly below eye level, so the eyelid covers more of the eye and it dries less quickly.
And the oldest advice in the field remains the best supported: every twenty minutes or so, look at something roughly twenty feet away for about twenty seconds. It relaxes the focusing muscle and prompts a few blinks, which is most of the battle.
What brightness does not cause
A bright screen can make you uncomfortable, tired and headachy. There is no good evidence that ordinary display use causes lasting damage to the eye. Blue light in particular has been the subject of a great amount of marketing and rather less clear science; its most credible effect is on sleep timing rather than on eye health, which is covered in the article on colour temperature.
Persistent pain, blurred vision or headaches that do not resolve when you leave the screen are worth taking to an optometrist or doctor rather than to a settings panel. An uncorrected prescription is a very common cause of symptoms that people blame on their monitor.